SEISMIC GYROTROPY AND ITS PHYSICAL REASONS

Abstract

A new concept, seismic gyrotropy, is presented in the context of both phenomenological and microlevel theory; comparison with the advanced theory of optical gyrotropy is given. Seismic gyrotropy is introduced as manifestation of spatial dispersion of elastic properties in a first approximation. Hooke's law (a material equation connecting stresses with strains) is used with additional terms proportional to strain derivatives; the factors of proportionality are components of the gyration tensor b, the tensor of fifth rank, invariant with respect to a group of rotations. The tensor b is given for following acenrtic groups of symmetry: ∞∞, ∞, ∞2, ∞m, 222, 2mm, 2, m; geological media with gyrotropic properties may belong to these groups. Using a medium of group symmetry ∞∞, it is shown that enantiomorphous media are «rotating». For «rotating» media, the nature of structure dissymetry on microlevel is found: microobjects of such media are situated in 3D-space accordingly to the principle translation plus azimuthal turn. Three types of micromodels for geological media are put forward which are constructed on this principle; these are models for sandy rocks, rocks with scattering inclusions and thin-layered media. As for the model of sandy rocks, numerical estimates of specific rotation are given, obtained by numerical simulation.