This work is based upon an extension to the coupling scheme of the Spectral Element Method (SEM) with a normal mode solution in spherical geometry. This extension allows us to consider a thin spherical shell of spectral elements between two modal solutions above and below. The SEM is based on a high order variational formulation in space and a second-order explicit scheme in time. It combines the geometrical flexibility of the classical finite element method with the exponential convergence rate associated with spectral techniques. In the inner sphere and outer shell, the solution is sought in terms of a modal solution in the frequency domain after expansion on the spherical harmonics basis. The SEM has been shown to obtain an excellent accuracy in solving the wave equation in complex media but is still numerically expensive for the whole Earth for high frequency simulations. On the other hand, modal solutions are well known and numerically cheap in spherically symmetric models. By combining these two methods we take advantage of both, allowing high frequency simulations in global Earth models with 3D structure in a limited depth range. Within the spectral element method, the coupling is introduced via a dynamic interface operator, a Dirichlet-to-Neumann (DtN) operator which can be explicitly constructed in the frequency and generalized spherical harmonics domain using modal solutions in the inner sphere and outer shell. The presence of the source and receivers in the top modal solution shell requires some special treatment. The accuracy of the method is checked against the mode summation method in simple spherically symmetric models and shows very good agreement for all type of waves, including diffracted waves traveling on the coupling boundary.
Simulations in a 3D D'' layer model based on the tomographic model SAW24B16 has been performed up to a corner frequency of 1/12 s The mesh used is presented on Fig. 37.1.
A description of the method with illustrations can be found on http://seismo.berkeley.edu/~yann
The computation were made using the computational resources of the NERSC, especially the IBM SP, under repo mp342.
We thank the Miller Insitute for their support.
Y. Capdeville, B. Romanowicz and A. To, Coupling Spectral Elements and Modes in a spherical earth: an extension to the ``sandwich'' case. Submitted to Geosphys. J. Int., 2002.
Capdeville Y., C. Larmat, J.P. Vilotte and J.P. Montagner, Numerical simulation of the scattering induced by a localized plume-like anomaly using a coupled spectral element and modal solution method. In press in Geoph. Res. Lett., 2001.
Capdeville Y., E. Chaljub, J.P. Vilotte and J.P. Montagner Coupling the Spectral Element Method with a modal solution for Elastic Wave Propagation in Global Earth Models. accepted in Geophys. J. Int., 2001.
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