By David Chase,Yuval Serfaty,Igor Belferand Zvi Koren

HOUSTONSeismic imaging tech-niques use background velocity modelsto migrate recorded seismic data fromthe acquisition time domain to commonimage gathers (CIGs) in the depth domain.Normally, these are of a lower dimen-sionality than is potentially accessible,with the extra dimensions being reducedby integration. As a consequence, muchinformation that could be of use in unique-ly characterizing subsurface geologicalfeatures is irretrievably lost. In order toretain this important information, the im-aging can be performed in two independentstages.

The first stage involves ray-based map-ping of the seismic events into subsurfacelocal angle domain (LAD) tables at eachimage point, retaining all the relevantdegrees of freedom. The LAD table isdefined by the reflection angles and theirazimuths, in addition to the two polarangles for the ray-pair directivity. Togetherwith the depth axis, this results in a 5-DLAD CIG. These are organized and storedin an in-line/cross-line lateral grid.

The second stage involves implement-ing different types of dedicated imagingoperators that work on these full-dimen-sional datasets to isolate desired subsurfacecharacteristics, such as structure continuity,discontinuous objects aligned near verticalplanes such as salt flanks or faults, andeven fracture systems. The method isdemonstrated on a real dataset

Seismic migrations use the recordeddata and an approximated backgroundvelocity model to back-propagate themany wave phenomena present in the

recorded data into their true subsurfacelocations, and by applying the rightimaging condition criteria, form the bestpossible image of the geological model.Among these wave phenomena, thereare those of crucial interest to the inter-preter, such as reflections, differenttypes of diffractions, and duplex/cornerwaves.

There are many challenges in thisprocess. The background velocity modelis a simplistic realization of the true geo-logical model that can include, for ex-ample, complex structural geometries andlayer velocities with different orders ofanisotropy, absorption and dispersion. Anaccurate geological model builder andappropriate wave/ray-based modeling areessential for simulating the wave propa-gation in such complex models. The im-aging condition applied at the subsurfacepoint is a critical operation that stronglyaffects the type and class of images ob-tained. Additionally, the acquisition affectsthe ability to optimally illuminate thesubsurface grid points from all direc-tions.

In seismic imaging, the recorded seis-

mic data are generally migrated directlyto depth-domain CIGs, which can betwo-dimensional (for example, depth ver-sus angle or offset), or more recently,three-dimensional (with the addition ofazimuthal dependence). Because thesegathers are of a lower dimensionalitythan is accessible during migration, themigrated physical events, having beenpotentially isolated in the imaging process,are rendered inseparable or even invisiblein the final image.

Novel ApproachThe novel approach introduced in

this article retains the integrity of thecollected data in a form that can be ofuse in achieving an unambiguous inter-pretation of the subsurface geology. Theinput seismic data are first mapped intothe full-dimensional decomposition foreach imaging point. This consists offour components of the local angle do-main. Dedicated imaging operators thencan be applied to form various desiredclasses of final images. This imagingapproach attempts to confront the manygeological and geophysical imaging chal-lenges encountered worldwide, and is

Approach Enhances Seismic Imaging

FIGURE 1Mapping Schema from Surface to Subsurface Domain (A), 5-D Gather

(B), Opening Angle (C) and Directional Angle (D) 3-D Gathers

A B DC

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an extension of ongoing work in imagingin the LAD.

The method consists of two independ-ent stages. The first stage involves ray-based data mapping of the surface seismicdata into the full subsurface space-angledomain. A point diffractor ray tracingoperator has been designed that shootsrays from the imaging point equally inall directions, and stores the required rayproperties for all of those that succeed inreaching the surface. The aim of this step

is to optimally associate the source andreceiver locations with the appropriatesubsurface directivities.

The permutations of the individualdiffracted rays form a system of reflectionray pairs (incident and scattered) thatenable the decomposition (binning) ofthe migrated seismic events into the insitu 4-D LAD table at each subsurfacepoint.

The in situ 4-D LAD table comprisestwo polar angles representing the direc-

tivity of the ray pairs (the sum of the in-cident and scattered slowness vectors)and two additional angles representingthe opening angle and opening azimuthbetween the two slowness vectors at theimage points. This type of data is storedas a 5-D CIG for each in-line/cross-linelocation. The size of the bins in the 4-DLAD table determines the resolution ofthe mapped input data. This subsurfacedomain also is the most appropriate fromthe physical point of view for many dataassessment and correction operations nor-mally performed in the input data do-main.

Specularity GathersThe second stage involves processing

the 5-D CIGs for both corrections andseismic property extractions using di-rectivity driven imaging. First, direc-tivity driven 3-D semblance or specu-larity gathers are computed for each5-D gather. The 3-D semblance valuesare computed for each directivity bin(dip/azimuth) over all depth points withina given window. As the specularity gath-ers are computed along the opening an-gle/azimuth traces, they indicate the co-herent energetic directions of the actualreflected events.

These directional gathers can be usedto form different types of weighted stackfilters. For example, to enhance structuralcontinuity of the subsurface reflectors,the specularity values are used directlyas weighted stack coefficients. This resultsin generating high-quality, full-azimuthangle gathers that stack to the optimalreflectivity image and considerably enrichthe information, and reduce uncertaintyin velocity model determination and am-plitude inversion analysis.

A further functionality is the separationof multiple specular events that overlapat a given imaging point, such as when acoherently reflecting fault crosses layeredhorizons, for example. Since the faultmay have a lesser overall seismic intensity,simple stacking will mask the fault. There-fore, it is essential to consider multiplespecular reflections and to construct theappropriate specular filter that will allowfor the unfolding of these unique seismicevents.

Another important option is diffrac-tion-enhanced imaging. Changes in theelastic properties of subsurface rocks ap-pear as seismic reflections. Diffractionsare generated by local discontinuities,which act as a point source, whereas re-

FIGURE 2Eagle Ford Sections from Regular Stack (A) And Specularity Enhanced (B) Volumes

A B

FIGURE 3Eagle Ford Depth Slices with Enhanced Specular Energy (Top)

And Enhanced Diffraction Energy (Bottom)

10,150 ft

15,400 ft14,200 ft

11,500 ft

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flections are generated by an extensivereflection boundary. This option involvesattenuating the high-specular energy, bydesigning a filter that is one minus thespecularity values. Furthermore, duplexwaves, which exhibit directivity azimuthaldependence, also can be extracted byconstructing the appropriate azimuthallydependant semblance filter.

The important implication is that onecan generate different types of images,each containing unique seismic informa-

tion without rerunning the heavy com-putational process of the first stage.Finally, the different images can be co-rendered to simultaneously visualize thedifferent geological characteristics.

Eagle Ford ExamplesThe examples presented in this article

are taken from the Eagle Ford Shaleplay. Figure 1A shows the schema of themapping system with the surface inputparameters and the respective subsurface

LAD domain parameters. The 5-D LADgather is uniquely visualized in the formof a 3-D display, where each of thelateral axes combines two polar angles:directivity dip/azimuth and opening an-gle/azimuth.

Figure 1B shows an example 5-Dgather for a specific in-line/cross-line co-ordinate from an advanced full-azimuthangle domain imaging and analysis systemdesigned to image, characterize, visualizeand interpret the total seismic wave fieldin all directions. The horizontal axes inFigure B are (the opening angle and itsazimuth as a continuous spiral) and (direction angle and its azimuth as a con-tinuous spriral).

Figure 1C shows a resulting 3-D open-ing angle/azimuth gather computed fromthe 5-D gather by integrating over the di-rectivity angles. Figure 1D shows a 3-Ddirectional gather computed from the 5-D gather by integrating over the openingangles and azimuths. Note the concen-tration of energy at low dip angles inFigure 1D associated with the generallyflat (zero dip) reflectors in this play.

Figure 2 shows the same section afterspecularity enhancement in the 5-D do-main. Note the removal of systematicnoise and of obvious diffractions. Theimage is much cleaner and more easilypicked. The gathers also are much cleaner,providing more stable and reliable move-out estimation and analysis results ofvariations in seismic velocities and am-plitudes with shot-receiver azimuth andincident angle.

Diffraction imaging allows local sub-surface heterogeneities to be detectedusing the diffraction enhancement pro-cedure. Figure 3 shows confirmation ofthis. The first set of images at the top ofFigure 3 shows different depth slices ofspecularly enhanced images. A smoothvariation of the wave field is visible withsmall disturbances, the form of whichcannot be determined using only reflectionwaves.

Diffraction energy is enhanced in theset of images at the bottom of Figure 3,resulting in slices that reveal a very dif-ferent picture from that shown by the re-flected field. A strong faulting plan isseen clearly and can be interpreted easily.Thus, significant additional geologicaldetail has become evident by enhancingthe diffraction energy.

A further example of the utility ofthis approach is illustrated in the EagleFord Shale depth slices in Figure 4, where

Eagle Ford Depth Slices with Enhanced Primary Reflections (Top) And Secondary Reflections from Muting Primary Reflections (Bottom)

FIGURE 4

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the separation of competing specularevents is shown. The primary specularevents in the directional gathers havebeen muted, allowing the secondary re-flections from the faults to become clearlyvisible.

In the top image in Figure 4, a depthslice of the specular enhanced image isshown at 17,880 feet. In the bottomimage, the primary specular mute is ap-plied. The dominant seismic energy fromthe near horizontal horizons is muted,leaving the weaker seismic energy fromthe fault. Thus, the major faults are de-lineated clearly in the muted image, al-lowing easy picking. The stacking masksthis coherent event, leaving the interpreterto attempt extraction in the post-stackdomain under adverse conditions.

The new approach considers migrationsas a general mapping procedure from thesurface acquisition domain to that of thephysically derived subsurface domain.The mapped CIG gathers can be consid-ered complete representations of thescattering event, and as such, can beprocessed subsequent to migration to cor-rect for defects in the input data andmodeling procedure, and to accentuateany physical aspect of interest.

The potential impact for seismic im-aging and interpretation is twofold. First,this mapped domain allows for data cor-rection normally performed before orduring the migration, such as data recon-struction and anti-aliasing. Second, newclasses of physically derived images be-come available to the interpreter. Thesecan greatly enhance the certainty of theinterpreters understanding of the geology.This has been demonstrated for direc-tionality derived imaging. The significantincrease in fault and fracture zone detectionopens new prospects for seismic processingand interpretation.

Editors Note: The authors acknowl-edge Raanan Dafni and Gali Dekel fortheir contributions to the preceding article.The Eagle Ford Shale data examples arecourtesy of Seitel.

DAVID CHASE

David Chase is project manager forParadigm Geophysicals EarthStudy360 full-azimuth angle domain imagingsystem. He is experienced in dealingwith challenging high-performance com-puting and very large datasets, as wellas in developing and implementing so-phisticated algorithms ranging fromapplied mathematics to image process-ing. Previously, Chase worked in thealgorithms department of Applied Ma-terials, as a project manager at Optimod,and at Intel. He holds a Ph.D. in theo-retical chemistry from Hebrew Univer-sity.

YUVAL SERFATY

Yuval Serfaty is a physicist workingin the EarthStudy 360 full-azimuth angledomain imaging system at Paradigm.He is involved in developing the com-putational infrastructure and resolvinggeophysical computational problems.Serfaty holds a bachelors in computerengineering and applied physics and amasters in applied physics from HebrewUniversity.

IGOR BELFER

Igor Belfer is an EarthStudy 360 ap-plications engineer at Paradigm. He isinvolved in developing and testing work-flows, specifically in the area of dif-fraction imaging in EarthStudy 360.Previously, Belfer worked as an algorithmdeveloper at GLUCON Medical, andprior to that was a senior geophysicistat the Geophysical Institute of Israel.He holds a Ph.D. from the Institute ofEarth Physics in Moscow.

ZVI KOREN

Zvi Koren is Paradigms chief tech-nology officer and research fellow. Hefounded the geophysical program atParadigm in 1990, and headed the teamresponsible for developing the ParadigmGeoDepth system for velocity modelbuilding, seismic modeling and depthimaging. Koren also led developmentof Paradigms EarthStudy 360 full-wave-field system based on full-azimuth angledomain imaging and analysis. He holdsa Ph.D. in geophysics from Tel AvivUniversity, and performed post-doctoralresearch at the Institute Physique duGlobe at Paris University.

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