Fryer, P., Pearce, J. A., Stokking, L. B., et al. , 1992 ... · on both the MCS profile and the...
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Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125 35. COMPARISON OF MULTICHANNEL SEISMIC REFLECTION DATA TO A SYNTHETIC SEISMOGRAM AND LITHOLOGY OF SITE 786 1 Michael S. Marlow, 2 Eric L. Geist, 2 Michael A. Hobart, 3 Gregory F. Moore, 4 and Brian Taylor 4 ABSTRACT A synthetic seismogram was constructed by using velocity and density logs collected at Ocean Drilling Program Site 786 in the Izu-Bonin forearc. Good agreement was found between the synthetic reflections and most of the observed reflections on a 96-channel seismic reflection profile across the forearc. Short and discontinuous deep reflections occur on the seismic reflection record that correspond to events from within the volcanic basement rocks below the site. These deep reflections correspond to variations in velocity and density observed on the downhole logs that result from different rock types and structures within the volcanic basement rocks. Aprominent and laterally continuous basement reflection corresponds to lithologic boundaries between a basalt flow and surrounding breccia layers. INTRODUCTION In preparation for Ocean Drilling Program (ODP) Leg 125, a series of geophysical transects were run in the Izu-Bonin forearc, using the Fred H. Moore. Descriptions of these data, including multichannel seismic (MCS) reflection sections, are given by Horine et al. (1990). The seismic reflection data were collected by using a 3065-in. 3 air-gun array and a 96-channel streamer. A 12.5-km-long segment of one of the MCS lines (Line 5, Fig. 1) was reprocessed and is shown in Figure 2. Standard processing techniques (listed in Table 1) were applied to the line except for an f-k migration of the data. No automatic gain control was applied to these data. SYNTHETIC SEISMOGRAM To create the synthetic seismogram, the velocity and density logs were first edited to remove erroneous values and to eliminate portions where no data were collected. Logs (Fig. 3) were then converted to time by using the time-depth relation calculated from the velocity log. Reflection coefficients were computed from acoustic impedance, accounting for vertical-incidence transmission loss through the sec- tion. The synthetic seismogram was then created by convolving the reflection coefficients with a 35-Hz Ricker wavelet and by resampling to 4 ms. The migrated section was wavelet processed to an approximately zero phase. The 35-Hz Ricker wavelet closely matches the wavelet extracted from the data. Both the synthetic seismogram and the migrated section were then depth-converted using smoothed velocities from the velocity log. Dissimilarities between the synthetic seismogram and the seismic reflection data arise because the synthetic seismogram contains higher frequencies than the seismic reflection data and because the lateral continuity of reflections in the reflection data is less than the Fresnel zone and thus is transitional, whereas in the log data and derived synthetic seismogram the lateral continuity of reflec- tions is well-defined across lithologic boundaries. 'Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125: College Station, TX (Ocean Drilling Program). 2 U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA94025, U.S.A. 3 Lamont-Doherty Geological Observatory, Palisades, NY 10964, U.S.A. 4 University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, U.S.A. COMPARISON OFSYNTHETIC SEISMOGRAM TO SEISMIC REFLECTION RECORD AND DOWNHOLE LITHOLOGY Reflections within the sedimentary section correlate well with the synthetic events between about 3130 and 3200 m (Fig. 2). At 3200 m, a strong and laterally continuous reflection separates overlying sedimentary strata from volcanic basement. This reflection is evident on both the MCS profile and the synthetic traces. This boundary separates the well-stratified, overlying Eocene through Pleistocene sedimentary section, which is characterized by laterally continuous reflections, from the Eocene volcanic basement rocks (Figs. 2 and 4; Fryer, Pearce, Stokking, et al., 1990), which are characterized by one laterally continuous reflection and by many short, discon- tinuous reflections. Well-correlated events occur at 3300 m within the volcanic base- ment rocks. Both the MCS data and the synthetic-seismogram traces show a strong and laterally continuous intrabasement reflection (Fig. 2). This strong reflection (at 200 mbsf in Fig. 4) corresponds to a basalt layer within two breccia units. Below about 3300 m, correlation between the MCS data and the synthetic-seismogram traces is dif- ficult, probably because reflections deep within the volcanic base- Table 1. Standard processing techniques. Processing sequence Edit noisy traces Resample to 4 ms Sort to 16.67-m common-midpoint (CMP) bins Trace balance Gain recovery Predictive deconvolution 32-ms gap 200-ms operator 3 design windows Time varying filter Water bottom 3-5-50-60 Hz End of record 3-5-30-40 Hz Discrete velocity analyses Normal moveout Stack f-k migration Water bottom mute Depth conversion 585
Transcript of Fryer, P., Pearce, J. A., Stokking, L. B., et al. , 1992 ... · on both the MCS profile and the...
Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 125
35. COMPARISON OF MULTICHANNEL SEISMIC REFLECTION DATA TO A SYNTHETICSEISMOGRAM AND LITHOLOGY OF SITE 7861
Michael S. Marlow,2 Eric L. Geist,2 Michael A. Hobart,3 Gregory F. Moore,4 and Brian Taylor4
ABSTRACT
A synthetic seismogram was constructed by using velocity and density logs collected at Ocean Drilling Program Site 786 inthe Izu-Bonin forearc. Good agreement was found between the synthetic reflections and most of the observed reflections on a96-channel seismic reflection profile across the forearc. Short and discontinuous deep reflections occur on the seismic reflectionrecord that correspond to events from within the volcanic basement rocks below the site. These deep reflections correspond tovariations in velocity and density observed on the downhole logs that result from different rock types and structures within thevolcanic basement rocks. Aprominent and laterally continuous basement reflection corresponds to lithologic boundaries betweena basalt flow and surrounding breccia layers.
INTRODUCTION
In preparation for Ocean Drilling Program (ODP) Leg 125, a seriesof geophysical transects were run in the Izu-Bonin forearc, using theFred H. Moore. Descriptions of these data, including multichannelseismic (MCS) reflection sections, are given by Horine et al. (1990).The seismic reflection data were collected by using a 3065-in.3 air-gunarray and a 96-channel streamer. A 12.5-km-long segment of one ofthe MCS lines (Line 5, Fig. 1) was reprocessed and is shown in Figure2. Standard processing techniques (listed in Table 1) were applied tothe line except for an f-k migration of the data. No automatic gaincontrol was applied to these data.
SYNTHETIC SEISMOGRAM
To create the synthetic seismogram, the velocity and density logswere first edited to remove erroneous values and to eliminate portionswhere no data were collected. Logs (Fig. 3) were then converted totime by using the time-depth relation calculated from the velocity log.Reflection coefficients were computed from acoustic impedance,accounting for vertical-incidence transmission loss through the sec-tion. The synthetic seismogram was then created by convolving thereflection coefficients with a 35-Hz Ricker wavelet and by resamplingto 4 ms.
The migrated section was wavelet processed to an approximatelyzero phase. The 35-Hz Ricker wavelet closely matches the waveletextracted from the data. Both the synthetic seismogram and themigrated section were then depth-converted using smoothedvelocities from the velocity log. Dissimilarities between the syntheticseismogram and the seismic reflection data arise because the syntheticseismogram contains higher frequencies than the seismic reflectiondata and because the lateral continuity of reflections in the reflection datais less than the Fresnel zone and thus is transitional, whereas in the logdata and derived synthetic seismogram the lateral continuity of reflec-tions is well-defined across lithologic boundaries.
'Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1992. Proc. ODP, Sci. Results, 125:College Station, TX (Ocean Drilling Program).
2U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, U.S.A.3Lamont-Doherty Geological Observatory, Palisades, NY 10964, U.S.A.4University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, U.S.A.
COMPARISON OF SYNTHETIC SEISMOGRAM TOSEISMIC REFLECTION RECORD AND DOWNHOLE
LITHOLOGY
Reflections within the sedimentary section correlate well with thesynthetic events between about 3130 and 3200 m (Fig. 2). At 3200 m,a strong and laterally continuous reflection separates overlyingsedimentary strata from volcanic basement. This reflection is evidenton both the MCS profile and the synthetic traces. This boundaryseparates the well-stratified, overlying Eocene through Pleistocenesedimentary section, which is characterized by laterally continuousreflections, from the Eocene volcanic basement rocks (Figs. 2 and 4;Fryer, Pearce, Stokking, et al., 1990), which are characterized byone laterally continuous reflection and by many short, discon-tinuous reflections.
Well-correlated events occur at 3300 m within the volcanic base-ment rocks. Both the MCS data and the synthetic-seismogram tracesshow a strong and laterally continuous intrabasement reflection (Fig.2). This strong reflection (at 200 mbsf in Fig. 4) corresponds to a basaltlayer within two breccia units. Below about 3300 m, correlationbetween the MCS data and the synthetic-seismogram traces is dif-ficult, probably because reflections deep within the volcanic base-
Table 1. Standard processing techniques.
Processing sequence
Edit noisy tracesResample to 4 msSort to 16.67-m common-midpoint (CMP) binsTrace balanceGain recoveryPredictive deconvolution
32-ms gap200-ms operator3 design windows
Time varying filterWater bottom 3-5-50-60 HzEnd of record 3-5-30-40 Hz
Discrete velocity analysesNormal moveoutStackf-k migrationWater bottom muteDepth conversion
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M. S. MARLOW ET AL.
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Figure 1. ODP Sites 782 through 786 in the Izu-Bonin forearc, as seen in schematic cross section (A) and plan view (B),and location of MCS Line 5 shown in Figure 2. Bathymetry in kilometers. Adapted from Fryer, Pearce, Stokking, et al.(1990).
ment are very short and discontinuous and commonly extend overonly a few traces. Most reflections correspond to lithologic boundariesobserved within the hole (Fig. 4). However, some reflections do notcorrespond to observed boundaries, but poor core recovery mayindicate that some boundaries were missed during drilling. Variationsin the reflections from igneous basement are to be expected becausethe diverse lithologic units recovered at Site 786 include massivebrecciated flows, ash flows, and intercalated sediment in the uppersection, as well as pillow lavas and dikes or sills in the lower section
(Fig. 4; Fryer, Pearce, Stokking, et al., 1990). Other diverse featuresdiscovered in the volcanic basement include conspicuous shear zonesand hydrothermal breccias, which may add to the complex reflectiongeometry observed in the seismic reflection data.
ACKNOWLEDGMENTS
We thank Michael A. Fisher, Richard Jarrard, and R. H. Wilkensfor their thoughtful reviews and comments.
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Figure 2. MCS reflection Line 5, with synthetic seimogram calculated from log data shown in Figure 3. Location of line is shown in Figure 1. V.E. = vertical exaggeration
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Figure 3. Logging data from Hole 786B, including density and velocity measurements, which were multiplied to producethe impedance data.
REFERENCES In Fryer, P., Pearce, J. A., Stokking, L. B., et al., Proc. ODP, Init. Repts.,125: College Station, TX (Ocean Drilling Program), 81-94.
Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init. Repts.,125: College Station, TX (Ocean Drilling Program). D a t e o f i n i t i a l receipt: 26 September 1990
Horine, R. L., Moore, G. F., and Taylor, B., 1990. Structure of the outer Date of acceptance: 18 March 1991Izu-Bonin forearc from seismic-reflection profiling and gravity modeling. Ms 125B-182
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SITE 786 SYNTHETIC SEISMOGRAM
Synthetic seismogram
Lithologic description
Sandstone (Hole 786A)
Bonlnite breccia
Basalt flow
Two andβsltβ flows
Andeslte breccia
Basalt breccia
Basalt bonlnite breccia
Dacltβ breccia
Andesite flow
Andesite brecciaDaciteBasalt flowSandstoneDacite breccia
Dacite flows
Dacite breccia
Boninite-andesitθ flows
Bonlnite flows
Rhyolite flowAndesite flowAndesite breccia
Basalt flowDacite flowBoninitβ flowsAndesite flows
Andesite brecciaBasalt flowBreccia and sandstone
Andesite flows
Andesite breccia
Basalt boninite-breccia
Bonlnite breccia
SandstoneBoninite-andesite pillows
Rhyolite dikeBreccia and sandBasalt and boninlteMetaandesiteMetarhyolite flowsMetarhyolite dikeMetaandesite flowBasalt dikeMetaandesiteBonlnite dikeMetaandesiteMetaandesite stockworkBasalt
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Hole 786B(Sea Floor = 3071 m)
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Explanation
Pillows and hyaloclastite
Breccia
Andesitic or basaltic flows
Andesitic or basaltic dikes
Boninitic flows
Rhyolitic flows
Boninitic dikes
Rhyolitic dikes
Stockwork
Sand, sandstone
Figure 4. Lithologic column from Hole 786B plotted against the synthetic seismogram shown in Figure 2. The top of the synthetic seismogramcorresponds to a depth of 3200 m shown on Figure 2.
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