Accurate absorbing boundary conditions for anisotropic elastic media. Part 1: Elliptic anisotropy

With the ultimate goal of devising effective absorbing boundary conditions (ABCs) for heterogeneous anisotropic elastic media, we investigate the accuracy aspects of local ABCs designed for tilted elliptic anisotropy in the frequency domain (time-harmonic case). Such media support both anti-plane and in-plane wavemodes with opposing signs of phase and group velocities (c"p"xc"g"x<0) that have long posed a significant challenge to the design of accurate (and stable) local ABCs. By first considering the simpler case of scalar anti-plane waves, we show that it is possible to overcome the challenges posed by c"p"xc"g"x<0 by simply utilizing the inevitable reflections occurring at the truncation boundaries. This understanding helps us to explain the ability of a recently developed local ABC - the perfectly matched discrete layer (PMDL) - to handle the challenges posed by c"p"xc"g"x<0without the need of intervening space-time transformations. PMDL is a simple variant of perfectly matched layers (PML) that is also equivalent to rational approximation-based local ABCs (rational ABCs); it inherits the straightforward approximation properties of rational ABCs along with the versatility of PML. The approximation properties of PMDL quantified through its reflection matrix is used to derive simple bounds on the PMDL parameters necessary for the accurate absorption of all outgoing anti-plane and in-plane wavemodes - including those with c"p"xc"g"x<0. Beyond the previously derived bound on the real parameters of PMDL sufficient for the absorption of outgoing propagating anti-plane wavemodes, we present bounds on the complex parameters of PMDL necessary for the absorption of outgoing propagating and evanescent wavemodes for both anti-plane and coupled in-plane pressure and shear waves. The validity of this work is demonstrated through a series of numerical experiments.

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