Transverse isotropy in exploration seismics *

Summary. Lamellations – layering on a scale small compared to the wavelengths – result in anisotropy (transverse isotropy) of elastic wave propagation. Since many sedimentary sequences can be regarded as lamellated, one might expect that exploration seismics would be beset with problems resulting from straightforward processing under the assumption of isotropy. Such problems have rarely been encountered. The absence of significant effects of anisotropy in reflection seismics can be explained by three facts: in exploration seismics, most observations employ rays that make but moderate angles with the axis of symmetry; until recently only compressional waves were used, and, provided the constituent lamellae have S/P ‐velocity ratios constrained to a narrow range, the wave‐front of compressional waves deviates significantly from a sphere only for larger angles. Anisotropy effects can, in principle, be inverted to give information concerning the details of lamellation. In order to observe such effects, one has to use shear‐wave observations. Any combination of observations that leads, under the assumption of isotropy, to inconsistencies can be used for the determination of anisotropy parameters. The simplest approach is to compare the depths determined with arrival times and stacking velocities from observations with polarization. For data of very high quality, the deviation of the reflection even from a hyperbola (or, more simply, the deviation of the corresponding t 2 / x 2 ‐plot from a straight line) can be exploited. To this end one has to use a technique that is akin to the determination of interval velocities from stacking velocities.