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Seismic Reflection Imaging at a Shallow Site1
J
i P. MilliganJ. RectorR. Bainer
This paperwaspreparedfor submittal to the
65th Socie@ of Expkwtion Geophysicists Annual ConventionHouston, TX
October 8-13,1995
January 1997
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DIY3.AIMER
lWdocumatwae prepuedu81rxWurrt ofworkqmUOlwJ byaIlagelwyofrheunited stat-governmentIUeitherthe United St8teeCovmunen t nor the University d Cdt&da na any d their employees,makeeany~W8V~h@da~ my w lieMutY or mponeibility for the Xcrlrq, Completaestbmw&lneeE of eny fnfornulion, qpemtue, producb a prucaedbcloed a qmeente thetiteusewould nor- @va* * righte. Re&encehereln toenyepedtk wmmerddpmdu~procen,a~bytieNow, trademark menufxturvr, or otherwise, does not neceeserilyconstitute or imply its endorsementmmmmembti~ a favoring by the United StateeGovernment or the University of California. The viewe andedau~~hmtiti~s-~ a--d~eutitisti~ c~mtmtheUntverdtyof Cel@rl@ andeheUnotbeueed faedvertMngapraductendoreementptupoe-
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Richmond Field Station: Shallow Site Lithology
Figure 1: Stratigraphk crm.+section of the sitq derived from well logs and pumping tests. The well we used for VSPwas located near the center of this section. Note the lack of continuous units Mow 12 IILindicating lateral
heterogeneity over distances of only a few meters. Pumping tests indicated lack of common aquifer connections.
The Richmond Field Station, on the east shores of Sarr Francisco Bay, is the site where we testedshallow seismic reflection imaging methods.
The lithology at thk site can be broadly described as fresh deltaic, consisting of unconsofidatcd andinterleaved lenses of sand and mud, overlying the Franciscan basement at depth 35 m.
Thk site had many pre-drilled boreholes, over about a 50 m. square area, that wert available for VSPdatacollection.All of theseboreholeshada maximumTD of 70 m., arrdwere6“ PVC cased.
The topology at this site was fairly fla~ so collection of surface seismic data would not be overlystrenuous.
The objective of our studies was to to determine the best seismic method to image these sediments,between the water table at 3 m depth to the basement at 35 m depth. Good cross-correlation betweenwell logs and the seismic data was also desirable, and would facilitate the trackkrg of knowrr lithologicalunits away from wells.. For instarrce, known aquifer control boundaries may then be mapped out over thetarget area. Velocity information from the seismic data would also be useful in defining physicalboundaries, and may be used in a joint inversion with reflectivity data and other non-seismicgeophysical data (eg: EM and gravity), to produce a 3-D image containing quantitative physicalproperties of the target area.
Seismic Reelection Imaging da Shallow S&e,
● Wallow VSP Technques using Hydmphone Arrays.
● Sbdlow Surfme Seismic Experiment.
Shallow Surface Seismic Experiment
Figure Z: End-on geophone line geometry for ‘walk-away’ noise tests. 120 geophones (40 Hz frequency), spaced 0.5 wand buried 15 cm were deployed.
To test the viability of using surface seismic methods to produce a reflection image of thk site, wecollected shot records from a surface geophone array. Common Shot Point Gathem (CSGS) wererecorded in a walkaway noise ,spread geometry, using 40 Hz natural frequency geophones spaced 0.5 m,and buried 15 cm. Burial lessened cultural and shot noise, while increasing the ground couplingcoefficient for increased frequency response. SP offsets ranged between 3 m and 63 m.
We had two types of sources available A simple hammer-on-plate, and a Betsy Gun firing 12 gaugeblank shells down a 1 m deep auger-drilled hole filled with water. We found that a single Betsy Gunshot produced higher SLNthan a stack of teo hammer blows, yielding higher bandwidth and significantlylower levels of airwave and ground roll noise.
With the 24 channel recording equipment available, it took us two days to acquire six 120-trace CSGS(720 traces total). Most of the time was spent surveying out and burying the geophones, troubleshootingtbe spread cables arrd auger-drilling the shot holes.
Figure 2: Walk-away surface common shot gather, using Retsy Gun source. Raw, AGC’d data is on the Iefl (a),bandpas filtered (80 Hz -600 Hz) AGC’d and deconvolved data is on the right (b). The zone where useful reflection
returns may be detected, without contamination by airwaves and first break refractions is shown on tbe right.
Shallow VSP techniques using HydrophoreArrays
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Figure 1: Simple hammer-on-plate source into a down-hole array of hydrophores.
2-D & 3-D Geometry, and data acquisitionBaffling the Tube WavesCommon Shot Gather (CSG)Wavefield Separation data processing2-D Imaging3-D Imaging
The principal arrivals in Figure 2 are the first breaks and sbot-generated airwave. Reflection arrivals,with their classic hyperbolic moveout, can only be reliably detected within an ‘optimum’ time/offsetwindow between the air waves and the first breaks (Hunter, et al, 1984). As can be seen in Figure 2, this‘optimum’ window limits useful reflection travel times to greater than 35 ms (> 25 m depth at this site).For less than 35 ms travel times, reflection arrivals are obscured by the refracted first break arrivals. ForSP offsets less than 12 m at this 35 ms fimit, reflection arrivals arc obscured by airwave and ground rollnoise. This means that CMP gathers will be missing the near offset traces for stacking, which will hurtthe S/N of the stack.
Large static shifts in the first break time are apparent in Figure 2, particularly at the longer offsets. Notethe short (12 m to 25 m offset) subset of first breaks which do not appear to have much significant timejitteq these offsets correspond to a well- packed gravel road with less lateral velocity variation than thesurrounding grass land. CMP stacking of this data would require accurate static corrections to ensureadequate stack performance at higher frequencies.
Flgurs3:f-k spectrs of the surface geophone CSG in Figure 2b. Note that the airwave is spatially afias-wt, and bas atutal bandwidth of abuut 400 Hz which exceeds tbe useftd data bandwidth of 200 HZ
In the above figures, reflection events are not immediately apparent in either the time domain CSG, or inthe ~-k spectra. We dld not persue the acquisition of multi-fold CMP data for reflection stacking, due tothe inability of the surface method to resolve reflections above 25 m depths, or with resolutions betterthan 3 m. The additional problem of static corrections due to the highly variable lateral velocityvariations in the weathered layer would also be a major problem in processing this surface data.
References:
Hunter, J.A., Pullan, S.E., Bums, R.A., Gagne, R., and Good, R., 1984, Shallow seismic reflectionmapping of the overburden bedrock interface with the engineering seismograph - some simpletechniques: Geophysics, 49, No. 8, 1381-1385.
Seissnis Re@ction Imaging ata Mallow Me.
● Shallow VSP Techniques using Hydrophore Arrays.
Shallow VSP Shooting Geometry
Figure 1: Multi shot point otYset layout for 2D data acquisition. A multi-element 0.5 m spaced, baffkd hydrophorearray was deployed. The gray zone is the depth slice into which reflection data is mapped during 2-D imaging, with
the maximum image offset being equal to half the maximum shot point otTset.
For our 2-D VSP dataacquisition,we set theshotpointspacingat 1.2 m. ‘Ilk allowedus to sort thetracesintothecommonreceiverdomainduringprocessing.MinimumusefulSP offsetwas2.4 m, andthemaximumoffsetwasset at 18.0 m, fimitingthemaximummappedoffsetfor reflectioneventsto 9.0m.
We used a 24 element hydrophore array with a 0.5 m element spacing for data acquisition. Closed-cellfoam baffles were inserted between the array elements to suppress tube wave generation and scattering.See the section on “Baffling the tube waves” for further details.
We deployed the hydrophore array at two different borehole levels to give a totaf of 48 channels ofcontinuous VSP data between 5 m and 29 m. This repositioning of the array required us to repeat all shotpoints.
We had two types of sources available: A simple hammer-on-plate, and a Betsy Gun firing 12 gaugeblank shells down a 1 m deep auger-drilled hole tilled with water. We found littfe difference infrequency content and S/N between the Betsy Gun source arrd the hammer sonrce. Typically, we foundthat a stack of 5-6 hammer blows were comparable to the S/N of a single Betsy Gun shot, so we usedthe hammer source for afl subsequent VSP dataacquisition.
The hammer-on-plate source shortened data acquisition time considerably compared to the Betsy Gunsource, which would have required auger-drilling of two shot holes at each shot point. The totafacquisition time to deploy the array and record 30 VSP shot records (720 traces) was less tharr 4 hours.See: “VSP Common Shot Gather” for an example of acquired data.
Hgure 2.’3D VSPshooting geometry multi-azimuth, multi-offset. Initially, we are using lfw increments between theradisl SP fin- and constant SP offset spacings (between 2 m and 6 m) along each radia$ though this may change as a
function of the required CDP fold-per-bin with increasing ofrset.
When laying out the SP geometry, it’s important to consider the increasing size of the reflection Fresnelzone with increasing offset. This Fresnel zone size maybe used as a basis for variable size binning ofthe data. Wavefield modeling of simple horizontal reflecton surfaces in an otherwize homogeneousvelocity background will determine the increase in size of the reflection Fresnel zone as a function of SPoffset. Based on these fesulfs, it will be possible to pick an azimuth step incremenq arrd a variable SPoffset function, so that there is a certain amount of Fresnel zone overlap over the entire zone of interest.
Overlapping reflection Frcsnel zones are not only desirable for nniform CDP coverage, but also forsorting the data into common receiver domains without spatial aliasing. This will be very importantduring migration processing.
Figure 3: Multi-well 3D VSPgeometry site map area coverage at a 10 m depth for a 20 m inter-weft spacing. A roughnde-of-thumh is the ahifity to image out to a lateral offset equal to the reflection depth, though this may vary
depending on the reflector dip.
Seismic Reelection Imaging at a Shalibw We.
Baffling the VSP Tube Waves
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FiPure 1: Raw VSP 24 channel record: left side (a) without baftles; the center (b) with inter-hydrophone baffles; the_.o.-.—–
right side (c) is the same as (b) but with individualtrace scalii for clarity. Note that hydrophores ‘blow No. 17 wereleft un-baflled. Hydrophore spacing is 03 m. TW labels the dominant tube waves. Baffling haa signifkantly reduced
tube wave amplitude and frequency. Tube wave velocity has been reduced from 630 mls to 130 mk
Note the dominant tube waves in the un-baffled record labeled TW in (a) with velocity 630 rrds. Thestrongest tube wave is generated by the direct P-wave arrival at the top of the fluid column (3 m depth).We can also see a reflection of this primary tube wave off the tail end of the hydmphone array, and offthe borehole base.
The addition of closed-cell foam baffles between hydrophores 1 and 17 (in Fig. 1 b &c) hassubstantially reduced the tube waves, in amplitude, frequency content, and velocity (130 mls). Therealso appears to be an apparent reduction in direct wave amplitude in the baffled section of the array, butin fact, the tube wave amplitudes have been reduced about 5-10 times more than the direct waveamplitudes.I
(1 Note that at the end of the baffled section (at hydrophore No. 17), new tube waves have been generated,
traveling both up and down the bore hole. This observation is in agreement with the theory that changesin the crossectional a.tea of the fluid column act as effective coupling points between outside boreholebody waves, allowing transfer of kinetic energy to generate tube waves in the borehole (Hardage, 1981,1983). There is indeed a change in crossectional area where the baffles stop: the baffles have a similaroutside diameter to that of the hydrophore elements (44 mm), while the inter- element cable has adiameter of only 12.5 mm. The baffling material was specifically chosen to have a similar outsidediameter for this reason.
Figure 2: f-k speetm of the 24 channelnon-haftledarmy shown in Figure la. Note how the useful frequency content(of the direct waves) does not exceed 300 HZ while that of the tube waves is 200 Hz. The tube wave velocity is also not
too dissimilar to that of the dbwct wave which would make wavefield separation and suppression of the tube waveenergy difficult during processing.
Figure 3:& speets’a of a fully haftled 24 channel array. Note the substasstird incrwtse in useful frequeney cnntent (byover 500 Hz). WMle the bandwidth of the tube waves has been reduced hy 50 H% for a dominant frequency of about
120 Hz. Hints of reflection wavefield energy can Ireseen in the negative wave number quadrantj with an expectedsimilar bandwidth to the direct waves. Wavetleld separation of tube wave ener~ in this case k relatively simple,
possibIy only by Inwcut filtering below 140 Hz.
We believethat the loss of high frequency P-wave response by the non-baffled array is due tointerference by tube wave arrivals. In the un-baffled army, there is a diameter change at the top andbottom of each hydrophore elemenq each element edge therefore is a potential body wave to tube wavecoupling point, as well as a tube wave reflection point. At any particular hydrophone of interes~ theincomming body wave must he summed with delayed tube wave arrivals, that have heen initiallygenerated by the same body wave incident at all tube wave coupling points afong the borehole. Providedthat the time delays are comparable to the dominant half period of the body wave, the overall effect ofthk sumation will be a Iowpass filtering of the incoming body wave. These filtering characteristics willchange as a function of these delays, which are in turn dependant on the incidence angle of the incomingbody wave with the borehole, the tube wave velocity, and the hydrophore element spacing. Figure 4below shows a dlagrans illustrating thk.
By slowing down the tube wave significantly (by a factor of 5), the inter-element baffles have increasedthe delay times in the linked tube wave arrivals enough to shift their low pass filtering, or notch filteringeffect, below the band of interest (140 Hz to 1 kHz).
Figure 4: f-k Ray pathdiagramillustratingthe first 4 defayedtube wave arrivals (in chronological order) at ahydrophore receiver, and their summation over time with the same converted bndy wave that ransed them.
Crnssectional changes in area of the horehole fluid column occur at the edges of all hydrophore element% and act asbody wave to tube wave coupling points. In this exampl% it is assumed that the tube wave velocity is always less than
the appam?nt velocity of the incoming bndy wave.
Figme 5 below shows the effect of increasing fluid head pressure on the ability of the closed-cell foamto attenuate and slow down tube waves. Tube wave velocities are slowed down to 106 m/sat the shrdlowdepth (3 m), and gradually increase to 620 mk at the deepest level (85 m), almost the same as theunbaffled tube wave velocity of 630 m/s. A foam material with a stiffer bulk modulus, ardor higher aircell pressure would be needed at deeper levels.
Figure 5: f-k A groupof 24 channelVSP bafned shot recordsj showing the effect of increasing depth on tbe baffte’sabilities tn attenuate and slow down tube waves. Note how tube wave amplitude and velocity increase with depth, as
the closed-cell foam collapses with increasing head pressure.
References:
Hardage, B.A., 1981, An examination of tube waves in vertical seismic profiling da~ Geophysics, 46,No. 6,892-903.
Hardage, B.A., 1983, Vertical Seismic Profiling, part A, principles: Geophysical Press, 450p.
Seismic Refiction Imaging at a Shallhw Site.
● Shah!owVSP Techniques using Hydrophore Arrays.● ShaUOwSurfwe Seismic Experiment.
● Shallow Richmond Ftiki Site.
VSP Common Shot Gather
Figure 1: VSP 4S channel common shot gather (CSG). Recorded from two 24 element baffled hydrophore armypositio~ and spliced together. The left side (a) shows the raw data after AG~ the right side (b) is the result ofbandpass filtering (BPF: 140 Hz -1200 Hz), AGC, and deconvolution. Note the two strong upgoing tube wavesgenerated at the tail end of each array in the raw data (veb)city -130 mls), and how these have been effectively
suppressed by tbe BPF and decon. Afler this minimaf processing, upgoing reflection arrivals can be readily seen, witha dominant period of about 1.6 ms. Hydropbone spacing is 0.5 m.
Figure 2: f-k spectra of the 4S charnel CSG shown in Figure 1 after BPF and decon. Note that we have over 900 Hz ofuseful bandwidth, resulting in sub-meter resolution abilities. Reflection wavefleld energy can he clearly seen in tbenegative wavenumher quadrant. The major reason for the substantial bydrophone bandwidth response is a direct
result of baffling the array, See tbe section on “BatTfing the tube waves” for further detaifs.
Seismic Rt?ff.sction I~”ng at a Sbalbw Si&.
● Shallow VSP Techniques using Hydrophore Arrays.
● Sha.!low Surfme Seikmic Eqwiment.
● Shallow Richmond Fie.M We.
VSPWavefield Separation Data Processing
Figure 1: Wavefield separation processing flow for baflled bydrophone VSPdata.
We used fairly conventional wavefield separation techniques for extracting reflection events from the 48channel common shot gather (CSG) baffled VSP data. A combination of bandpass filter (140 Hz -1100Hz) and velocity filtering in the f-k domain were sufficient to suppress the tube waves.
After tube wave separation. amplitude balancing was done to account for weak shot points adorreceivers. The (hammer) somce signatures were then deconvolved from the data in a surface consistentmarmer,
To separate the downgoing direct waves, we compared diffenmt multi-channel filters (median, f-k, t-p,and eigenvector) applied after di~ct arrival alignment and equalization. The eigenvector filter yieldedresults with the smalIest edge effects.
Re-sorting the traces into the common receiver domain (CRG) was frequently used. For example, foramplitude balancing of ensembles, coherency filtering of n+lection events, and design of the tlmaldeconvolution operator.
Figure 2: An example of separated upgoing reflections from a shot point offset of 7.2 m.
Seismic Refiction Imaging&a Shallow Site.
● SkaUo w VSP Techn~ues using Hydrophore Arrays
● Shallow Surface Seismic Experiment.
● Shaffow Richmond FiM Site.
VSP 2-DReflection Imaging
Figurv 1: The VSP-CDPmappingalgorithm.The depth point (x,z) to which the trace value at time t is mappeddepends on the trajectory that the velocity field V(X,Z)imposes on the process. See Figure 1 in: “2-D Geometry”
section for a diagram of the imaged zone (slice).
The VSP-CDP mapping process we used is a simple (time) point to (depth) point algorithm. Themaximum offset that a point may be mapped is limited to half the maximum SP offset. Note that thisrdgorithm can only correctly map (specular) reflections from horizontal layerv reflection events fromdipping layers will be laterally mispositioned down-dip. This was not thought to be a major problem inthe case of our data from Richmond, as there was not expected to be any overall dipping trend more than10 deg. at this site. It was hoped that stacking of the mapped traces from different shot point offsets,with their differently orientated Fresnel zones would help to prevent any overall mispositioning ofreflection boundaries, but some smearing, or bad focusing, couldn’t be helped using this method. We arecurrently working on 2-D and 3-D migration algorithms to produce a more well focused reflectionimage.
Figure 2: P-wave intervalvelocitygraphsfromthethree nearest 5P offset..,correkded with the induztion (resistivity)weft Iogj and a driller’s log from a welI only 2 m away. Note the correlation between high velocity and sancflgravel
unitsj which are associated with fresh water aquifer units when their resistivity is high.
Before tltk VSP-CDP mapping can be applied, we must first construct a velocity map of the zone to beimaged. This was done by inversion of the first break time picks from the dirtct wave arrivals. In this
way, we generated a depth-velocity function for each SP offset. We started with a near-offset (2.4 m to4.8 m) velocity function by averaging the inversion results from the three closest offset shot gathew, thkaverage was then raytrace matched to the first break picks of the remaining shot gathers by variation ofonly the weatherd layer velocity. In this way, we generated depth-velocity functions independently foreach shot point offset.
Figure 3: An example of VSP-CDP mapped reflections from a shot gather at offset 7.2 m.
Figure 4: A flow diagramshowingthe necessary2-D reflectionimagingsteps.
The VSP-CDP mapped traces from individual shot gathers of ‘clean’ upgoing reflection events werestacked after amplitude balancing and mirror residual static corrections (less than 0.2 m).
Figure 5: Final stacked 2-D image from the Richmond Field Station.
Logs from two cone penetrometer test (CPT) logs have been spliced into the stacked image of Figure 5.Both of these CPT holes were no more than 1.5 m out of the reflectivity image depth plane. Each logconsists of a bearing load trace, a sleeve friction trace, and a DC resistivity trace. Increasing darkness inall three CPT traces simultaneously indicates intersection of a fresh water saturated sand/gravel unit.
The CPT1 log stopped at 21 m depth, unable to penetrate through a gravel layer due to too high abearing load; this depth correlates well with a strong reflectivity horizon which is almost flat out to the 9m offwt limit of the image.
Them is a strong reflectivity horizon at about 11 m depth, which ties in well with the CPT1 log, butaPPe~ to be dipping slighfly (6 deg.), md fails to correlate exactly with the CFT 4 log high (dark) by amistie in depth of about 0.5 m; this could be due to minor elevation changes between the wellhead andthe top of each CPT hole. Neither CPT hole was surveyed in mrd corrected to a common elevationdatum.
There is rdso a steeply down-dipping (about 30 deg.) horizon at 16 m depth, that is quite strong at zerooffset, but fades out after 5 m. Since this event does not seem to correlate with the CPT I log in anyway, it is reasonable to assume that this is an out-of-plane wave, or converted wave, or possibly adiffraction.
Of particular interest in Figure 5 is the apparent pinch-out of a unit at 28 m depth, less than 1 m awayfrom the bore hole. This unit may be a sand lens.
The Franciscan basement rock in thk image is intwpreted to be the strong reflection horizon at 35 mdepth.
SeLwmk Re~ction Imaging &a Shulbw Site.
● Shallow VSP Techniques using Hydrophore Arrays.
● shaflow Surface Seismic Experiment,
● Sha/.!Ow Richmond Fiei%iSite.
VSP 3-DReflection Imaging
Figure 1: The proposedalgorithm for 3D VSPdata imaging of refleetivitie% and their inversion into quantitativesedimenthock properties. Note that this method will repeatedly iterate on the velocity rontrol loop, until a reffeetion
event or horizon is well focussed enough in that part of the imaged zone that is currently being worked on.
An initial velocity map will be generated by inversion of first break arrival times in the direct wave.Parts of this map will then be used for the migration and mapping of several key (strong)reflection/diffraction events in the VSP data. Each mapped sub-image will be analysed for coherency(focus), and the relevant portions of the velocity map will be parametrically modified until the mappedsub-image is well focused enough. This may be performed in a combination of ‘layer stripping’, and‘cylinder stripping’ modes, stinting at the borehole, and working outward. Thus the accuracy of thisvelocity map (3D volume) will be higher close to the borehole, with decreasing inaccuracy further away.The accuracy of the final 3D image will afso inherit this decrease in accuracy with increasing offset.
For a description and diagranr on the 3D acquisition geometry, see the section on Shallow VSP shootinggeometry.
Note: work is in progress for the 3D migmtion, velocity inversion, and mapping of wavefield separatedVSP data.
Seismic Rejlect&m Imaging at a ShalC%wS&.
● Wallow Surfiie Seismic Expm”ment.
● SM1OW Richmond FieLi Site.
Technical Inform
ation Departm
ent • Lawrence Liverm
ore National Laboratory
University of C
alifornia • Livermore, C
alifornia 94551
