On the theory of the microseismic sounding method

Abstract

Based on the theory of surface waves in a media with weak lateral heterogeneities, an explanation is given for the peculiarities that are considered as prospecting indicators in the microseismic sounding method (MSM). According to this theory, during the propagation of surface waves in such media, their local characteristics are the same as in the case of propagation in a horizontally homogeneous medium characterized by the same vertical velocity section as underlying a given point. Since the wave energy flux through a semiinfinite vertical strip is conserved, the amplitude of the wave is redistributed along the vertical. Based on this principle and simple physical considerations, it is shown that above a low-velocity domain, the amplitudes of microseisms should increase at wavelengths approximately exceeding the depth of the anomaly by a factor of three and decrease at small wavelengths. The model calculations show that variation of spectral amplitudes only allows a depth to the anomaly to be estimated rather than its average extent. Practically identical spectra characterize the anomalies in which the product of the vertical extent of the anomaly and the velocity contrast is approximately similar.