Interest in velocity anisotropy increased sharply among exploration seismologists following publication in 1986 of evidence for azimuthal anisotropy in sedimentary rocks. This increased level of interest generated a need for a glossary of anisotropy terminology in the language of geophysicists. This glossary is to: • help geophysicists find proper words • promote correct usage. The introduction preceding the alphabetically organized glossary focuses on concepts that have special importance for anisotropy but may be unfamiliar to most geophysicists. Symmetry concepts receive special emphasis because of their fundamental importance.

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A shear‐wave (S‐wave) VSP experiment was performed at Lost Hills Field, California, in an attempt to detect hydraulic fractures induced in a nearby well. The hydrofrac well was located between an impulsive, S‐wave source on the surface and a receiver well containing a clamped, three‐component geophone. Both direct and scattered waves were detected immediately after shut‐in, when the hydraulic pumps were shut off and recording started. The scattered energy disappeared within about an hour, which is consistent with other measurements that indicate some degree of fracture closure and leak‐off within that period. Although S‐wave splitting was evident, no change was detected in the fast wave (polarized parallel to the fracture). However, the slow wave (polarized perpendicular to the fracture) did change over a period of about an hour, after which the prehydrofrac wavelet shape was recovered. The fact that only the wave polarized perpendicular to the fracture was affected is a dramatic confirmation of both theoretical predictions and laboratory observations of S‐wave behavior in a fractured medium. Subtracting the prehydrofrac wavelet from the wavelets recorded within the first hour after shut‐in revealed scattered wavelets that were diminished and phase‐rotated versions of the incident (prehydrofrac) wavelet. Arrival times of the direct and scattered waves were matched by ray tracing. We accounted for the scattered‐wave amplitudes by using numerical solutions of S‐wave diffractions off of ribbon‐shaped fractures. Amplitudes derived from full‐wavefield Born scattering, however, did not match recorded amplitudes. The phase of the scattered wavelets was matched very well by Born scattering when the incident wavelet was input, but only for fracture lengths no larger than half those predicted from fracture‐simulator models. These results show that a carefully controlled experiment, combined with accurate modeling, can provide important information about the geometry of induced fractures.

Depths calculated from S-wave stacking velocities and event times almost always exceed actual depths, sometimes by as much as 25 percent. In contrast, depths from corresponding P-wave information are often within 10 percent of actual depths. Discrepancies in depths calculated from P- and S-wave data are attributed to velocity anisotropy, a property of sedimentary rocks that noticeably affects S-wave moveout curves but leaves the P-wave relatively unaffected. Two careful studies show that discrepancies in depths, and hence in constituent layer thicknesses, correlate with lithology. Discrepancies ranged from an average of 13 percent (Midland basin) to greater than 40 percent (Paloma field) in shales, but were within expected errors in massive sandstones or carbonates. Hence anisotropy effects are indicators of lithology. Analysis of seismic data involved determining interval velocities from stacking velocities, calculating layer thicknesses, and then comparing layer thicknesses from S-wave data with thicknesses from P-wave data. When the S-wave thicknesses were significantly greater than the P-wave (i.e., outside the range of expected errors), I concluded the layer was anisotropic. I illustrate the technique with data from the Paloma field project of the Conoco Shear Wave Group Shoot.