We perform a technique called multi-shot processing on a section of 12-channel sonic
logs in order to better resolve compressional and shear velocities. The data are from
the ODP Leg 102 cruise, which occupied drill site 418A near the Bermuda Rise in 1985.
Multi-shot processing has been done on a 9 meter section of this data, using different
combinations of numbers of shots vs. numbers of receivers in an attempt to compare
the vertical resolution and stability of this processing method. The method is stable
only with certain shot-to-receiver subarray combinations. This paper demonstrates
that the optimum combinations using this set of data are 4 shots with 6 receivers
apiece, and 3 shots with 8 receivers each. While a combination using 5 shots with 4
receivers is possible, the method produces spurious results. This may be because of
spatial aliasing over too few receivers, or it may be a result of poor outside control
over the entire experiment (ship heave, etc.). It is hoped that an optimum subarray
combination can be used to resolve velocities over shorter array lengths using the
redundancy in the sonic data. This would result in a greater ability to characterize
fracturing and alteration in the oceanic crust, since velocity variations have been shown
to correlate with fracture zones.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumNational Science Foundation (U.S.) (Grant OCE89-00316

Burns, Dan

Thompson, Delaine

DSpace@MIT

MULTI-SHOT PROCESSING FOR BETTERVELOCITY DETERMINATIONbyDelaine ThompsonEarth Resources LaboratoryDepartment of Earth, Atmospheric, and Planetary SciencesMassachusetts Institute of TechnologyCambridge, MA 02139Dan BurnsWoods Hole Oceanographic InstitutionWoods Hole, MA 02543ABSTRACTWe perform a technique called multi-shot processing on a section of 12-channel soniclogs in order to better resolve compressional and shear velocities. The data are fromthe ODP Leg 102 cruise, which occupied drill site 418A near the Bermuda Rise in 1985.Multi-shot processing has been done on a 9 meter section of this data, using differentcombinations of numbers of shots vs. numbers of receivers in an attempt to comparethe vertical resolution and stability of this processing method. The method is stableonly with certain shot-to-receiver subarray combinations. This paper demonstratesthat the optimum combinations using this set of data are 4 shots with 6 receiversapiece, and 3 shots with 8 receivers each. While a combination using 5 shots with 4receivers is possible, the method produces spurious results. This may be because ofspatial aliasing over too few receivers, or it may be a result of poor outside controlover the entire experiment (ship heave, etc.). It is hoped that an optimum subarraycombination can be used to resolve velocities over shorter array lengths using theredundancy in the sonic data. This would result in a greater ability to characterizefracturing and alteration in the oceanic crust, since velocity variations have been shownto correlate with fracture zones.INTRODUCTIONThe structure of the ocean crust has been an object of scientific study for many years.The results of this study have revealed the layered structure which characterizes most of160 Thompson and Burnsthe earth's ocean basins. This structure is simply modelled as pelagic sediments (Layer1) overlying basaltic pillows and flows (Layer 2A). These in turn overlie metamorphosedpillows (Layer 2B), which grade into metamorphosed intrusive dykes (Layer 2C) (Mooset aI., 1986). Numerous sites drilled by the Ocean Drilling Program (ODP) and th~Deep Sea Drilling Project (DSDP) have penetrated Layer 2, including Well 418A onthe Bermuda Rise and Well 504B near the Costa Rican Rift zone. These two drillsites provide an ideal opportunity to study the chemical, lithological and structuralcharacteristics of deep ocean crust. One aspect of the crust to be characterized is theamount and type of fracturing in Layer 2 as a function of depth, lithology and age.To accomplish this, good velocity estimates gleaned from multi-channel sonic (MCS)logging data can be correlated with fracture zones (Moos and Zoback, 1983; Paillet,1980; Moos, 1988; Anderson et aI., 1984). The presence of fracture zones will affectboth v" and Vs ; however, the interpretation of velocity data will depend heavily onthe vertical resolution and signal to noise (SjN) ratio achieved through the processingtechnique. Multi-shot processing (Rsu and Chang, 1987; hereafter referred to as MSP)uses subarrays of the receiver section and the small moveup between source firingsto combine velocity information over a depth interval. Thus both goals of verticalresolution and better SjN ratio may be achieved by using this technique.A set of 30 depth points covering a 9 meter depth interval from the well site 418Ain "old" oceanic crust has been processed using the MSP method. The data wereprocessed using subarrays of 4 shots with 6 receivers, and 3 shots with 8 receiversfor all depth points. Also, some runs were made using 1 shot with 12 receivers, and 5shots with 4 receivers. Neither one of these combinations is ·satisfactory; 1 shot with 12receivers does not take advantage ofthe method, and using 5 shots with only 4 receiversproduces unstable results. By determining the optimum shot number-receiver numbercombination, the benefit of the method can be maximized. A larger subset of the MCSdata can then be processed using this optimal combination in the hope that velocityvariations in rapidly changing lithology will be better resolved.PROCEDUREThe multichannel sonic log is a full waveform array-type logging tool which utilizes12 receivers at .15 m .(.5 ft) spacing to record the entire waveform at each sourcefiring. Since there are multiple receivers, a robust technique such as semblance can beused to give accurate estimates for the compressional, shear and Stoneley velocities.MSP makes use of the inherent redundancy in the MCS data. With an inter-receiverspacing of .15 m, and a source moveup of .3 m (1 ft), there is an overlap betweenup to 5 successive firings of certain receivers at common depth points. MSP takesadvantage of this overlap by stacking the semblance estimates from a subarray ofreceivers covering the same depth interval. Figure 1 shows the shot moveup andMulti-Shot Processing 161receiver-shot combinations possible in the MSP technique.Semblance is a method used to estimate slowness from well logs (Kimball andMarzetta, 1984). The semblance is a quantity defined as the normalized coherentenergy in a time window of length Tw , at arrival time To on trace 1, with linearmoveout given by the slowness S (Kimball and Marzetta, 1984):(1)t=To+Tw [N ] 2t~o f; Aj(t +(j - 1)t.zS)p(S,To) = t=To+Tw [N ]N t~o f;A;(t+(j-1)t.zS)where:N = number of receivers,Aj = amplitude of trace j,t.z = receiver offset.Each subarray of the MCS data over a selected depth interval is processed using thistechnique. Characteristic plots of the semblance values as a function of arrival time(To) and slowness (S) for two different subarrays are shown in Figure 2. The majorpeaks are easily identified as the compressional, shear, and Stoneley arrivals. Sinceeach subarray corresponds to a different offset range, the arrival time varies for eachmode within different subarrays. However, since the slowness for each mode should bethe same (averaged over the length of the subarray), the semblance data can be stackedby projecting the semblance onto the slowness axis. From Hsu and Chang (1987):Pi,n(S) = maxT(Pi,n(S, T) (2)where:i = source position,n = subarray number,S '= slowness'(sec/m),T = arrival time at receiver 1 (sec),P = projected semblance statistic,P = semblance statistic.162 Thompson and BurnsOnce the semblance from each subarray has been projected onto the slowness axis,the projected values from all the selected subarrays are combined via an algebraicmean. In this manner, velocity estimates as a function of depth can be found. Thevertical resolution of these estimates depends on the length of the subarray chosen,and the SIN ratio depends on the number of shots used in the stacking procedure.PROCESSING RESULTSA subset of the MCS data gathered. by the ODP Leg 102 shipboard party has beenprocessed using the MSP technique described in the previous section. The sectionchosen is located at a depth of 6300.7 meters below rig floor (mbs) which correspondsto a depth of 781.7 meters below the sea floor (mbsf). The depth interval spanned is9 meters. It corresponds to an interval in the borehole characterized by pillow basaltsgrading into massive basalts (see Figure 3 for a reproduction of the MCS data results).Using Figure 1 as a guide, it is clear that only certain shot number-receiver numbercombinations are available for use in the MSP procedure. In order to provide a basisfor comparison, many different combinations were processed: 4 shots with 6 receiverseach (a vertical resolution of .75 m), and 3 shots with 8 receivers each (a verticalresolution of 1.15 m). In addition to these combinations, 5 shots with 4 receivers each(a vertical resolution of .45 m), and semblance using the full array of 12 receivers wereprocessed. The latter combination was uninteresting in that it did not utilize any of thepower of the method of MSP, so we concentrate'd on the results the other combinationsproduced.Semblance values are calculated for each subarray; the program scans for velocitiesbetween 1.3 km/s and 7.5 km/s, and arrival times between 0 and 1800 J1.sec. In Figure4, the output following the projection of the semblance values for three different shot-receiver number combinations is shown, along with the combined semblance statistic.A peak along these curves indicates that an event is present at a particular slowness.It is obvious from this figure that while the 4-6 and 3-8 combinations produced fairlyconsistent results from the semblance calculations, the 4-5 shot-receiver combinationis highly uneven and unstable. The final combined semblance statistic for that com-bination reveals little information. Thus it seems that the optimal combinations forthis particular data set (with a source moveup twice that of the inter-receiver spacing)are either 4 shots - 6 receivers, or 3 shots - 8 receivers. With another data set, othercombinations might be possible, but below and above a certain threshold, the resultsusing MSP apparently become degraded.Figure 5 shows the results of processing all 30 depth points for compressional, shear,and Stoneley slownesses. This processing was done for both the 4-6 combination andthe 3-8 combination. The results are fairly similar with greater variability visible onMulti-Shot Processing 163the 4-6 results. This may be an indication of greater vertical resolution, but this cannotbe verified until further data have been processed.CONCLUSIONSThe method known as multi-shot processing (MSP) has been applied to ODP data fromwell 418A, with the result that certain combinations of numbers of shots-numbers ofreceivers produce more consistent, stable values for velocity estimation. While eachsubarray independently produced good semblance values, the combination of subarraysacross a depth interval tended to degrade above a certain number of shots. Thus anoptimal combination is necessary to produce reliable results. In the work done so far,the combinations of 4 shots with 6 receivers or 3 shots with 8 receivers produce thebest results. Above that number of shots, a smearing of the semblance peaks occurs,and the results are not reliable. It is possible that with better outside control of thelogging experiment (knowledge of ship position, better instrument control, etc.), moreshot number - receiver number combinations will be possible with MSP. In the future,this method may be useful in characterizing velocity variations in a rapidly changinglithological zone. Further, the type .and amount of fracturing in various zones may beverifiable using multi-shot processing.ACKNOWLEDGEMENTSThis work was supported in part by the Full Waveform Acoustic Logging Consortiumat M.LT. and by National Science Foundation grant No. OCE89-00316.164 Thompson and BurnsREFERENCESAnderson, R.N., H. O'Malley, and R. 1. Newmark, Nuclear, multichannel sonic, ultra-sonic analyses for determination of fracturing and alteration in a fast formation:the deep oceanic crust, Trans. SPWLA 25th Ann. Logging Symp., Paper Y, 1984.Hsu, K. and A. B. Baggeroer, Multiple-shot processing of array sonic waveforms, Geo-physics, 52, 1376-1390, 1987.Kimball, C. V. and T.L. Marzetta, Semblance processing of borehole acoustic arraydata, Geophysics, 49, 274-1, 1984.Leg 102 Shipboard Scientific Party, Looking down an old hole, Nature, 316, 682, 1985.Leg 102 Shipboard Scientific Party, Site 418-Bermuda Rise, in Salisbury, M. H., Scott,J. H., Auroux, C. A. et aI., Proc. Init. Repts. (Pt. A), ODP, 102,1986.Moos, D., Elastic properties of 110 Ma oceanic crust from sonic full waveforms inDSDP hole 418A, in Salisbury, M. H., Scott, J. H., Auroux, C. A. et aI., Proc. Init.Repts. (Pt. A), ODP, 102,1986.Moos, D. and M. D. Zoback, In-situ studies of velocity in fractured crystalline rocks,J. Geophys. Res., 88, 2345-2358, 1983.Moos, D., D. S. Goldberg, M. A. Hobart, and R. N. Anderson, Elastic wave velocitiesin layer 2A from full waveform sonic logs at hole 504B, in Leinen, M., Rea, D.K.et aI., Init., Repts. DSDP, 92, 1986.Paillet, F. 1., Acoustic propagation in the vicinity of fractures which intersect a fluidfilled borehole, Trans. SPWLA 21st Ann. Logging Symp., Paper DD, 1980.Multi-Shot Processing••• •X SOURCE • •• • •• RECEIVER • • •• • • •• • • •• • • • •• • • • •• • • • • •• • • • • 0• • • • • • ...Jo't m «:• • • • • >0.75 m a:- - --w1.15 m - - - - l-• • • •~:cI-a.• • •wCl• •• 4 'k•l~ :k:)165Figure 1: Successive source locations for a 12-receiver tool, assuming an inter-receiverspacing of .15 m and a source moveup of .3 m. Some various source number-receivernumber combinations to be used in MSP are shown (after Rsu and Chang, 1987).166Veloci ty (KTTl/S)7.06.0Thompson and Burns5.04.03.02.01.307.08>6.05.04.03.02.01.3oArrival Time (microsec) 18001800Figure 2: Example output from semblance calculations from the MCS data. Thesemblance values are contoured in the arrival time-slowness plane. At the top arethe results from processing an array of 12 receivers; at the bottom are the resultsfrom a subarray using 6 receivers. The compressional and Stoneley arrivals areeasily seen.\Multi-Shot Processing 167"')I, )(~,J~ ~~)' <~f;i t~ )~ :~~t~ .t-l..U.iI ', II-Id-~i.'' ..",-'.-..,-- - _... _...::.:::'-'.... - .._-Figure 3: MCS data from drill site 418A. The section shown is from near the bottomof the well (from Leg 102 Shipboard Scientific Party, 1986). An arrow points tothe approximate starting depth of the data processed in this paper.168 Thompson and Burns4 shots . 6 receivers0.8• 0.7u"•:0 0.6E•<J> 0.5."•c 0.4:0E0 0.3u0.20.00 0.20 0.40 0.60 0.80Slowness (s/km)3 shots . 8 receivers0.8• 0.7u" 0.6•:0E 0.5•<J>0.4."•"0.3:0E 0.20u0.10.00.0 0.2 0.4 0.6 0.8Slowness (s/km)4 shots • 6 receivers3 shots • 8 receivers0.9.,--------------,0.80.70.60.50.40.30.20.10.0 +-~---,-- ....~___,-~.,..--r_~_10.00 0.15 0.30 0.45 0.60 0.75 0.90Slowness (s/km)0.20.40.9.,--------------...,0.80.70.60.5."•u.~ 0.3c:•uc•:0E•<J>."•u•IQ.•u"•:0E•<J>0.1 +-~-r~-,.._-..,...-....,--.,...-_i0.00 0.150.300.450.600.750,90Slowness (s/km)•u"•:0E•<J>."•u•'0c:5 shots • 4 receivers0.9.,-------------....,0.80.70.60.50.40.3 +--__._---r----,r-~.,...-__._-_10.00 0.15 0.30 0.45 0.60 0.75 0.90Slowness (s/km)0.8•u"• 0.7:0E•<J>." 0.6•":0E0 0.5u0.40.05 shots • 4 receivers0.2 0.4 0.6Slowness (s/km)0.8Figure 4: On the left are the projected semblance values as functions of slowness forvarious shot number - receiver number combinations. On the right are the resultsfrom combining the projected semblance curves through an arithmetic mean. Theestimates are taken from depth point 6302.1 mbs.Multi-Shot ProcessingSlowness .vs. Depth (4-6)169,.- ...... , .""'- ....,....., ........,.- ...., ..._-- ....,- ...,.-"0.7-0.6E'"-0.5'"-'"0.4'"" 0.3l:::0 0.2U>0.10.0780 782 784 786 788Depth (mbsf)790 792P slownessS slownessStoneley slownessSlowness .vs. Depth (3-8)---,,-----~'-----------~~,-",,- ... , ... .- ......_-- ... - ............_--_..__ ......0.70.6E'"0.5u;- 0.4'"'""0.3l:::0 0.2(J)0.10.0780 782 784 786 788Depth (mbsf)790P slowness_._._.-._. S slowness- - - - - Stoneley slowness792Figure 5: The estimates of compressional, shear, and Stoneley velocities as a functionof depth below the sea floor for (top) 4 shots with 6 receivers each, and (bottom)3 shots with 8 receivers. Note the many similarities in the results.170 Thompson and Burns

Multi-Shot Processing For Better Velocity Determination

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Multi-Shot Processing For Better Velocity Determination

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