Abstract
We present 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 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 3-D structure in a limited depth range. Within the spectral element method, the coupling is introduced via a dynamic interface operator, a Dirichlet-to-Neumann 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 travelling on the coupling boundary. A first simulation in a 3-D D″-layer model based on the tomographic model SAW24b16 is presented up to a corner frequency of 1/12 s. The comparison with data shows surprisingly good results for the 3-D model even when the observed waveform amplitudes differ significantly from those predicted in the spherically symmetric reference model (PREM).
Highlights
While it has long been known that the top layers of the Earth’s interior, the crust and the uppermost mantle, are strongly heterogeneous, with lateral variations of structure commonly exceeding 10 per cent, it has only recently been recognized that such strong variations may be present at the bottom of the mantle, in the D region
Recent global tomographic models of S velocity clearly show the distinctive character of lateral variations of structure at the top and at the base of the mantle (e.g. Su et al 1994; Li & Romanowicz 1996; Masters et al 1996; Grand et al 1997; Liu & Dziewonski 1998; Ritsema et al 1999; Megnin & Romanowicz 2000)
Because of its singularities at each frequency of d, this operation cannot be performed by a traditional fast Fourier transform (FFT)
Summary
While it has long been known that the top layers of the Earth’s interior, the crust and the uppermost mantle, are strongly heterogeneous, with lateral variations of structure commonly exceeding 10 per cent, it has only recently been recognized that such strong variations may be present at the bottom of the mantle, in the D region. The spectrum of lateral heterogeneity changes from being ‘white’ in the bulk of the lower mantle, to being dominated by long wavelengths in the upper mantle as well as in the last 500 km above the core–mantle boundary (CMB), as reflected in more organized spatial patterns These patterns have been confirmed in studies focused on the global 2-D analysis of CMB diffracted waves (e.g. review Garnero 2000)
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