Lithospheric imaging from teleseismic data by frequency-domain elastic full-waveform tomography

We shall present a 2D elastic frequency-domain full-waveform tomography method suitable for lithospheric imaging from teleseismic data. In the teleseismic configuration, the source is a plane wave impinging the base of the lithospheric target located below the receiver netwok. The plane-wave source is implemented in the frequency-domain forward problem using a scattered-field formulation. The wave modeling is performed with a finite-element discontinuous Galerkin method on unstructured triangular meshes. The inverse problem is solved in the frequency-domain using a quasi-Newton LBFGS optimization and the adjoint-state method. Preliminary applications in the framework of the acoustic approximation were presented to highlight the resolution improvements provided by the inversion of topside reflections after the first reflection at the free surface. Theses shorter-aperture converted phases increase dramatically the high-wavenumber coverage in the model space which would have been rather poor otherwise. We assess in a realistic teleseismic setting for a 0.2-2 Hz source bandwidth the frequency sampling required for avoiding wraparound of lithospheric reflectors, which result from the narrow aperture illumination provided by plane wave sources when temporal frequencies are not sufficiently finely sampled. Before considering application to real data, obliquity of plane wave sources with respect to the imaged section must be adressed either by implementation of the 2.5D wave equation or by applying empirical corrections to velocities.