HR: 1340h
AN: S33A-0296    [Abstracts]
TI: Near-Source Scattering of Explosion-Generated Rg: Insight From Difference Spectrograms of NTS Explosions
AU: * Gupta, I
EM: gupta@multimax.com
AF: Multimax, Inc., 1441 McCormick Drive, Largo, MD 20774 United States
AU: Chan, W
EM: winston@multimax.com
AF: Multimax, Inc., 1441 McCormick Drive, Largo, MD 20774 United States
AU: Wagner, R
EM: wagner@multimax.com
AF: Multimax, Inc., 1441 McCormick Drive, Largo, MD 20774 United States
AB: Several recent studies of the generation of low-frequency Lg from explosions indicate that the Lg wavetrain from explosions contains significant contributions from (1) the scattering of explosion-generated Rg into S and (2) direct S waves from the non-spherical spall source associated with a buried explosion. The pronounced spectral nulls observed in Lg spectra of Yucca Flats (NTS) and Semipalatinsk explosions (Patton and Taylor, 1995; Gupta et al., 1997) are related to Rg excitation caused by spall-related block motions in a conical volume over the shot point, which may be approximately represented by a compensated linear vector dipole (CLVD) source (Patton et al., 2005). Frequency-dependent excitation of Rg waves should be imprinted on all scattered P, S and Lg waves. A spectrogram may be considered as a three-dimensional matrix of numbers providing amplitude and frequency information for each point in the time series. We found difference spectrograms, derived from a normal explosion and a closely located over-buried shot recorded at the same common station, to be remarkably useful for an understanding of the origin and spectral contents of various regional phases. This technique allows isolation of source characteristics, essentially free from path and recording site effects, since the overburied shot acts as the empirical Green's function. Application of this methodology to several pairs of closely located explosions shows that the scattering of explosion-generated Rg makes significant contribution to not only Lg and its coda but also to the two other regional phases Pg (presumably by the scattering of Rg into P) and Sn. The scattered energy, identified by the presence of a spectral null at the appropriate frequency, generally appears to be more prominent in the somewhat later-arriving sections of Pg, Sn, and Lg than in the initial part. Difference spectrograms appear to provide a powerful new technique for understanding the mechanism of near-source scattering of explosion-generated Rg and its contribution to various regional phases.
DE: 7219 Seismic monitoring and test-ban treaty verification
SC: Seismology [S]
MN: Fall Meeting 2005