SEISMIC GYROTROPY AND ITS PHYSICAL REASON S

A new concept, seismic gyrotropy, is presented in the context of both phenomenological and microlevel theory; comparison with the advanced t heory of optical gyrotropy is given. Seismic gyrotropy is introduced a s manifestation of spatial dispersion of elastic properties in a first approximation. Hooke's law (a material equation connecting stresses w ith strains) is used with additional terms proportional to strain deri vatives; 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: infinity,infinity, infinity, infinity 2, infinity m, 222, 2m m, 2, m; geological media with gyrotropic properties may belong to the se groups. Using a medium of group symmetry infinity infinity, it is s hown that enantiomorphous media are <<rotating>>. For <<rotating>> med ia, the nature of structure dissymetry on microlevel is found: microob jects of such media are situated in 3D-space accordingly to the princi ple translation plus azimuthal turn. Three types of micromodels for ge ological media are put forward which are constructed on this principle ; these are models for sandy rocks, rocks with scattering inclusions a nd thin-layered media. As for the model of sandy rocks, numerical esti mates of specific rotation are given, obtained by numerical simulation .