Seismic Characterization of the November 8, 10, and 11, 1999 Dead Sea Underwater Chemical Calibration Explosions Using Cepstral Modeling and Inversion

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. This low bandwidth limits the resolution of short delay signals from water layer bounces but is adequate for resolving the bubble pulses from these events. The cepstral peaks were observed in all of recorded phases, and similar delay times were observed at two different stations for the largest explosion. The cepstral peak quefrencies (time delays) correspond to bubble pulse periods of 400, 550, and 800 ms. We estimate yields 650 ±140kg, 1950±750 kg, and 4200±500 kg for the 500, 2000, and 5000 kg explosions, respectively. Depth estimates of about 80 m from the combination of the bubble pulse period and interpreted surface reflections are not as well resolved, because of the shallow depths of the explosions and the limited bandwidth of the IMS data. However, the results are consistent with the known depths of 70 m. These results show that accuracy of this method is limited by the seismic bandwidth, particularly for small, shallow events. Overall, this study points out the importance of using seismic data for detection and characterization of underwater explosions in inland seas. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE SEP 2000 2. REPORT TYPE 3. DATES COVERED 00-00-2000 to 00-00-2000 4. TITLE AND SUBTITLE Seismic Characterization Of The November 8, 10, And 11, 1999 Dead Sea Underwater Calibration Explosions Using Cepstral Modeling And Inversion 5a. CONTRACT NUMBER