Two-layer earth model corrections to theMLTWA estimates of intrinsic- and scattering-attenuation obtained in a uniform half-space

Abstract

Following the numerical scheme of Yoshimoto we synthesized seismogram envelopes in the multiple scattering framework.We supposed the earth model constituted by a inhomogeneous crust overlying a transparent mantle. In this model velocity is assumed depth-dependent through a continuous function of the depth, v = v(h); Moho discontinuity is approximated by a sharp increase of the velocity around the crust–mantle boundary; inhomogeneity in the crust is parametrized through a depth-dependent scattering coefficient (the inverse of mean free path) g = g0 f (h), with f (h) function of depth, and g0 the scattering coefficient at zero depth; intrinsic attenuation is parametrized in terms of the intrinsic attenuation coefficient, ηi , that is assumed independent of depth. Generating a suite of energy envelopes as a function of lapse time and distance, for reasonable values of B0 , the seismic albedo and Le−1, the extinction length inverse (which are functions of g0 and ηi ), we span a wide range including most of the measurements done through the world. Then, we apply the ordinary MLTWA technique to these synthetic envelopes. In this application, we assume a constant g and a constant velocity, v = <v(h)> which equals the average of v(h) calculated in the depth range characteristic of the volume encompassed by the scattered waves. In this way, we obtain the estimates of B0, and Le−1, for a constant half-space. The relationship between the estimates of B0 and Le−1, obtained assuming half-space, and the correspondent values used in the simulation, results to be well approximated by a second-order polynomial. Then, evaluating the best fit polynomial coefficients, we obtain a correspondence map between attenuation parameters retrieved for a uniform model with those characteristic of a more realistic structure. This map is useful to reinterpret all the couples B0 and Le−1 already calculated through the world in geological structures similar to the one adopted in our simulation. Results show that scattering and intrinsic-attenuation coefficients estimated using MLTWA in the assumption of a uniform half-space are always overestimated