SEISMIC PROPAGATION SIMULATION IN COMPLEX MEDIA WITH NON-RECTANGULAR IRREGULAR-GRID FINITE-DIFFERENCE

This paper presents a finite-difference(FD)method with spatially non-rectangular irregular grids to simulate the elastic wave propagation.Staggered irregular grid finite difference oper- ators with a second-order time and spatial accuracy are used to approximate the velocity-stress elastic wave equations.This method is very simple and the cost of computing time is not much.Complicated geometries like curved thin layers,cased borehole and nonplanar interfaces may be treated with non- rectangular irregular grids in a more flexible way.Unlike the multi-grid scheme,this method requires no interpolation between the fine and coarse grids and all grids are computed at the same spatial iteration.Compared with the rectangular irregular grid FD,the spurious diffractions from'staircase' interfaces can easily be eliminated without using finer grids.Dispersion and stability conditions of the proposed method can be established in a similar form as for the rectangular irregular grid scheme.The Higdon's absorbing boundary condition is adopted to eliminate boundary reflections.Numerical simu- lations show that this method has satisfactory stability and accuracy in simulating wave propagation near rough solid-fluid interfaces.The computation costs are less than those using a regular grid and rectangular grid FD method.

Accelerating the discontinuous Galerkin method for seismic wave propagation simulations using multiple GPUs with CUDA and MPI

We have successfully ported an arbitrary highorder discontinuous Galerkin method for solving the threedimensional isotropic elastic wave equation on unstructured tetrahedral meshes to multiple Graphic Processing Units （GPUs） using the Compute Unified Device Architecture （CUDA） of NVIDIA and Message Passing Interface （MPI） and obtained a speedup factor of about 28.3 for the single-precision version of our codes and a speedup factor of about 14.9 for the double-precision version. The GPU used in the comparisons is NVIDIA Tesla C2070 Fermi, and the CPU used is Intel Xeon W5660. To effectively overlap inter-process communication with computation, we separate the elements on each subdomain into inner and outer elements and complete the computation on outer elements and fill the MPI buffer first. While the MPI messages travel across the network, the GPU performs computation on inner elements, and all other calculations that do not use information of outer elements from neighboring subdomains. A significant portion of the speedup also comes from a customized matrix-matrix multiplication kernel, which is used extensively throughout our program. Preliminary performance analysis on our parallel GPU codes shows favorable strong and weak scalabilities.

Spectral-element simulations of elastic wave propagation in exploration and geotechnical applications

We apply the spectral-element method（SEM）,a high-order finite-element method（FEM） to simulate seismic wave propagation in complex media for exploration and geotechnical problems. The SEM accurately treats geometrical complexities through its flexible FEM mesh and accurately interpolates wavefields through high-order Lagrange polynomials. It has been a numerical solver used extensively in earthquake seismology. We demonstrate the applicability of SEM for selected 2D exploration and geotechnical velocity models with an open-source SEM software package SPECFEM2D. The first scenario involves a marine survey for a salt dome with the presence of major internal discontinuities,and the second example simulates seismic wave propagation for an open-pit mine with complex surface topography. Wavefield snapshots,synthetic seismograms,and peak particle velocity maps are presented to illustrate the promising use of SEM for industrial problems.

Global SH-wave propagation in a 2D whole Moon model using the parallel hybrid PSM/FDM method

We present numerical modeling of SH-wave propagation for the recently proposed whole Moon model and try to improve our understanding of lunar seismic wave propagation. We use a hybrid PSM/FDM method on staggered grids to solve the wave equations and implement the calculation on a parallel PC cluster to improve the computing efficiency. Features of global SH-wave propagation are firstly discussed for a 100-km shallow and900-km deep moonquakes, respectively. Effects of frequency range and lateral variation of crust thickness are then investigated with various models. Our synthetic waveforms are finally compared with observed Apollo data to show the features of wave propagation that were produced by our model and those not reproduced by our models. Our numerical modeling show that the low-velocity upper crust plays significant role in the development of reverberating wave trains. Increasing frequency enhances the strength and duration of the reverberations.Surface multiples dominate wavefields for shallow event.Core–mantle reflections can be clearly identified for deep event at low frequency. The layered whole Moon model and the low-velocity upper crust produce the reverberating wave trains following each phases consistent with observation. However, more realistic Moon model should be considered in order to explain the strong and slow decay scattering between various phases shown on observation data.

Studies on seismic waves

The development of seismic wave study in China in the past four years is reviewed. The discussion is divided into several aspects, including seismic wave propagation in laterally homogeneous media, laterally heterogeneous media, anisotropic and porous media, surface wave and seismic wave inversion, and seismic wave study in prospecting and logging problems. Important projects in the current studies on seismic wave is suggested as the development of high efficient numerical methods, and applying them to the studies of excitation and propagation of seismic waves in complex media and strong ground motion, which will form a foundation for refined earthquake hazard analysis and prediction.

A Preconditioned 3-DMulti-Region Fast Multipole Solver for Seismic Wave Propagation in Complex Geometries

The analysis of seismic wave propagation and amplification in complex geological structures requires efficient numerical methods.In this article,following up on recent studies devoted to the formulation,implementation and evaluation of 3-D single-and multi-region elastodynamic fast multipole boundary element methods(FM-BEMs),a simple preconditioning strategy is proposed.Its efficiency is demonstrated on both the single-andmulti-region versions using benchmark examples(scattering of plane waves by canyons and basins).Finally,the preconditioned FM-BEM is applied to the scattering of plane seismic waves in an actual configuration(alpine basin of Grenoble,France),for which the high velocity contrast is seen to significantly affect the overall efficiency of the multi-region FM-BEM.

Seismic-Wave Scattering,Imaging,and Inversion

Ru-Shan Wu has made seminal contributions in many research areas in geophysics,such as seismic-wave propagation,scattering,imaging,and inversion.We highlight some of his research in holography imaging,diffraction tomography,seismic-wave scattering and its applications to studying Earth’s heterogeneity,oneway wave propagation and one-return wave modeling,beamlet and dreamlet applications,strong non-linear full-waveform inversion,and direct envelop inversion.

A Stable Finite Difference Method for the Elastic Wave Equation on Complex Geometries with Free Surfaces

A stable and explicit second order accurate finite difference method for the elastic wave equation in curvilinear coordinates is presented.The discretization of the spatial operators in the method is shown to be self-adjoint for free-surface,Dirichlet and periodic boundary conditions.The fully discrete version of the method conserves a discrete energy to machine precision.