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Seismic reflection profiling in deep water: avoiding spurious reflectivity at lower-crustal and upper-mantle traveltimes
ELSEVIER
Tectonophysics
232 (1994) 425-435
Seismic reflection profiling in deep water: avoiding spurious reflectivity at lower-crustal and upper-mantle traveltimes J.H. McBride a, T.J. Henstock b, R.S. White b, R.W. Hobbs a a British Institutions Reflection Profiling Syndicate, Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 OEZ, UK b Bullard Laboratories, Department of Earth Sciences, Universify of Cambridge, Madingley Road, Cambridge CB3 OEZ, UK (Received
January
4, 1993; revised version
accepted
May 29,1993)
Abstract
The acquisition of seismic reflection data in deep water can be complicated by persistent multiple reflections from the seabed and sediments that continue to contaminate subsequent records beyond their initial source point. These multiples may create both severe and subtle artefacts that could easily be misinterpreted as sub-horizontal primary reflections from the lower crust or uppermost mantle. Contamination is particularly severe for shot repetition rates less than about 40 s (i.e. for shot-points spaced less than 100 m> and in areas with a sedimentary cover that is thick and contains reflection coefficients that are large relative to the seabed. Under these circumstances, shots fired at constant time increments maximise the likelihood of contamination in the stacked section. In this study, we use seismic reflection data from three sites in the North Atlantic to show how, and under what conditions, such multiples are produced and how they may mislead the interpreter. We also outline a field-recording procedure to minimise their effect on the common mid-point stacked profile.
1. Introduction The success of deep seismic reflection profiling over the continental shelf has encouraged the extension of the reflection technique into the deep ocean basins (NAT Study Group, 1985; Peddy et al., 1989; White et al., 1990; Rosendahl et al., 1991; Banda et al., 1992). Because of the much greater water depths involved, the manner in which reflections from the lower crust or uppermost mantle are recorded may require modification in order to mitigate the effects of persistent and high-amplitude multiples generated by a previous shot. The inadvertent recording of a 0040-1951/94/$07.00 0 1994 Elsevier SSDI 0040-1951(93)E0252-P
Science
multiple series from a previous shot, which we call “multiple wrap-around”, can produce spurious, but highly coherent, events at lower-crustal and upper-mantle traveltimes that masquerade as primary reflections (e.g. Tucker and Yorston, 1973). Using deep seismic reflection data from the Cape Verde abyssal plain, from the Western Approaches off southwestern England, and from the Blake Spur fracture zone area off the southeastern United States (Fig. 11, we assess the importance of the multiple wrap-around problem, show how contamination occurs and can simulate genuine primary reflectivity, and suggest a scheme to reduce or prevent its impact.
B.V. All rights reserved
426
2. The “multiple
J.H. McBride et al. / Tectonophysics 232 (1994) 425-43.5
wrap-around”
problem
As the water depth increases to thousands of metres, multiple reflections from the seabed and/or from a thick, well-layered sedimentary sequence travel increasingly long paths in the water column. In deep water, the resulting highamplitude series of simple multiples can bounce within the water column for long periods without the loss of amplitude that would otherwise accrue in shallow water (Taner, 19801. The relatively high repetition rate of reflection off a shallow seabed causes the amplitude of the multiple series to die out quickly compared to the shot repetition rate (Fig. 2). If the time delay between successive shots is much less than the time required for the multiple series to lose significant amplitude relative to the primaries, then contamination of a shot record by a multiple series from the previous shot will occur (Fig. 3). The time interval over which these multiples will arrive on the subsequent record depends on the water depth and the shot time delay. 2.1. OCEAN reflection survey A useful “laboratory” for studying the effects of deep-sea multiples is provided by the 578 km of deep seismic reflection profiles collected by
Fig. 2. Schematic demonstration of how the amplitude of multiples is lost rapidly in shallow water due to repeated reflection off the seabed and sea surface, compared to how the amplitude of multiples in deep water is retained over the same time interval. The decay of amplitude is directly proportional to the power of the number of bounces.
BIRPS (British Institutions Reflection Profiling Syndicate) as part of Project “OCEAN” over the oceanic crust beneath the Cape Verde abyssal plain in approximately 4900 m of water (Fig. 1). The profiles were recorded in the autumn of 1991 using GECO-PRAKLA’S M/V Bin Hai 511, producing a combination of strike and dip lines spaced at 4 km. The record length for the seismic pro-
0
‘*....,_,
--..
. . . . ...*._
M2 ..........
*‘*‘-* M3 ,.,.,_
5000
Water
Fig. 1. Location map for three deep seismic reflection surveys used in this study. BSFZ = Line 719 near Blake Spur fracture zone; CVA = BIRPS OCEAN survey over Cape Verde abyssal plain; WAM = BIRPS WAM survey over oceanic crust southwest of Western Approaches. Bathymetric contour interval is 1 km.
depth
(m)
Fig. 3. Arrival times for multiples from a previous shot (M2M,) based on a shot delay of 21 s (corresponding to a shot point interval of 50 m) and allowing a record length of 18 s, as used in the BIRPS OCEAN survey. Assumed seawater mean seismic velocity is 1500 m s-t. For this example, the zero-offset Moho reflection arrives 3 s after the seafloor reflection. BSFZ, CL%, and WAM indicate average water depths for Blake Spur fracture zone area, Cape Verde abyssal plain, and Western Approaches margin, respectively. M, is first seabed multiple, Mz is second, etc.
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Noise
test
Scaled
Amplitude
_ _ Hd B
lo-20Hz
0
20
40
60
80
100
20-40Hz
120
Fig. 4. (a) Noise test record (shot point 1589, OCEAN Line ll), made by recording without a shot, showing the high-amplitude and coherent wrap-around multiple reflections. The third and fourth multiples (Ms and M4) have arrived from the previous shot fired 21.66 s earlier. For display purposes, the record has been bandpass filtered 30-60 Ha, a linear gain has been applied, and a variable area-no wiggle trace mode used. Record is displayed for all 180 channels (group spacing = 25 m). It is sobering to realise that each shot is recorded against this “background” noise. (b) Amplitude decay curves for trace 10 from the record shown in (a) for two bandpass filters, (A) lo-20 Hz (at 7.5 and 25 dB/octave), and (B) 20-40 Hz (at 17 and 45 dB/octave). (c) Frequency spectrum for MY multiple. Note difficulty of isolating multiples Ms and M4 in frequency. Multiples from sediments just below the seabed tend to be much higher in amplitude than multiples from the seabed itself.
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files is 18 s (two-way time) and the shot-point interval is 50 m. Primary navigation was by Global Positioning System (GPS) satellites. The airgun source had a total capacity of 7118 in3 ( - 117 1) at 2000 psi (- 13.8 MPa), corresponding to 116.6 bar-m peak-to-peak amplitude (for O-128 Hz). Shooting conditions for the survey were very quiet with ambient root-mean-square pressure on the 4500 m, 180-channel receiver cable typically less than 2.0 pbar. For the OCEAN reflection survey, no deepwater static delay was used, which allowed us to record and study multiple reflections reverberating within the water column. Noise tests, produced by making an 18 s recording in sequence without firing the airguns, clearly show the resilience of the wrap-around multiples (Fig. 4a). An amplitude-frequency analysis (Fig. 4b,c) of the noise record illustrates the high energy retained even after a multiple has been travelling for over 30 s (Figs. 3 and 4b) and shows that the multiples contain a broad band of frequencies making them difficult to remove by band-pass filtering. During the cruise, an experiment was carried out to examine the multiple wrap-around problem by varying the shot delay time from 19.0 to 23.0 s at 0.5 s intervals, which would include possible delays for a 50 m shot interval at an absolute ship’s speed of about 5.1 to 4.2 knots, respectively. The results indicate that a poorly chosen shot delay time may bring the arrival of wrap-around multiples at traveltimes corresponding to the lower crust or uppermost mantle (e.g. Fig. 5). Because these multiples travel for such long times in the water column, they cannot easily be discriminated from primary reflections on the basis of normal move-out, so they are not susceptible to attenuation by stacking after velocity analysis (Levin, 1971). This exercise also indicates that the multiple from the seabed does not necessarily carry the highest amplitudes: the more complex multiples from the underlying thick well-layered sedimentary sequence may be much stronger (note position of the expected arrival of M3 and M4 from the seabed in Fig. 5 compared to the observed peak multiple energy). If the sedimentary section contains reflectors with a large reflection coefficient, then the resultant
M3+5s
-
Spurious Event:
Fig. 5. Shot record 1600 from OCEAN Line 11 with first 120 channels displayed out of 180. Note contamination from the third and fourth multiples CM, and M4) arriving from the previous shot fired 22.44 s earlier. M, and M4 have travelled 26.11 s and 32.64 s, respectively. Strong hyperbolic arrival at - 6.5 s is from the seabed; reflectivity between 8 and 10 s is from the igneous oceanic crust, including strong diffraction from the top of basement. Display mode as in Fig. 4a.
complex interaction of multiples generated in the section could account for our observation that the peak in amplitude occurs below the seabed multiple (Fig. 4b) (see also Hardy et al., 1989). 2.2. WAM reflection sumey The contamination by wrap-around multiples as described above can create spurious events in the common mid-point (CMP) domain which may invite interpretation as genuine primary reflectiv-
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ity. This will happen if the s~chronisation from shot-to-shot of primary events matches that of the wrap-around multiples. For example, the BIRPS WAM (Western Approaches Margin) deep reflection line 1 (Fig. 0, which was shot to 15 s using a 50 m source interval off the southwest of England in a water depth similar to OCEAN, suffered from the same multiple wrap-around problem as did OCEAN. Also like OCEAN, WAM was shot over oceanic crust with a significant, if somewhat thinner, sedimentary cover (Peddy et al., 1989; Hardy et al., 19901. Examination of successive shot records reveals a contamination problem similar to that observed for OCEAN, again from M3 and M4 multiples (Fig. 6). In this case, the multiples contain a strong periodicity generated by the sediment cover. Shot records from areas that lacked a sediment cover
over igneous basement show much less corruption from wrap-around multiples, although they are still present (Fig. 7). The 1985 WAM survey employed a nominally constant shot spacing without a constant shot time interval but because the primary navigation was limited to LORAN C, the degree of error in timing was probably sufficient to at least partially randomise the timing between successive arrivals of wrap-around multiples (Fig. 6). For most areas along the line, the M3 and M4 multiples appear to have been cancelled by destructive interference after the CMP sort and stack. The net effect on the WAM stack (Fig. 8) was to produce only very limited, if any, contamination caused by incompletely canceiled multiples over short regions of the line between about 10.5 and 12.5 s (corresponding to the interval over which the M4 series arrives; Fig. 6). Al-
5
SEA BED
M4 zone I
SP
649
Fig. 6. Shot records from WAM Line 1 with first 30 channels displayed out of 60. Note ~nta~nation multiples CM3 and M4) arriving from the previous shot. Display mode as in Fig. 4a.
656 from the third and fourth
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.I.H. McBride et af. / Tectonophysics 232 f1994) 425-435
though coherent contamination by wrap-around multiples is limited in this case, they still can represent a significant increase in the background level of incoherent noise.
SP 5
1101
2.3. Blake Spur fracture zone reflection survey A serious form of multiple contamination in the CMP domain can occur if the survey is shot using a strictly constant time interval, as is often used for deep sea surveys. As part of a wide-angle and multichannel seismic reflection programme over the Blake Spur fracture zone southwest of Bermuda (Fig. 1) (White et al., 1990; Minshull et al., 1991; Morris et al., 19931, Line 719 was collected using an approximately 5980 in3
BIRPS WAM
sEABED
551
552
Fig. 7. WAM Line 1 shot records over small basement high shown in Fig. 8. Note marked reduction of multiple contamination over expected &fe zone compared to areas with thick sedimentary cover (Fig. 6). Display mode as in Fig. 4a.
u... 0
__..--.i !O km
Fig. 8. Portion of CMP-stacked section from WAM-1. Time interval marked is where contamination from M4 multiple would be expected. Diamonds are lined beneath the area from where the shot records in Fig. 6 are displayed. We see essentially little or no contamination over this interval (except occasionally very short events) because the error in timing associated with small navigation errors probably effectively randomised the shot delays. M = Moho. Section has been processed with a predictive de~onvolution, frequency-wavenumber filter, and a bandpass frequency filter as used for displaying the shot records (Figs. 6 and 7).
( N 98 1) airgun source and a 240-channel, 3000 m receiver cable recording to 12 s. Line 719, which was shot with a 20 s constant shot-time delay along most of the line, in places contains a strong series of wrap-around multiples on the CMP stack similar in appearance to the series observed on the OCEAN and WAM shot records. Fig. 9a shows one of the more severe examples of these multiples on the CMP stack with a series starting at 9.80 s. For an observed two-way traveltime to the seabed of 7.23 s and a known constant shottime delay of 20 s, the M7 multiple series, beginning with the seabed multiple itself, should start at 8.92 s and the M4 series at 16.15 s, harmlessly beyond the end of the 12 s record. The two questions that must be answered are: why does the traveltime onset of the observed multiple series not match that of the predicted M3; and why are the wrap-around multiples so severe along this part of the line? Because any simple multiple series generated from a given sequence of pri-
J.H. McBride et al. / Tectonophysics
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232 (1994) 425-435
where the primary series is approximated by the observed deconvolved reflection from the sediments, we can compute M3 directly from the CMP traces as an autoconvolution. The results of this exercise (Fig. 10) show clearly that the seabed multiple contributes little to the multiple series. A similar conclusion is suggested by the observed amplitudes of the wrap-around multiples on the OCEAN reflection survey. The four main traveltime onsets in M3 observed on the single trace
and on the CMP stack (Figs. 9a and 10) match closely those produced on the autoconvolved single trace (Fig. 10). Computing autoconvolutions from other areas along the line (e.g. from the section in Fig. 9b) where the sedimentary section is thinner and/or sediment reflections less coherent and continuous, and comparing them with the observed section suggests that the strength of the multiple series is enhanced by a thicker, more continuous and reflective sedimentary section overlying basement. Although most workers would at least suspect the more severe example in Fig. 9a to be an artefact of some kind, more common and insidious are wrap-around multiple series such as those shown in Fig. 9b from along the same line which could easily be interpreted as primary reflections from the lower crust or upper mantle. After migration, additional filtering, and
(a) 7
6)
mary reflectors can be expressed as the appropriate combination of convolutions c”> or M, = primary series * primary series M2 = M, *primary series M3 = M2 *primary series = (M, *primary series) * primary series = M, * M,
I-
2 km
1 0 -2 -
_ -
-
_-
km
Fig. 9. (a) Portion of migrated CMP section from Line 719 in the Blake Spur fracture zone area (for location see Fig. 1) showing strong wrap-around multiples. A, B, C, and D refer to multiple phases identified in Fig. 10. (b) Elsewhere along same line showing wrap-around multiples (arrows) that could easily be misinterpreted as primaries. Line 719 was shot with a strictly constant shot time delay. Principal processing by Lamont Doherty Earth 0bseIvatot-y.
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J.H. McBride et al. / Tectonophysics 232 (1994) 425-435
depth conversion, these artefacts may become almost indistinguishable from the real thing.
0
3. Mitigating wrap-around multiples in the CMP domain
. SEA BOTTOM PRIMARY
i
I I
For the OCEAN reflection survey, we took a two-step approach to reducing the effect of multiple wrap-around. While primarily controlling the shot distance interval on the basis of GPS navigation, we first imposed a nominally constant shottime delay of 21 s in order to monitor the multiples and to place them where they would cause
6-
L 0 1
-
Moho
lst MULTIPLE
20
-
-
2nd MULTIPLE
b
5
SEABED primary sediment series
SECOND SHOT
TC
: -8
3rd MULTIPLE SEA BOTTOM PRIMARY
:L‘I
10? XI P I
-
Moho
k 4th MULTIPLE
S
cc-
1st MULTIPLE CF SECOND SHUT
,tu
s
L I
@L
20
L,
2nd MULTIPLE OF SECmD
SHOT
a: QFig. 11. Diagram showing arrival times of various sea bottom multiples through two consecutive shot recordings using the fixed shot time delay of 21 s chosen for the OCEAN reflection survey where water depths are about 4900 m.
(IL-
L E-L-_.-_
_&
y’ :
zz%
-_.-_-
12 -
M3 = Ml
* MI
-
ObservedCMP trace
Fig. 10. Single stacked CMP trace (4440) from Line 719 (Fig. 9a). M, * M, used to synthesize the M, wrap-around multiple from portion of observed trace using primary series from sediment reflections. Four arrows show close match of traveltime onsets in M, with those observed on single trace and on stack (Fig. 9a). t,, is start time of M, * M, synthesized from primary sediment series.
the least contamination of primary reflections. As seen in Fig. 11, we placed M, where the peak arrivals would be within the water column above the strongest primaries; M4 is just above the M, of the current shot and is well below the anticipated arrival of the Moho reflection (Fig. 12a). As noted in Fig. 12b, the multiple wrap-around
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J.H. McBride et al. / Tectonophysics 232 (1994) 425-435
problem could have been more or less avoided by using a 100 m shot spacing, since during the intervening N 40 s, most of the multiple energy would have died away; however, this would have halved the fold of cover for the CMP stack and
produced potential aliasing problems for dipping reflectors. The second step was to introduce a pseudo-random time delay (l-255 ms) trigger device, designed and built at Bullard Laboratories, inserted between the ship’s navigation computer
(a)
(b) 0
0
:
~
r ._ .:-.
.-_
i--. :
._ .:-:_ .__ _:
tCM4
Fig. 12. (a) Typical shot record from OCEAN survey (Line 11). Note arrivals of M, and MJ in water column (above 6.5 s) and lack of a strong M4 in lower part of record (lo-13 s) as opposed to prominent M4 in Fig. 5 (exact arrival of multiples is not known due to the random time delay). M4 is more or less absent because its strongest portion (i.e. as much as - 1 s after its onset time) occurs beyond the end of the record. Constant delay was 20.70 s plus a random time delay. (b) Shot record (1590, Line 11) immediately after noise test (when no shot was fired; Fig. 4a). The energy from the wrap-around multiples has died away during the approximately 42 s shot delay. Refer to Fig. 5 for explanation of primary features.
.I.H. McBride et al. / Tectonophysics 232 (I 994) 425-435
434
8
S 10
0
-”
BIRPS
OCEAN
Line
11
Fig. 13. Constant-velocity (1525 m s- ‘) finite difference migration of a portion of OCEAN Line Il. Section is displayed variable area, no wiggle trace with a linear gain and a frequency-wavenumber filter. M = Moho reflection.
output and the profiling system. Superimposing this delay on the 21 s shot time delay helped to increase the likelihood that previous shot multiples would tend to cancel one another when summed into the CMP domain. As a result of these two steps, high-amplitude coherent wraparound multiples such as observed from parts of the Blake Spur and other deep reflection surveys are not observed on the OCEAN CMP sections (e.g. Fig. 13). In a few instances on the OCEAN sections, principally below small synforms on the surface of the igneous crust, noise cancellation is incomplete in the 11-13 s interval on the stack; however, the randomisation has reduced the level of relative amplitude, periodicity, and coherency to well below that which would otherwise have developed, such as on the Blake Spur example (Fig. 9a). This emphasises the importance of having controlled the shot time interval during which such artefacts arise. Shooting with a strictly constant time delay synchronises primary and wrap-around multiple events between shots, so that corruption of the CMP stack is likely if the shot delay time is chosen so that multiples arrive during the recording time. If Line 719 had been shot with an 18 s,
instead of a 20 s (in places, 22 s), delay, then the worst part of the multiple corruption would have arrived near the bottom of the record and not within the zone of interest. Once at sea, practical considerations such as the minimum time that the compressor can be recharged and the minimum ship speed at which the receiver cable can be controlled will ultimately limit the available choices of shot time delay. Shooting with a positioning-based navigation system may effectively randomise the shot delays, as seems to have happened for WAM (Fig. 8); however, it may be risky to depend on this, especially when a loss of the primary navigation system leads to the temporary measure of shooting on a constant or only slowly varying time interval.
4. Conclusions As reflection profiling programmes move into the deep water of the ocean basins, contamination from multiple wrap-around can be a serious problem in producing artefacts that could be misinterpreted as reflectivity from the lower crust and uppermost mantle. We use examples from the North Atlantic to demonstrate how this problem develops for three different acquisition schemes, and suggest a procedure for alleviating contamination without requiring significant alteration of the original programme. For deep sea ( > 2-3 km water depth) surveys, contamination is a problem for shot-point spacings of less than 100 m at the usual ship speed and is particularly severe in areas with thick and strongly reflective sedimentary cover. Under these circumstances, recording with a strictly constant time delay will maximise the likelihood of contamination.
Acknowledgements
Data acquisition for the BIRPS project is made possible through grants from the Natural Environment Research Council and through the BIRPS Industrial Associates [Amerada Hess Ltd., Amoco Production Co., BP Exploration Co. Ltd., Chevron UK Ltd., Conoco (UK) Ltd., Enterprise
J.H. McBride et al. / Tectonophysics 232 (1994) 425-435
Oil plc, Lasmo North Sea PLC, Mobil North Sea Ltd., and Shell UK Exploration and Production]. R.W. England provided invaluable assistance as a client observer at sea during the BIRPS OCEAN cruise. We thank GECO-PRAKLA and Party Chief Rolf Henriksen for helping to make the BIRPS OCEAN cruise a success. Data processing for this study was performed on the GECO-PRAKLA Seismic Kernel System operating at Bullard Laboratories. Reviews by F. Marillier and two anonymous referees substantially improved the paper. This is University of Cambridge Department of Earth Sciences Contribution No. 3109.
References Banda, E., Ranero, C.R., Danobeitia, J.J. and Rivero, A., 1992. Seismic boundaries of the eastern Central Atlantic Mesozoic crust from multichannel seismic data. Geol. Sot. Am. Bull., 104: 1340-1349. Hardy, R.J.J., Hobbs, R.W. and Warner, M.R., 1989. Labeling long-period multiple reflections. Geophysics, 54: 122-126. Hardy, R.J.J., Warner, M.R. and The BIRPS Group, 1990. Imaging the lower crust in deep water. In: J.H. Leven, D.M. Finlayson, C. Wright, J.C. Dooley and B.L.N. Ken-
435
nett (Editors), Seismic Probing of Continents and their margins. Tectonophysics, 173: 141-143. Levin, F.K., 1971. Apparent velocity from dipping interface reflections. Geophysics, 36: 510-516. Minshull, T.A., White, R.S., Mutter, J.C., Buhl, P., Detrick, R.S., Williams, C.A. and Morris, E., 1991. Crustal structure at the Blake Spur fracture zone from expanding spread profiles. J. Geophys. Res., 96: 9955-9984. Morris, E., Detrick, R.S., Minshull, T.A., Mutter, J.C., White, R.S., Su, W. and Buhl, P., 1993. Seismic structure of oceanic crust in the Western North Atlantic. J. Geophys. Res., 98: 13,879-13,903. NAT Study Group, 1985. North Atlantic transect: A wideaperture, two-ship multichannel seismic investigation of the oceanic crust. J. Geophys. Res., 90: 10,321-10,341. Peddy, C., Pinet, B., Masson, D., Scrutton, R., Sibuet, J.-C., Warner, M.R., Lefort, J.P. and Schroeder, I.J. (BIRPS and ECORS), 1989. Crustal structure of the Goban spur continental margin, northeastern Atlantic, from deep seismic reflection profiling. J. Geol. Sot. London, 146: 427-437. Rosendahl, B.R., Groschel-Becker, H., Meyers, J. and Kaczmarick, K., 1991. Deep seismic reflection study of a passive margin, southeastern Gulf of Guinea. Geology, 19: 291-295. Taner, M.T., 1980. Long period sea-floor multiples and their suppression. Geophys. Prospect., 28: 30-48. Tucker, P.M. and Yorston, H.J., 1973. Pitfalls in Seismic Interpretation. Sot. Explor. Geophys., Tulsa, OK, Monogr. Ser., 2, 50 pp. White, R.S., Detrick, R.S., Mutter, J.C., Buhl, P., Minshull, T.A. and Morris, E., 1990. New seismic images of oceanic crustal structure. Geology, 18: 462-465.
Tectonophysics
232 (1994) 425-435
Seismic reflection profiling in deep water: avoiding spurious reflectivity at lower-crustal and upper-mantle traveltimes J.H. McBride a, T.J. Henstock b, R.S. White b, R.W. Hobbs a a British Institutions Reflection Profiling Syndicate, Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 OEZ, UK b Bullard Laboratories, Department of Earth Sciences, Universify of Cambridge, Madingley Road, Cambridge CB3 OEZ, UK (Received
January
4, 1993; revised version
accepted
May 29,1993)
Abstract
The acquisition of seismic reflection data in deep water can be complicated by persistent multiple reflections from the seabed and sediments that continue to contaminate subsequent records beyond their initial source point. These multiples may create both severe and subtle artefacts that could easily be misinterpreted as sub-horizontal primary reflections from the lower crust or uppermost mantle. Contamination is particularly severe for shot repetition rates less than about 40 s (i.e. for shot-points spaced less than 100 m> and in areas with a sedimentary cover that is thick and contains reflection coefficients that are large relative to the seabed. Under these circumstances, shots fired at constant time increments maximise the likelihood of contamination in the stacked section. In this study, we use seismic reflection data from three sites in the North Atlantic to show how, and under what conditions, such multiples are produced and how they may mislead the interpreter. We also outline a field-recording procedure to minimise their effect on the common mid-point stacked profile.
1. Introduction The success of deep seismic reflection profiling over the continental shelf has encouraged the extension of the reflection technique into the deep ocean basins (NAT Study Group, 1985; Peddy et al., 1989; White et al., 1990; Rosendahl et al., 1991; Banda et al., 1992). Because of the much greater water depths involved, the manner in which reflections from the lower crust or uppermost mantle are recorded may require modification in order to mitigate the effects of persistent and high-amplitude multiples generated by a previous shot. The inadvertent recording of a 0040-1951/94/$07.00 0 1994 Elsevier SSDI 0040-1951(93)E0252-P
Science
multiple series from a previous shot, which we call “multiple wrap-around”, can produce spurious, but highly coherent, events at lower-crustal and upper-mantle traveltimes that masquerade as primary reflections (e.g. Tucker and Yorston, 1973). Using deep seismic reflection data from the Cape Verde abyssal plain, from the Western Approaches off southwestern England, and from the Blake Spur fracture zone area off the southeastern United States (Fig. 11, we assess the importance of the multiple wrap-around problem, show how contamination occurs and can simulate genuine primary reflectivity, and suggest a scheme to reduce or prevent its impact.
B.V. All rights reserved
426
2. The “multiple
J.H. McBride et al. / Tectonophysics 232 (1994) 425-43.5
wrap-around”
problem
As the water depth increases to thousands of metres, multiple reflections from the seabed and/or from a thick, well-layered sedimentary sequence travel increasingly long paths in the water column. In deep water, the resulting highamplitude series of simple multiples can bounce within the water column for long periods without the loss of amplitude that would otherwise accrue in shallow water (Taner, 19801. The relatively high repetition rate of reflection off a shallow seabed causes the amplitude of the multiple series to die out quickly compared to the shot repetition rate (Fig. 2). If the time delay between successive shots is much less than the time required for the multiple series to lose significant amplitude relative to the primaries, then contamination of a shot record by a multiple series from the previous shot will occur (Fig. 3). The time interval over which these multiples will arrive on the subsequent record depends on the water depth and the shot time delay. 2.1. OCEAN reflection survey A useful “laboratory” for studying the effects of deep-sea multiples is provided by the 578 km of deep seismic reflection profiles collected by
Fig. 2. Schematic demonstration of how the amplitude of multiples is lost rapidly in shallow water due to repeated reflection off the seabed and sea surface, compared to how the amplitude of multiples in deep water is retained over the same time interval. The decay of amplitude is directly proportional to the power of the number of bounces.
BIRPS (British Institutions Reflection Profiling Syndicate) as part of Project “OCEAN” over the oceanic crust beneath the Cape Verde abyssal plain in approximately 4900 m of water (Fig. 1). The profiles were recorded in the autumn of 1991 using GECO-PRAKLA’S M/V Bin Hai 511, producing a combination of strike and dip lines spaced at 4 km. The record length for the seismic pro-
0
‘*....,_,
--..
. . . . ...*._
M2 ..........
*‘*‘-* M3 ,.,.,_
5000
Water
Fig. 1. Location map for three deep seismic reflection surveys used in this study. BSFZ = Line 719 near Blake Spur fracture zone; CVA = BIRPS OCEAN survey over Cape Verde abyssal plain; WAM = BIRPS WAM survey over oceanic crust southwest of Western Approaches. Bathymetric contour interval is 1 km.
depth
(m)
Fig. 3. Arrival times for multiples from a previous shot (M2M,) based on a shot delay of 21 s (corresponding to a shot point interval of 50 m) and allowing a record length of 18 s, as used in the BIRPS OCEAN survey. Assumed seawater mean seismic velocity is 1500 m s-t. For this example, the zero-offset Moho reflection arrives 3 s after the seafloor reflection. BSFZ, CL%, and WAM indicate average water depths for Blake Spur fracture zone area, Cape Verde abyssal plain, and Western Approaches margin, respectively. M, is first seabed multiple, Mz is second, etc.
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Noise
test
Scaled
Amplitude
_ _ Hd B
lo-20Hz
0
20
40
60
80
100
20-40Hz
120
Fig. 4. (a) Noise test record (shot point 1589, OCEAN Line ll), made by recording without a shot, showing the high-amplitude and coherent wrap-around multiple reflections. The third and fourth multiples (Ms and M4) have arrived from the previous shot fired 21.66 s earlier. For display purposes, the record has been bandpass filtered 30-60 Ha, a linear gain has been applied, and a variable area-no wiggle trace mode used. Record is displayed for all 180 channels (group spacing = 25 m). It is sobering to realise that each shot is recorded against this “background” noise. (b) Amplitude decay curves for trace 10 from the record shown in (a) for two bandpass filters, (A) lo-20 Hz (at 7.5 and 25 dB/octave), and (B) 20-40 Hz (at 17 and 45 dB/octave). (c) Frequency spectrum for MY multiple. Note difficulty of isolating multiples Ms and M4 in frequency. Multiples from sediments just below the seabed tend to be much higher in amplitude than multiples from the seabed itself.
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files is 18 s (two-way time) and the shot-point interval is 50 m. Primary navigation was by Global Positioning System (GPS) satellites. The airgun source had a total capacity of 7118 in3 ( - 117 1) at 2000 psi (- 13.8 MPa), corresponding to 116.6 bar-m peak-to-peak amplitude (for O-128 Hz). Shooting conditions for the survey were very quiet with ambient root-mean-square pressure on the 4500 m, 180-channel receiver cable typically less than 2.0 pbar. For the OCEAN reflection survey, no deepwater static delay was used, which allowed us to record and study multiple reflections reverberating within the water column. Noise tests, produced by making an 18 s recording in sequence without firing the airguns, clearly show the resilience of the wrap-around multiples (Fig. 4a). An amplitude-frequency analysis (Fig. 4b,c) of the noise record illustrates the high energy retained even after a multiple has been travelling for over 30 s (Figs. 3 and 4b) and shows that the multiples contain a broad band of frequencies making them difficult to remove by band-pass filtering. During the cruise, an experiment was carried out to examine the multiple wrap-around problem by varying the shot delay time from 19.0 to 23.0 s at 0.5 s intervals, which would include possible delays for a 50 m shot interval at an absolute ship’s speed of about 5.1 to 4.2 knots, respectively. The results indicate that a poorly chosen shot delay time may bring the arrival of wrap-around multiples at traveltimes corresponding to the lower crust or uppermost mantle (e.g. Fig. 5). Because these multiples travel for such long times in the water column, they cannot easily be discriminated from primary reflections on the basis of normal move-out, so they are not susceptible to attenuation by stacking after velocity analysis (Levin, 1971). This exercise also indicates that the multiple from the seabed does not necessarily carry the highest amplitudes: the more complex multiples from the underlying thick well-layered sedimentary sequence may be much stronger (note position of the expected arrival of M3 and M4 from the seabed in Fig. 5 compared to the observed peak multiple energy). If the sedimentary section contains reflectors with a large reflection coefficient, then the resultant
M3+5s
-
Spurious Event:
Fig. 5. Shot record 1600 from OCEAN Line 11 with first 120 channels displayed out of 180. Note contamination from the third and fourth multiples CM, and M4) arriving from the previous shot fired 22.44 s earlier. M, and M4 have travelled 26.11 s and 32.64 s, respectively. Strong hyperbolic arrival at - 6.5 s is from the seabed; reflectivity between 8 and 10 s is from the igneous oceanic crust, including strong diffraction from the top of basement. Display mode as in Fig. 4a.
complex interaction of multiples generated in the section could account for our observation that the peak in amplitude occurs below the seabed multiple (Fig. 4b) (see also Hardy et al., 1989). 2.2. WAM reflection sumey The contamination by wrap-around multiples as described above can create spurious events in the common mid-point (CMP) domain which may invite interpretation as genuine primary reflectiv-
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ity. This will happen if the s~chronisation from shot-to-shot of primary events matches that of the wrap-around multiples. For example, the BIRPS WAM (Western Approaches Margin) deep reflection line 1 (Fig. 0, which was shot to 15 s using a 50 m source interval off the southwest of England in a water depth similar to OCEAN, suffered from the same multiple wrap-around problem as did OCEAN. Also like OCEAN, WAM was shot over oceanic crust with a significant, if somewhat thinner, sedimentary cover (Peddy et al., 1989; Hardy et al., 19901. Examination of successive shot records reveals a contamination problem similar to that observed for OCEAN, again from M3 and M4 multiples (Fig. 6). In this case, the multiples contain a strong periodicity generated by the sediment cover. Shot records from areas that lacked a sediment cover
over igneous basement show much less corruption from wrap-around multiples, although they are still present (Fig. 7). The 1985 WAM survey employed a nominally constant shot spacing without a constant shot time interval but because the primary navigation was limited to LORAN C, the degree of error in timing was probably sufficient to at least partially randomise the timing between successive arrivals of wrap-around multiples (Fig. 6). For most areas along the line, the M3 and M4 multiples appear to have been cancelled by destructive interference after the CMP sort and stack. The net effect on the WAM stack (Fig. 8) was to produce only very limited, if any, contamination caused by incompletely canceiled multiples over short regions of the line between about 10.5 and 12.5 s (corresponding to the interval over which the M4 series arrives; Fig. 6). Al-
5
SEA BED
M4 zone I
SP
649
Fig. 6. Shot records from WAM Line 1 with first 30 channels displayed out of 60. Note ~nta~nation multiples CM3 and M4) arriving from the previous shot. Display mode as in Fig. 4a.
656 from the third and fourth
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though coherent contamination by wrap-around multiples is limited in this case, they still can represent a significant increase in the background level of incoherent noise.
SP 5
1101
2.3. Blake Spur fracture zone reflection survey A serious form of multiple contamination in the CMP domain can occur if the survey is shot using a strictly constant time interval, as is often used for deep sea surveys. As part of a wide-angle and multichannel seismic reflection programme over the Blake Spur fracture zone southwest of Bermuda (Fig. 1) (White et al., 1990; Minshull et al., 1991; Morris et al., 19931, Line 719 was collected using an approximately 5980 in3
BIRPS WAM
sEABED
551
552
Fig. 7. WAM Line 1 shot records over small basement high shown in Fig. 8. Note marked reduction of multiple contamination over expected &fe zone compared to areas with thick sedimentary cover (Fig. 6). Display mode as in Fig. 4a.
u... 0
__..--.i !O km
Fig. 8. Portion of CMP-stacked section from WAM-1. Time interval marked is where contamination from M4 multiple would be expected. Diamonds are lined beneath the area from where the shot records in Fig. 6 are displayed. We see essentially little or no contamination over this interval (except occasionally very short events) because the error in timing associated with small navigation errors probably effectively randomised the shot delays. M = Moho. Section has been processed with a predictive de~onvolution, frequency-wavenumber filter, and a bandpass frequency filter as used for displaying the shot records (Figs. 6 and 7).
( N 98 1) airgun source and a 240-channel, 3000 m receiver cable recording to 12 s. Line 719, which was shot with a 20 s constant shot-time delay along most of the line, in places contains a strong series of wrap-around multiples on the CMP stack similar in appearance to the series observed on the OCEAN and WAM shot records. Fig. 9a shows one of the more severe examples of these multiples on the CMP stack with a series starting at 9.80 s. For an observed two-way traveltime to the seabed of 7.23 s and a known constant shottime delay of 20 s, the M7 multiple series, beginning with the seabed multiple itself, should start at 8.92 s and the M4 series at 16.15 s, harmlessly beyond the end of the 12 s record. The two questions that must be answered are: why does the traveltime onset of the observed multiple series not match that of the predicted M3; and why are the wrap-around multiples so severe along this part of the line? Because any simple multiple series generated from a given sequence of pri-
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where the primary series is approximated by the observed deconvolved reflection from the sediments, we can compute M3 directly from the CMP traces as an autoconvolution. The results of this exercise (Fig. 10) show clearly that the seabed multiple contributes little to the multiple series. A similar conclusion is suggested by the observed amplitudes of the wrap-around multiples on the OCEAN reflection survey. The four main traveltime onsets in M3 observed on the single trace
and on the CMP stack (Figs. 9a and 10) match closely those produced on the autoconvolved single trace (Fig. 10). Computing autoconvolutions from other areas along the line (e.g. from the section in Fig. 9b) where the sedimentary section is thinner and/or sediment reflections less coherent and continuous, and comparing them with the observed section suggests that the strength of the multiple series is enhanced by a thicker, more continuous and reflective sedimentary section overlying basement. Although most workers would at least suspect the more severe example in Fig. 9a to be an artefact of some kind, more common and insidious are wrap-around multiple series such as those shown in Fig. 9b from along the same line which could easily be interpreted as primary reflections from the lower crust or upper mantle. After migration, additional filtering, and
(a) 7
6)
mary reflectors can be expressed as the appropriate combination of convolutions c”> or M, = primary series * primary series M2 = M, *primary series M3 = M2 *primary series = (M, *primary series) * primary series = M, * M,
I-
2 km
1 0 -2 -
_ -
-
_-
km
Fig. 9. (a) Portion of migrated CMP section from Line 719 in the Blake Spur fracture zone area (for location see Fig. 1) showing strong wrap-around multiples. A, B, C, and D refer to multiple phases identified in Fig. 10. (b) Elsewhere along same line showing wrap-around multiples (arrows) that could easily be misinterpreted as primaries. Line 719 was shot with a strictly constant shot time delay. Principal processing by Lamont Doherty Earth 0bseIvatot-y.
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depth conversion, these artefacts may become almost indistinguishable from the real thing.
0
3. Mitigating wrap-around multiples in the CMP domain
. SEA BOTTOM PRIMARY
i
I I
For the OCEAN reflection survey, we took a two-step approach to reducing the effect of multiple wrap-around. While primarily controlling the shot distance interval on the basis of GPS navigation, we first imposed a nominally constant shottime delay of 21 s in order to monitor the multiples and to place them where they would cause
6-
L 0 1
-
Moho
lst MULTIPLE
20
-
-
2nd MULTIPLE
b
5
SEABED primary sediment series
SECOND SHOT
TC
: -8
3rd MULTIPLE SEA BOTTOM PRIMARY
:L‘I
10? XI P I
-
Moho
k 4th MULTIPLE
S
cc-
1st MULTIPLE CF SECOND SHUT
,tu
s
L I
@L
20
L,
2nd MULTIPLE OF SECmD
SHOT
a: QFig. 11. Diagram showing arrival times of various sea bottom multiples through two consecutive shot recordings using the fixed shot time delay of 21 s chosen for the OCEAN reflection survey where water depths are about 4900 m.
(IL-
L E-L-_.-_
_&
y’ :
zz%
-_.-_-
12 -
M3 = Ml
* MI
-
ObservedCMP trace
Fig. 10. Single stacked CMP trace (4440) from Line 719 (Fig. 9a). M, * M, used to synthesize the M, wrap-around multiple from portion of observed trace using primary series from sediment reflections. Four arrows show close match of traveltime onsets in M, with those observed on single trace and on stack (Fig. 9a). t,, is start time of M, * M, synthesized from primary sediment series.
the least contamination of primary reflections. As seen in Fig. 11, we placed M, where the peak arrivals would be within the water column above the strongest primaries; M4 is just above the M, of the current shot and is well below the anticipated arrival of the Moho reflection (Fig. 12a). As noted in Fig. 12b, the multiple wrap-around
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problem could have been more or less avoided by using a 100 m shot spacing, since during the intervening N 40 s, most of the multiple energy would have died away; however, this would have halved the fold of cover for the CMP stack and
produced potential aliasing problems for dipping reflectors. The second step was to introduce a pseudo-random time delay (l-255 ms) trigger device, designed and built at Bullard Laboratories, inserted between the ship’s navigation computer
(a)
(b) 0
0
:
~
r ._ .:-.
.-_
i--. :
._ .:-:_ .__ _:
tCM4
Fig. 12. (a) Typical shot record from OCEAN survey (Line 11). Note arrivals of M, and MJ in water column (above 6.5 s) and lack of a strong M4 in lower part of record (lo-13 s) as opposed to prominent M4 in Fig. 5 (exact arrival of multiples is not known due to the random time delay). M4 is more or less absent because its strongest portion (i.e. as much as - 1 s after its onset time) occurs beyond the end of the record. Constant delay was 20.70 s plus a random time delay. (b) Shot record (1590, Line 11) immediately after noise test (when no shot was fired; Fig. 4a). The energy from the wrap-around multiples has died away during the approximately 42 s shot delay. Refer to Fig. 5 for explanation of primary features.
.I.H. McBride et al. / Tectonophysics 232 (I 994) 425-435
434
8
S 10
0
-”
BIRPS
OCEAN
Line
11
Fig. 13. Constant-velocity (1525 m s- ‘) finite difference migration of a portion of OCEAN Line Il. Section is displayed variable area, no wiggle trace with a linear gain and a frequency-wavenumber filter. M = Moho reflection.
output and the profiling system. Superimposing this delay on the 21 s shot time delay helped to increase the likelihood that previous shot multiples would tend to cancel one another when summed into the CMP domain. As a result of these two steps, high-amplitude coherent wraparound multiples such as observed from parts of the Blake Spur and other deep reflection surveys are not observed on the OCEAN CMP sections (e.g. Fig. 13). In a few instances on the OCEAN sections, principally below small synforms on the surface of the igneous crust, noise cancellation is incomplete in the 11-13 s interval on the stack; however, the randomisation has reduced the level of relative amplitude, periodicity, and coherency to well below that which would otherwise have developed, such as on the Blake Spur example (Fig. 9a). This emphasises the importance of having controlled the shot time interval during which such artefacts arise. Shooting with a strictly constant time delay synchronises primary and wrap-around multiple events between shots, so that corruption of the CMP stack is likely if the shot delay time is chosen so that multiples arrive during the recording time. If Line 719 had been shot with an 18 s,
instead of a 20 s (in places, 22 s), delay, then the worst part of the multiple corruption would have arrived near the bottom of the record and not within the zone of interest. Once at sea, practical considerations such as the minimum time that the compressor can be recharged and the minimum ship speed at which the receiver cable can be controlled will ultimately limit the available choices of shot time delay. Shooting with a positioning-based navigation system may effectively randomise the shot delays, as seems to have happened for WAM (Fig. 8); however, it may be risky to depend on this, especially when a loss of the primary navigation system leads to the temporary measure of shooting on a constant or only slowly varying time interval.
4. Conclusions As reflection profiling programmes move into the deep water of the ocean basins, contamination from multiple wrap-around can be a serious problem in producing artefacts that could be misinterpreted as reflectivity from the lower crust and uppermost mantle. We use examples from the North Atlantic to demonstrate how this problem develops for three different acquisition schemes, and suggest a procedure for alleviating contamination without requiring significant alteration of the original programme. For deep sea ( > 2-3 km water depth) surveys, contamination is a problem for shot-point spacings of less than 100 m at the usual ship speed and is particularly severe in areas with thick and strongly reflective sedimentary cover. Under these circumstances, recording with a strictly constant time delay will maximise the likelihood of contamination.
Acknowledgements
Data acquisition for the BIRPS project is made possible through grants from the Natural Environment Research Council and through the BIRPS Industrial Associates [Amerada Hess Ltd., Amoco Production Co., BP Exploration Co. Ltd., Chevron UK Ltd., Conoco (UK) Ltd., Enterprise
J.H. McBride et al. / Tectonophysics 232 (1994) 425-435
Oil plc, Lasmo North Sea PLC, Mobil North Sea Ltd., and Shell UK Exploration and Production]. R.W. England provided invaluable assistance as a client observer at sea during the BIRPS OCEAN cruise. We thank GECO-PRAKLA and Party Chief Rolf Henriksen for helping to make the BIRPS OCEAN cruise a success. Data processing for this study was performed on the GECO-PRAKLA Seismic Kernel System operating at Bullard Laboratories. Reviews by F. Marillier and two anonymous referees substantially improved the paper. This is University of Cambridge Department of Earth Sciences Contribution No. 3109.
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nett (Editors), Seismic Probing of Continents and their margins. Tectonophysics, 173: 141-143. Levin, F.K., 1971. Apparent velocity from dipping interface reflections. Geophysics, 36: 510-516. Minshull, T.A., White, R.S., Mutter, J.C., Buhl, P., Detrick, R.S., Williams, C.A. and Morris, E., 1991. Crustal structure at the Blake Spur fracture zone from expanding spread profiles. J. Geophys. Res., 96: 9955-9984. Morris, E., Detrick, R.S., Minshull, T.A., Mutter, J.C., White, R.S., Su, W. and Buhl, P., 1993. Seismic structure of oceanic crust in the Western North Atlantic. J. Geophys. Res., 98: 13,879-13,903. NAT Study Group, 1985. North Atlantic transect: A wideaperture, two-ship multichannel seismic investigation of the oceanic crust. J. Geophys. Res., 90: 10,321-10,341. Peddy, C., Pinet, B., Masson, D., Scrutton, R., Sibuet, J.-C., Warner, M.R., Lefort, J.P. and Schroeder, I.J. (BIRPS and ECORS), 1989. Crustal structure of the Goban spur continental margin, northeastern Atlantic, from deep seismic reflection profiling. J. Geol. Sot. London, 146: 427-437. Rosendahl, B.R., Groschel-Becker, H., Meyers, J. and Kaczmarick, K., 1991. Deep seismic reflection study of a passive margin, southeastern Gulf of Guinea. Geology, 19: 291-295. Taner, M.T., 1980. Long period sea-floor multiples and their suppression. Geophys. Prospect., 28: 30-48. Tucker, P.M. and Yorston, H.J., 1973. Pitfalls in Seismic Interpretation. Sot. Explor. Geophys., Tulsa, OK, Monogr. Ser., 2, 50 pp. White, R.S., Detrick, R.S., Mutter, J.C., Buhl, P., Minshull, T.A. and Morris, E., 1990. New seismic images of oceanic crustal structure. Geology, 18: 462-465.