Physical seismic modelling of anisotropic wave propagation: Latest results and future directions

R. James Brown, Donald C. Lawton, Scott P. Cheadle, Giovanni B. Marchisio

In this paper we report the results of continued scaled physical model experiments in the laboratory wherein ultrasonic elastic waves are propagated through an anisotropic medium of orthorhombic symmetry. Whereas earlier experiments consisted for the most part in sending and receiving on opposite faces of a small cube of the phenolic material, the results at hand are from multiple-offset profiles run parallel and at 45 degrees to principal directions on a larger slab of phenolic.

The variation of NMO (or stacking) velocity with offset (or angle of incidence) has been determined for compressional and transverse shear waves for the two principal directions on the 3-face (parallel to laminations) of the slab. These velocities are compared with the corresponding values calculated from Thomsen s approximate equations. The two sets of velocity values agree quite well, maximum differences being less than 1% for qP and about 2% for qSH.

Shot records were acquired for a variety of source and receiver polarizations, initially for profiles along principal directions. These records, for which there is sagittal symmetry, do not display any fundamental differences to records acquired over transversely isotropic media. The effect of the shear-wave window and the variation of the hyperbolic NMO parameter with offset are clearly seen. For profiles shot at 45 degrees to principal directions, we see clear shear-wave splitting at and near zero offset. In addition, we see more complicated wave effects, such as one or another wave phase dying out with increasing offset, which could be due to cusping of wave surfaces or rapid changes of amplitude and/or polarization with ray direction, consequences of nearby shear-wave singularities.