3D Orthorhombic Earth Model Effects on Seismic Source Characterization

Abstract:

Most earth materials are seismically anisotropic, especially so in materials such as shales, tectonic fabrics, or where oriented fractures are present. However, the base assumption for many numerical simulations is that earth materials are treated as isotropic media. This is done for the sake of simplicity, the apparent weakness of anisotropy in the far field, and the lack of well-characterized anisotropic material properties for input into numerical simulations. One approach for addressing the higher complexity of actual geologic regions is to model the material as an orthorhombic medium. An orthorhombic medium is characterized by three mutually orthogonal symmetry planes comprising a dense system of vertically-aligned microfractures superimposed on a finely-layered horizontal geology, which can be reduced to an elastic stress-strain constitutive relationship containing nine independent moduli comprising a set of nine, coupled, first-order, linear, inhomogeneous partial differential equations. These moduli can be determined by observing (or prescribing) nine independent P-wave and S-wave phase speeds along different propagation directions. We have developed an explicit time-domain finite-difference (FD) algorithm for simulating three-dimensional (3D) elastic wave propagation in a heterogeneous orthorhombic medium.

The objective of this research is to investigate the errors and biases that result from modeling a non-isotropic medium as an isotropic medium. This is done by computing “observed data” by using synthetic, anisotropic simulations with the assumption of an orthorhombic, anisotropic earth model. Green’s functions for an assumed isotropic earth model are computed and then used in moment tension inversion with the “observed” data. One specific area of interest is how shear waves, which are introduced in an anisotropic model even for an isotropic explosion, affect the characterization of seismic sources when isotropic earth assumptions are made. This work is done in support of the modeling component of the Source Physics Experiment (SPE), a series of underground chemical explosions at the Nevada National Security Site.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2020-7231 A.