3D Hybrid Seismic Modeling: Ray-FD rr DWN-FD Approach for Elastic Models With Locally Complex Structures

A 3D hybrid approach allowing to compute seismic wavefield in elastic models containing a complex local structure embedded in a large, but considerably simpler, structure is designed. This hybrid method combines either the ray or discrete wave number (DWN) method in the large simple structure with the finite difference (FD) method in the local complex structure. Each method is to be used within its limits of applicability: DWN in 1D (not too large) models  with respect to the wavelength, ray method in 1D, 2D, or 3D large scale but smooth models in which a few simple body waves are dominant (surface or guided waves can be neglected), FD for local complex structures with high velocity contrasts and frequencies up to 5Hz.

The hybrid method is based on two successive steps. In the 1st one, the wavefield propagating in the large simple structure is computed. This wavefield, calculated by the ray or DWN method, is incident at the points along a two-fold formal boundary (excitation box,EB) surrounding that part of the model which is to be replaced by the complex medium in the 2nd step.

The 3D ray-FD method is tested on several models in which the locally complex structure is the well-known Volvi lake basin, embeded by various 1D structures. The wavefield is excited by the point source situated outside the basin. Despite the structure outside the EB may be less dimensional (2D, 1D, homogeneous) but the whole problem is actually 3D due to the 3D features of the structure inside the EB, 3D shape of the EB and arbitrary source-EB configuration.
Simple (1D) structures outside EB allow for comparison with the alternative hybrid DWN-FD results. The results from both approaches show a very good agreement for realistic crustal and local structural models