The winds of change Anisotropic rocks—their preferred

3D PP field data showing azimuthal variation of fre-quency. The next two figures present an analysis of field datafrom the Wind River Project, a mid-1990s study of a natu-rally fractured gas reservoir. Figure 14 depicts one NS dipline extracted from two 3D limited-azimuth volumes. Thedata are post DMO-stack-migration. The EW raypaths stackand the NS raypaths stack are shown. There is a fault blockin which the EW raypaths are attenuated and the NS ray-paths are not. This one fault block is interpreted to have NSgas-filled fractures. The CMP gathers were checked for properNMO in each case. The EW gathers showed flat NMO butlow-amplitude, high-frequency reflections. The NS gathersshow high-amplitude, low-frequency reflections that requirea slower NMO than the EW velocity to be flattened.

Figure 15 is the color frequency plot of the data in thered boxes on Figure 14. Specialized techniques to achievehigh-frequency resolution in short time windows must beemployed. Queen and Theophanis (2000) published on howto corender both a frequency and its amplitude. The fielddata exhibit a large anomaly in the yellow oval: a lack ofreflection energy in the fault block for EW raypaths, butplenty of reflection energy in the NS raypaths.

The field data exhibit a small anomaly in the right faultblock (red oval in Figure 15). The red low-frequency EWreflection differs from the blue NS high-frequency reflection,even though both arise from the same boundary in the earthat the same CMP bin.

Azimuthal tuning might be present in this thin bedreflection, wherein vertical aligned fractures change thelocal interval velocity by azimuth. When the velocity ischanged by azimuth, then the frequency which has max-ium amplitude (tuning) is different by azimuth.

The asset team: Geologist, engineer, geophysicist, petro-physicist. The anisotropic rocks that give rise to azimuthallyvariant seismic signatures exhibit variable permeability sig-natures. The engineers document the pressure and fluidflow properties through time, and the geologist documentsthe geologic history of these rocks. The geophysicist mapsthe heterogeneity (porosity, fluid distributions) and theplumbing (high permeability streaks within the reservoir).The nonunique nature of the seismic data requires that theseendeavors are collaborative in nature.

What people say about “how field data should look” issimply a function of the community’s understanding ofhow seismic waves propagate through heterogeneousanisotropic media, and therefore often matters little. Whatdoes matter very seriously is what your own field data sayabout the rocks of your reservoir or under your leases, andthe symmetries exhibited within your data. Therefore, myrecommendation is that you test all the assertions of thispaper against your own field data, and inquire if your ownfield data exhibit the symmetries and the relationships pre-sented herein.

1258 THE LEADING EDGE DECEMBER 2004

The winds of changeAnisotropic rocks—their preferred direction of fluid flow and their associated seismic signatures—Part 2

HELOISE B. LYNN, Lynn Incorporated, Houston, Texas, U.S.

A Summary of Part 1 and Introduction to Part 2

Our complex earth has a continuum of all scales ofordered heterogeneities. Ordered heterogeneities give riseto three types of seismic observations, depending upon therelationship between the wavelength and the scale lengthof the ordered heterogeneity:

1) Reflections that arise from layers 3/8ths wavelengththick or greater.

2) Azimuthal scattering, azimuthal attenuation, andazimuthal dispersion that arise from aligned orderedheterogeneities with scale lengths ~0.01 - ~0.4 wave-length.

3) Azimuthal anisotropy = azimuthal variations of trav-eltimes (and amplitude) that show a 90° variationbetween fast-to-slow on offsets approximately equal totarget depth that arise from aligned ordered hetero-geneities that are very small relative to the wavelength.Unequal horizontal stresses and/or aligned orderedheterogeneities (fractures) can cause azimuthalanisotropy. Equivalent media theory and “their penny-shaped cracks” are used to model and explain theseobservations. In this category, little or no change ofphase over azimuth is observed and little or no changeof frequency with azimuth (fixed offset) is observed.Azimuthal variation of frequency (fixed offset) doesoccur and is observed, but this is probably an azimuthalscattering effect, and as such is more properly locatedunder item 2.

Standard industry seismic imaging (prestack time,prestack depth) routinely does a good job with reflectionsfor limited azimuthal ranges. Our industry also has soft-ware to measure, quantify, and output the azimuthal vari-ations in traveltimes. But, the effects listed in item 2(azimuthal dispersion, azimuthal attenuation, andazimuthal phase) are routinely ignored and obliterated.Trace-by-trace spectral whitening is the epitome of whaterases this signal. The intent of “The winds of change” isto invite a reconsideration of how we ought to be treatingazimuthal phase/scattering/dispersion/attenuation/spec-tra for this is where we find the information on the longscale lengths of fractures with the wider apertures thatgovern fluid flow in the reservoir.

In Part 1 of this paper (TLE, November 2004), I sum-marized the effects that ordered heterogeneities can haveon seismic energy given the relative scales of the orderedheterogeneities and the seismic wavelengths. The dataexamples focused on the effects on P-wave data. In Part 2,I continue with effects on P-wave energy and then considerthe effects of seismic anisotropy on S-wave energy and thechallenge of corendering multiple seismic attributes to usefor reservoir characterization.

Anisotropic rocks. The most powerful seismic diagnosticconcerning vertical aligned fractures is that shear-wavessplit into two shear-waves upon encountering these frac-tures. In the presence of unequal horizontal stresses and/oraligned vertical fractures, an arbitrarily polarized vertically-propagating shear-wave will split into the fast shear-wave,polarized parallel to the stiff direction of the rock, and theslow shear-wave, polarized perpendicular to the fast shear-wave. The polarization (particle motion direction) of thesesplit shear-waves documents the stiff direction of the rockand the compliant direction of the rock. The local geologywill give clues as to why one azimuth should look stiff andthe other azimuth (90° different) should look compliant.

The PP variations in traveltime, amplitude, frequency,phase, coherence, etc. are conventionally understood toexhibit a 90° variation between fast-slow-fast-slow, or bright-dim-bright-dim, or etc. The simplest of all rocks will havethe principal azimuths of the PP reflections coincide with

the principal azimuths of the PSor SS split shear-waves, in the 0-360° analyses.

Azimuthal variations in PPseismic signatures can beambiguous as to whether het-erogeneity (different rocks alongthe different raypaths) oranisotropy is causing the signa-ture. Therefore many peopleprefer to look at split shear-waves, for they have traveled(reasonably) the same path andany difference in their ampli-tude, traveltime, phase, or fre-quency is due to the aniso-tropy.One could call the split shear-waves the “gold standard”against which the PP azimuthalsignatures are measured.

In the presence of verticalaligned fractures, the largestanomaly of all, in principle, is inthe attenuation. Our industryhas rather eased itself intoanisotropy by first going afterazimuthal traveltimes, becausewe feel most comfortable withthose, and then entered intoazimuthal amplitudes, but nowwe are finally willing to thinkabout azimuthal attenuation orazimuthal phase.

Shear-wave splitting. DonWinterstein presented, at the1988 SEG/EAGE SummerResearch Workshop, multicom-ponent data from a shear-waveVSP. Figure 16 shows the data asrecorded in the acquisition coor-dinate system (which has noth-ing to do with the earth’scoordinate system). Note the dif-ference in the shape of thewavelets, and the profound dif-ference in the spectra.

Winterstein correctly under-stood that this indicated shear-

wave splitting, and rotated his geophones into the naturalcoordinate system (Figure 17). Now the two wavelets areremarkably similar, although not identical, and the spectraare much more similar (but, significantly, also not identi-cal). The slow shear-wavelet is 10 ms delayed behind thefast shear-wave. Please note carefully that the fast shear-wave wavelet, the solid curve, is very close to zero phase,in that the two side troughs are just about equal in ampli-tude and width.

However, the slow shear-wave, the dotted wavelet,clearly exhibits a phase delay, or backloading, in that thetrailing trough is much deeper than the initial trough. Thisphase delay is an indication of dispersion; different fre-quencies have traveled at different velocities, or the phaseof the different frequency elements is not zero phase. Thephase signature of the split shear-waves is not noise—it issignal.

Also worthy of note are holes in the S2 spectrum (cir-

DECEMBER 2004 THE LEADING EDGE 1259

Figure 14. Migrated line taken from identical traverse through migrated data comprised of E-W-ori-ented traces (left) and N-S-oriented traces (right). Data are from the DOE Wind River Basin Project.Note the difference in the fault block image around 2.5 s. The data highlighted by the red box are exam-ined in Figure 15.

Figure 15. Frequency spectra of the boxed data in Figure 14. The comparison underscores that the PPreflection frequency is different in the EW and NS directions. Red = 10.0-31.4 Hz; green = 24.3-45.7Hz; and blue = 38.6-60.0 Hz. (Figure courtesy of John Queen.)

Line 165 — Deeper Horizon

Trace display

— Line 165

cled in red). These holes in the S2 spectra can be interpretedas indicating the frequency-wavelength (velocity) at whichthe destructive interference of the backscattered energy hascancelled out those elements of the signal. The (periodic)spacing of the compliant ordered elements is some even divi-sion of the wavelength.

Frequency-dependent shear-wave splitting, or frequency-dependent anisotropy. Different frequencies have differentwavelengths. The relationship of the wavelength to the frac-ture parameters (spacing, height, aperture, etc.) governs theeffective shear-wave velocity and the splitting. The offsetand the azimuth of propagation also perturb the velocity.Dispersion in split shear-waves is being studied to provideinsight into the fracture size (Chapman et al., 11th Inter-national Workshop on Seismic Anisotropy).

Iceland spar: Calcite crystals. Pure calcite crystals are opti-cally birefringent and so provide hands-on experience withbirefringence.

As shown in Figure 18, one image becomes two images,when the image is viewed through a pure piece of Icelandspar. The two images are orthogonally polarized. The opti-cal birefringence arises from the ordered arrangement of thedifferent atoms within the crystalline lattice.

Double splitting can be viewed with two Iceland sparcrystals, when their optic axes are not aligned. This is sim-ilar to the double splitting that can happen in the earth, whentwo different layers have different open fracture azimuths(different by ~20-70°). If the open fracture azimuth just flipsby 90°, no doublesplitting occurs but a much simpler situ-ation does—the fast shear-wave becomes the slow shear-wave, and the slow shear-wave becomes the fast shear-wave.Layer-stripping or other similar technique enables the recon-struction of the multiply split shear-waves.

Time-lapse seismic. If in the course of production practices,the in-situ stress state is altered, then the fracture popula-tions are altered. The multiazimuth and/or multicompo-nent 3D seismic signatures provide better documentation ofthe changes in the in-situ stress state, the change of fracturepopulations, and the fluid-distributions, than narrow-azimuth 3D PP surveys. As Andrey Bakulin stated at the 11thInternational Workshop on Seismic Anisotropy, repeatedseismic measurements will be the leading method of learn-ing more about the reservoir, and thereby extending ourunderstanding of anisotropic seismic signatures. Repeatedseismic measurements can be “conventional” time-lapse 3D


Figure 16. Early example of shear-wave splitting (Winterstein,Snowbird SEG/EAGE Summer Research Workshop, 1988). Therecorded signal and spectra differ because the horizontal phones werenot in alignment with the region’s natural coordinate system.

Figure 17. Same data as in Figure 16, but the phones have been mathe-matically rotated to align with the regional natural coordinate system.Note the relative front loading of the S1 (solid line) wavelet and therelative back loading of the S2 (dotted line) wavelet. This is signal, notnoise. The phase characteristics of S1 are not identical to the phase ofS2. The difference in phase is telling us about the internal structure ofthe medium through which the S-wave has passed. The slightly loweramplitude spectrum of S2 compared to S1 in the circled areas is relatedto the size of ordered heterogeneity.

Figure 18. Kerr’s 1959 book Optical Mineralogy illustrates opticalbirefringence through Iceland spar-calcite.

surveys, or passive seismic mon-itoring surveys during flooding,using either borehole 3-Creceivers, or surface seismic 3-Creceivers.

The preferred direction of fluidflow. There are at least two pos-sible fractured-reservoir mod-els: (1) horizontal permeabilityanisotropy arises from essen-tially one interconnected set offractures or (2) flow anisotropyarises from two or more fracturesets of varying azimuth. If onefracture azimuth dominates,then the seismic azimuthalanisotropy will be large and pro-portional to fracture density.

If two or more fracture azi-muths are open and flowing flu-ids, then there is a diminishedPP azimuthal variation signa-ture and a decrease in the inter-val velocity (VP), and usually adeviation of the fast intervalvelocity off the regional (theregional fast VP is usually themaximum horizontal stressdirection).

The case of one fracture setbeing open is usually interpreted in the context ofthe maximum horizontal stress parallel to it, and theasset team should be aware of the World Stress Map(University of Karlsruhe, Germany), for it predictsthe direction of the maximum horizontal perme-ability for the simplest of earth models. The WorldStress Map was initiated by Mary Lou Zoback andher colleagues in the U.S. Geological Survey (USGS)in the San Francisco area and Stanford University.

Figure 19 presents an example of one World StressMap. The topographic relief is the shading of thebackground. The icon points in the maximum hori-zontal stress direction, with its size proportional tothe quality of the input data. Normal fault regimesare red; strike slip fault regimes are green; thrustfault areas are blue; and unknown regimes are black.The input data include focal mechanisms, breakouts,drilling-induced fractures, hydraulic fractures, andgeologic indicators (recent dikes, etc.). Recent dis-cussions have focused upon the local “scatter” in thedata. There are two points of view. The older pointof view expected homogeneity in the stress field,and called the scatter in azimuth noise. The newerpoint of view permits local heterogeneities to affectthe orientation of the maximum horizontal stress, andthe scatter is signal, not noise.

The second case (two or more fracture sets thatpermit fluid flow through interconnected fracturenetwork) has at least three seismic indicators. The P-wave interval velocity decreases; the azimuthal vari-ation in the interval velocity decreases; and theazimuth of the fast P interval velocity swings off theregional. Therefore, we can recognize the deviationoff regional as part of the signal we seek. The coren-dering of all three numbers into one glyph (or icon)is the preferred method to visually track three num-


Figure 20. NMO-corrected CMP gather showing far-offset traces (5000-7000ft) as a function of source-receiver azimuth. This example shows about a 0.5%azimuthal variation in RMS velocity. The fast directions (red) are N50E andN230E; the slow directions (blue) are N140E and N320E. (Data for Figures 20-26 courtesy of Devon; processing for Figures 20-26 by GMG/Axis).

Figure 19. Regional stress map of China and SE Asia. The expected direction of fluid flow is usuallyparallel the “max hor stress” indicated by the direction of the bar.

bers throughout a mapped region.

Fluid flow (as seen through the eyes of the engineers).Reservoir engineers use interference tests, planned orunplanned, that document fluid connectivity at the inter-well scale (the scale that is typically examined through seis-mic data). Early breakthroughs, pressure tests, and knowndepths of documented fluid flow (gas, oil, or water) are whatthe geophysicist would like the engineer to bring to thetable. Many times, the engineers “blow the whistle” on thereservoir, when flow rates and flow volumes are far largerthan those deemed possible by matrix porosity and matrixpermeability.

Jim Gaiser has pointed out that “fracture denial” is ram-pant in some regions, in some companies. Nearly all rocksare fractured, and nearly all reservoirs are affected by thelocal fractures, to one degree or another.

Oriented cores, crossed-dipole shear-wave logs run inan uncased hole, FMI/FMS logs, resistivity at the bit images,and multicomponent VSPs are all valuable datasets to pro-vide ground truth for seismic interpretation. The end result

of all seismic azimuthal analysesis usually the generation ofmaps that are put into the reser-voir simulator. The mapping ofthe heterogeneity and theanisotropy gained from the seis-mic “isn’t worth a flip unless itis put in a map and used in asimulator”, according to severalengineers I’ve spoken with.

Will, Archer, and Dershowitz(2003) have published on the useof azimuthal seismic attributesto help build the discrete frac-ture network (DFN) model forthe reservoir simulators. BobParney and his colleagues havealso published on this matter.

Associated seismic signatures.It is now fairly routine to mea-sure (in wide-azimuth surveys)the azimuthal variation in the

PP traveltimes and AVO gradi-ents. Ambiguity, though, remainson whether heterogeneity oranisotropy is causing theobserved variation.

The split mode-converted waves (P down, S up) removethat ambiguity; for the same downgoing P-wave path at 45°to the natural coordinate system (the fractures’ azimuthand/or the maximum horizontal stress), the upgoing S trav-els on just about the same travel path, and the only differ-ence is the polarization of the two split shear-waves.Therefore, the difference between the split shear-waves isdue solely to anisotropy.

Attenuation is in principle the largest magnitude anom-aly of all. We in the seismic business started with traveltimes(the easiest to measure!) and then progressed to amplitudes(AVO) that are moderately straightforward to process andinterpret. However, even though azimuthal variation inattenuation ought to be the much larger magnitude anom-aly, we routinely destroy any azimuthal variation in atten-uation and phase (3D PP) by imposing trace-by-trace spikingdeconvolution, followed up with prestack spectral whiten-ing.

A moderate azimuthal variation in PP traveltime isshown in Figure 20. The far offsets (5000-7000 ft), whereinoffset is about equal to target depth are shown after normal


Figure 22. The same azimuthal interval velocity information from Figure 21 corendered into a singleicon—in this case an arrow. The direction of the arrow is the direction of the maximum Vint; the colorrepresents the magnitude of the max Vint and the length of the arrow is proportional to the differencebetween the maximum and minimum Vint.

Figure 21. Processing to preserveazimuthal attributes requires at leastthree attributes: the direction of theattribute’s maximum magnitude, themaximum magnitude, and the differ-ence between the maximum and mini-mum attribute magnitude. These areshown as three individual plots for theinterval velocity determined from the3D volume composed of data from thearea shown in Figure 20 in the timezone between approximately 1200 and1300 ms. Figure 22 shows how thesethree attributes can be corendered intoa single icon.

moveout with one velocity and stacking. The minimumtraveltime is the fast direction (here N50E) and the maxi-mum traveltime is the slow direction (N140E). The varia-

tion every 90° between fast andslow is consistent with ortho-rhombic symmetry (i.e., flat lay-ers and vertical fractures).

Once the decision is made toevaluate the azimuthal variationin the PP traveltimes, at leastthree quantities must be mea-sured: the fast interval velocity,the azimuth of the fast intervalvelocity, and the azimuthal dif-ference in the interval velocity.Figure 21 presents a small area(about 110 inlines � 110 cross-lines) where these three quanti-ties are displayed in color, forthe reservoir interval. The con-tour lines of the background arethe time structure of the base ofreservoir. The large fault trendsNE and separates the high side(SE) from the low side (NW).

Figure 22 shows these threemaps corendered into one mapvia a colored icon, here a col-ored arrow. The color is the mag-nitude of the interval velocity(blue is fast and less likely forhydrocarbons, and red is slow

and more likely for hydrocarbons). The direction of the iconis the fast direction. The length of the icon is the azimuthalvariation (fast interval velocity minus slow interval veloc-


Figure 23. Inline extracted from a wide-azimuth 3D PP survey (onshore) illustrating how faulting canaffect the local stress field, which in turn affects azimuthal seismic attributes. A closer view of the leftand center panels is shown in the next two figures.

ity). The shaded gray of the background is a shaded reliefof the base of reservoir reflector’s structure, with a sun anglefrom the SE, 45° off horizontal. From inspection of this fig-ure, it is clear that on the high side of the fault, the slowerinterval velocity rocks have their fractures parallel the bigNE fault. On the other hand, the low side of the fault haslocally very fast rocks with their fractures oriented perpen-dicular to the fault. This is reminiscent of faults with somestrike-slip movement as well as the normal faulting.

In-situ stress perturbations in the vicinity of faults. It isfairly common to observe the in-situ stress field perturbed

in the vicinity of faults. This per-turbation is picked up in thetraveltimes and/or the ampli-tudes.

Figure 23 shows an NS lineextracted from a 3D PP survey(left) with a normal fault, trend-ing N45E down to the right. Thecenter panel shows the fast rmsvelocity azimuth. Red indicatesN45E, which is the fault-paral-lel azimuth. The right panelshows rms Vfast minus Vslow,with warm colors indicatinglarger azimuthal variations.Figure 24 is an enlargement ofthe center panel, with its associ-ated wiggle trace seismic (PP).Figure 25 shows the fast Vrmsazimuth superimposed on theseismic. The feature is directlyassociated with the offset in thereflectors.

Usually, the interval veloc-ity carries more precise localinformation than rms velocity.Figure 26 presents another linein this survey where the top andbase of the reservoir are shownas blue and black lines (leftpanel) and a fault is present(down to the left). This faulttrends N45E. The fast azimuthVint is parallel to the fault trend,indicating that the seismic fastdirection (PP) is parallel to thesmall-scale fractures parallel tothe big fault. The azimuth of thefast rms velocity, though, isN90E at the base of reservoir,and needs the interval velocitycalculation to present it moreaccurately as N135E (left panel).The right panel shows the largeazimuthal difference in intervalvelocity at the top of the reser-voir adjacent to the fault.

Historical development. TheU.S. Department of Energy seis-mic study for the naturally frac-tured gas reservoir of the UpperGreen River, Bluebell-Altamontwas comprised of two 2D, 9-Clines shot in the natural coordi-

nate system (parallel and perpendicular the fractures). Thedata were acquired in April 1994, and have been written upextensively. Figure 27 documents the local (orthogonal) frac-ture azimuths seen in outcrop, the NW maximum horizon-tal stress direction from borehole elongation, the NWpaleo-maximum horizontal stress direction (gilsonite dikes),the NW azimuth of S1 (VSP), and NW fracture orientationsin core, in relation to the field layout. Various wells (con-nected with colored lines) indicate seismic stratigraphicmodel sections we constructed. At the tiepoint, the PP AVOgradient depended upon the azimuth: parallel the fractureswe saw dimming gradients, but perpendicular to the frac-


Figure 24. Closeup of the center panel of Figure 23 and its associated seismic traces.

Figure 25. The fast rms velocity azimuth is overlain onto the seismic wiggle traces from Figure 24.

tures, we saw brightening gra-dients, for some reflectors, butnot for all (Figure 28). The focusof the study was Top GreenRiver to Mahogany Bench.Below Mahogany Bench, deeperreflectors exhibited azimuthalvariations in PP re-flectionstrength on the near offsets notexplainable with current theo-ries; we need better theories toaccount for observations likethese, which are not uncommon.

Figure 29 presents the S2-S2and S1-S1 reflection data at thetiepoint, next to the color AVOgradient display (PP). AGC hasbeen applied to the S-wave datain order to clearly pick travel-times. The S-wave traveltimesshow varying amounts of shear-wave splitting, as are posted inFigure 31. But, before we get toFigure 31, it is necessary to em-phasize that the largest ano-maly in the shear-wave data wasnot in the traveltimes but in theamplitudes. Figure 30 shows theS-wave reflection data displayedrelative true amplitude (withoutthe AGC). The S1-S1 reflector atZ is very bright, while the S2-S2reflector at Z is about 12 dblower in amplitude. The largestazimuthal difference in PP AVOwas at the Z reflector, circled inred in Figures 29 and 30.

In our original study, we pre-sented the PPAVOaz magnitude(Figure 31) as proportional to theshear-wave birefringence in thelayer. Subsequently, Ruger andTsvankin (1997) derived anexpression for the PP AVOazwherein the PP AVOaz is pro-portional to the boundary’s con-trast in shear-wave birefringence(gamma) and the contrast indelta (the Thomsen anisotropicparameter). Figure 32 presentsour data’s results graphed toshow the contrast in S-wavebirefringence against theazimuthal difference in PP AVOgradient (absolute value). Thesomewhat linear trend is con-sistent with the prediction ofRuger-Tsvankin. Figure 31 canbe interpreted as indicating that,in this area, the contrast in deltaexerted a lesser control upon the PP AVOaz, while the con-trast in S-wave birefringence at the boundary exerts a verystrong control. In addition, it has been suggested that the con-trast in delta is proportional to the contrast in shear-wave bire-fringence (Bakulin, 2000).

The other anisotropic parameter, epsilon, that relates thehorizontal VP to the vertical VP, also plays a role, for epsilon

can vary azimuthally. If epsilon in the maximum and the min-imum horizontal stress direction were derived from far-off-set PP reflection data (a dual scan of PP NMO and eta (thenonhyperbolic moveout on the far offsets), with well controlto determine delta), then the ratio of the epsilons is indicativeof the max/min horizontal stress ratio.


Figure 26. Interval velocity attributes from another line in the same survey as the three previous figures.This figure compares the rms velocity results (center) with the interval velocity results in the zone ofinterest (top and base are posted in the left panel). The right panel shows the difference between the fastand slow interval velocity. Note the large (pink) change in the vicinity of the fault.

Figure 27. Outline of Bluebell-Altamont 1994 study area. Two 9-C 2D lines were acquired. Line 1 wasparallel to the fractures and line 2 was perpendicular to the fractures. A 9-C VSP identified the SSreflectors and determined the S-wave time-depth velocity function.

Pitfalls to avoid in azimuthal studies. Dip causes a 90°azimuthal variation in velocity, and in reflection times, thedowndip reflections take more time than the updip reflec-tions. Coherent noise that is azimuthally varying might notbe as well removed in some azimuths as in others. Spatialirregularities in fold and offset will affect S/N and have noth-ing to do with the subsurface geology. The question of

azimuthal PP signatures arisingfrom heterogeneity or anisotropyhas been discussed. Finally, thenonuniqueness of cause—verti-cal aligned fractures and/or un-equal horizontal stresses—isalways present in PP reflectionseismic.

The seismic anisotropy sig-natures are evaluated in the con-text of your geologic andengineering data. The base of thereservoir is usually a more fruit-ful place to hunt for PP AVOazanomalies, for one has the trans-mission effect as well as the reflec-tion effects (Sayers et al., 1998;Luo et al., 2004). PPAVOaz is sen-sitive to the contrast of fracturedensity at the boundary; it saysnothing about which layer ismore fractured. If both layers areequally fractured, the PP AVOazsignature might be small.

Additional seismic and geo-logic information must be used inconjunction with PPAVOaz stud-ies. Modeling and the intervalproperties of the overlying layerand the underlying layer are partof the additional information weseek. Either the magnitude ofthose layers’ shear-wave bire-fringence is posted, or, in substi-tution, the azimuthal varition inthe PP interval velocity can beposted, for joint evaluation of PPAVOaz with the layer’s proper-ties (shear-wave birefringence, orinterval VP azimuthal variation).This “canonical icon map” pre-sents nine numbers in the con-text of local structure—threenumbers for the Vint upper layer,three for PPAVOAz, and three forVint lower layer numbers, whichcomprise the minimum of whatmust be considered when evalu-ating PPAVOaz (Lynn et al., 2003.Page two of this paper detailsthese three numbers.) Dan Coxhas written up a Mathcad modelfor the Ruger-Tsvankin equationsthat is worth looking into.

Erika Angerer of CGG is tak-ing azimuthal data to azimuthalimpedance, in order to get backto layer properties. This approachoffers a method to combine theazimuthal traveltime and azi-

muthal amplitude information, in limited azimuth sectors, toget impedance (elastic impedance or acoustic impedance),and thereby simplify the icon maps. By simplifying the iconmaps, additional information can be added more easily—azimuthal coherence and azi-muthal phase. Fairly sophisti-cated mathematics is necessary to understand the symmetriesof the azimuthal variations of amplitude and the azimuthal


Figure 28. Comparison of CMP supergathers at the tie of lines 1 and 2. Line 1 runs parallel to thecracks and line 2 runs perpendicular to the cracks. Red ovals highlight areas of difference in AVO behav-ior; blue ovals highlight areas of similar AVO behavior (Data processing by Pulsonic).

Figure 29. Bluebell-Altamont DOE data. (left) Line 1 S2-S2 and line 1 S1-S1 data at tie point. Thetime variant traveltime mistie is due to shear-wave splitting. The data have had AGC applied to empha-size the traveltime misties. (right) The PP AVO gradient for line 1 and line 2 at tie point. Note thelarge difference at the Z reflector—just above 2.0 s.

variations of interval velocity, forthey do not necessarily track eachother! Mark Chapman, and hiscolleagues of EdinburghAnisotropy Project, presentedgood insights at the 2004SEG/EAGE Summer ResearchWorkshop, wherein the large PPAVO gradient was neither in thefast nor slow direction, but at 45°to the fast direction when the frac-tures were large enough to affectthe permeability. This is a signif-icant new development in theproposed interpretation stagethat would require additionalthought as to how to translate thisinto azimuthal impedance, aswell as field data studies to testthis hypothesis.

The future? I hope that there aresome who will consider that thispaper is a “look into the future”of what our industry couldaccomplish. But for the real lookinto our future, I see more andmore renewable energy sourcescoming that will enable familiesand communities to be stand-alone energy-sufficient. The localsetting will dictate the balance ofcontributions from wind, solar,tidal, hydroelectric, and/or geot-hermal. Countries like Norwayand Denmark, already buildinginfrastructure for the posthydro-carbon age, are leading our pro-gress into the sustainable future.Now those are the winds ofchange.

Summary. The added dimen-sion of azimuth for 3D PP (andPS) data provides additionalinformation concerning alignedcompliant connected pore space.If you are interested in the“plumbing” of your reservoir,then you are interested in theazimuthal variation in seismicsignatures.

3D PP wide-azimuth fielddata habitually contain threemajor types of signal:

1) Reflections from thick layers (3/8ths wavelength thickand greater).

2) Azimuthal variations in reflection traveltimes and ampli-tudes, that are to some part explainable by equivalentmedia theory, wherein our seismic traveltimes behave asthough there were “penny-shaped cracks much smallerthan the wavelength” that are influencing the velocityfields.

3) Azimuthal attenuation, scattering (coda), azimuthal dis-persion, wherein some property(s) of the fracture popu-lation (spacing, height, length, apertures, etc.) is on the

order of the wavelength. This means larger than thosepenny shaped cracks of equivalent media theory. Anyfracture set that has property scale lengths approachingthe seismic wavelength are inherently of more interestthan fracture sets with scale lengths much much smallerthan the wavelength.

Items 1 and 2 are routinely understood and honored inour data processing flows, with specialized and powerful soft-ware routines. Item 3 is routinely destroyed in standard pro-cessing, even in some azimuthal processing sequences. Item3 should not be routinely destroyed in standard processing,but examined to see what useful insights are present.


Figure 30. (left) Line 2 S1 and S2 sections at tie point, displayed in relative amplitude. (right) Same asin Figure 29.

Figure 31. The azimuthal difference of the P-wave AVO gradient was originally viewed as propor-tional to the S-wave birefringence measured in the layer.

Azimuthal attenuation is knownfrom laboratory studies to be thelargest of the azimuthal anom-alies, when the scale lengths ofthe fractures approach that of 0.1wavelength (or wavelengthscales). Studies have docu-mented the power-law relationswithin fracture populations(Marrett, 1999, 1997, 1996).Furthermore, many PP AVOazanomalies routinely observedare of much larger magnitudesthan the 20-30% magnitudesthat the Ruger-Tsvankin (equiv-alent media theory) equationspredict; therefore, it is logical toask, “Could some of item 3 beinfluencing the magnitude ofour unexplainedly large PPAVOaz anomalies?” While thispaper has been focused uponthe PP reflections, the commentsalso apply to PS reflections,which in general are far moresensitive to aligned ordered het-erogeneities than PP waves.

Suggested reading. “Estimationof fracture parameters from reflec-tion seismic data, Part II: Fractured models with ortho-rhom-bic symmetry” by Bakulin et al., (GEOPHYSICS, 2000). “Detectionand analysis of naturally fractured gas reservoirs: Multiazimuthseismic surveys in the Wind River Basin, Wyoming” by Grimmet al. (GEOPHYSICS, 1999). “P-wave azimuthal variations in atten-uation, amplitude, and velocity in 3D field data: Implicationsfor mapping horizontal permeability anisotropy” by Lynn andBeckham (SEG 1998 Expanded Abstracts). “P-wave and S-waveazimuthal anisotropy at a naturally fractured gas reservoir,Bluebell-Altamont Field, Utah” by Lynn et al. (GEOPHYSICS,1999). “Extent of power-law scaling for natural fractures inrock” by Marrett et al. (Geology, 1999. “Permeability, porosity,and shear-wave anisotropy from scaling of open fracture pop-ulations” by Marrett (Fractured Reservoirs: Characterization andModeling Guidebook, Rocky Mountain Association Geologists,1997). “Aggregate properties of fracture populations” by Marrett(Journal of Structural Geology, 1996). “Color display of the local-ized spectrum” by Theophanis and Queen (GEOPHYSICS, 2000).“Using AVO for fracture detection: Analytic basis and practi-cal solutions” by Ruger and Tsvankin (TLE, 1997). “How shear-wave properties relate to rock fractures: Simple cases” byWinterstein (TLE, 1992). “Velocity anisotropy terminology forgeophysicists, by Winterstein (GEOPHYSICS, 1989). “P-wave AVOinterpretation needs the input of additional information” byLynn et al. (SEG 2003 Expanded Abstracts). TLE

Acknowledgments: Grateful thanks to:

• Devon Energy, and especially Mike Ammerman, for permission toshow their field data, and their sustained interest and support through-out many years.

• To the acquisition and processing contractors who made this work pos-sible—WesternGeco (Jim Gaiser, Richard Van Dok), Axis (Ed Jenner,

Marty Williams), Schlumberger (especially Allan Campbell for beingthe world’s best processor of S-wave VSPs!), Pulsonic (Peter Carey,now at Sensor), and all the multicomponent multiazimuth industryplayers for building the infrastructure for this work.

• The U.S. Department of Energy for funding the three naturally-frac-tured gas reservoir projects that I worked on. The InternationalWorkshop on Seismic Anisotropy community, which for the last 20+years have been bravely going where no one has gone before.

• Dan Cox, Delta-Sigma, for coding up a MathCad module of the Ruger-Tsvankin equations that enable easy quick modeling of PP AVOaz.

• John Queen (formerly Conoco Research, now High-Q), for providingcolor-rendering of the wavelet’s frequency, for azimuthal frequency stud-ies. His research in anisotropy for over 20 years has driven the fieldforward.

• Andrea Grandi, for coding up the commercially available Frac-Dip toeasily model PP and PS azimuthal reflections in anisotropic dippingmedia. David Taylor, and ANISEIS, for enabling seismic modeling inanisotropic media.

• My alma mater, Stanford University, especially professors GeorgeThompson and Jon Claerbout, for providing depth and breadth to myseismic studies.

• Richard Bates, now at University of St. Andrew, Scotland, for pro-viding project leadership and world-class field science for theDepartment of Energy studies.

• Yves Simon and Michele Simon, for maintaining a long associationwith Lynn Incorporated and their fine geophysical contributions.

• Don Robinson, Resolve GeoSciences, for providing a wonderful long-term professional association and friendship.

• And most significantly of all—Walter S. Lynn, beloved husband, stal-wart friend, and distinguished colleague.

Corresponding author: [email protected]


Figure 32. Data in Figure 31 graphed to show the the contrast in S-wave birefringence against theazimuthal difference in PP AVO gradient (absolute value). The somewhat linear trend is consistent withthe prediction of Ruger-Tsvankin (1997).

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