A NEW MECHANISM FOR SHAPE INDUCED SEISMIC ANISOTROPY

In a medium with regions of relatively high and low wavespeed, rays will predominantly bend towards the regions with a higher velocity. Consequently, the observed wave velocity, derived from the traveltime between a source and receiver, appears to be higher than the mean intrinsic wave velocity of the medium. The smaller the length scale of the heterogeneities, the more effective the bending of the wave paths will be. If the characteristic length scale of the heterogeneities in a medium is different in different normal directions, the wave velocity appears to be angle dependent. This form of anisotropy is called shape induced anisotropy. In this paper shape induced anisotropy is studied by ray perturbations in a random medium with 2 different normal length scales that are prescribed by a stochastic autocorrelation function. Ray bending results in a higher apparent velocity, if the gradient of the slowness perpendicular to the wavepath is larger. This occurs if the ray propagates in the direction of the largest correlation length. Contradictionary to the smooth angle dependence of the wave velocity in a medium with intrinsic anisotropy, in shape induced anisotropy the variations with the angle of incidence of the apparent velocity are much more pronounced. Because of the ray approximation, the presented mechanism for shape induced anisotropy is valid for heterogeneities much larger than one wavelength.