11. Marine Acoustic Techniques

11. MARINE ACOUSTIC TECHNIQUES

F. N. Spiess Marine Physical Laboratory Scripps Institution of Oceanography University of California, San Diego La Jolla, California 92093

1. Introduction : The Ocean Acoustic Environment A variety of sonar systems have been found to be useful in marine geology and geophysics. Several types will be treated in this section from the perspectives of their design and use. Whether they are swath mapping echo sounding, bottom imaging, or acoustic positioning systems, their forms and their limitations are driven primarily by the characteristics of the mediaocean water and seafloor-with which they interact. Sound absorption, background noise, sound velocity, and the nature of the seafloor all play major roles in controlling system design and operation. Sound absorption, in conjunction with geometric spreading (modified by refractive effects) as acoustic energy travels through the water, determines how much of what is transmitted will be available for any useful purpose, while the background noise provides the competitive sound field within which the desired signal must be detected. The nature of the seafloor determines the fraction of the transmitted sound that will be redirected by reflection and scattering to return to the receiver, bringing with it some element of information of use to the seagoing geologist or geophysicist. The speed of sound enters in a ubiquitous manner, determining the wavelength for any given frequency, setting the parameters for measurement of travel time, and, by its spatial variability, controlling the directions in which the sound may go. All of these characteristics have been the subject of much research and will only be treated briefly here. More detailed treatments can be found in textbooks on this subject (see, e.g., Urick, 1983). The systems to be treated in this chapter all operate at frequencies above about 1 kHz-a distinction usually recognized in geological and geophysical literature by calling them sonars, while those operating below 1 kHz are dubbed seismic systems. The line of demarcation is not clear, and the terminology really reflects a much earlier time when the only useful acoustic I1 METHODS OF EXPERIMENTAL PHYSICS Vol. 24, Part B

Copyright 0 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

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energy available for studying the earth’s crust was that originating from earthquakes, while active underwater acoustic systems (the original “sonars”) were emphasizing the ultrasonic regime. As sound travels through seawater its intensity decreases as energy spreads out in space, most simply in inverse proportion to the square of the distance from the source. This energy is not lost, it is merely diluted. At the same time, however, there is actual dissipation, with three different mechanisms controlling the process, each being dominant in its own particular frequency range. From the lower frequencies up to nearly 10 kHz the absorption is caused primarily by the presence of boric acid [B(OH)3], whose dissociation has a time constant such that energy-absorbing shifts in its equilibrium cannot take place in times shorter than a few tenths of a millisecond. From just below 10 kHz to considerably above 100 kHz a similar but more complex multistate dissociation process associated with magnesium sulfate (MgS04) in solution in the sea produces the energy losses. Finally, above a few hundred kilohertz, seawater behaves in the same manner as fresh water, with energy dissipation resulting from viscous effects. Figure 1 shows the resulting situation as summarized by Fisher and Simmons (1977). In each of the three regimes the absorption per unit length of path increases about as the square of the frequency, with a less rapid increase in each transition zone. These values are relevant at 1 atmosphere, but they do, indeed, exhibit a dependence on pressure. Over the range with which we will be concerned, laboratory measurements show that the absorption decreases by a factor of per atmosphere, without measurable change in the relaxation 8.1 x frequency of the controlling MgS04 dissociation process (Hsu, 1981). For a high-resolution sonar, operating near the seafloor at 80-100 kHz, the attenuation would be about 20 dB/km at 1 atmosphere and only 12 dB/km at a normal deep ocean depth of 5000m. This change is thus quite substantial. In the lower-frequency portion, say at the 12 kHz typical of echo sounder and transponder operating frequencies, the attenuation is more like 1 dB/km at 1 atmosphere and 0.6 dB/km at 5000 m. The background noise against which the reflected or backscattered energy must be detected and measured is of three kinds : ambient noise, platform noise, and flow noise. The term ambient noise is used to describe the background acoustic field that would exist if the sonar and the vehicle on which it is mounted were not present. For frequencies from 1 to about 50 kHz this background is produced primarily at, or very close to, the sea surface by the action of the wind and related effects (e.g., air turbulence, bubbles, and hydrodynamic processes associated with breaking waves). While the mechanisms involved are not well understood, the underwater acoustics community has generally relied on the World War I1 work of Knudsen and

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FIG.I . Sound absorption in seawater as a function of frequency (Fisher and Simmons, 1977).

others (Eckart, 1946) for its description. The classic Knudsen curves based on omnidirectional hydrophone data show spectral levels at any particular sea state decreasing with a slope of 5 dB per octave over this frequency range and increasing from a 10-kHz value of 28 dB at 1 micropascal ( pPa) in a 1-Hz band at sea state 0 to 45 dB at sea state 2 and 50 dB at sea state 4. While these

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curves are usually used in setting sonar design parameters, it is clear that the situation will vary with water depth, proximity to shore, sonar directional sensitivity, receiver operating depth, etc. In particular, at the high-frequency end, although no observational data are available, one will expect that the surface-generated noise will be absorbed by the intervening water column, with appreciable resulting reductions in spectral levels above 10 kHz at depths of a few kilometers. These effects are usually estimated simply by applying the laboratory-measured attenuation (see, e.g., Hsu, 1981) for the appropriate path length to the Knudsen spectral levels. Depending on sea state and operating depth there will be a transition from surface-generated noise to simple thermal noise, which will dominate from about 50 kHz on up in frequency, rising at about 6dB per octave. This represents the temperature-dependent absolute lower limit on the input to any underwater acoustic receiving system. Most acoustic systems are operated from a ship, submersible, towed body, or other type of moving platform. In these cases the actual background against which signal reception must take place may not be the ambient noise of the ocean but the local noise generated by motion of the vehicle and by its propulsion and auxiliary machinery. These noise fields are extremely vehicle-specific, involving a wide range of sources : bubbles (generation, oscillation and collapse), turbulence, structural vibrations, unbalanced rotating machinery, diesel engine and turbine noises, etc. (Ross, 1976). In an ideal world these aspects would be measured and included in the sonar design or even controlled during the design of the vehicle itself. In the real world, however, the oceanographic ship is usually built with scant consideration for sonars and the sonar design only takes the noise into account in a very general manner (usually by choice of operating frequency, since higher-frequency systems are less noise-prone). This leaves only two major degrees of freedom : choice of where on the vehicle the receiving elements should best be placed and introduction of signal processing to mitigate the effects of the noise. On most ships the three major sources of interference are the obviously noisy pieces of machinery (whose noise will enter both through the water and through the surrounding structure), the propulsion units (propellers, thrusters), and bubbles generated primarily at the bow but often swept along under the ship. Placement of receiving transducers as far as possible from these noise sources becomes a matter of compromise, since they are usually numerous and distributed from the bow thruster to the stern propeller. Detailed measurements under realistic operating conditions provide the only sensible approach. These are not always easy to make since they involve placement of acoustic receivers having relevant frequency and directional responses in locations that may not be easily accessible. When such measurements are made they may also include observations relevant to the bubble

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sweepdown problem, which has been an important limitation for some hullmounted hydrophone array systems (e.g., USNS Wyrnan). A measurement program of this type was, for example, undertaken on a number of oceanographic research ships as a prelude to installation of Sea Beam multibeam swath mapping echo sounder systems (Tyce, 1980). Both the bubble sweepdown problem and the direct production of pressure fluctuation noise due to turbulence incident on the transducer faces can be mitigated to a considerable extent by the use of sonar domes or fairings within which the water is quiet-the noise sources are thus moved away from the immediate vicinity of the active elements. Highly directional systems using large transducers or distributed arrays have some advantages in this context, since the nearby bubbles or turbulence field will produce noise which is not coherent over the entire receiving surface ;thus their contributions will add powerwise at the different sensing elements, rather than in proportion to amplitude, and there will be a signal-to-noise gain. Ship operating conditions can also be manipulated to reduce interfering background, particularly if lower-speed or lower-power operation can be countenanced, since most of these effects increase with speed and with engine load. Some pieces of machinery or structure will have resonances which produce strong acoustic or vibrational outputs under particular combinations of speed and loading and which can be avoided by judicious choices of load vs. rpm. Under some conditions (particularly when small ships are involved) it is desirable to tow the acoustic transducer system in order t o provide both remoteness from ship’s machinery and, by operating the sonar somewhat deeper than the ship’s hull, to reduce the effects of turbulence and bubbles. With regard to towed systems in general the first-order problem usually is produced by flow-excited mechanical vibration of various fittings on or close to the vehicle. These sounds can usually be eliminated at the source by careful checking of the condition of vehicle appurtenances before each run. The most difficult point to quiet is the actual attachment fitting between the tow cable and the vehicle, since good practice generally requires swivels and pins to prevent the oscillatory motions of the tow body from destroying the wire at the connection. Fairings similar to sonar domes should be used to shield transducers at towing speeds of more than a few knots. For small tow bodies these can most easily be built to encompass the entire vehicle and provide some drag reduction as well. The principal design consideration from the sonar viewpoint is to push the boundary between the outside ocean and the water trapped in the fairing as far from the transducer elements as possible. Finally, one can resort to signal processing techniques to eliminate identifiable noise components. This is particularly effective in combating impulsive,

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spiky noise or very steady single-frequencynoise sources. Essentially one can fairly easily reduce the contributions which have time constants either very short or very long in comparison to the pulses which constitute the signal. The first step in this process is the use of receiving filters with bandwidths matched to the signal. Beyond that one can introduce limiters to combat very short noise impulses and notch rejection filters to eliminate very steady narrowband components such as may be produced by rotating machinery. Although strictly speaking it is not a type of noise, any active seafloororiented sonar must detect its signals to some extent against reverberation of its own transmission from other parts of the surrounding environment, or at the very least the operator must be aware of such effects and eliminate them at some phase in processing the data. Volume reverberation and reflections from the vehicle are (with one exception noted below) generally only a problem if the sonar is operated close enough to the seafloor that echoes are expected to return within a few milliseconds of the transmitted pulse. Under such circumstances the only responses are to use a low enough transmitted power or receiver gain that the combination of signal and reverberation lies within the dynamic range of the system and to use directionality to reduce the input from directions other than those from which the signal is expected to return. Interfering volume reverberation from more distant sources arises primarily from patchy distributions of biological material in the water. The principal predictable sources are the deep scattering layers which are present in most of the world’s oceans at daytime depths from tens of meters to a few hundred meters. With a few exceptions the scatterers in these assemblages migrate on a diurnal basis, being close to the surface at night and making their transitions up and down during the evening and morning twilight periods. Although the number of scatterers per unit volume is small, they are sharply stratified in depth, so with a conventional broadbeam echo sounder they generally produce a fairly well-defined horizontal trace. Unless the water depth is close to that of the scattering layer it is possible to gate out the return from these layers and effectivelyeliminate their contribution. Since the scatterers are small and few in number per unit volume they do not appreciably attenuate the sonar rays which traverse them. Fish, either singly or in schools, also constitute important sources of volume reverberation. These sources usually can be perceived and eliminated since they are very localized in space. They can, however, be dense enough when schooling to deflect an appreciable fraction of the incident energy and thus obscure geological features below them. Reverberation from the sea surface is potentially a much more severe problem, since the air-water interface is such a good reflector of sound. Fortunately, in this instance the use of directional transmitters and receivers acts to reduce the direct aspects of this effect by several orders of magnitude

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in most systems. In shallow water, however, the effect still must be taken into account because appreciable energy will be received after having gone to the bottom, back to the surface, and then reflected a second time to come from below into the down-looking directional receiver. This is not a problem for echo sounding since one is only concerned with the first return and the remainder can be ignored. If one is looking for subbottom features or for backscattering to image seafloor roughness at longer ranges, these twice (or even thrice) reflected returns may be stronger than the direct signals from the features of interest, and thus will interfere strongly with system operation. The only recourse here is based on the fact that, with the water depth and a clean version of the first reflection, one can knowledgeably subtract the power introduced into the further returns. The speed of propagation of sound waves in the ocean varies from point to point depending on the local pressure P , temperature T, and salinity S , increasing as any one of these increases. Empirical relationships between the sound speed and P , T, and S are based on laboratory measurements. The resulting equations have evolved over the years as the quantity and quality of the measured data have improved. The best formulations available at this time are those of Lovett (1978), based primarily on laboratory measurements by Wilson (1960) and Del Grosso (1974), with input from at-sea sound propagation data as analyzed by Anderson (1971). Of the three equations he derived, Lovett prefers one which requires a third-degree polynomial in T, a second-degree polynomial in P , and is linear in S, plus six cross terms involving various combinations and powers of T, S , and P . It is estimated that, over the ranges of variation encountered in the world’s oceans, this equation should approach an accuracy of 1 part in lo5. The magnitudes of the various effects can be obtained by inspection of the leading terms. For near-freezing water of normal salinity near the sea surface the velocity is approximately 1450 m/sec with a temperature effect of about 5 m/sec per degree Celsius, a salinity effect of 1.33 m/sec per part per thousand, and a depth effect of about 0.016 m/sec per meter; at 6000 m depth the temperature effect increases to about 5.5 m/sec per degree Celsius, salinity 1.36 m/sec per part per thousand, and depth about 0.018 m/sec per meter. In nearly all situations relevant t o use of sonar systems in marine geology and geophysics the spatial variations in sound speed are primarily in the vertical; thus in any given region and time, the effects of sound velocity variation can be characterized on the basis of the nature of vertical profiles of sound speed measured directly or, more often, by the profiles of temperature and salinity. For near-vertical paths this means that the primary effect will be on the acoustic travel time or the effective sound velocity. If the vertical coordinate is z and the sound speed c(z), then the effective velocity (often called the sounding velocity) E between z = 0 and z = 21

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is given by l / c = 1/21

s:

dZ/C(Z) .

If high accuracy is required one must naturally recall that pressure measurements are often used as a description of depth and that the two quantities (because of variations in water density caused by temperature and pressure and by the variation of the earth's gravitational attraction with depth) are not strictly proportional to one another (Saunders, 198 1). Various versions of the sounding velocity and the relationship between c and T, S, and P have been used in marine geological work. The most common are the Matthews tables (Matthews, 1939). Matthews divided the oceans of the world into approximately 50 areas and tabulated the effective sound speeds and resulting sounding corrections as a function of depth for each area. In the 1960s Wilson's equation (Wilson, 1960) was used, and today the equations recommended by Lovett (1978), discussed above, seem most appropriate. For cases in which propagation occurs at a significant angle from the vertical, the effects of the vertical sound velocity gradient affect the directional aspects as well as travel time. The result can be the production of shadow zones in the vicinity of a boundary (surface or bottom) when the velocity decreases as one moves away from the bounding surface. The simplest situation occurs near the seafloor when the water is isothermal and isohaline. Then the velocity increases at a rate of about 0.018 m/sec per meter (at 5000 m depth). In a region of constant gradient all the resulting rays are arcs of circles and it is easy to calculate the range to the seafloor point at which the shadow begins. For a source a distance h off the bottom, sound speed co at the level seafloor, and a sound speed gradient g (c = co - gz, where z is the distance measured upward from the seafloor), the horizontal distance to the shadow zone is r = 2coh/g

We will return to this point in Section 4. From a system design viewpoint the speed of sound enters in a very fundamental manner since it sets the relationship between frequency and wavelength and thus has to do with the limits on angular resolution. The simplest form of the resolution criterion is the thumb rule that the angular width of a transmitted or received sonar beam is, in radians, the inverse of the number of wavelengths across the transducer or hydrophone array. Given an approximate sound speed of 1500 m/sec, this means that a 1-m aperture results in a beamwidth of about 60" at 1.5 kHz and 0.6" at 150 kHz. The

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range of variability of sound speed in the ocean is so small that for most situations it can be ignored in this context. Unlike the acoustic properties of the water, those of the solid earth are quite diverse, following from the more complex and varied nature of the materials involved. Even though geology/geophysics-oriented sonars are concerned only with the uppermost few hundred meters of the earth beneath the sea, this diversity of properties has led system designers simply to use a worst-case approach and has led users to ignore all but the simplest types of acoustic measurements (e.g., travel times and directions as opposed to quantitative aspects of reflected or backscattered energy). The amount of energy reflected or backscattered from the seafloor or from beneath it depends obviously on the incident energy levels, but also on the angle of incidence (controlled by gross bottom slope), local roughness of the reflecting interface, and actual material of which the various layers are composed. Of these three the easiest to relate to some relevant “ground truth” is the seafloor material, which is available from various types of coring activities. Relationships between physical parameters of sediments and the properties of sound velocity and attenuation have thus been investigated intensively by a few workers. A useful starting point for anyone entering this field is embodied in two journal issues resulting from a workshop on this topic held in 1979 for the geology/geophysics/underwater acoustics community by the U.S. Naval Ocean Research and Development Activity (Gorsline and HoIcombe, 1980). The keynote paper (Hamilton, 1980) summarizes existing knowledge of relationships between sediment composition and porosity and the acoustically important properties of density and sound velocity, including estimates of sound velocity gradients beneath the seafloor. From the sound speed and bulk density, the reflectivity of a smooth, plane seafloor can be computed (Rayleigh, 1894). At normal incidence this can be written as (p2C2 -

p1C1)/@2Cz

+ p1C1)

where pc is the acoustic impedance-the product of density and sound velocity. The ratio of sediment sound velocity to water velocity in abyssal areas, as given by Hamilton and Bachman (1982), ranges from 0.98 for clays in a red clay zone to 1.04 for calcareous clayey sand. The range of densities is much larger (1.2-1.6) and thus, considering the scatter in measured values, to a good approximation there is alinear relation between the acoustic impedance and sediment density ranging from 1.8 x lo6 to 2.3 x lo6 kg/m2 sec. Using pc = 1.5 x lo6 kg/m2 sec for the overlying water gives a range of normal incidence reflectivities of 0.008 to 0.04, representing losses at the reflecting surface of 13-21 dB, independent of frequency.

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While these numbers provide a useful guide, the real seafloor is not usually devoid of fine-scale roughness, which introduces frequency-dependent effects. For example, in moving from a bare siliceous sediment deep-sea location to a nearby manganese nodule-covered region the normal incidence reflectivity at 4.5 kHz increased by 8 dB (Spiess et a/., 1984). Although the process has not been applied appreciably in marine geology, the frequency dependence of the average reflectivity in an area can, in principle, be used to determine the probability distribution function of the elevations over a rough seafloor (Clay and Medwin, 1977). The variations of acoustic impedance with depth may be such as to define layers with clear interfaces from which internal reflections can take place in the same manner as at the water/sediment interface. In addition, in some sediments there may be smaller-scale variations whose spatial separation can match the wavelengths of the incident sound, giving rise to constructive interference and apparent interfaces whose depths depend on the acoustic frequencies involved, as will be discussed further below (Mayer, 1979).

Since backscattering aspects other than normal incidence are of importance in some geological sonar applications (e.g., side-looking sonars and swath mapping sounders), the backscattered energy for a variety of grazing angles is of interest. Again, there is scant information available. Urick (1983) summarizes results from a number of sets of shallow-water observations, while McKinney and Anderson (1964) give a detailed account. The most recent contribution on this topic, by Spiess and Weydert (Weydert, 1985), provides data for bare sediment and manganese nodule-covered areas over the frequency range 9-160 kHz and grazing angles from 15" to 90". Attenuation of sound traveling through marine sediments plays a role (along with the strength of successive reflectors) in determining the depth of penetration of sound into the sediment. The theoretical treatment of this problem by Biot indicates that attenuation in water-filled porous sediments should increase as frequency squared for low frequencies and as the square root of frequency at the high end. Experimental evidence over the range of interest here indicates a constant loss per wavelength (attenuation proportional to frequency) (Hamilton, 1980). Oceanic data measured at 4 kHz give values of 0.38 dB/m in the Southern California Borderland, 0.19-0.28 dB/m on a variety of deep-sea clays, and 0.12 dB/m in a carbonate sediment region (Tyce et al., 1980). The variability of these seafloor properties (scattering, reflectivity, sound velocity, and attenuation) and the lack of unique relationships between all of them and the actual nature of the seafloor point up both the strengths and weaknesses of sonar systems in a geological context. They are most effective when several sonar types are used in combination with other observations

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(e.g., photography, bottom samples). They can then be used to determine the extent of regions having common properties and can provide guidance to the most useful places to take samples or photographs.

2. Echo Sounders In their simplest form, echo sounders are among the most familiar instruments to almost everyone who goes to sea, yet their very simplicity leads to complexities in interpretation and a drive toward more sophisticated forms. They also provide good examples of the applications of most underwater acoustics and sonar design principles. If one sets out to design a sounder or to select one to buy from among the alternatives available, some questions must immediately be answered : over what depth range must it operate, what degrees of space and time resolution are required, and how should the data be displayed and logged? These will be followed by other questions concerning navigation and data processing. Choice of depth range in general dictates the acoustic frequency at which the system will operate. The frequency dependence of sound attenuation in seawater dictates that lower frequencies will be transmitted more effectively than higher ones. Unfortunately, the nature of acoustic transducers is such that as one goes down in frequency the available bandwidth (and thus time resolution) decreases and the transducer size required for efficient generation of the in-water signal increases. In addition, the natural acoustic background noise and the noise from one’s own ship both increase as one goes lower in frequency. A further effect is that the transducer array size must be greater at lower frequencies to achieve a given transmit and receive beamwidth for spatial resolution and noise rejection. Over the years of use of sounders certain operating frequencies have survived and, without any particular legislative or scientific decision, have become standard. Shallow-water systems tend to be in the 30-and 60-kHz regions, while deep-ocean equipment is almost invariably very close to 12 kHz. The low-frequency member of the family is about 3 i kHz, sacrificing spatial and temporal resolution in favor of ability to penetrate into the seafloor sediments and reveal internal reflectors in the subbottom. Since both the terrain to be mapped and the operating conditions of the survey ship are quite variable, the parameters of pulse length (and related receiver bandwidth) are left to be varied at the user’s discretion. This makes it possible for the inevitable trade-off between signal-to-noise ratio and range resolution to be in the operator’s hands. A short pulse (high resolution) must be matched with a corresponding broad frequency transmit and receive band (approximately the inverse of the pulse length for simple systems). The broad

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receive band means higher background noise, and thus under adverse weather or ship’s machinery operating conditions one will lengthen the pulse and narrow the receive band in order to have adequate signal-to-noise ratio at the expense of resolution. An alternative which has been used in a few systems is to transmit a longer-duration signal which occupies a broad band and cross-correlate a replica of the transmission with the returning echo with resulting improved signal-to-noise ratio without loss of time resolution. An aspect of time resolution which is often taken for granted in today’s systems was the subject of one of the first deep-ocean echo sounder system improvements following the intensive sonar development work of World War 11. Investigations of the extensive abyssal plains in the Atlantic Ocean led workers to realize that a slow drift in the echo sounder’s time base could bias the ability to determine the degree to which these surfaces are level. With today’s reference frequencies good to 1 part in 10’ over long periods, the limitations on zero-slope determination arise primarily from variability of the sound velocity profile and tidal effects. In spite of numerous attempts to build signal recognition systems which would allow direct digital encoding and logging of the round-trip acoustic travel-time data, the primary display and storage approach for single-beam systems is some form of wet or dry paper facsimile recorder. In most installations the recorder provides the trigger signal for the output pulse and records the intensity of the signal out of the receiver. A stylus sweeps across the recorder paper at a rate chosen by the operator. In most deep-ocean work one pass across the recording paper (30-50 cm wide) is made in 1 sec. As the stylus goes it darkens the paper in relation to the strength of the received signal. Usually the leading edge of the return from the seafloor can be determined to a part of a millimeter, corresponding to depth resolution of 1 m. Higher stylus speeds are usually available to improve accuracy, but only rarely will a deep-water echo be crisp enough to warrant pushing for such resolution. There are two reasons why this recording approach, which requires a subsequent digitizing process before data reduction is completed, is still used. First, there is geological insight to be gained by seeing the detailed structure of the return. Second, with broad-beamwidth sounders there are often several overlapping returns and some judgment is then required to identify the one which most nearly approximates the depth under the ship. It is this aspect which has in general been the downfall of direct digitizing schemes. Narrow-beamwidth systems, to be discussed below, minimize this problem and thus are usually built to include direct echo recognition and digitization of travel time. A considerable body of echo sounder data has been collected using

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transducers with fairly broad transmit and receive beams. These typically are 30" or more, and as a result the systems interact with a seafloor zone which may have a diameter equal to as much as half the water depth. This seafloor area is often called the footprint of the system. The primary reason for using such broad beams was not a lack of understanding or a desire to avoid a requirement for large dimensions (at 12 kHz a 6" beam could be achieved with a 1.25-m-diameter transducer) but rather was the fact that any narrower beam would have to be stabilized to compensate for the ship's roll and pitch. The primary consequences of use of broad beams are loss of lateral resolution and introduction of "side echoes." With such sounders it must be recalled that one is essentially measuring the range to the reflecting surface closest to the ship-not necessarily the distance straight down to the seafloor. The most obvious results of this (beyond loss of ability to delineate features smaller than the footprint) are illustrated by considering two simple extreme cases. The first is a plane seafloor tilted by an angle 6 to the horizontal. In this case every sounding will be shallower than the depth directly under the ship by a factor of cos 6, since the first, specular, arrival will come from a point uphill from the ship where the incident sound strikes the bottom at 90". In a survey made over seafloor approximating this condition the angle 6 can be deduced and the necessary correction made if it is significant. The second simple example is a point reflector (e.g., the top of a small pinnacle). If the ship is maintaining constant course and speed the resulting trace will be hyperbolic. A variety of computational schemes can be used to convert successions of such hyperbolae into a better approximation of the real topography ;however, all of them require substantial assumptions since there is no sure way (considering the local variations in specular reflectivity) to deduce how far to the side of the survey track each reflector may be. The most direct means for coping with these effects is to reduce the beamwidth of the sounder and in some manner solve the resulting beam stabilization problem. The first systems to do this used approximately 6" beamwidths and a gimballed support in order to maintain the transducer orientation approximately along the vertical. The transducers, while notably larger than the conventional broad-beam ones, were not unmanageable. The most common of the systems in use today have beamwidths in the 2-3" range and are electronically stabilized to a vertical reference gyroscope. These are installed on a number of U.S. survey and research ships. They do not have the full dimensions required of a simple circular disk transducer, which would be between 2.5 and 3 m in diameter. Instead, they use the crossed line (Mills cross) approach. Several small transmitting units are mounted under the ship to form a fore and aft line about 3 m long, and a second, similar line of receiving units is mounted athwartships. The relative phases of the 12kHz signals at the several transmitters are adjusted to approximate the time

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delays for the individual units in order to maintain the major, fan-shaped lobe of the resulting beam pattern in a vertical, athwartships plane in spite of the pitching motion of the ship. The elements of the transverse receiving array are fed into a delay line phase-shifting network, which is continuously adjusted in accordance with the vertical reference to maintain that fan beam in the vertical fore-and-aft plane. The resulting beam pattern intersection concentrates on a patch 2-3" square on the seafloor directly below the ship. While this does not have as good side lobe suppression as would be the case for a completely filled aperture for both transmit and receive, it represents an effective practical compromise in terms of numbers of transmitting elements and processing complexity. This approach will be discussed further below in connection with swath mapping sounders. One other approach which is, again, an approximation to the ideal is particularly useful at lower frequencies, where transducer dimensions become a problem. This involves the application of finite-amplitude acoustic techniques. While the detailed treatment of this topic is fairly complex, the principlesare straightforward (Naval Undersea Systems Center, 1984). If one transmits two very strong collinear beams of sound at frequenciesf~and f2 into the sea there will be appreciable second-order effects in the water near the transducer which produce acoustic energy at the difference frequency fi - f 2 . Whilefi andf2 are generated at the electroacoustic transducer, the difference signal is generated as the two primary ones interact in the water. The result is that the low-frequency signal behaves as if it were coming from a long, low-frequency end-fire line sound source, producing a narrow beam without need for a transducer any larger than that required to produce a narrow beam at the much higher primary frequencies. This effect, though it has poor electrical-to-acoustic conversion efficiency, has been used to produce a mechanically stabilized, narrow, low-frequency transmitted beam, whose reflection from the seafloor is then received in a conventional manner with a nonstabilized, wide beam receiver. In spite of the improvements resulting from narrowing the echo sounder's beam, its primary shortcoming is that, even when receiving energy from points off track to either side, it lacks the capability to do more than provide an estimate of depth along a single line track. Particularly in exploratory investigations it is frustrating to pass clear side-echoes and not be able to determine whether they are to the right or left. Realizing this, and having a requirement to do fine-scale seafloor surveys to provide a base for submarine navigation on bottom topography, the U.S. Navy sponsored the development, procurement, and use of the first effective multibeam swath mapping sonars, which were installed in the survey ships Dutton and Bowditch in the mid-1960s and have been in use ever since (Glenn, 1970). Commercially availableversions of this development have also been on the market for about

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10 years. The first commercial unit purchased for research was installed in the French ship Jean Charcot (Renard and Allenou, 1979) and subsequent installations have now been made on other ships including the R/Vs Sonne, Thomas Washington, Surveyor, Polarstern, Atlantis II, and Conrad. The in-water portion of this system is identical with that of the electrically stabilized narrow single-beam systems described above and installed in survey ships such as the Navy's Kane and Bent. The transmit beam pattern is a single fan stabilized to lie in the vertical/athwartships plane. The multiple receiving units of the transverse array, however, are fed into a multichannel beamformer, which produces 16 beams whose patterns intersect the athwartships transmitted beam at 2 t 0 intervals, generating 16 received signals for each ping sent from the transmitter. This geometry is shown schematically in Fig. 2. An interpolation scheme then acts on the 16 rectified low-passed outputs to produce 15 which are stabilized against a vertical reference gyro and which are set at 0 (vertical) and multiples of 25" on either side. This means that for successive pings the data points are consistent in orientation to the vertical, but, given the roll of the ship, will be distributed unequally with more on one side than on the other for any given pulse. The system estimates the effective time of arrival of the seafloor echo in each beam and calculates the lateral coordinate (relative to the ship's position) and water depth for each of the

FIG. 2. Sea Beam swath mapping sounder system-schematic representation.

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15 signals. These data are recorded digitally for subsequent processing and are used to generate three displays : (1) a conventional single-channel graphic recorder display for the vertical (or any other selected) beam, (2) an oscilloscope display of all 15 points in a form representing a profile of the seafloor across the ship's track, and (3) a computer-generated strip-chart plot of selected contour intervals against time. The speed of advance of the paper can be set to approximate the speed of the ship so that, if the ship holds constant course, an approximately correct strip contour chart is produced in very nearly real time. In addition, the individual sounding values are recorded in digital form and are available for further processing. The nature of the hydrophone array and the beamformer set the fundamental performance parameters of the system. Array dimensions and acoustic frequency (12 kHz) set the size of the footprint from which each data point is derived. To a reasonable approximation this is a patch about onetwentieth of the water depth across (e.g., 300 m in 6000 m of water). Features having transverse dimensions less than this will be only mariginally resolved at best. The beamformer dictates that the centers of the 15 final beams (not necessarily the points from which the effective sonar returns come) are separated by 2f" intervals across the track. This means that the swath width, from the center of one outermost beam t o the center of the other, is 3 7 f " , which translates into a swath width equal to two-thirds of the water depth (4 km in 6 km of water). The fact that both footprint and swath width depend on water depth means that some care must be taken in planning surveys when the topography has substantial relief, since the swath width may vary strongly from one part of the area to another. For example, if a seamount has an elevation equal to half the depth of the surrounding water, then the swath width as one crosses the crest will be only half that at the base, and additional passes must be made in the vicinity of the peak to obtain full coverage. The beamformer also sets a limit on the pulse repetition rate since it cannot tolerate the local reverberation which occurs whenever a new pulse of sound is transmitted. This means that the repetition period must be somewhat greater than the longest round-trip travel time for any beam in the group, and the distance traveled by the ship between pings becomes, like the footprint and the swath width, roughly proportional to the depth, with the constant depending on ship's speed. Experience with R/V Thomas Washington indicates that speeds in the 10-12-knot (5-6-m/sec) regime (depending on sea state) can be used. With a 10-sec repetition period the data density along the track is then about five times the across-track density. Like any other complex system, this one can, under some circumstances, create artifacts (Kleinrock et al., 1984; de Moustier, 1985b). In this system those that have been identified arise because of the overlap of energy from a strong specular reflection into the other beams. Although some efforts are

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made within the system circuitry to subtract out these effects, they are not always successful. The result is that occasionally, over a flat, highly reflective seafloor, the returns in the outer beams are biased toward an early return and the resulting depths appear shallower than they really are. Also, if the nonspecular backscatter is weak and the background noise is high (e.g., due to bubbles going past the receiving array in heavy weather), the system will generate its own soundings within the restrictive range gate used to eliminate unrealistic returns. Another type of artifact arises when the track orientation relative to sloping topography is such that the specular return is from a point forward or aft of the athwartships zone covered by the set of receiving beams. The most obvious situation occurs when the ship is traveling at right angles to the contour lines. There is then a tendency, under some circumstances, for the center beams to receive some energy from the specular (uphill) return and thus to be biased in the shallow direction. Since there is a tendency to orient survey tracks across the strike of major features in order to take advantage of the somewhat higher along-track data density, this means that the results should be watched carefully and if an along-track central ridge emerges its presence should be verified by a swath made with the ship’s track lying along the contour lines. Given today’s technology, real skill (insight and experience) is required t o carry out an effective open-ocean bathymetric investigation ; the reason is simple-lack of high-precision navigation. Whether using a conventional single-beam echo sounder or a swath mapping system, one must control the ship’s track primarily by dead reckoning during the data collection period in such a way that key features are not missed and adequate ties between lines are made, all without wasting precious ship time on excessive overlapping coverage. Once the data (soundings, satellite fixes, and ship’s courses and speeds) are in hand, there is a postprocessing operation in which the best possible (and most logical) adjustments are made to the originally estimated ship’s track and speed to bring various sounding lines or swaths into consonance with one another. Since one has close to complete coverage in many Sea Beam surveys, this is both a challenging and a rewarding task. Given the much larger amount of data it is essential that this task be computerized and a number of groups have developed the necessary capability (Renard and Allenou, 1979; Tyce, 1986; Abbott et al., 1986). The need for these skills should be markedly reduced with the advent of the newly emerging Global Positioning System (GPS). This system will provide continuous navigational data with substantially smaller uncertainties than the Sea Beam footprint. An initial feel for this possibility can be obtained from the results of a survey made in 1982 by R/V Thomas Washington in the eastern equatorial Pacific with a GPS system on board. Since only a fraction of the total planned number of satellites are in

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operation, there was good coverage for only a few hours each day. During those periods, however, swath sounding data were obtained on tracks in rough terrain which required no adjustment to produce full agreement where swaths crossed or overlapped one another. Conversion of travel time into distance (depth) has generated problems in the use of echo sounder data. First, the original data are actually measurements of travel time even though they are nearly always expressed as depth. Charts made with acoustic depths are usually labeled “uncorrected.” When the measurements are made with instruments using metric dimensions, time is translated into depth with a conversion factor of 750 m/sec (round-trip travel time). When the sounder is calibrated in English units (usually fathoms) the conversion factor is 400 fm/sec. This means that if one wants to translate between charts made on one type of instrument or the other, before taking any account of the actual sound speed profile, one must use 750/400m (acoustic) per fathom (acoustic) or a factor of 15/8 (1.875) rather than the 1.8288 used to convert real fathoms to meters. One must therefore be particularly suspicious of “uncorrected” charts in which the units of depth (travel time) are not those in which the data were initially described. The second problem arises when depths are corrected. The nature of the correction process is usually not described in the chart itself, but is more often given in some obscure part of the accompanying paper. In much of the literature the approach is to use the easiest way out, which is to convert with the Matthews tables (Matthews, 1939) described in Section 1. This at least has the advantage that it is relatively simple for the users to convert back to travel time and then make the corrections with their favorite hydrographic data and sound velocity formulation. Fortunately, in most geological interpretive situations the shapes of features are the most important aspect, and these are usually not appreciably distorted by the conversion process. When a particularly accurate actual physical depth is required (as might be the case for installation of a critically designed mooring), it is best to return to the original acoustic data, use the best available hydrographic data relevant to the time and place of the observations, and calculate the conversion with one of the equations given by Lovett (1978). Ifthehydrographicdataareofgoodqualitythisshouldresult in a conversion good to at least 1 m, which in general will be as good as the accuracy of the sounding or knowledge of the open-ocean tidal effects. Low-frequency single-beam sounders (3-4 kHz typically) have provided most of the high-resolution information we have with regard to the sediment cover on the seafloor. Again, the choice of operating frequency is a compromise between penetration (since attenuation for sediments increases roughly in proportion to frequency in this range) and range resolution (which decreases, for a variety of reasons, about inversely with frequency). These systems, in most oceanic sediments, will provide penetration of 50-200 m,

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depending both on the nature of the attenuation of sound and on the strengths of the reflectors (seafloor and internal) since any single strong reflector robs the layers below of available acoustic energy. The much lower frequency (10-200 Hz) seismic reflection and refraction systems, striving for penetration as opposed to resolution, are treated elsewhere in this book, since they involve a large lore with regard to both hardware (sound sources and receivers) and data processing which differs considerably in detail from that of the higher-frequency systems treated in this chapter. It is of historic interest to see how design choices for sonar systems of this kind have been made. As the 12-kHz single-beam systems were perfected and used widely it became apparent that their signals were, in some areas (particularly the so-called transparent sediment regions in the western Pacific), penetrating into the sediments and revealing at least one internal reflector. At the same time, work with impulsive sources (seismic reflection systems) was revealing layered structure with implications of more detail than these low-frequency systems could resolve. As the desire for penetration with good resolution grew, one might have expected a variety of systems t o emerge. For example, an octave decrease in frequency from 12 to 6 kHz would have represented a substantial penetration gain with modest loss of resolution. Systems in the 1-2-kHz regime would have represented, as the geometric mean between 12 kHz and 100 Hz, the most obvious attempt to fill the gap. At that time, however, the U.S. Navy began t o build a series of long-range active antisubmarine sonars operating at about 3+ kHz for design reasons only indirectly related to marine geology and geophysics. Since this sonar was expected to make use of propagation paths reflected from the seafloor (bottom bounce sonar), acoustic properties of sediments at this frequency became very interesting to both underwater acousticians and the Navy. Transducers for 3t-kHz sounding became easily available and the choice of the intermediate between 12 kHz and 100Hz was made on pragmatic grounds. The interpretation of records from these broad-beam single-channel systems requires additional care beyond that required for the higherfrequency sounders. It should not be assumed without question that all the subbottom traces seen on a 3t-kHz sounder record represent discrete internal reflectors, even though that is usually the case. Two other types of reflectiongenerating processes must always be considered. The first is the possibility of reflections from nearby surface features. In most cases this effect can be recognized by the skilled interpreter from the shape of the trace as it develops in time in relation to other reflectors representing layering in the sediment. When operating in conjunction with other sonar systems (side-looking sonars or swath mapping sounders), the existence of the surface features which could produce side echoes of this type can often be established independently.

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The second interpretation problem is more subtle. These sonars typically use pulses of the order of 4 or more cycles long. If the sediment has layered variations of acoustic impedance with a vertical scale comparable to or somewhat less than the acoustic wavelength, there may be some depth range in which the resulting multiplicity of small echoes will add coherently and produce a return resembling that from a discrete layer. This effect was investigated in detail by Mayer (1979) using the Marine Physical Laboratory Deep Tow system and transponder navigated coring. With the assistance of Hamilton and co-workers (1982), he determined the variations of acoustic impedance in carbonate sediment cores. Convolving the profile of impedance vs. depth in sediment with a replica of the Deep Tow 4-kHz sounder output pulse then produced a reflection sequence which matched the Deep Tow data taken at the coring site. The existence of sediment having these properties can be recognized by shifting the sounding frequency and looking for a change in the pattern of reflectors. This was done using the Deep Tow 4/6-kHz capability in a region of terrigenous sediment on the continental side of the East Pacific Rise near 20"N, with the result shown in Fig. 3. In principle, the return from a multifrequency sonar could be inverted to obtain the acoustic impedance profile in the sediment in such regions. This might then be related to short-term variations in the geological situation (e.g., successive episodes of glaciation-Mayer, 1979). One final aspect of echo sounding may become important in the geological context (no treatment of scattering layers, fish schools, and other biologically important phenomena will be included in this chapter). In general the systems INTENSITY

EQUIV. PLANE WAVE PRESSURE 0 Uiz

0

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I

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20

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W A V E LENGTHS

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,

8

I

,

t

~

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FIG.3. Near-bottom seismic records at 4 and 6 kHz showing different patterns over the same continuous profile.

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discussed above are designed and operated to optimize the determination of round-trip travel time. In fact, the Sea Beam system makes the judgments with regard to travel time internally and separates the user from any knowledge of the intensity and structure of the echoes, except for the one beam selected for conventional graphic recorder display. On the other hand, it has been known for many years, starting at least with the work of Hersey and Breslau in the 1950s (Breslau, 1967), that correlations can be made between the echo structure and energy and the nature of the seafloor. A recent example of this occurred in connection with a Sea Beam topographic survey of a manganese nodule-covered area at 15"N in the eastern Pacific. Auxiliary equipment was provided to record the nature of the returning echoes digitally. As a first step in using the data, the level of the maximum return for any beam from each pulse was plotted. In spite of limitations of dynamic range in the echo processor, the intensity map clearly delineated both nodule-covered and bare mud areas (de Moustier, 1985a) as later determined by Deep Tow near-bottom observations (Spiess et al., 1984). Similar observations have been made at several sounder frequencies by other groups (see, e.g., Sumitomo, 1982). The Sea Beam system appears to be a particularly good candidate for making intensity measurements since it allows determination of local bottom slope and includes backscattered returns over a range of angles of incidence. Combining both intensity and reflection angle dependence should provide additional insight into the geological nature of the seafloor, particularly when the data are used to interpolate between or to guide taking of bottom photographs and samples. A comprehensive approach to the use of intensity data from Sea Beam sounding systems has been started by de Moustier (1985b), including both angular dependence and statistical analyses of the fluctuation of the returns.

3. Bottom-Imaging Sonars The seafloor typically exhibits variability in texture and slope. These combine to produce contrasts in acoustic backscattering properties which can be viewed as sonar images in much the same sense as optical images. While the analogy is basically valid, the inherent properties of the two media are such that there are profound practical differences between photographic and acoustic pictures. The combination of attenuation of high acoustic frequencies in the ocean (limiting most geologically relevant systems to below 1 MHz) and the propagation speed of underwater sound (setting the correspondingly shortest wavelengths in the 2-mm range) leads t o a much more gross angular resolution for practical aperture sizes than is the case for optical systems. The low propagation speed, however, makes it quite easy

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to incorporate range measurement ; thus, in contrast to usual optical images in which both dimensions are essentially angular, sonar images usually have angular resolution along one axis and range resolution along the other, making them more comparable to radar imagery (although the latter is useless under water). Most underwater acoustic systems used for imaging in marine geology and geophysics employ a single-beam approach, moving the beam across the terrain either by sweeping in azimuth (scanning sonars) or by directing the beam to one side and letting the forward progress of the vehicle produce the sweep (side-looking sonar). Typical beam patterns are narrow in the horizontal direction and broad in the vertical (approximations to the patterns of horizontal line transducers or arrays) and operate at rather low grazing angles, using a fairly short pulse. The resulting pictures to some approximation present a plan view of the seafloor. The side-looking sonars are generally preferred as a complement to other survey or mapping systems (echo sounders, subbottom profilers, magnetometers, etc.) since they are better matched to a continually progressing track than the azimuthally scanning systems. In this sense they are quite comparable with echo sounders, although the goal in this case is to reproduce the variations of energy backscattered from the transmitted signal rather than to accentuate the timing of a few discrete arrivals. The fact that energy is expected to be returned over a considerable range interval dictates that the primary signal processing included in these systems is time-varied gain (TVG). The simplest of the TVG circuits compensates for the expected change of backscattered sound intensity with range from some particular assumed height off bottom and seafloor properties, while others simply provide for manual adjustment of the TVG sequence curve to match particular operating conditions. More sophisticated versions rely on digital processing, changing the gain on a point-by-point basis. For example, in the Sea MARC I (Chayes, 1983) the TVG is generated and stored using a microprocessor and compensates for the inevitable deviations of the transducer’s vertical beam pattern from the ideal, as well as for the expected changes of received intensity as a function of range. An adaptive approach was implemented in the Deep Tow system on an experimental basis by simply digitizing each return, averaging from ping to ping (with correction for varying height off bottom), and displaying the difference between each individual return and the long-term average. The most common method of diplaying side-looking sonar data is through use of a facsimile recorder. In its simplest form this is done by using the backscattered signal (after time-varied gain) to control the darkening of the trace for each successive stylus sweep across the recorder paper. In this case, as in a photographic negative, the dark areas of the image represent regions

1 1.

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U

INTENSIFIED RETURN

FIG.4. Schematic diagram of side-looking sonar concept. The upper portion is a section through the ocean at right angles to the vehicle’s track and in the plane of the sonar’s narrow beam. As the transmitted pulse travels outward it first strikes the seafloor directly below the vehicle, returning the initial portion of the prolonged echo, which continues as the wave front expands. An intense return comes from the reflection from the face of the small ridge. After that there is no return until the reverberation begins again as sound strikes the bottom at the far edge of the shadow. The strip in the lower portion of the figure shows how this would look on the graphic recorder display, with the paper darkened in relation to the strength of the signal returning at any given instant after the outgoing pulse is transmitted. The approximate height h of the ridge can be calculated from the shadow length S,the total range R from sonar to far edge of shadow, and the height D of the sonar off bottom, as h = SD/(R + S ) .

producing strong returns and light areas correspond to “shadows” or regions of low backscatter. Some systems, however, utilize an intermediate step and produce hard copy corresponding to a photographic positive, with lighter zones representing the “brighter” returns. One must thus take some care when looking at a record from an unfamiliar system. Interpretation of side-looking sonar imagery requires insight and experience, and benefits greatly from the existence of complementaryinformationparticularly echo sounder traces and bottom photographs. The first sonars of this type were designed to detect mines in harbors or channels and relied strongly on the fact that objects projecting above the smooth seafloor would cast a shadow and detection would be made on the basis of the resulting high contrast in the image. Figure 4 illustrates the principle, which is basically different from the situation for a photographic image, since the shadow in the sonar case lies in the range dimension of the image rather than the angular one. The distinction is most obvious when one realizes that in the sonar case the sound source and the receiver are at the same position-no shadows would be seen in a conventional photograph having this source/receiver relationship, but there are ranges from which no energy is returned and these are the shadow zones of the sonar picture.

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Shadow dimensions provide the simplest form of quantitative information which can be derived from side-looking sonar images. On an otherwise horizontal seafloor the ratio of the shadow length to the total range from source to far end of shadow is the ratio of the object height to the height of the sonar off the bottom. These dimensions are easily available from the facsimile recorder image, unless it has been rectified by the schemes which will be discussed below. Similarly, the downward displacement of a vertical step can be determined from the ratio of the shadow length to the slant range from source to leading edge of shadow, times the sonar height off bottom. Certain qualitative aspects are apparent-a strong reflection immediately preceding a shadow usually represents a feature projecting above its surroundings, while a shadow immediately followed by a strong reflection usually represents a depression in the terrain. Any simplistic interpretations, however, must clearly be tempered with geological insight from other sources. A very good example of this is given by Fig. 5 . As we first began to see this image from our Deep Tow side-looking sonar, we imagined barchan dunes, with the white zones as the shadows behind them. Examination of the complementary narrow-beam Deep Tow sounder traces, however, showed no relief of the steepness one would expect from the shadow characteristics ; with the aid of transponder-navigated bottom photographs, the final interpretation was that the areas of light return (white zones) were produced by the low backscattering from the very low relief dune materials, lying as crescentic wisps of sand on a much higher backscattering pavement (Lonsdale and Malfait, 1974). A second example is drawn from a survey in a manganese nodule-covered area in the equatorial Pacific. Figure 6 shows images in which there are large blank areas. These are not shadows but are instead patches in which there are no nodules exposed on the seafloor, and the returns come only from the local clay surface, which has a much smaller backscattering coefficient than the surrounding heavily nodule-covered areas (Spiess et al., 1978). In short, one must be sure to take into account the possible existence of large contrasts in backscattering due to seafloor textural variability as well as the effects of shadowing and changes in bottom slope associated with topographic irregularities. When one is considering inclusion of side-looking sonar as an element of a seafloor investigation, it must be realized that all systems are by no means equivalent. The two most important parameters to consider are range (swath width) and beamwidth (along track resolution), and these vary substantially from one system to another. Swath width in particular can vary significantly depending on the nature of the features one expects to see. For example, the Deep Tow SLS (Spiess and Lonsdale, 1982) can image contrasts in manganese nodule coverage out t o ranges of 300-400 m, while large outcrops or

1 1. MARINE ACOUSTIC TECHNIQUES

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FIG.5. Side-looking sonar image of barchan dune-like sediments on manganese pavement (Lonsdale and Malfait, 1974).

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100

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300

Range. rn

400

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FIG.6. Side-lookingsonar imagery over a manganesenodule field, illustratingthe delineation

of bare patches. Dark areas represent strong backscattered returns and light areas are low intensity (Spiess et a/., 1978).

bare features at spreading centers will produce useful reflections from 600-700m and large pieces of wreckage have been detected at a range of nearly 1 km. Along-track resolution again is a function of range, since most of today’s systems are operating in a far-field mode at the longer-range end of their capability and thus resolution will decrease (patch width increase) with range in proportion to the angular width of the beam. This in turn is controlled approximately by the ratio of the acoustic wavelength to the width of the SLS transducer. Again, for example, the Deep Tow SLS, with an 80-wavelength-long transducer, has its transition from near field to far field

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at about 70 m and has an along-track patch width of 5 m at 400 m range and 10 m at 800 m. At short range the along-track patch size can usually be taken to be of the same order of magnitude as the length of the transducer. This leads, for simple systems, to a limitation on area coverage rate if one desires full coverage, since one pulse must be sent out every time the vehicle moves ahead one patch width, and only one pulse can be in the water at a time. If vehicle speed is u, patch width w , speed of sound c, maximum range R , and rate of area coverage a, then (for a two-sided system operating at a low grazing angle) w- 2R _ v

c

and

a 3 2vR = cw

Thus any combination of maximum range and vehicle velocity which will give full coverage results in the same rate of area coverage. This basic search rate law can be circumvented by use of a side-looking sonar equivalent of a multibeam system in which the transmitted signal is sent out with a rather broad beam and several adjacent receiving beams are used (either from separate receive transducers or formed electronically by inserting the various segments of the receiving array into a beam-forming matrix). In the case of n beams the speed of advance can then be 2R ncw

u=-

and

a = ncw

At the long-range end of the distribution of existing SLS systems are the deep-ocean, near-surface towed units. Only two such systems exist at this time. One is a series of systems called Gloria, built and operated by the U.K. Institute of Oceanographic Sciences (10s) starting in the late 1960s. The 1980 version operates with a 4-sec FM pulse at a frequency of about 6.5 kHz and uses a transducer about 5 m long (resulting in a 2.7" beamwidth), towed 20-50 m below the sea surface, with a maximum swath wdith (two sides) of 60 km. This system and its predecessors have been used in almost every ocean of the world and have been particularly effective in delineating the large-scale fabric of the seafloor (see, e.g., Searle and Hey, 1983). The other long-range system is a new arrival (Sea MARC 11), built by International Submarine Technology (IST) for the University of Hawaii in 1982 (Blackinton ef al., 1983; Hussong and Fryer, 1983). Its operating frequency is 11 kHz on one side and 12 kHz on the other, its transducer length is 3.8 m, and from an operating depth of 50-100 m it covers a swath out to a range of 5 km on each side in 5 km of water. Both systems record their data in digital form for postprocessing and provide for a real-time graphic recorder display. The Sea MARC I1 SLS includes a phase difference swath mapping capability of a type

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which will be discussed later in this section. Long-range antisubmarine (ASW) search sonars can, under some conditions, be used as long-range side lookers simply by training them abeam, or selecting the returns from the 90" or 270" directions out of the scan with appropriate sampling switches (Andrews and Humphrey, 1980). A number of SLS units have been built as parts of deep-ocean near-bottom search and survey systems. The earliest to our knowledge was a short-lived single-purpose search system built by Clay and others at Columbia University's Hudson Laboratories at the time of the search operations for the submarine Thresher (Spiess and Maxwell, 1964). In 1966 our group at the Marine Physical Laboratory added an SLS subsystem t o our already existing Deep Tow complex. This system is of the simplest possible design, transmitting simultaneously at 110 kHz to both sides and displaying the returns directly on a graphic recorder after application of a simple, operatoradjustable TVG. Its 3/4" beamwidth limits smearing of a point reflector to less than 10 m at its extreme range of about 800 my and it generates a useful backscattered return from typical deep-sea sediment out to a range of at least 300 m at a 10" grazing angle. Buchanan's group at the U.S. Naval Research Laboratory added a side-looking sonar system to their near-bottom search system in the late 1960s; that system was retired from use, however, about 10 years later. The two remaining Navy systems containing SLS are the longstanding Naval Oceanographic Office Teleprobe and the recently procured STSS based in San Diego at Submarine Development Group I. Both of the Navy systems operate at about 160 kHz. One other deep-operating SLS system is Sea MARC I, built by IST for Ryan's group at Lamont-Doherty Geological Observatory. This is an intermediate-range system (3 km for strong reflectors) operating at about 30 kHz with a long enough transducer to produce a beam 1.7" wide in the horizontal plane (Chayes, 1983). While the deep-operating side-looking sonars are usually specially designed and built as parts of more complex multisensor seafloor/benthic boundary layer search and survey systems, a large number of single-purpose shallow-operating SLS systems have been manufactured (by Klein, EDO, Sonatech, and Interocean, among others). These typically operate at frequencies of the order of 100-200 kHz and have beamwidths ranging from 1/2 to 3" or more; again, one must take some care in the integration of any of these into a research or survey situation to be sure that the resolution and swath width are matched to the goals of the planned operation-mere specification of "side-looking sonar" may not result in a cost-effective system selection. When side-looking sonars are operated on the same vehicle as other survey devices the simple choice of maximum range may not be the most appropriate. Often the other instruments merely produce single-line data (e.g., magnetometer, subbottom profiler) and the line spacing may be chosen

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on the basis of area complexity t o produce useful maps of the magnetic field or subbottom which may be compatible with the choice of a shorter-range, higher-resolution SLS. More than any other sonar system type, side-looking sonars include on-line processing between the output of the acoustic receiving transducers and the nearly real-time displays. The first step is the time-varied gain (discussed above), which can compensate for attenuation, spherical spreading, variation of seafloor backscattering properties with grazing angle, and variations of acoustic transmit and receive sensitivity as a function of depression angle, all of which lead to variations in output intensity with range which may differ as the sonar height off bottom changes. A second step arises because in the simplest displays the distance scale is different for the two dimensions on the recorder image. Across the recorder image the scale is set by choice of stylus sweep speed and the speed of sound, while along the image it is set by the recorder paper feed rate. This leads to the introduction of some systems for adjusting the paper feed rate (either manually or automatically) in relation to the vehicle’s forward speed so that the along- and across-track distance scales are approximately the same. The third processing step which may be introduced compensates for the fact that, in a simple system, the across-track image is generated initially as the slant range from the sonar to each element of the seafloor. Since one is essentially interested in a plan view image of the seafloor one must be able, mentally, to make an appropriate transformation between the simple slant range image and the real seafloor. In some systems this is facilitated by making a “slant range-corrected” presentation of the image. This is done by assuming that the seafloor is a horizontal plane and moving each pixel of the original image to the somewhat shorter range it would have if it were at the proper horizontal distance from the track under the plane assumption. Thus if the sonar is at height h off bottom, a point at slant range r is moved in to a point corresponding to an off-track distance x, using the relationship x= As long as the seafloor is, in fact, approximately horizontal, this is a useful thing to do. Some care must be taken, however, if there is a significant departure from this assumption. For example, if there is an appreciable across-track slope (say 8, not always known), then the correct transformation equation is

e.

x = (r2 - h2)”2 cos 8 f h sin 8

Here h is the “height” based on the travel time for the initial arrival and x the across-track distance measured from the point vertically below the vehicle. The first term transforms the slant ranges into horizontal distances away from the specular reflection point and the second term corrects for the

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amount that the horizontal component of the displacement of the specular point itself differs from the seafloor point directly below the vehicle track. From this viewpoint the ideal situation is to know the actual depth difference between the vehicle and the reflection point for each value of slant range. With that number the correct lateral location of each element of the image can be calculated. The next, and most complex, manipulation is the production of a composite image in which data from adjacent or overlapping sweeps are combined to produce a larger area picture. The simplest approach is to produce a corrected playout from the system with the graphic recorder, plot the vehicle’s track on the appropriate scale, and paste up sections of the recorder playback in their proper relative positions. This approach works best for well-navigated surveys in which the vehicle is either hull-mounted or towed at shallow depth, since under these conditions the track can be controlled to give primarily straight segments. When the vehicle is towed with large scope the track may be less easy to match by this method. The more elegant approach is to work from digitized data. Given the availability of digital equipment, an increasing number of systems now include direct on-line digitization and recording of the output data prior to making any corrections beyond an initial TVG to keep the signal within the (usually large) dynamic range of the receiver amplifiers and analog-to-digital converter. With digitized sonar output on tape this can be merged with the navigated track, vehicle heading, and whatever quantitative topographic data may be available. Each picture element can then be assigned a particular x-y or latitude-longitude coordinate and the result printed out on a high-quality graphic recorder or photographic medium. Digital recording of the uncorrected data is useful from viewpoints other than the production of mosaics. If it is done correctly it can preserve much greater dynamic range than can be encompassed in any graphic recording medium. This makes it possible to replay the data for sections of the record in which the settings of the real-time display were not properly matched to the amplitude or contrast in the signals actually being received. With a good digital record one can use the variety of image processing and display techniques (contrast compression or expansion, differentiation, artificial coloring, etc.) which have become commonplace in analyzing other types of data as well as processing with TVG characteristics different from those used in the real-time display. Looking into the future, the availability of digital outputs should lead users to build and operate their systems in an amplitude-calibrated mode and then to make quantitative use of the output data. As this aspect develops, the absolute values of acoustic backscattering from the seafloor will become

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identified with certain ranges of seafloor composition and roughness and the need for dense ground truth data (e.g., photography and sampling) will decrease with increasing ability to use the inherently more rapid area coverage which sonar systems can provide. A beginning of such analyses, particularly oriented toward manganese nodule resource assessment, has been made by Spiess and Weydert (Weydert, 1985). Sonars which produce images in a more or less horizontal plane by scanning in azimuth can have a wide range in both complexity and cost. Most of the more sophisticated versions have been developed and built for military applications-detection of submarines in the upper layers of the water or mines which may be near the seafloor. They may be hull-mounted or towed and their receiving systems may be multibeam or scanned mechanically or electronically. Mechanical and some electronically scanned systems sweep relatively slowly in azimuth-essentially dwelling within one beamwidth for the entire time used for sound to travel from source to maximum range and back. Other electronically scanned systems transmit energy in a broad azimuthal sector and scan the receiver across that entire sector once for every pulse length. If one thinks in terms of a range r and azimuth t$ plane, for the simple systems the scan is slow in t$ in comparison with r, while in the scanwithin-pulse systems the scan in t$ is rapid relative to that in r. If the sonar beamwidth is $ 0 , pulse length t o , and sound speed c, the time to scan 360°, T, the scanning time, will be limited on the one hand to T > (360"/#0)(2r/c) and on the other to T < t o . From the signal processing viewpoint the latter case is wasteful of the transmitted energy since only the fraction t$0/360is being used to detect what may lie in any given (r, 4) resolution cell. Because of their rather poor match to mapping or large-area surveys (compared with side-looking sonars), azimuthally scanned systems in geology and geophysics have generally been used in rather specialized situations, such as viewing the terrain for navigational or instrument placement purposes (e.g., on small submersibles). Most of the systems used on undersea vehicles have been of the mechanically scanned variety. They are thus essentiallyside-looking sonars whose transducers can be rotated relative to the vehicle. The fact that they are used primarily as navigational tools, plus the fact that they essentially generate a new complete plan view image with every pulse and thus much of the information is redundant from one ping to the next, has led to a display philosophy emphasizing impermanent oscilloscope displays, relying on analog (e.g., oscilloscope photographs, magnetic tape) or digital recording to be able to store information which might be of long-term value. Data processing for these systems has all the options discussed above for the side-looking systems. One further simple sonar is often used from near-bottom vehicles. This is essentially a side-looking sonar turned to point its fan-shaped beam forward,

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providing the operator of a near-bottom vehicle with early warning of any obstacles the vehicle might be approaching. It was in this context that the first of the phase difference bottom-mapping sonars was produced (Nickles and Anderson, 1968). The concept is a simple one. In addition to having a single horizontal line transmit/receive transducer, one uses a second similar element to receive only. The second unit is mounted parallel to the first and separated from it vertically by a known amount, which is at least a significant fraction of a halfwavelength at whatever acoustic frequency may be involved. As the energy returns from various elements of the seafloor one measures and records not only the sound intensity but also the electrical phase difference between the signals as received at the two receiving hydrophones. Knowing the hydrophone separation and the acoustic wavelength, the electrical phase difference can be converted into a measure of the vertical physical angle of arrival of that element of sound relative to the direction of the normal to the plane defined by the hydrophone pair. That is, for spacing s, electrical phase difference y, physical angle 8, and wavelength A, one has sin t9 = yA/2ns Onecan thus calculate a seafloor profile in whatever direction the hydrophone pair is pointing (particularly ahead for obstacle avoidance or abeam for swath mapping). If the vertical distance below the sonar is y and the horizontal distance out from it x , then the coordinates of the bottom profile points are

In this type of system the accuracy of determiningy increases as A/s decreases for any given ability to measure the phase difference. System complexity increases, however, as the range over which y varies becomes large, since usually one can measure w only over a 2n range; thus for large values y = 2nn + y', with y' measured directly and n (the number of whole cycles) determined from the continuity of the data on any given ping or from some auxiliary measurement system. A particularly simple version of this type of system can be produced by using a spacings which is several wavelengths and simply producing two SLS displays-one a conventional one and the other a similar display but using the sum or difference of the two hydrophone outputs. The latter image will be similar to the former except that it will show white bands at the particular ranges at which the two arrivals cancel one another, as shown schematically in Fig. 7. At those ranges the water depths (relative to the depth of the sonar will be y = nAr/s

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PAIR OF RECEIVING HYDROPHONES

FIG. 7. Schematic view of “interferometer” side-looking sonar. The summed pair of receiving hydrophones produces an image with white stripes (interference nulls) corresponding to depression angles for which the path difference for the arriving sound is an odd multiple of a half-wavelength.

The more sophisticated approach, as initially implemented by Anderson (Nickles and Anderson, 1968), in which one measures iy’ as a function of time after ping and calculates the resulting profile is currently operational in only one system (Sea MARC 11), in a postprocessing mode (Blackinton el al., 1983). When comparing systems of this kind in a swath mapping context with systems such as Sea Beam, one must be very careful to distinguish their intrinsic capabilities. While any phase difference SLS mapper operated near the surface can probably cover a wider swath, its footprint at the longer ranges will be substantially wider and its depth resolution much poorer. The depth resolution is related to the electrical phase resolution b y dy = (rU2ns)diy

Values of diy/277 much better than 0.01 are difficult to achieve in this context. Thus if sis only about one wavelength, then the depth resolution is only about 1% of the slant range. This is 100 m at 10 km. Phase difference-measuring systems do not have the advantages of angular discrimination that are included in the beam-forming systems. This inflicts a signal-to-noise ratio or power requirement penalty, often resulting in “noisy” depth data requiring the use of sophisticated averaging methods (Matsumoto et al., 1985). It also leads to a curious type of spurious data

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points. If at some range the vertical fan beam receives energy from two different depression angles, as would be the case if the local radius of curvature of the bottom were less than the range in the vicinity of a specular reflection point or if a school of fish were in the beam at a slant range greater than the water depth, then the returning signal will have two (or more) components and the effective phase will be controlled not only by the relative phases but also by the relative amplitudes of the signal components, with the resultant lying somewhere between the true angles for the extreme contributing components. The principal advantages of this type of system are that it requires minimal athwartship dimensions for the transducer system and substitutes a single phase-measuring channel for the multiple-beam beamformer. In a hullmounted configuration it would still require compensation for pitching motion to keep the fan vertical. In a towed configuration, if the body can be kept reasonably stable, then transmit beam forming is not required.

4. Acoustics for Position Determination The sonar systems discussed above all, in some sense, provide direct information about the seafloor-its depth, texture, and subbottom structure. This section is concerned with acoustic techniques which contribute in a different manner, primarily by providing the dimensions which make it possible to correlate not only the acoustic measurements, but others (magnetics, photography, sampling) as well, into maps from which the shape of the seafloor and patterns of other observations can be better understood. In general, acoustic means for determining positions in the ocean are useful primarily in fine-scale studies of the bottom in which it is important to keep position uncertainties less than 100 m for mapping, for correlating different types of measurements, or for careful placement of objects on the seafloor. Of the three attributes of an acoustic signal-arrival time, direction, and amplitude-only the first two have been used to any appreciable degree for this purpose. The simplest acoustic navigation devices are free-running, pulsed sound sources (pingers). These have been used extensively as markers to which a diver or small submersible might home, or which could be used to track a submersible. In this application a directional receiver is normally used so that the relative direction of the source can be determined. The other application of pingers has traditionally been to determine the height off bottom of some device being lowered near or actually onto the seafloor. In this context the direct and bottom-reflected sounds are displayed on a graphic recorder, usually using the receiving and display capabilities of the ship’s

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echo sounder. Separation between the direct and echo returns can then be used as a measure of height off bottom. Most pinger applications in geology and geophysics use very simple units in which the repetition period is only roughly controlled. For many years, however, some local navigation systems (e.g., the Navy’s Dabob Bay tracking range) have made use of pingers with good enough time bases that the one-way travel time could be measured to the level of milliseconds (ranges to 1 or 2 m) by assuming that the transmit instant was known at the receiver. Although pingers are simple and fairly inexpensive, their usefulness is limited, and nearly all acoustic position determination systems use transponders as the primary remote element. These listen continuously in some acoustic band and, when a signal of predetermined type arrives, it is recognized and a reply is transmitted. In almost all systems the signal used for navigation is a simple pulse, 2-10msec long, at some predetermined frequency. The frequency choice is based on the distance over which the system is required to operate, the power requirements, and the timing accuracy. For most intermediate-range work (e.g., 1-20 km) the choice lies in the 5-20-kHz range. This a range in which the sound absorption is reasonable (about 1 dB/km), the available bandwidths of transducers can support timing to a millisecond, and at the same time they can operate at a variety of frequencies in order to distinguish one transponder from another. Transponder systems are used in two distinctly different modes. In one (usually called long baseline) the primary frame of reference is provided by several transponders emplaced on the seafloor with their relative positions accurately known. The various elements to be tracked interrogate the transponders and the resulting acoustic travel time measurements are converted to ranges and used to determine the positions of the interrogators. In the other mode (short or ultrashort baseline) the primary frame of reference is at the interrogate-receive end of the system, where a small array of receivers, or precise directional receivers, determine the location of any individual transponder on the seafloor or on an object to be tracked relative to the receiving system by a combination of range and angle measurements. In the earliest short-baseline systems (Applied Physics Lab., 1958) the interrogation unit and the localized receiving array were located on the seafloor and hard-wired to a shore station. These systems were motivated by a requirement to track torpedoes in test firings in restricted bodies of water (e.g., Dabob Bay and St. Croix test ranges). In most geology-geophysics applications, however, the interrogating transducer and the receiving array are located on a ship; thus a second distinction is usually made that the primary reference frame for long-baseline systems is fixed to the seafloor while that for short-baseline systems is fixed t o the ship. This has led to a

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preference for long-baseline systems for deep ocean bottom-oriented operations which cover an appreciable area (e.g., 10 x 10 km) and to the use of short baselines where the positioning of the ship in a very localized area is the primary concern (e.g., dynamic positioning for drilling ships) or in short-range localized operations. For remote vehicle tracking, short-baseline systems can naturally be indexed to the seafloor by tracking transponders placed on the bottom while simultaneously making position determinations of another transponder attached to the remote vehicle. Short-baseline systems on ships are limited in their accuracy by the fact that the reference frame is attached to a moving platform; thus the interrelationships among successive positions are dependent on the ability to know how the ship's position and orientation change between observations. For the case of station-keeping nearly on top of a transponder this implies a need for a good vertical reference, while if the horizontal component of the separation between the ship and the transponder is large, a good compass is required (i.e., 0.2" for 10-m accuracy at 3 km). On the other hand, if the purpose is to track a remote vehicle relative to a seafloor reference point and if the signals from both transponders are received nearly simultaneously, then their relative positions can be determined with only the inherent limitations of the acoustic system itself (except for uncertainties due to motion of the interrogation point between transmit and receive). Installation of a short-baseline system on any ship requires some care in mounting the in-water components since lack of knowledge of their exact positions translates directly into systematic errors in determination of the horizontal and vertical components of the position angle. The short-baseline system trades care in the initial shipboard installation for ease of operation on arrival in the work area, where only one transponder need be placed on the bottom and work can begin. In the long-baseline case, some time must be spent installing the several transponders with some appropriate geometry in the work area. Since a larger work area is usually contemplated than is the case for short-baseline operations, however, the transponder launching is often integrated with an initial reconnaissance phase of operations. Placement of transponders for long-baseline navigation of near-bottom vehicles requires three types of consideration. First is the general formnumber of transponders and their separations. This is determined by the effective range of the individual transponders in the particular application and the position accuracy required. Since this type of system operates on range information from each reference point, the effectiveness of coverage in any given part of the area can be judged by the angles of intersection of arcs (spheres) drawn from each proposed transponder position (Lowenstein and Mudie, 1966). Some redundancy of coverage is also desirable, since this

11.

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2501

I

0

I

I

I

2

I

3 Horizontal range

I

4

'

,j N. miles

FIG. 8. Sound rays from a near-bottom omnidirectional source in a typical isothermal, upward-refracting environment (Spies, 1966).

can be used to improve the accuracy with which the internal geometry of the array can be determined. The second consideration has to do with how far off bottom the transponder hydrophones are to be moored. If the only function is to track surface craft in deep water, this is unimportant. However, for tracking near-bottom objects (towed vehicles, submersibles, sampling devices) there must be a direct path between the hydrophones of both the transponders and the tracked objects. In the nearly isothermal conditions encountered deep in the ocean, the effects of upward refraction produce a shadow zone (Fig. 8) near the seafloor. In addition, topographic features often block the direct path of sound from source to receiver. The first-order solution to these problems is to buoy the transponder (or at least its acoustically active elements) up off the seafloor. Since there is almost always a slight near-bottom current with a time-varying component of a few centimeters per second (0.1 knot), this means that the internal geometry of the network will be distorted by currentinduced transponder displacements. One must thus make a compromise between height of the transponder off bottom, mooring stiffness (buoyancyto-drag ratio, multiple leg moorings), and system accuracy. While a height of about 90 m has emerged as a common compromise value, some users vary this to meet special requirements. Our group has used values from 15 to 180 m, depending on the circumstances. One element which can enter here

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is the use of transponders in conjunction with other measuring devices, particularly self-contained current meters. In some instances (e.g., complex bottom topography and suspected strong near-bottom currents) it is desirable to know the exact position of the current meter. It thus is often expeditious to place it on the same mooring with the transponder, putting both close to the seafloor. The third consideration, usually implemented after arrival in the area but susceptible to detailed advance planning if a Sea Beam map of the area is available, is modification of the ideal array configuration to match the topography. Placement of transponders on high points and avoidance of valleys is usually preferred, although occasionally it is necessary to place a unit in a low place to ensure coverage for operations very close to the bottom in that particular part of the area. In most systems the transponders receive at one frequency and reply at another. While this could mean that each unit would have its own pair of frequencies, that is usually not the case. For long-baseline systems two styles of operation are in use, each providing a different type of simplicity as a result of its design choice. One style uses a single frequency to interrogate all units, with each replying at its own particular identifiable frequency. The other transmits at various frequencies with all units replying at the same frequency. The former approach allows travel-time measurement circuitry to be used with timing for the whole transponder array starting with the single transmit pulse and, given a receiving complex which listens independently in each frequency band, stops the clock for each transponder independently as its return arrives. With the latter method the transponder replies, all being at 12 kHz, are easily received by most research ships through their echo sounder receiving systems. The former method, which waits until all returns are out of the water before initiating the next interrogation, is well matched to automatic operation under good signal-to-noise conditions. The latter system, which uses an echo sounder-type display to measure the acoustic ranges, can interrogate its transponders more frequently, usually once per second. The resulting continuity of traces on the graphic recorder provides reliable operation at low signal-to-noise ratios or under severe multipath conditions, as well as a better ability to interpolate to pick off ranges to all transponders simultaneously. A primary consideration for long-baseline systems is how the depth of the vehicle being tracked is determined. Three transponders can be used, with the resulting intersection point of the three range spheres determining the three coordinates of the point being tracked. The other alternative is to measure the vehicle depth independently, using a precision pressure gauge or an up-looking echo sounder. The latter approach is to be preferred when the point being tracked is nearly at the same depth as the transponders. In

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particular, if the tracked point is very close to the plane defined by the three transponders, the depth accuracy becomes very poor. In addition, of course there is a requirement for an additional transponder. The problem of shadowing is particularly severe in areas (e.g., spreading centers, seamount peaks) in which the vehicle must operate close to very abrupt changes in seafloor elevation. A method for coping with this is to place the transducer to be tracked some useful, known distance above the vehicle. While this has not been implemented for small submersibles, we have used it with our Deep Tow system by securing an auxiliary acoustic transducer 100-200 m up the wire. Since the vehicle is heavy relative to its drag the wire angle just above it is very nearly vertical and thus tracking the auxiliary transducer produces a good approximation to the horizontal coordinates of the towed body. Two different implementations of this approach were used. The first involved a battery-powered package which could be attached to the towing wire and which was connected by a highfrequency (about 100 kHz) directional telemetry link to the vehicle below. When the vehicle’s own normal transponder interrogatiodreceive transducer was shadowed, the auxiliary system was interrogated from the vehicle ; it then transmitted the appropriate normal interrogation signal (10, 10.5, or 11 kHZ) and received the 12-kHz return, which then modulated the highfrequency carrier and was thereby transmitted down to the vehicle, detected, and telemetered to the ship up the normal transponder channel of the towing coaxial cable. During the position computation process the additional known acoustic path length was subtracted from the range and calculations carried out as usual. The second method involved use of a second slip ring channel at the vehicle. The electrical signals (transmit and receive) then used that channel and passed through a light coaxial cable married to the tow cable up to an acoustic transducer clamped to the wire. In this case the transponder system was simply switched back and forth between the transducer mounted on the vehicle and the one secured above it on the wire. Since the self-powered remote acoustic unit was much larger and heavier than simply the auxiliary hydrophone and represented a substantially larger financial investment, the second method has become the chosen approach. While the normal situation for long-baseline systems is to make a direct range measurement by having the transmit and receive function at the same point, other configurations are also useful. Figure 9a shows the normal case in which a vehicle (ship, submersible, or remote vehicle connected electrically to its operator) transmits and receives the acoustic signal. Figure 9b shows a situation that can often be convenient in which both the towing ship and the towed vehicle interrogate the transponders, but reception is only at one of the tracked points-ship or remote (but electrically connected) vehicle. This is particularly useful if the ship is operating in a very noisy condition

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FIG.9. Three commonly used transponder navigation configurations. In (a) the fish sends out the interrogate pulse, triggered through the conducting cable from the ship, and receives the return signal, which is telemetered back up the wire to the ship. Transponder and fish depths are determined independently and the horizontal range is approximated by using the resulting right triangle geometry. Arrangement (b) allows determination of the slant range from ship to transponder without having to receive a signal at the ship. The ship transmits the interrogation and the fish receives the signal, telemetering it up the wire. Twice the elapsed time for that circuitous path minus the round-trip travel time between fish and transponder equals the ship-transponder-ship travel time. Approach (c) is used if the suspension cable does not have a conducting core. The ship interrogates both the transponder and the relay. The relay in turn interrogates the transponder. The ship receives the signal from the relay as well as the two replies from the transponder, and from these can separate out the distance between relay and transponder.

while the towed body presents a quiet receiving environment. For this geometry one measures travel times t f for the fish-transponder-fish path and t , for the ship-transponder-fish path. The appropriate time to use for the ship-transponder range is then

r, = 2t,

- tf

Figure 9c shows a third useful configuration. A relay transponder (Boegeman et af., 1972) is clamped to a normal coring or dredging wire to track the position of a bottom sample or measurement probe. The relay is itself a type of transponder. On receipt of an interrogation signal from the ship, it transmits signals which interrogate the transponders and also are heard at the ship. In addition, the ship ranges directly on the transponders.

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In this case three different kinds of travel times are measured : to for ship to relay to ship, tt for ship to transponder to ship, and tr for ship to relay to transponder to ship. The travel time to be used for tracking the relay is then T, = 2tr - to

- tt

With the acoustic travel times in hand, one can proceed to calculate the position of the point being tracked (presuming knowledge of the transponder positions, which will be discussed below). The first step is to convert the travel times to distances. This requires knowledge of the sound velocity structure and the ray path between transponder and vehicle. In any normal situation this is done on the assumption that the sound speed varies only in the vertical direction and thus that the ray is uniquely defined throughout the area by the depths of the end points and the travel time. Properly speaking, one proceeds by finding the appropriate linking ray and from that extracting the difference between the horizontal coordinates of the end points, even including effects of the earth’s curvature if very high precision is needed (Spiess, 1985a). These horizontal ranges can then be used as the inputs to the geometric positioning problem. In practice, however, if accuracies of only 1 in lo3 to lo4 are required, one can use a sounding velocity appropriate to the depths at the two end points, assume straight line propagation paths, and use right triangle geometry to calculate the horizontal separation. We can examine the error which this introduces in a situation in which the sound velocity profile is linear; that is, the velocity at an elevation z above the level of the transponder is given by c = co + gz. Under these conditions (ignoring curvature of the earth) the relationship between the round-trip travel time T, the difference in depth z between the vehicle and the transponder, and the horizontal component r of the slant distance R between the two is

Since gT/4 will be small compared with 1 in the deep ocean, where g 1.8 x and T < 20 sec, we can expand the sinh term, giving

-

g2T2 g4T4

’ (1 + - + -96+ - . .

R = d m = G 2-

15,360

Out t o ranges of about 12 km the first correction term will be less than 1 part in 1000, and closer than 4 km the error will be less than 1 m. In situations in which one is working with both deep-operated and surface vehicles in the same array, the best means of finding a good estimate of a sounding velocity for this purpose is to determine the geometry of the

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transponder array (see below) using the deep vehicle and the sound velocity at its depth based on temperature, pressure, and salinity. One can then use that array geometry for navigation of the surface vehicle and adjust the effective sound velocity for near-surface navigation to minimize the resulting position errors. If there are only two transponders, the next step is the simple one of finding the intersection point for two circles. There will be two such points and the ambiguity is usually resolved either by the gross geometry or by watching the direction of motion of the intersection points as time passes and choosing the one which produces the more plausible track. Since accuracy is always poor along the line joining the two transponders (the “base line”), one nearly always uses an array of at least three transponders, arranged so that when the vehicle is close to the baseline for a particular pair the third one will provide the control in the across-baseline direction. If one routinely has three or more transponders in range it is preferable to use all the data rather than simply picking one pair. In this case, since the real observed range rings will not in general all intersect in a point, one must make a decision as to the coordinates to be designated as the fix point for a given set of observed ranges. This is best done by a least-squares fitting approach in which the x , y coordinates are chosen t o minimize the sum of the squares of the range errors. That is, if the observed ranges to the transponders are T i , x and y should minimize

E2 =

C[ i

~ i

J(x-

+ ( y - yi)’]’

where x; and yi are the coordinates of the respective transponders. Not only does this multiple-transponder approach eliminate the ambiguity of the twotransponder case, but also the resulting minimum value of E provides a measure of the goodness of the data and, as discussed below, opens up a method for refining one’s knowledge of the coordinates of the transponders. The equations which result from the minimization formalism are nonlinear ; however, by making suitable initial approximations they can be linearized and their solution carried out numerically, for example, by the method described by Lowenstein (1966). A number of methods exist for determining the relative coordinates of the transponders in any given network. All but one of these involves making observations at various locations near the array. The one that does not requires use of transponders which are more sophisticated than those normally available. In this approach there is a capability, on command, for the transponders to interrogate one another. One could then, from some point, query each unit of a pair directly and also have one unit, on receipt of a special signal, interrogate the other. One would then have the two direct

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travel times tl and f~ as well as a time f 1 2 from ship to unit one to unit two and back to the ship. The round-trip travel time corresponding to the base line length would then be TIZ= 2t12 - t l - f 2 . It should be noted that in this process the determination of the resultant base line length depends only on the sound speed along the base line and does not require knowledge of the conditions along paths between the interrogating ship and the individual transponders. The only difficulty with this approach (aside from the more expensive transponders) is that each transponder must have a clear path to at least two others. This does not present a significant problem when the units are buoyed well above the seafloor, but if for some reason (e.g., maintenance of very close position control in the face of near-bottom currents) the units are close to the bottom, then upward refraction and topographic shadowing are important. This approach could support large-area coverage with a small number of high-powered units. They could be buoyed quite far off the seafloor and the changing geometry due to fluctuating currents could be compensated by using repeated redeterminations of the continuously changing array geometry. Particularly for large transponder separations, errors may arise if the interrogating vehicle moves appreciably between initial signal transmission and final reception. The more conventional approach is to use range data from the vehicles being tracked to refine the initial, approximately known, geometry. Naturally, the better the first approximation the more smoothly matters will proceed. In this connection the advent of the new Global Positioning System will support more efficient operation. The partial system has been used to provide the initial positions for two surveys to date and in each instance the subsequent surveys had quite low position errors from the start. The simplest means for determining array geometry is by using base line crossings. If one continuously ranges on a transponder pair while cruising through the area, the sum of the horizontal ranges to each of two units will be a minimum (and equal to the base line length) when one crosses the base line between the units, while the difference will be a maximum if the base line is crossed external to the members of the pair. Since these observations are essentially single ones for each crossing, this method does not allow for reduction of array position errors through averaging. The other approach is to work with multiple transponder fixes and extend the least squares approach described above. If rij is the observed horizontal range component for the j t h observation of the ith transponder, then one wants to minimize

E’ =

C C [rij - d ( X i i

j

- Xj)’

+ (ui - ~ j ) ~ ] *

by adjusting all of the xi, yi, Xj, andyj. This problem can be solved iteratively

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by a simple extension of the process for deriving a single position as described earlier. One starts with approximate transponder locations and the observed ranges for a number of multitransponder fixes (100 or more usually). After calculating the fix positions one inverts the process, assuming that the fix positions are known and calculating the best-fitting transponder position. The process is again inverted to find a better-fitting set of vehicle positions, and so forth. At each step in the looping process the rms range residuals measure the goodness of the fit and the looping process can be continued until that measure no longer decreases. It should be emphasized that these procedures can be carried out in parallel with the survey operation and that usually it is not necessary to allocate time specifically for array calibration. Using this method in deep-ocean situations with a near-bottom vehicle usually results in rms range error numbers of 1 or 2 m, which is commensurate with the resolution of the input data. While these are in a sense a measure of position accuracy, some care must be taken in their interpretation since there can be situations in which the arc-crossing geometry is not good and the resulting positions may be poorly determined along some particular direction. The method has also been used with simulated data derived from exact positions perturbed by random errors in order to study the possibility of using this approach to determine strain buildup in local areas such as spreading centers. In this case, for a 10-cm standard deviation of the range errors and a reasonable spatial distribution of 300 observation points, the rms base line length errors for a four-transponder array ranged from 0.5 to 2cm for 20 different realizations. The method could thus be used in a geodetic context with precision transponders and careful in situ determination of sound velocity (Spiess, 1985a). Relating acoustic transponder coordinates to latitude and longitude requires use of some other navigation system to provide the tie. The usual approach is to make a least-squares fit between sets of points for which acoustic and satellite coordinates are both measured. The results when the Transit system is used usually show residual errors comparable to those expected for the satellite observations. With GPS the acoustic and satellite error contributions may be of comparable magnitude and geodetic measurements of subdecimeter accuracy may be obtainable relative to seafloor points (Spiess, 1985b).

5. A System Example In the initial section of this chapter it was noted that acoustic techniques are best used in combination with one another and in conjunction with other observational and sampling approaches. En musse, these produce powerful

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systems for investigating the seafloor. An example of such a multipronged system is the Deep Tow system (Spiess and Lonsdale, 1982) assembled and used by the author and his associates over a period of 20 years. The system has grown incrementally by the addition of successive new subsystems and capabilities rather than having been built initially as a complete system. It is anticipated that further capabilities will be added in the future. The primary element is an instrument package (Fig. 10) which can be towed very close to the deep seafloor by an electromechanical cable whose coaxial conducting core transmits power, control signals down, and telemeters up the outputs of the various sonars (plus magnetometer readings ; television pictures ;water temperature, pressure, and conductivity; light transmission ; etc.). All the sonar types discussed above are represented. A precision, 3" beamwidth, 125-kHz echo sounder provides high-resolution topographic profile information. Its footprint at a normal operating elevation of 40 m off bottom is about 2 m across; thus only rarely does it display side echoes. The 1 10-kHz side-looking sonar provides images of exposed rock features out to ranges of 500-800 m with a 3/4" beamwidth. The subbottom penetration sonar operates at either 4 or 6 kHz and provides penetration to 100 m in most deep-ocean situations in which that thickness of sediment is present. It can

FIG. 10. Deep Tow vehicle showing sensors for the various subsystems.

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resolve sediment cover as thin as 2 m. The purpose of the alternate frequency capability is to make it possible to distinguish between discrete reflecting horizons and interference effects such as those referred to in Section 2. In addition to the primary acoustic data collection systems, three other sonars provide supporting information. These are the up-looking sounder, the obstacle avoidance unit, and the transponder navigation system. The uplooking unit (30" at 23 kHz) complements the precision sounder, with the sum of their ranges giving the actual water depth. This approach is limited in accuracy by the fact that the round-trip travel time for the up-looking acoustic path is usually 4-7 sec, and during that period the towed vehicle may be moved vertically (by wave action on the towing ship) by as much as 2 m. To improve performance the vehicle depth is measured by a precision pressure gauge, which in turn is calibrated by means of the up-looking sounder, using appropriate averaging. The obstacle avoidance sonar is a simple device operating at 40 kHz and projecting a fan beam ahead which provides early warning of the approach of significant small-scale features. Large-scale aspects of the topography (sea peaks, major scarps) are usually mapped out in an adequate approximate way from preliminary surveys with the ship's hull-mounted sounder. The transponder navigation system, which provides information for ship control and to tie observations together, is of the long-baseline type. Transponders are normally interrogated from both fish and ship, with replies recorded from the fish since it is usually the quieter listening location. In this system we use three different interrogation frequencies (10, 10.5, and 11 kHz) and a single reply frequency (12 kHz). Data are handled as described in Section 4, with initial approximate array geometry being refined as the survey progresses by using an iterative least-squares fitting process to achieve residual range errors of the order of 1-2m. Relay transponders are also available to provide control of bottom sampling in relation to features mapped with the various Deep Tow systems. The sonars are complemented by a stereo photographic system, which is particularly useful in providing a somewhat finer-scale view of the seafloor than that provided by the side-looking sonar. In rocky, spreading-center terrain it provides the detail of lava flow forms, and in areas of rippled or furrowed sediments it provides knowledge of the shorter-wavelength details. The proton magnetometer provides observations good to about 1 nT (1 gamma) and benefits from the data from the sonars, particularly the subbottom penetration system, which reveals the depth of the basement rock in rise flank areas where it may be covered by modest amounts of sediment. While it is clear that all these systems, in a general way, contribute to building a comprehensive picture of the geology of a particular site, there are instances in which the interaction is fruitful on a more basic level in

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connection with the first-order interpretation of the data from a particular system. A good example is the analysis of the subbottom penetration system records in regions in which the sediment may be patchy or pierced by frequent outcrops. That system is essentially omnidirectional, achieving its lateral resolution simply from its proximity to the seafloor. Nearby rocks can thus produce side echoes which under some circumstances may be difficult to distinguish from subbottom features. In this situation the side-looking sonar provides support by displaying any nearby outcrops, with range information which can be compared with the 4-kHz sounder record. An example of the usefulness of the navigation system in providing a tie between samples and acoustic data occurred during our investigations of carbonate sediments (Mayer, 1979). We were able to put our piston cores within 10 m of the track along which the subbottom records had been made. We were thus confident that the measurements on the core materials were relevant to the acoustic data. Similarly, the transponder-based side-looking sonar maps of mining vehicle tracks in a 4500-m-deep manganese nodule area made it a straightforward matter to photograph them on subsequent passes through the area and then to box core within 30 m of the tracks to complete the data collection for an environmental impact study (Spiess et al., 1984). Complementarity with various other systems naturally exists as well. The Deep Tow system has been used simultaneously with the Sea Beam swath mapping sounder system. Having the latter in operation not only provides the necessary larger-scale survey information but also facilitates the transponder placement process. Operation of the near-bottom narrow-beam sounder system over the same area as Sea Beam provides ground truth on the resolving power of the swath mapping system. This turns out to be as careful analyses would predict : features having a horizontal extent comparable to only a few footprints are not accurately rendered. In particular, steep slopes which have modest horizontal extent are always underestimated. Finally, it should be noted that new subsystems are being added which will allow quantitative determination of backscattering properties of the adjacent seafloor (Weydert, 1985). It is hoped that this will be only one of many moves to make use of quantitative measurements of acoustic return intensity with resulting expansion of the usefulness of acoustic techniques in marine geology. References J. L. Abbott, S. M. Smith, J. S. Charters, P. G. Downes, T. Hylas, R. L. Moe, J. M. Moore,

and D. V. Stuber, Scripps seagoing computer centers: Real-time data acquisition and processing. Proc. Working Symp. Oceanogr. Data Syst., 4th pp. 123-129. IEEE Comput. SOC.Press, New York, 1986.

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E. R. Anderson, Sound speed in sea water as a function of realistic temperature-salinitypressure domains. U.S. Nav. Undersea Res. Dev. Cent. TP 243 (1971). J. E. Andrews and P. B. Humphrey, Swathmap: Long range sidescan sonar mapping of the deep seafloor. Mar. Geod. 4, 141-159 (1980). Applied Physics Laboratory, “An Introduction to the Three Dimensional Underwater Tracking Range,” Rep. 58-3. Univ. of Washington, Seattle, 1958. J. G. Blackinton, D. M. Hussong, and J. Kosalos, First results from a combination side-scan sonar and seafloor mapping system (Sea MARC 11). Proc.-Annu. Offshore Technol. Conf. 15, 307-311 (1983). D. E. Boegeman, G. J. Miller, and W. R. Normark, Precise positioning for near-bottom equipment using a relay transponder. Mar. Geophys. Res. 1, 381-396 (1972). L. Breslau, Classification of seafloor sediments with a shipborne acoustical system. Woods Hole Oceanogr. Inst. Contrib. No. 1678 (1967). D. N. Chayes, Evolution of Sea MARC I. IEEE Proc. Working Symp. Oceanogr. Data Syst., 3rd pp. 103-108. IEEE Comput. SOC.Press, New York, 1983. C. S . Clay and H. Medwin, “Acoustical Oceanography,” pp. 338-344. Wiley, New York, 1977. V. A. Del Grosso, New equation for the speed of sound in natural waters. J. Acoust. Soc. Am. 56, 1084-1091 (1974). C. de Moustier, Inference of manganese nodule coverage from Sea Beam acoustic backscattering data. Geophysics 50, 989-1001 (1985a). C. de Moustier, Deep seafloor acoustic backscattering measurements using Sea Beam. Ph.D. Thesis, Univ. of California, San Diego, 1985b. C. Eckart, “Principles and Applications of Underwater Sound,” NDRC Summary Report, 1946. (Reprinted by Dep. Navy, Headquarters Nav. Materiel Command, Washington, D.C., 1968.) F. H. Fisher and V. P. Simmons, Sound absorption in sea water. J. Acoust. SOC. Am. 62, 558-564 (1977). M. F. Glenn, Introducing an operational multi-beam array sonar. Int. Hydrogr. Rev. (Monaco) 47, 35-39 (1970). D. S . Gorsline and T. L. Holcombe, Interpretive modeling of deep ocean sediments and their physical properties-a foreword. J. Acoust. SOC.Am. 68, 1311-1312 (1980). E. L. Hamilton, Geoacoustic modelling of the sea floor. J. Acoust. SOC.Am. 68, 1313-1340 (1980). E. L. Hamilton and R. Bachman, Sound velocity and related properties of marine sediments. J. Acoust. SOC. Am. 72, 1891-1904 (1982). E. L. Hamilton, R. T. Bachman, W. H.Berger, T. C. Johnson, and L. A. Mayer, Acoustic and related properties of calcareous deep-sea sediments. J. Sediment Petrol. 52, 733-753 (1982). C.-C. Hsu, Differential sound absorption technique and effect of ion-pairing and pressure on sound absorption in seawater and aqueous mixtures of magnesium sulfate and sodium chloride. Ph.D. Thesis, SIO Ref. 81-34, Univ. of California, San Diego, Scripps Inst. of Oceanogr., 1981. D. M. Hussong and P. Fryer, Submarine volcanoes in the Mariana Arc: Early results of the Sea MARC I1 seafloor mapping system. EOS, Trans. Am. Geophys. Union 64, 627-632 (1983). M. C. Kleinrock, R. N. Hey, and C. de Moustier, The Omega deception in Sea Beam data. EOS, Trans. Am. Geophys. Union 65, 1104 (1984). P. Lonsdale and B. Malfait, Abyssal dunes of foraminifera1 sand on the Carnegie Ridge. Geol. SOC.Am. Bull. 85, 1697-1712 (1974). J. R. Lovett, Merged seawater sound-speed equations. J. Acoust. SOC.Am. 63, 1713 (1978).

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