Seismic Characterization Of The November 8, 10, And 11, 1999 Dead Sea Underwater Calibration Explosions Using Cepstral Modeling and Inversion

摘要

We have developed a cepstral simulation and inversion algorithm for analysis of regional seismic recordings of underwater explosions that determines the yield and depth of the explosions. The spectra of seismic recordings of the underwater blasts have strong time-independent spectral scalloping produced by the correlated bubble pulses and echoes of acoustic reflections from the water surface and in the water column. Signed cepstra, computed from the fourier transform of the log-amplitude spectra of regional seismic phases (Pn, Pg, Sn, and Lg), contain positive peaks produced by the bubble pulse correlation and negative peaks from the water-surface reflection. Theoretical cepstra of underwater explosions with assumed depths and yields are computed from simulated time sequences of bubble pressure pulses and water column reflections. The pulses are modeled as minimum-phase wavelets with the amplitudes of the bubble pulses set by the source physics of underwater explosions and the reverberation amplitudes determined from the assumed reflection coefficients of the surface and bottom of the water column. A prototyped inversion algorithm, that determines explosion depth and yield, matches the synthetic model cepstra to data cepstra, stacked across all seismic phases and channels, by means of exhaustive search and downhill simplex and simulated annealing optimization methods. We have tested this algorithm on seismic recordings of three Defense Threat Reduction Agency's (DTRA) calibration chemical explosions detonated in the Dead Sea on November 8, 10, and 11, 1999. The three explosions all detonated at about 70 m and with yields of 500, 2000, and 5000 kg were recorded at the regional seismic stations EIL and MRNA on the high-frequency channels sampled at 40 Hz.