Abstract

The presence of vertically oriented fractures and/or unequal horizontal stresses has created an azimuthally anisotropic earth, in which shear-wave (SH) data collected along the principal axes of the anisotropy display time and reflection amplitude anomalies.Amoco shot two crossing shear-wave (SH) lines that were approximately parallel to the orthogonal principal axes of the azimuthal anisotropy. At the tie point, these crossing SH lines display a time-variant mis-tie. The tie point also displays reflection-coefficient anomalies, attributable to azimuthally dependent shear-wave velocities. Field mapping documented a set of fractures striking N69E which are approximately parallel to the line that exhibited greater traveltimes. Time-variant mis-ties and reflection coefficient anomalies are two of the seismic responses theoretically expected of an azimuthally anisotropic earth, i.e., one in which the shear-wave velocity depends upon the polarization azimuth of the shear wave.

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