Seismic Sources and Wave Propagation in 3-D: Radiation from Cylindrical Cavities

We apply the boundary element method (BEM) to study seismic wave radiation from explosions in cylindrical cavities. This approach simulates wave propagation by replacing the boundary between the cavity wall and the surrounding formation by a distribution of fictitious sources that produce the same composite wavefield as the original model. The cavity wall is discretized into elements in the numerical implementation, and the fictitious source amplitudes on each element are determined by solving boundary condition equations. In order to accomplish this, the interactions of all elements must be computed as well as the wavefields radiating from the sources to the receiver positions. A parallel computer implementation of the algorithm is therefore advantageous, since the frequency domain computations are easily parallelized for rapid solution of the problem. We also compute discrete wavenumber synthetic seismograms at regional distances from the cavity source, using an equivalent moment tensor model of the explosion in the cylindrical cavity. Results for the radiation patterns show that at large aspect ratios (the ratio of cavity length to cavity diameter), the source has a strong variation in signal strength at high frequency. However, at lower frequencies typical of regional wave propagation, the P-wave radiation pattern is more nearly isotropic, and the S-wave amplitude is not so large. In addition, we find the interesting result that the radiation pattern of a large aspect ratio cavity in a transversely isotropic medium can be more like a classical point source than an a similar isotropic example. Incorporation of the cavity source into simulations of regional wave propagation using discrete wavenumber techniques shows that even though the shear wave is relatively weak at the source, it can still lead to the generation of a significant transverse component Lg wave This article is from 'Proceedings of the Annual Seismic Research Symposium on Monitoring a Comprehensive Test Ban Treaty (17th) Held in Scottsdale, Arizona on 12-15 September, 1995', 1996 0607 035, p374-383.