Linear Equivalent Seismic Sources from Simulations of Underground Chemical Explosions

Underground explosions nonlinearly deform the surrounding earth material and can interact with the free surface to produce spall. However, at typical seismological observation distances the seismic wavefield can be accurately modeled using linear approximations. Although nonlinear algorithms can accurately simulate very near field ground motions, they are computationally expensive and unnecessary for far field wave simulations. Linearized seismic wave propagation codes, on the other hand, are unable to accurately model the very near field effects but are computationally efficient. To exploit the strengths of both types of algorithms, we have coupled Sandia's CTH shock physics code to a linearized elastic wave propagation code using time-varying boundary conditions, to pass information from the nonlinear domain (CTH) to the linear one. As part of these efforts, we are investigating linear equivalent seismic sources, where we find the purely linear seismic moment tensor and source time functions that optimally fit the waveforms produced by the nonlinear source. This is accomplished by employing standard linear seismic source inversions in the frequency domain with Greens Functions produced using the linear elastic algorithm throughout. We will present results of these investigations showing the effects of geological material and the free surface on linear source parameters for simple earth models with simulated chemical explosions at various scaled depths of burial. We will also discuss how well the linear source models are able to reconstruct the waveforms from the nonlinear source in the various scenarios.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & 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.