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Shear waves from 3-D–9-C seismic reflection data

Have we been looking for signal in all the wrong places?

4th Wave Imaging Corp., Laguna Beach, California, U.S.

Milo Backus

Bureau of Economic Geology, The University of Texas at Austin, U.S.

Corresponding author: Jim.Simmons@4thwaveimaging.com

The first 20% of the full text of this article appears below.

Nine-component (9-C) 3-D seismic reflection data are acquired using orthogonal shear-wave sources and orthogonal horizontal geophones. Shear-wave sources are oriented inline and crossline (S_I and S_X) to the receiver lines, as are the horizontal geophones (R_I and R_X). Four shear-wave data sets result (S_I,R_I; S_I,R_X; S_X,R_I,S_X,R_X). These inline-crossline coordinates are referred to as field coordinates. A main interpretation emphasis for these data has been to infer the presence of vertical cracks using shear-wave birefringence. Nonzero crossterms (S_I,R_X; S_X,R_I) have been used as an indicator that shear-wave birefringence (splitting) is present.

It does not seem to be widely recognized that field coordinates implicitly record a mixture of SH, SV, and P-waves. The degree of mixing depends on source-receiver azimuth. Shear-wave data processing, signal recovery, and interpretation are unnecessarily complicated (and often questionable) in field coordinates because of this spatially variable mixing of wave modes.

Shear-wave data analysis is simplified if prestack data are rotated to a radial-transverse coordinate system using the measured source-receiver azimuth. The radial-source radial-receiver component (S_R,R_R) contains predominantly SV waves and P-waves, while the transverse-source transverse-receiver component (S_T,R_T) contains only SH waves in an isotropic flat-layered earth. SH data are simplified in that SH waves convert only to SH upon reflection and transmission, unlike SV propagation, which is coupled with P. The amplitude-versus-offset (AVO) response for SH reflections is generally very different from that of SV reflections. SV reflections generally reverse polarity at local incidence angles of 20–30°, while SH reflections are much more constant at precritical angles.

A 9-C–3-D data set acquired with a square recording template illustrates the . . . [Full Text of this Article]