Seismic waveform modeling and surface wave tomography in a three-dimensional Earth: asymptotic and non-asymptotic approaches

We investigate the impact of the theoretical limitations brought by asympto tic methods on upper-mantle tomographic models deduced from long-period sur face wave data(period > 80 s), by performing a synthetic test using a non-a symptotic formalism, This methodology incorporates the effects of back and multiple forward scattering on the wave field by summing normal modes compu ted to third order of perturbations directly in the 3D Earth, and models th e sensitivity to scatterers away from the great-circle path. We first compa re the methods we used for the forward problem, both theoretically and nume rically. Then we present results from the computation of 7849 synthetic Lov e waveforms in an upper mantle model consisting of two heterogeneities with power up to spherical harmonic degree 12. The waveforms are subsequently i nverted using a 0th order asymptotic formalism (equivalent to a path-averag e approximation in the surface waves domain). We show that the main structu res are retrieved, but that the theoretical noise on the output model is of the same order as the noise due to the path-coverage and a priori constrai nts. (C) 2000 Elsevier Science B.V, All rights reserved.