Shear Wave Generation by Decoupled and Partially Coupled Explosions
Abstract
The objective of this project is to investigate the sources of shear wave generation by decoupled and partially coupled explosions, and the differences in shear wave generation between tamped and decoupled explosions. A perfectly spherical explosion at the center of a perfectly spherical cavity large enough to fully decouple the explosion would generate no shear waves, so all shear waves from decoupled explosions are due to asymmetries in and near the cavity, and to scattering and conversions. We quantify the effect of different sources of asymmetry through numerical modeling, and take advantage of unique features of different data sets to eliminate particular mechanisms and so constrain the possible sources of observed shear waves. We present numerical calculations, and analytic solutions for different pressure functions, of the problem of shear wave generation by an explosion offset from the center of a spherical cavity, which causes the shock wave to vary in amplitude and arrival time around the cavity surface. The offset explosion has a dipole component and can generate significant shear waves with a modest offset from the center. The numerical calculations produce cavity reverberations that are strongest along the axis of the source offset. We develop a physically reasonable pressure function that models the pressure reverberations and show that it leads to longer duration waveforms with more low frequency content, again with a dominant dipole contribution. We have shown previously that the radiation pattern of the near field, initial, shear-wave arrivals from the decoupled explosion Sterling could be reproduced by modeling that included the flat floor (due to melted and recrystallized salt) of the cavity. The observations also have substantial coda, which are not reproduced by the calculations. Similarity of coda waveforms with distance indicates their source is at or very near the cavity.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 30, 2008
- Accession Number
- ADA487728
Entities
People
- G. E. Baker
- Heming Xu
- Jeffry L. Stevens