A combined acoustic and electromagnetic wave-based techniques for bathymetry and subbottom profiling in shallow waters

Journal of Applied Geophysics 68 (2009) 203–218

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Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o

A combined acoustic and electromagnetic wave-based techniques for bathymetry and subbottom profiling in shallow waters Y.-T. Lin a, C.C. Schuettpelz b, C.H. Wu a, D. Fratta c,⁎ a b c

Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA Golder Associates, Denver Office, 44 Union Boulevard, Suite 300; Lakewood, CO 80232 Geological Engineering Program, University of Wisconsin-Madison, Madison, WI 53706, USA

a r t i c l e

i n f o

Article history: Received 7 April 2008 Accepted 11 November 2008 Keywords: Sub-bottom profiler Ground penetrating radar Shallow waters Bathymetry Sublayer imaging

a b s t r a c t Acoustic-wave based sub-bottom profiler (SBP) and electromagnetic-wave based ground penetrating radar (GPR) are two complementary geophysical tools that were used to map bathymetry and sediment sublayers in shallow waters. Near shore regions in Great Lakes, inland lakes, and rivers in Wisconsin, USA were examined using both geophysical tools. In areas with high silt and clay contents, such as Lake Superior, the SBP was able to image the sediment sublayers, whereas in the areas with sand cover and vegetation, the GPR provided sediment stratigraphic information. The higher vertical and horizontal resolutions of the SBP surveys provided more accurate and detailed bathymetry information than GPR surveys. In the Yahara River, SBP surveys imaged blurring contrasts between sublayers due to the gradual deposition of sediments; however, GPR provided sharp delineations of sediment layers but was only able to image the top two sublayers because of the high silt and clay electromagnetic wave attenuation. To confirm these findings, Shelby tubes and hydraulic jetting were used to collect ground-truth information. Sublayer thicknesses are estimated by evaluating acoustic and electromagnetic wave velocities using a mixture-based equation. In Lake Michigan, both techniques show similar sediment stratigraphy, indicating that the average sediment particle size ranges between silt and fine sand. Three-dimensional maps of bathymetry and subbottom sediments are also constructed. Overall it is shown that the combination of GPR and SBP techniques compensates each survey's strengths in an effective methodology for imaging bathymetry and sub-bottom profiles in shallow water environments. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Accurate surveys of both bathymetry and subbottom layers in lakes, rivers, and oceans provide crucial information pertaining to a number of environmental and coastal issues (Lawler, 1993, Lane et al., 1994, Van Rijn,1997). For example, successive surveys of bathymetry and sublayers before and after storm events help quantify the magnitude of bottom sediment erosion, deposition, and redistribution processes (Page et al., 1994; Walling et al., 1998; Ogston et al., 2000; Lee et al., 2004). Bathymetry and sublayer information helps in the evaluation of the role of cohesive sediments on lakebed downcutting and bluff recession along Great Lakes shorelines (Davidson-Arnott and Ollerhead 1995). In general, there are several in situ and geophysical techniques used to evaluate bathymetric or sublayer geometries. These techniques differ both in terms of their applicability and spatial and temporal scales. Two traditional methods of collecting quality data from a stationary boat include soundings obtained with a weight attached to a line and sediment coring that can be associated to GPS coordinates (Przedwpjski ⁎ Corresponding author. E-mail addresses: [email protected] (Y.-T. Lin), [email protected] (C.C. Schuettpelz), [email protected] (C.H. Wu), [email protected] (D. Fratta). 0926-9851/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2008.11.010

et al., 1995; Kalff, 2002). Sediment coring has the added advantage of allowing the collection of samples that can be used to validate geophysical and other non-intrusive surveying techniques. However, the number of sampling points is usually limited due to considerable logistical efforts. Airborne scanning laser altimetry (LIDAR — Irish and White, 1998) can be also used to map bathymetry in very clear water environments but cannot resolve sublayers. As an alternative to the traditional surveys described above, geophysical techniques offer cost-effective and non-destructive methods that allows for continuous mapping of bathymetry and subsurface information. Geophysical techniques also provide larger penetration depths while maintaining a substantial survey area size (Scholz, 2001; Cagatay et al., 2003; Bradford et al., 2005). Acoustic-based geophysical methods are commonly used in aquatic environments (i.e., side scanning sonar and subbottom profiling techniques — Garcia et al., 2004; Nitsche et al., 2004; Schrottke et al., 2006) to image the bathymetry and subbottom sediment layers. Other techniques are based on the monitoring of electrical and magnetic properties of water and sediments at different frequencies. These techniques include electrical resistivity (ER), electromagnetic methods (EM), and ground penetrating radar (GPR). These techniques are used to map water depth and sublayers. In ER surveys, floating electrodes are

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utilized to measure the potential field caused by sublayers. Installing electrodes underwater at the water–sediment interface is undesirable due to difficulties inherent when working below the water line (Kwon et al., 2005; Rinaldi et al., 2006). EM methods induce time varying magnetic fields that trigger a response from conductive sublayers. The amplitude and the attenuation of the eddy currents created in sediment layers are an indication of the electrical conductivity of the different layers (Telford et al., 1990). GPR surveys image dielectric properties of water and subbottom sediments by carrying or towing the receiver and transmitter antennae in or behind a boat (Annan and Davis, 1977; Mellett, 1995; Yang et al., 2006). Previous studies have documented the usefulness of each geophysical technique for different ranges of resolution, penetration depths and other physical parameters related to the measurement of subsurface sediment layer types (Davis and Annan, 1989; Pilkington and Grieve, 1992; Zelt and Smith, 1992). Resolution and penetration depths vary with signal frequency and instrument configuration. In general, the resolution of ER is on the order of meters and decreases with depth. The resolution of SBP and GPR can be as fine as centimeters depending on excitation frequency and the elastic or electromagnetic wave velocity of each material; however, ER methods provide penetration depths much greater than the SBP and GPR methods. SBP signals have difficulty penetrating through coarse-grained sediments (e.g., sand and gravel), and glacial till due to low energy transmission and signal scattering (Morang et al., 1997). The EM signal strength in GPR surveys attenuates quickly in high conductivity materials, penetrating only a few centimeters in clayey sediments and sea water (Annan, 2005). Due to advantages and disadvantages offered by each geophysical technique, the combined use of testing methodologies provides a more complete picture of the subsurface and allows for results to be compared against one another (McCann and Forster, 1990). Recent studies have adopted the use of combined geophysical tools, mainly in geological applications: evaluating near-surface stratigraphy, monitoring sediment fills in the subsurface, estimating water content and water conductivity, and locating the position of the water table and bedrock (Garambois et al., 2002; Schwamborn et al., 2002; Gabriel et al., 2003; Sass 2006; Yang et al., 2006; Sloan et al., 2007). Other successful applications include the use of geophysical techniques to estimate beach thicknesses and volumes (Gunn et al., 2006) and to monitor landslide areas (Bruno and Martillier, 2000). The majority of these studies were conducted on land. Typically, geophysical techniques were applied separately and results were combined and compared at a later time. Although Sellmann et al. (1992) and Schwamborn et al. (2002) have already combined GPR and SBP to conduct surveys in lakes, efforts were paid to survey relatively deep waters (N5 m). Sellmann et al. (1992) conducted SBP and GPR surveys at two separate dates in summer, which may have not been performed on the same survey lines due to effects of waves and currents and limitations imposed by GPS resolution. Schwamborn et al. (2002) ignored seasonal effects to lakebottom sublayers, performing SBP surveys in summer and GPR surveys in winter to map the uppermost basin fill in Lake Nikolay in Siberia, Russia. However, seasonal variations are important to beach profiles (Komar, 1997) and sediment transport processes are especially dynamic in shallow rivers and lakes during stormy seasons (Madsen et al., 1993; Russell, 1993). In view of these previous studies, it is recognized that simultaneously using two different geophysical tools would help obtaining reliable estimates of bathymetry and subbottom sedimentary layers in shallow water environments. In this study, bathymetry and sublayer structure in shallow waters (water depths b5 m) are of interest. Study sites in Wisconsin, USA include: mouth of the Yahara River in Lake Mendota, South shoreline of Lake Superior, and West shoreline of Lake Michigan. The Yahara River site has soft sediment layers that are easily affected by storm events. Lakes Superior and Michigan sites have exposed bluffs onshore and cohesive lakebed sediments in shallow, near shore waters. Both the bluffs and shallow water sediments are sensitive to seasonal weather

conditions. The vertical lowering of near shore sediments is expected to control long-term bluff recession rates (Davidson-Arnott and Ollerhead, 1995). Due to instrument resolution, penetration depth, and sediment characteristics in the study sites, the complementary use of acousticwave SBP and EM wave-based GPR instruments are applied. The objective of this paper is to validate the concepts and feasibility of combined GPR and SBP techniques, address the associated accuracy and uncertainty of both techniques in comparison with ground-truth data, and to assess the necessity of using combined techniques in shallow waters. 2. Methods 2.1. Study sites An evaluation of the potentials and limitations of SBP and GPR techniques in shallow water was tested at three fresh water sites: Lake Superior (Bayfield County), Lake Mendota (Dane County), and Lake Michigan (Ozaukee County) in Wisconsin, USA. The geographic location of each of the three study sites is shown in Fig. 1 and described briefly below. 2.1.1. Lake Superior site The Lake Superior site consists of a cohesive lakebed extending along the southern shoreline in Bayfield County, WI with prominent coastal bluffs. The bluff soils include clay, ripple-marked sand, cobbles, and boulders, and the material in the foreshore areas is sand (Swenson et al., 2006). By using available aerial photographs with resolution of 1 m/ pixel, the bluff recession rate between 1966 and 1998 was estimated to be 0.17 m/year (Swenson et al., 2006). The underlying glacial till is composed primarily of clayey sediments. Erosion and vertical lowering of the cohesive lakebed, also called lakebed downcutting, is an important factor when considering the long-term bluff recession rate (Davidson-Arnott and Ollerhead, 1995; Davidson-Arnott and Langham, 2000). Downcutting is common along cohesive shoreline bluffs in the Great Lakes. The recession of the cohesive bluff is primarily a function of the irreversible process of downcutting. Fine sediments (silt and clay) are eroded and deposited in deep waters; whereas the sand material usually remains in the neashore zone (USACE, 2002). However, if downcutting is prevented, the bluff toe would not recede and the bluff would remain stable. Near shore sediments typically consist of several sandy bars that migrate back and forth over a firm glacial till deposit. The downcutting process probably occurs if there are not sufficient sand supplies to protect the till lake bed from wave actions. The migration of these sand bars and other longshore sediment transport processes can be monitored with combined SBP and GPR images. When surveyed over the course of a few years, the downcutting rate of the firm glacial till can be determined. The downcutting rates can then be used to build a relationship between long-term shoreline erosion and bluff recession in the Great Lakes. 2.1.2. Lake Mendota site Lake Mendota is a 3940-Ha freshwater lake located adjacent to and north of the city of Madison, WI (Fig. 1). It is the largest of a four-lake system that also includes Lakes Monona, Waubesa, and Kegonsa. These four lakes are joined by the Yahara River that feeds and drains the lake system from North to South. Two locations in Lake Mendota were surveyed. The first location is at the mouth of the Yahara River where the river enters Lake Mendota. This site was selected due to the presence of soft sediments carried into Lake Mendota. Sediment transport processes in rivers are especially dynamic during and after storm events which induce erosion in the surrounding watershed, raise the river stage (causing both higher velocity flows and increased erosion of river banks) and high winds (leading to more turbid underwater currents — Soranno et al., 1997; Stoner et al., 1998; Merritt et al., 2003). The increased movement of sediment during inclement weather may

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Fig. 1. Locations of the three study sites in Wisconsin, USA.

dramatically affect both the bathymetry and stratigraphy of the uppermost sublayers. The other survey location is near Picnic Point, a nearly mile-long peninsula in the southern part of the lake. This study focused on a sandy shoreline along the west side of Picnic Point. 2.1.3. Lake Michigan site The city of Mequon (Ozaukee County, WI) lies along the Wisconsin shoreline of Lake Michigan. Concordia University occupies a one halfmile long stretch of the shoreline in Mequon, WI (Fig. 1). The university is located on top of a 130-foot-high bluff extending to the lake. This bluff is suffering erosion and slope failures while waves continuously undercut the bluff base. A spring also adds to the instability of the bluff. Similar to the problems described for the Lake Superior site, the clayey sediments along the Lake Michigan shoreline, sandy and silty lake sediments in midslope of the bluff, the clayey till at the top of the bluff, and the downcutting of the near shore sediments need to be considered when studying the long-term bluff recession rate (Brown et al., 2005). The bluff retreat rates estimated by applying aerial photographs were 0.63 m/year from 1956 to 1995 (Brown et al., 2005). The survey was conducted directly off the coast of Concordia University campus. Storm events are identified as a key bluff erosion factor (Carter, 1976), but the

nearshore bathymetry measurements after major storm events in these sites are still lacking. 2.2. Imaging instrument: sub-bottom profiler The subbottom profiler (SBP) uses sonic and ultrasonic waves to image the water-sediment interface and underlying sediment layers by detecting changes in the mechanical impedance through reflections from the subbottom stratigraphy (Schock et al., 1989; Ballard et al., 1993; Schock, 2004). Typically, SBP systems send narrow-angle acoustic waves with two different scanning frequencies. The high-frequency signal (e.g., 200 kHz) yields high-resolution returns from the water–sediment interface and a strong bathymetric response, while the low-frequency signal (e.g., 20 kHz) can penetrate into the sub-bottom formations for mapping of sediment layers. The effectiveness of subbottom profiling depends on the nature of sediments. Sand and other coarse-grained sediments provide stronger bathymetric reflections and poor penetration into subbottom layers; however, clay and other fine-grained sediments yield weaker bathymetric reflections and deeper penetration into subbottom layers. The resolution of the technique depends on the signal frequency (f)

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Fig. 2. Deployment of GPR and SBP systems for shallow water applications. (a) Sketch of the combined subbottom profiling setup. (b) SBP and GPR setup system used in Lake Michigan and Lake Superior. The GPR and SBP were mounted directly on the boat. (c) SBP and GPR setup system used in Lake Mendota. The SBP (not visible) was mounted on the stern of the boat while the GPR system was mounted in two inflatable rafts and towed behind the boat.

and the P-wave velocity (VP) in water and sediment layers. The resolution of the low-frequency (f = 20 kHz) signal in saturated soft sediments (VP ≈ 1500 m/s — Santamarina et al., 2005) is approximately equal to a quarter of the wavelength:

the sediment layers. For example, for a loaded 200 MHz antenna, which has a central frequency of ~100 Hz, the resolution with the sediment EM wave velocity VEM = 108 m/s is: GPR

Vp 1500 m=s SBP = 0:018 m SBP resolution = Res ≈ 4
20000 Hz 4
f

Res ð1Þ

The SBP used in this study is a Tritech SeaKing Parametric Subbottom Profiler. This narrow-beam system emits a range of ultrasonic signals and captures reflections from layers and structural elements at high frequency (200 kHz) and low frequency (selectable 6.7 to 30 kHz) to map the bathymetry and subbottom layers below the surface of the water. In this study, the low frequency is set to 20 kHz, to maximize the output power of the system. The SBP is connected to a Trimble Ag114 GPS receiver with 1 m resolution to record real-time position. 2.3. Imaging instrument: ground penetrating radar In areas where the data obtained with the SBP are limited due to low penetration, GPR provides a complementary alternative. The GPR system sends EM waves into a medium and monitors reflections at interfaces with contrasting electromagnetic impedance, i.e., contrasts in the dielectric permittivity of the subbottom sediments. One of the greatest limitations of GPR surveying is the presence of layers with high electrical conductivity, e.g. clay sediments or sea water where the EM wave is highly attenuated (Annan, 2005). However, in fresh water environments with coarse-grained sediment layers, the EM wave undergoes little attenuation and effectively images the depth of the lake or river bed and sediment layers underneath. GPR resolution depends both on the antenna frequency and the EM wave velocity in

≈

VEM 108 m=s = = 0:25 m GPR resolution 4
f 4
100
106 Hz

ð2Þ

Because the GPR system is portable, of high resolution, and efficient in field settings, it has become a useful tool to map shallow substrates (Annan and Davis,1992; Neal, 2004). Annan (2005) presents a detailed discussion of the GPR technique. A Sensors & Software pulseEKKO 100 GPR system with a 400 V source was used in this study. Both 100 and 200 MHz antennae were adopted based on the balance between the penetration depth (low frequency antenna) and the fine resolution (high frequency antenna). In shallow water environments (i.e., less than 3 m), 200 MHz antennae were selected. In deep water environments (i.e., more than 5 m), 100 MHz antennae were used. Since GPR surveys were conducted in continuous profiling mode, large numbers of stacked signals increased acquisition time and smeared the horizontal resolution of the acquired data. In this study, 64 stacks per trace were adopted for both 100 and 200 MHz collection to reduce data smearing. 2.4. Combined geophysical techniques: SBP and GPR surveys Combined SBP and GPR surveys were performed at each of the three sites. The SBP and GPR systems were mounted on or towed behind a boat to collect data simultaneously. Fig. 2 shows pictures of the SBP and GPR system-boat setup. The SBP was always clamped on the stern of the boat and a GPS receiver was connected to provide continuous position data for survey lines. The GPR system was

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mounted on an inflatable Zodiac boat (Lakes Superior and Michigan Sites) or towed behind the main boat in two small inflatable plastic boats (Lake Mendota Site). The change in setup configuration was necessary to avoid interferences with the hull of the boat used in Lake Mendota. 2.5. Limitations of the SBP and GPR systems Each of the two geophysical techniques has a range of applications that depends not only on the system performance but also on the physical properties of the water and near surface sediments. A qualitative determination of the penetration depth of the techniques is given by the skin depth. The skin depth Sd is defined as the depth a wave travels before attenuating to 1/e of the initial amplitude (where e is the Neperian logarithms base): SBP

=

GPR

=

Sd

Sd

1 1 Vp = αp 2πf D

SBP skin depth

ð3Þ

1 1 VEM
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi = rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  GPR skin depth ð4Þ α EM 2πf  2 σ eff 1 1 + − 1 2 2πf
κ Ve 0

where αP and αEM are the attenuation coefficients acting on the waves generated by the SBP and GPR respectively, D is the elastic wave damping coefficient, σeff is the effective electrical conductivity, κ′ is the real relative permittivity, and εo (=8.85 · 10− 12 F/m) is the permittivity of the free space. As shown in Eqs. (3) and (4), the skin depths depend on the wave frequency, the material damping (P-waves) and electrical conductivity (EM waves). The applicable penetration depth of the SBP in sea and fresh water is not limited by the skin depth (the attenuation coefficient and damping is quite low in water), but rather by the geometric spreading as the wave propagates away from the source. The application depth of GPR in off-shore environments is greatly limited by skin depth in sea water (high electrical conductivity and attenuation coefficient) but can be applied easily in freshwater where low electrical conductivity and attenuation coefficients are more typical. The quality of subbottom data also depends on the magnitude of the reflection coefficient at the water-sediment interface and within different sediment layers. The reflection coefficient for normally incident waves is: R=

I2 − I1 I2 + I1

ð5Þ

where Ii is the impedance of layer i. The mechanical impedance is defined as I =Vp ·ρ wherepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ρ is the layer density and the electromagnetic impedance is I = ðμ 0 = eÞ where μo = 4π· 10− 7 H/m is the magnetic permeability of free space and ε is the dielectric permittivity of the layer i. In the case of SBP applications, the P-wave velocity in water (~1500 m/s) is similar to the P-wave velocity in saturated sediments (1450–1900 m/s — Santamarina et al., 2005). Therefore, the contrast in mechanical impedance is mainly controlled by the change in density. Coarse sediments (e.g., sands and gravels) tend provide sharper density and velocity contrasts than soft sediments (e.g., clays and fine silt) with respect to water. Therefore, coarse sediments yield stronger bathymetric reflections but lower penetration into subbottom sediments (less transmitted energy reaches the subbottom). On the

Table 1 Comparison of SBP and GPR bathymetry and subbottom imaging applications. Technique Fine-grained sediments Sediment type (e.g., clays and clayey silts) SBP GPR

Coarse-grained sediments (e.g., sands and gravelly sands)

Low attenuation, high penetration High attenuation, low penetration High attenuation, low penetration Low attenuation, high penetration

Fig. 3. Schematic of underwater camera-aided hydraulic jetting system.

contrary, soft sediments yield weaker reflections from the bathymetry, allowing greater energy to reach subbottom sediments and improved images of subbottom sediments. This situation is different in the case of GPR surveys and the propagation of EM waves. Softer sediments generally have higher electrical conductivity, greater energy absorption, and lower signal penetration. Coarse sediments (low conductivity) allow EM waves to penetrate deeper into subbottom sediments, providing more useful information about sublayers. These analyses are summarized in Table 1 and show the complementary information provided by both SBP and GPR. The field surveys were designed to verify these analyses. 2.6. Ground-truth data collection: Shelby tube sampler Ground-truth data were used to confirm the sediment types obtained with SBP and GPR surveys. Sediment coring techniques including the standard Shelby tube (7.6 cm outside diameter) were used to collect bottom sediments. The thin-wall tube is used for obtaining less undisturbed soil samples. To facilitate the hand deployment of the Shelby tubes from the small boats used in this study, light aluminum tubes were used instead of the traditional steel tubes. After collecting the sediment, the tubes were covered on the both sides with a plastic cap and duct-tape and carefully transported back to the laboratory for characterization and analysis. The hand-deployed Shelby tubes provide a simple and economical way to collect soft sediment samples but limited the coring length to less than 1 m. For obtaining longer cores or coring in stronger, consolidated sediments, the machinedriven vibracore is a useful, more expensive alternative (Morales, 1997). 2.7. Ground-truth data collection: underwater camera-aided hydraulic jetting system Hydraulic jetting is used to directly measure the thickness of sandy sediments. This technique can be used as an alternative to the Shelby tube coring technique (Rukavina and Lahaie, 1991). The equipment is portable and can be operated from a boat (Fig. 3). The jetting pipe was assembled and coupled to a water pump through a flexible hose. The water jet fluidized the sandy sediment column as the jetting pipe advanced into the sediment column. Penetration depth is read using a

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Fig. 4. GPR data processing: (a) Raw GPR data. (b) GPR data after background subtraction. (c) GPR data after gain application. (d) Picking of the first time arrival at the lake bottom. (e) Moving average application for the elimination of wave effects on data. (f) Complete processed data (Depth calculated based on EM wave velocity in water).

graduated marking on the pipe. Penetration continued until the jet encountered bedrock or firm glacial sediments. The depth of jetting pipe penetration is the thickness of the sandy sediment layer.

resolutions, signal footprints, and source/receiver directivity functions. Data processing procedures followed for SBP and GPR results presented here are shown in Figs. 4 and 5.

3. Data processing

3.1. Systematic noise

The proper interpretation of SBP and GPR information requires addressing several issues: systematic noise, wave action noise, horizontal smearing due to signal stacking, and differing equipment

Both SBP and GPR data show a number of systematic wavelets that are related to ringing of the transmitter head (SBP measurements) and multiple reflections between transmitter and receiver

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Fig. 5. SBP data processing: (a) Raw data. (b) Amplitude versus depth and evaluation of strong reflectors. (c) and (d) Evaluation of wave period and selection of moving average period. (e) Wave action noise removed from survey traces (Depth is calculated based on the acoustic wave velocity in water).

antennae (GPR measurements). Furthermore, EM wave reflections from the boat, batteries, and SBP profiler appeared in the collected signals and lead to spurious reflections. These false reflections were eliminated using a spatial filter since all the spurious wavelets appear at the same time and with the same amplitude in each trace. By subtracting from a single trace the average of surrounding traces, the spurious reflections were removed (Fig. 4b). This type of spatial filtering procedure is known as a background subtraction filter. Gains were applied to the resulting traces to enhance the presence of bottom and subbottom reflectors (Fig. 4c).

3.2. Wave action noise Wave action displaced each trace by a certain amount depending upon whether the trace was gathered in a wave crest, in a wave trough, or somewhere in between. The sequence of processing steps used to remove the wave action is based on the following procedure: i) Convert the travel time to water depths by using water acoustic and EM wave speeds (VPwater =1500 m/s and Vwater EM =0.033 m/ns). In both cases, the velocity of wave propagation in water is well known.

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ii) Search and pick the time of the reflection from the water– sediment interface in each GPR or SBP trace. The picked arrivals create a time series that reflects the magnitude and period of wave action. Adjacent reflection arrivals may deviate from one another substantially due to the presence of waves on water surface. iii) Remove wave action by applying a moving average to the time series described in step (ii), obtaining a smooth curve of arrival times from the lake bottom. The number of traces over which to apply the moving average is based on the wave period, which can be observed from SBP data. SBP traces were acquired at a higher frequency (7.22 traces/s) than GPR traces (1.01 traces/s)

and allowed for the detection of the wave period in all surveys performed for this research. iv) Calculate the difference between reflection times determined for each individual trace and the reflection times calculated using the moving average at the same location. v) Shift each trace by the time difference calculated in step (iv). The wave action is then eliminated from GPR and SBP surveys and a clear view of the bathymetry and sublayers appears. Datasets gathered over Lake Superior were used as an example of this algorithm since roughest wave actions were experienced while

Fig. 6. Survey results of site P1 in Lake Superior. (a) Geographic location of the survey line, (b) Shoreline photograph, (c) SBP survey results, and (d) GPR survey results. Results show the position of two distinct troughs (T1 and T2) and sand bars (Bar1 and Bar2).

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performing these surveys (see Figs. 4a and 5a). After picking an arrival from the lake bottom for each trace (Figs. 4d and 5b), the bottom of the lake can be delineated and used to determine wave period and height. The SBP survey shows a wave period equal to 1.67 s and wave heights between 0.1 and 0.2 m (Fig. 5c). Therefore, a 2-s movingaveraging is sufficient to smooth the original SBP and GPR arrival times. Figs. 4e and 5d show the bathymetry before and after the removal of wave action, while Figs. 4f and 5e show both corrected bathymetry and sublayer information. 4. Results Several SBP and GPR profiles were collected in Lakes Superior, Mendota, and Michigan to characterize shallow water sediments. This section summarizes these results.

211

Due to the presence of silt and clay in the upper layer, the GPR signals seem to attenuate rapidly, unable to penetrate through the uppermost conductive sediments. The glacial till formations found in the near shore environment also correspond to results from previous studies in cohesive lakebeds along the Great Lakes (Kamphuis, 1990; Skafel and Bishop, 1994). Fig. 7a provides the geographic location of the P2 survey line. A photograph of the shoreline inland is shown in Fig. 7b. The processed GPR and SBP surveys are presented in Fig. 7c and d. The results show two prominent longshore bars between 80 and 150 m from the shoreline. The thickness of the top layer can be estimated by assuming the speed of acoustic and electromagnetic waves in sediments without ground truth information. Acoustic wave velocity is assumed to be around 1800 m/s in sediments. The EM velocity for low conductive, non ferromagnetic materials is: c VEM = p0ffiffiffi κ

4.1. Lake Superior site The survey sites were located at a cohesive lakebed area extending offshore from an area with coastal bluffs and near the mouth of the Brule River in Lake Superior (Fig. 1). Survey line P1 is shown in Fig. 6a, with a photograph of shoreline conditions shown in Fig. 6b. The bathymetry and sublayer information gathered in line P1 is shown in Fig. 6c and d. The actual water depth measured using a line attached to a weight was 1.31 m (approximate measurement accuracy: 0.5%), which was 2.1% smaller than the depth obtained with SBP data and 3.8% smaller than the depth obtained with the GPR data. The average difference in elevation of bathymetry between the SBP and GPR is 0.090 m with average water depth of 2.15 m (i.e.; a difference of 4.3%). Along survey line P1, the SBP profile indicates the presence of a twolayer structure in subsurface, while the GPR profile only identifies several minor reflectors (Fig. 6c and d). These reflectors could have been boulders or other point reflectors that do not identify any visible layer systems (see Fig. 6d — large boulders were seen from the boat during the survey). There was one 0.25 m-long sediment core collected to provide ground truth to the SBP and GPR profiles (Fig. 6c). Physical properties and soil type classification of the collected sediments are documented in Table 2. The collected sediments were classified according to the Unified Soil Classification System (ASTM designation D-2487). Kamphuis and Hall (1983) and Davidson-Arnott and Ollerhead (1995) documented the presence consolidated glacial tills nearshore cohesive bluffs in Great Lakes. These consolidated glacial tills have shear strengths ranging between 6.2 and 22.5 kPa. This high shear strength prevented the collection of a longer sediment cores using the hand-held Shelby tube sampler. The collected core showed that the uppermost part of the sediment (0–20 cm) contains 30% silt and clay, and that the silt and clay content decrease with depth and the lower part of the core (20–25 cm) has approximately 75% gravel and sand. The P-wave velocity in each layer can be estimated based on effective stress and porosity (Santamarina et al., 2005) and the reflection coefficient can be obtained using Eq. (5). The strong reflection coefficient at the 20-cm core sediment depth seems to confirm the two-layer structure shown in SBP survey results.

ð6Þ

where c0 (=0.3 m/ns) is the speed of light in free space, and κ is the relative real dielectric permittivity of the material. The relative real dielectric permittivity can be estimated using a mixture model (Wharton et al., 1980): β

β

β

β

κ = ð1 − nÞκ s + nSr κ w + nð1 − Sr Þka

ð7Þ

where ks (=3–8), kw (=80), and ka (=1) are the relative real dielectric permittivities of the solids, water and air phases, n is porosity, Sr is the degree of saturation, and β is an experimental constant assumed to be 0.5. Porosity is assumed to be 0.25 (using the ground truth data obtained on survey line P1) and Sr is set to 1 for saturated lakebed sediment. Therefore, the relative real dielectric permittivity is estimated to be k = 15.31 (assumed ks = 5). This relative permittivity yields radar velocity of 0.076 m/ns. The calculated thicknesses of the top layer are provided in Fig. 7c and d. However, the acoustic wave speed in saturated sediments with porosity of 0.25 may range from 1756 to 1843 m/s (Santamarina et al., 2005). And the EM wave speed may vary between 0.069 to 0.085 m/ns for the sediment dielectric permittivities ranging from 3 and 8. Therefore, the estimated thickness of top two layers using SBP and GPR surveys vary as documented in Table 3. The collection of sediment cores along survey line P2 was not successful. Instead, the hydraulic jetting system was used as the ground-truth method to compare with geophysical results. Hydraulic jetting was applied at the two marked locations in Fig. 7d. Glacial till prevented the jetting from penetrating subsurface sediments at Location 1. At Location 2, the hydraulic jet advanced approximately 20 cm into the uppermost sediments, an indication of the amount of sand overlying the glacial till (USACE, 2002). The hydraulic jetting can be compared with sublayer information gathered from GPR results in Fig. 7d. 4.2. Lake Mendota

Table 2 Sediment physical properties for ground-truth data in the Lake Superior survey site. Sample depth

Bulk density (g/cm3)

Porosity

⁎Reflection coefficient

0–5 cm 5–10 cm 10–15 cm 15–20 cm 20–25 cm

2.12 2.17 2.33 2.19 2.40

0.28 0.26 0.20 0.27 0.17

0.012 0.085 − 0.108 0.134

Sediment size (%)

Group name

GR

SA

SI

CL

2 0 3 4 7

54 55 67 62 69

27 27 20 21 21

17 18 10 13 3

Sandy lean clay Sandy lean clay Sandy silt Sandy silt Silt with sand

Symbols: GR = gravel; SA = sand; SI = silt; CL = clay. ⁎ These reflection coefficients were calculated using Eq. (5) while the P-wave velocity is obtained using the method proposed by Santamarina et al. (2005).

Lake Mendota surveys were performed at the mouth and inside the Yahara River (northern Lake Mendota) and near Picnic Point (southern Lake Mendota — Fig. 1). Fig. 8 shows the results of SBP and GPR surveys in the Yahara River. These results are compared to the ground-truth data water depth of 1.57 m (approximate measurement accuracy of 0.4%). Both SBP and GPR show approximately 2.8% larger water depth measurements. That is, the average difference in water depth between geophysical surveys and ground-truth information is 4.54 cm when water depth averages 1.65 m. In Fig. 8c and d, the dotted lines indicate the subbottom layers as determine by the SBP and GPR surveys. The ground-truth data collected using Shelby tube sampling showed organic-rich sediments (loss on ignition values indicate 40–50% organic

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Fig. 7. Survey results of site P2 in Lake Superior: (a) geographic location of the survey line, (b) shoreline photograph, (c) SBP survey results, and (d) GPR survey results. Hydraulic jetting system was used at locations 1 and 2. Results show a complicated near shore zone with several bars and troughs compared with Site P1 (Fig. 6).

content). Based on the physical properties and soil classification of the sediments collected in the Yahara River (see Table 4), the sediments can be divided as three separate units with 10 cm thickness. This observation matches the SBP survey measurements (Fig. 8c) while the GPR survey shows only two layers (see Fig. 8d). The results seem to be an indication of the presence of least 79% silt and clay in the top ~30 cm layer of sediment. This layer would highly attenuate the electromagnetic waves

and the penetration depth of the GPR signals was limited. Therefore, the GPR profile can only map few of these layers as shown in Fig. 8d. The second survey line in Lake Mendota was conducted west of Picnic Point along a sandy shoreline (Fig. 9a). Due to the sandy nature of the sediment, most of the SBP signal energy was reflected at the water–sediment interface instead of penetrating into subbottom sediments and only a few sublayers were visible along the profile (see

Y.-T. Lin et al. / Journal of Applied Geophysics 68 (2009) 203–218 Table 3 Thickness of top layer estimated by using SBP and GPR data in the Lake Superior survey site (see Figure 7). SBP data

GPR data

Velocity in Location 1: Location 2: Velocity in Location 1: Location 2: sediments top layer top layer sediments top layer top layer thickness thickness thickness thickness Upper limit 1843 m/s 0.69 m Lower limit 1756 m/s 0.66 m 1800 m/s 0.67 m Velocity adopted in this study

0.83 m 0.79 m 0.81 m

0.085 m/ns 0.68 m 0.069 m/ns 0.56 m 0.076 m/ns 0.61 m

0.96 m 0.78 m 0.86 m

213

data can map areas with underwater vegetation at the expense of attenuating signal strength. Therefore, the lake bottom cannot be seen clearly. The Underwater Camera-aided Hydraulic Jetting System provides images of the vegetated areas (see Fig. 9b). However, vegetation appears transparent to the GPR signals. Since sand dominated sediments at the water–sediment interface, Shelby tube sampling was not possible. Instead, the hydraulic jetting system helped to estimate the thickness of sandy layer from the bathymetric surface to the glacial till or bedrock. The thickness was 0.86 m and is marked in Fig. 9d. 4.3. Lake Michigan

Fig. 9c). In contrast, the GPR reflections were well-defined. GPR provided excellent images of the presence of sublayers under the sandy beach (see Fig. 9d). One very interesting feature is that the SBP

Surveys were performed near Concordia University in Lake Michigan (Fig. 1). Survey lines were run back and forth perpendicular to the shore to provide three-dimensional coverage of the nearshore

Fig. 8. Survey results at the mouth of the Yahara River in Lake Mendota. (a) Survey Line in the mouth of the Yahara River of Lake Mendota. (b) Bathymetry from SBP and GPR surveys. (c) SBP survey results. (d) GPR survey results.

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Table 4 Sediment physical properties for ground-truth data in Yahara River (Lake Mendota survey site). Sample depth

Bulk density (g/cm3)

Porosity

Sediment size (%) SA

SI

CL

0–10 cm 10–20 cm 20–30 cm

1.10 1.20 1.18

0.91 0.74 0.85

20 6 21

26 50 35

54 44 44

Group Name

Organic clay with sand Organic clay Organic silt

Symbols: SA = sand; SI = silt; CL = clay.

bottom and subbottom sediments (see Fig. 10a). The bathymetry obtained using SBP and GPR provided good estimates of water depth when compared with water depth ground-truth (errors 1.5% for SBP

survey and 2.6% for GPR survey). The average difference between SBP and GPR data was 4.67 cm with average water depth of 2 m (Fig. 10b). The sublayer results are presented in Fig. 10c. These images show that similar sublayers were delineated with SBP and GPR data, which may indicate that the mean grain size of bottom sediments in the foreshore areas ranges between sand and silt, a combination that may evenly favor both geophysical techniques. Using a zigzag survey profile, 3-D topographic images of the foreshore areas were constructed (Fig. 11). The approximate location of the sand bar and longshore trough can be estimated and one sublayer is delineated in deeper waters offshore (water depth is approximately 2.5 to 3.0 m). Ground-truth data could not be collected and only hydraulic jetting system was used in this site (see Fig. 11). The hydraulic jetting system yielded a top layer the thickness of 0.37 m and 0.61 m. These results compare with the 0.36 and 0.6 m

Fig. 9. Survey results near Picnic Point in Lake Mendota. (a) Survey Line west of Picnic Point in Lake Mendota. (b) An underwater photograph of the vegetation along the survey line (c) SBP survey results (d) GPR survey results.

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215

Fig. 10. Lake Michigan survey. (a) Geographic location of the survey. (b) Bathymetry from SBP and GPR surveys. (c) Summary of survey results. Traces emphasize the presence of subbottom layers.

thickness calculated using the SBP data assuming 1800 m/s sediment acoustic wave velocity. Therefore, in the area the thickness of the uppermost subsurface layer increases from 0.36 m to 0.6 m as the survey progressed north. 5. Discussion 5.1. Vertical resolution Bathymetry provided by both techniques typically differed from ground-truth information by less than 4%. The errors are mainly attributed to three reasons: (1) The acoustic speed in water is affected by temperature, salinity, hydrostatic pressure, suspended particulate matter and bubbles (Chen and Millero, 1977; Bilaniuk and Wong, 1993; Richards, 1998). As an example, if temperature varies between 15 and 25 °C, the acoustic speed has a maximum error about 2% when a velocity of 1500 m/s is assumed. Electromagnetic wave

velocity for GPR is controlled by the real dielectric permittivity of water (Eq. (6)), which is also a function of temperature and decreases as temperature increases (Hasted, 1972). The surveys were conducted between July and October, and the corresponding water temperature in survey sites ranged from 15 to 25 °C. Given the dielectric permittivity is set to 80, the maximum error is 1.4%. (2) The theoretical vertical resolutions of the SBP (200 kHz) and GPR (200 MHz) are 1.8 cm and 25 cm. In shallow water, errors associated with geophysical surveys are higher since a smaller depth is taken into consideration. (3) Travel time arrival determinations and data processing also leads to added errors. The travel time picks of bottom reflections were determined at the maximum amplitude signal response for both SBP and GPR traces. In the data processing procedures, the average wave period was adopted for moving average manipulation. For some waves with longer period, the results affected by wave motions may not be completely eliminated. Therefore, vertical elevation differences in bathymetry

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Fig. 11. Surveyed bathygraphy and subbottom topography along the Lake Michigan shoreline at Concordia University.

between SBP and GPR data acquired in this research project are acceptable. 5.2. Horizontal resolution The horizontal resolution can be determined by the Fresnel-based horizontal resolution: sffiffiffi t Reshorizontal = 2
r = V
ð8Þ f where r is the radius of the first Fresnel's zone, V is the wave velocity, t is the two-way travel time, and f is the frequency of the wave (Schwamborn et al., 2002). The calculated horizontal resolutions for SBP and GPR are 0.12 m and 1 m, respectively. However, the horizontal resolution is also dependent on the data smearing caused by the speed of the boat (~0.7 m/s), the beam angle (i.e. the footprint), pulse frequency, the transducer array, and the bandwidth of the source of the SBP and GPR systems (Arcone, 1995; Quinn, 1997; Schwamborn et al., 2002). In this study, the trace acquisition speed of the SBP (7.52 trace/m) is better than that of the GPR (1 trace/m). As a result, some small scale geological features in the subsurface (texture) may be smeared by the GPR survey. 5.3. Signal penetration When attempting to gather sublayer information, the SBP and GPR are dominant in different sediment environments due to the physics of propagation of acoustic and electromagnetic waves. In the bluff areas along Lake Superior (survey line P1) where clayey sediments dominate the subbottom, the SBP showed a two layer structure (silt/sand) while GPR signals were highly attenuated (see Fig. 6). On the other hand, in survey line P2 where a gradual beach shoreline replaced the bluff, both techniques defined the same sublayer stratigraphy (see Fig. 7). Without ground-truth data along survey line P2, the differences in penetration depth of the electromagnetic wave between survey lines P1 and P2 cannot be confirmed. However there is strong indication that survey line P2 may be located in an area with coarse-grain sediment deposition because it was located near the mouth of Brule River. At the mouth of Yahara River in Lake Mendota, ground-truth data showed that physical properties such as sediment density, and particle size distribution changed gradually with the depth. The gradual variations in sediment type are characteristic of fluvial deposits in lakes (Schwamborn et al., 2000). These types of sediments do not show much change in density with depth. Therefore, SBP information, which relies on changes in density to image sublayers, does not provide much

useful information. Conversely, the variation in porosity of the finegrained sediments can be relatively large (see Table 4), leading to larger contrasts in dielectric permittivity and stronger reflections from electromagnetic wave propagation (Davis and Annan, 1989). However, because the majority of the soil consists of silt and clay, the GPR signal was attenuated significantly, and thus in some locations, there was no sublayer detection. The sandy soils near Picnic Point prevented the SBP signal from being able to map sediment stratigraphy. Furthermore, the presence of vegetation further inhibited propagation of the acoustic wave and provided only weak images of bathymetry. However, GPR works well in sandy environments and is not affected by plant growth, providing excellent reflections of bathymetry and subbottom sediment layers. Shelby tube sampling failed in the sandy soils near Picnic Point, but the hydraulic jetting system was used obtained an approximate thickness of uppermost sand layer of 0.86 m. By converting two way travel time to the thickness of sandy layer (0.86 m), the radar speed was estimated as 0.057 m/ns, near to the value of saturated sand (0.06 m/ns -held Shelby tube coring technique was used to collect shallow sediment cores. However, the methodology has several limitations. For example, the technique could not be used to collect cores from consolidated bottom sediments like the ones found in the Lakes Superior and Michigan sites. In these sites only geophysical data were collected. However the combined geophysical techniques provide an approximate but acceptable understanding of the nature of sediment sublayers. 6. Conclusions The combined SBP and GPR techniques provide consistent bathymetry information in three study sites. Collecting GPR and SBP data concurrently also eliminates several logistical and technical problems. Combined surveying removes the possibility of surveying differing line positions with different equipment, reduces data acquisition time in the field, and eliminates seasonal effects. SBP and GPR provide complementary information because of differences in the way acoustic and electromagnetic waves propagate through different sediments. In clayey sediments (Lake Superior survey line P1 and Yahara River in Lake Mendota), GPR was unable to image deeper sublayers because of: (1) high electrical conductivity in silty and clayey sediments inhibit propagation of the electromagnetic waves and (2) boulders scatter and disrupt returning electromagnetic waves. When sediment permittivity varies more than mass density (such as at the mouth of Yahara River), GPR can effectively image subbottom layers. In sandy, vegetated soils (Picnic Point, Lake Mendota), SBP was not helpful, but GPR provided much useful information because of the ability of

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electromagnetic waves to travel through low conductivity soils and plant material. Therefore, in order to obtain complete sublayer information in different sediment conditions, combining SBP and GPR techniques are necessary and useful. Mapping of the subbottom environment can be directly obtained using ground-truth data or by estimation using geophysical tools. While ground-truth data were not always available, the hydraulic jetting system was used to estimate the thickness of the uppermost layer of sediment to the bed rock or glacial till layer below. The thickness of sublayers is estimated by assuming the speeds of acoustic and electromagnetic waves. The velocity of an acoustic wave in soil varies over a narrower range than that of an electromagnetic wave and gives a more accurate estimate of layer thickness. Since geophysical tools can be applied quickly over a large aerial extent (Concordia University, Lake Michigan), three-dimensional images of bathymetry and subbottom sediments can be effectively constructed for near shore regions. Acknowledgements This research was funded in part by the Wisconsin Coastal Management Program (WZMP-AD07956-008.37) and the National Science Foundation (CMMI-0553765). We thank Mr. Gene Clark at the University of Wisconsin Sea Grant Institute for his valuable suggestions and advice on this research. The continuous support of Mr. Michael Friis and Ms. Angel Kathleen at the WCPM is also greatly appreciated. References Annan, A.P., 2005. Ground penetrating radar. In: Butler, K. (Ed.), Near Surface Geophysics, pp. 357–438. Annan, A.P., Davis, J.L., 1977. Impulse radar applied to the thickness measurements and freshwater bathymetry. Geol. Surv. Can., Paper 77–1b, 63–65. Annan, A.P., Davis, J.L., 1992. Design and development of a digital ground penetrating radar system. In: Pilon, J. (Ed.), Ground Penetrating Radar. Geol. Surv. Can. Pap., vol. 90 (4), pp. 15–23. Arcone, S.A., 1995. Numerical studies of the radiation patterns of resistively loaded dipoles. J. Appl. Geophys. 33, 39–52. Ballard, R.F., Sjostrom, K.J., McGee, R.G., Leist, R.L., 1993. A rapid geophysical technique for subbottom imaging. Geophysical Techniques for Site and Material Characterization. GA, Atlanta, pp. 117–128. Bilaniuk, N., Wong, G.S.K., 1993. Speed of sound in pure water as a function of temperature. J. Acoust. Soc. Am. 93 (3), 1609–1612. Bradford, J.H., McNamara, J.P., Bowden, W., Gooseff, M., 2005. Measuring thaw depth beneath peat-lined arctic streams using ground-penetrating radar. Hydrol. Process. 19, 2689–2699. Brown, E.A., Wu, C.H., Mickelson, D.M., Edil, T.B., 2005. Factors controlling rates of bluff recession at two sites on Lake Michigan. J. Great Lakes Res. 31 (3), 306–321. Bruno, F., Martillier, F., 2000. Test of high-resolution seismic reflection and other geophysical techniques on the Boup landslide in the Swiss Alps. Surv. Geophys. 21 (4), 333–348. Cagatay, M.N., Gorur, N., Polonia, A., Demirbag, E., Sakinc, M., Cormier, M.-H., Capotondi, L., McHugh, C., Emre, O., Eris, K., 2003. Sea-level changes and depositional environments in the Izmit Gulf, eastern Marmara Sea, during the late glacial-Holocene period. Mar. Geol. 202 (3–4), 159–173. Carter, C.H., 1976. Lake shore erosion, Lake County, Ohio: setting, processes, and recession rates from 1876–1973. Ohio Div. Geol. Surv. Report No. 99. Chen, C.T., Millero, F.J., 1977. Speed of sound in seawater at high-pressure. J. Acoust. Soc. Am. 62 (5), 1129–1135. Davidson-Arnott, R.G.D., Ollerhead, F., 1995. Nearshore erosion on a cohesive shoreline. Mar. Geol. 122, 349–365. Davidson-Arnott, R.G.D., Langham, D.R.J., 2000. The effects of softening on nearshore erosion of a cohesive shoreline. Mar. Geol. 166, 145–162. Davis, J.L., Annan, A.P., 1989. Ground penetrating radar for high resolution mapping of soil and rock stratigraphy. Geophys. Prospect. 37, 531–551. Gabriel, G., Kirsch, R., Siemon, B., Wiederhold, H., 2003. Geophysical investigation of buried Pleistocene subglacial valleys in Northern Germany. J. Appl. Geophys. 53 (4), 159–180. Garambois, S., Senechal, P., Perroud, H., 2002. On the use of combined geophysical methods to assess water content and water conductivity of near-surface formations. J. Hydrol. 259 (1–4), 32–48. Garcia, G.A., Garcia-Gil, S., Vilas, F., 2004. Echo characters and recent sedimentary processes as indicted by high-resolution sub-bottom profiling in Ria de Vigo, NW Spain). Geo Mar. Lett. 24, 32–45. Gunn, D.A., Pearson, S., Chamber, J., Nelder, L., Lee, J., Beamish, D., Busby, J., Tinsley, R., Tinsley, W., 2006. An evaluation of combined geophysical and geotechnical methods to characterize beach thickness. Q. J. Eng. Geol. Hydrogeol. 39, 339–355. Hasted, J.B., 1972. In: Frank (Ed.), Liguid water: Dielectric properties in Water: A comprehensive treatise, vol. 1. Plenum Press, New York, pp. 255–309. 1972.

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