Multicomponent seismic recording (measurement with vertical-and horizontal-component geophones and possibly a hydrophone or microphone) captures the seismic wavefield more completely than conventional single-element techniques. In the last several years, multicomponent surveying has developed rapidly, allowing creation of converted-wave or P-S images. These make use of downgoing P-waves that convert on reflection at their deepest point of penetration to upcoming S-waves. Survey design for acquiring P-S data is similar to that for P-waves, but must take into account subsurface V P /V S values and the asymmetric P-S ray path. P-S surveys use conventional sources, but require several times more recording channels per receiving location. Some special processes for P-S analysis include anisotropic rotations, S-wave receiver statics, asymmetric and anisotropic binning, nonhyperbolic velocity analysis and NMO correction, P-S to P-P time transformation, P-S dip moveout, prestack migration with two velocities and wavefields, and stacking velocity and reflectivity inversion for S-wave velocities.Current P-S sections are approaching (and in some cases exceeding) the quality of conventional P-P seismic data. Interpretation of P-S sections uses full elastic ray tracing, synthetic seismograms, correlation with P-wave sections, and depth migration. Development of the P-S method has taken about 20 years, but has now become commercially viable.
Converted seismic waves (P-to-S on reflection) are being increasingly used to explore for subsurface targets. Rapid advancements in multicomponent acquisition methods and processing techniques have led to numerous applications for P-S images. Uses that have arisen include sand/shale differentiation, carbonate identification, definition of interfaces with low P-wave contrast, anisotropy analysis, imaging through gas zones, shallow high-resolution imaging, and reservoir monitoring. Marine converted-wave analysis using 4-C recordings (a three-component geophone plus a hydrophone) has generated some remarkable images. BACKGROUND
An industrial laminate, Phenolic CE, is shown to possess seismic anisotropy. This material is composed of laminated sheets of canvas fabric, with an approximately orthogonal weave of fibers, bonded with phenolic resin. It is currently being used in scaled physical modeling studies of anisotropic media at The University of Calgary. Ultrasonic transmission experiments using this material show a directional variation of compressional-and shear-wave velocities and distinct shear-wave birefringence, or splitting. Analysis of group-velocity measurements taken for specific directions of propagation through the material demonstrates that the observed anisotropy is characteristic of orthorhombic symmetry, i.e., that the material has three mutually orthogonal axes of two-fold symmetry. For P waves, the observed anisotropy in symmetry planes varies from 6.3 to 22.4 percent, while for S waves it is observed to vary from 3.5 to 9.6 percent.From the Kelvin-Christoffel equations, which yield phase velocities given a set of stiffness values, expressions are elaborated that yield the stiffnesses of a material given a specified set of group-velocity observations, at least three of which must be for offsymmetry directions.
Seismic anisotropy in dipping shales causes imaging and positioning problems for underlying structures. We developed an anisotropic depth‐migration approach for P-wave seismic data in transversely isotropic (TI) media with a tilted axis of symmetry normal to bedding. We added anisotropic and dip parameters to the depth‐imaging velocity model and used prestack depth‐migrated image gathers in a diagnostic manner to refine the anisotropic velocity model. The apparent position of structures below dipping anisotropic overburden changes considerably between isotropic and anisotropic migrations. The ray‐tracing algorithm used in a 2-D prestack Kirchhoff depth migration was modified to calculate traveltimes in the presence of TI media with a tilted symmetry axis. The resulting anisotropic depth‐migration algorithm was applied to physical‐model seismic data and field seismic data from the Canadian Rocky Mountain Thrust and Fold Belt. The anisotropic depth migrations offer significant improvements in positioning and reflector continuity over those obtained using isotropic algorithms.
A scaled physical model was constructed to investigate the magnitudes of imaging errors incurred by the use of isotropic processing code when there is seismic velocity anisotropy present in the dipping overburden. The model consists of a block of transversely isotropic (TI) phenolic material with the TI axis of symmetry dipping at an angle of 45°. Its scaled thickness is 1500 m, and it is intended to simulate the dipping clastic sequences found in many fold-thrust belts. A piece of isotropic Plexiglas, affixed to the underside of the anisotropic block, has a step function in it to simulate a target reef edge or fault. The anisotropy parameters of the material are 8 = 0.1 and e = 0.24.On zero-offset data the imaged position of the target is shifted laterally 320 m in the updip direction of the beds, whereas on time-and depth-migrated multichannel sections the shift is 300 m. The lateral shift is offset dependent, with the amount of shift in any commonmidpoint gather decreasing from 320 m on the near offsets to 280 m on the far offsets. Prestack depth-migration velocity analysis based upon obtaining consistent depth images in the common-offset domain results in the base of the anisotropic section being imaged 50 m (about 3%) too deep.
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