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Three-Component Amplitude vs. Offset Analysis

Deborah Miles, Gary Gassaway, Laurie Bennett, Richard Brown

Amplitude changes in compressional waves (P waves) from a specific reflector are a function of their angle of incidence and the elastic constant contrast at the boundary. Zoeppritz, in 1909, quantified this relationship in his simplification of Knott's 1899 equations. Beginning about 1976, P-Previous HitwaveNext Hit amplitude changes with angle of incidence on single-component seismic data were analyzed through modeling or inversion to obtain shear Previous HitwaveNext Hit (S Previous HitwaveNext Hit) velocities, and Poisson's ratio. Using three-component seismic data and Zoeppritz equations, one can also analyze the S-Previous HitwaveNext Hit amplitude changes with offset by solving for the P-Previous HitwaveNext Hit velocity. Thus, with three-component seismic data, the amplitude vs. offset inversion (AVO) of the P-Previous HitwaveNext Hit gathers (SAMPLE3TM) provides an S-wav velocity and Poisson's ratio, and the AVO inversion of the S-Previous HitwaveNext Hit gather yields P-Previous HitwaveNext Hit velocities. Since they are done at the same CDP, the two solutions must agree and thereby tie the P-Previous HitwaveNext Hit reflectors to the S-Previous HitwaveNext Hit reflectors. The amplitude changes with offset of the converted waves are also predicted by Zoeppritz equations and can be used to check the predicted P- and S-Previous HitwaveNext Hit velocities.

Processing three-component seismic data for AVO analysis generally follows a processing flow similar to the processing flow for single-component data. Just as in single component AVO, the processor must be sure to preserve the amplitude relative to the other traces. However, the processing must also preserve the relative amplitudes between components. After processing to a P-Previous HitwaveNext Hit, common offset gather, corrections for the free surface effects, source arrays, receiver arrays, and spherical divergence are applied to the amplitudes. When doing AVO inversions (SAMPLETM) on single-component seismic data, the corrections are calculated for z only with the assumption that there is no energy in the transverse direction. However, with three-component data, the corrections for spheri al divergence and the source and receiver arrays must be calculated in terms of Previous HitwaveNext Hit fronts, not just ray paths, and for all three components. Geophone arrays are generally unnecessary for three-component seismic since the OMNIPHONE polarization filter removes much of the groundroll from three-component seismic data either in a processing center or in the field using a single OMNIPHONE.

Using AVO inversion (SAMPLE3) to tie the P-Previous HitwaveNext Hit, S-Previous HitwaveNext Hit, and converted-Previous HitwaveNext Hit data together gives the interpreter expanded interpretation capabilities. First, by identifying the top and bottom of a specific zone, one can use P-Previous HitwaveNext Hit and S-Previous HitwaveNext Hit time isochrons to calculate Poisson's ratio. Changes in Poisson's ratio or Vp/Vs ratios are indicative of changes in pore fluids and lithologies. Second, geologic features such as reefs may be stronger events on the S-Previous HitwaveNext Hit or converted-Previous HitwaveNext Hit data than on the P-Previous HitwaveNext Hit data, or vice versa. Therefore, by comparing the P, shear, and converted Previous HitwaveTop data, one may locate previously unknown responses.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.