Ð During the period from 1975 to 1979, the former Soviet Union conducted a series a six nuclear explosions in a water-®lled cavity in salt which was created in 1968 by a tamped 27 kt explosion at a depth of 597 m at the Azgir test site at the north end of the Caspian Sea. Broadband, near-regional seismic data recorded from these tests have been processed and analyzed in an attempt to characterize the seismic source characteristics of these explosions and assess their relevance to the cavity decoupling evasion scenario. The results of these analyses indicate that the explosions in the water-®lled cavity were not decoupled, but rather show evidence of enhanced seismic coupling with respect to that which would be expected from tamped explosions of the same yields in salt. Theoretical ®nite dierence simulations of these tests have been conducted in which the complex, nonlinear interactions between the shock eects in both the water and surrounding salt medium have been explicitly modeled. The results of these simulations indicate that the most prominent yield dependent features of the observed seismic source functions can be largely explained by the dynamic interactions between the expanding and contracting steam bubbles generated by the explosions in water and the shock-wave re¯ections from the cavity wall. More speci®cally, it has been found that the shock-wave re¯ection from the cavity wall retards the expansion of the steam bubble in a yield dependent fashion relative to that expected in the open ocean, resulting in a smaller maximum bubble radius and a shorter bubble oscillation period.
Experimental observations of the seismic efficiency of a decoupled nuclear explosion in a salt dome are presented. The energy decoupling factor was calculated from local, regional and teleseismic data using the ratio of the amplitudes of the tamped (well coupled) explosion that had created the cavity and that of the decoupled one. To calculate the amplitudes of the seismic waves of a well coupled explosion of the same yield as the decoupled one, a scaling law for explosions fired within the same region was used. An energy decoupling factor near 30 is consistent with local, regional and teleseismic data. It is concluded that decoupling was only partial for the event described. Numerical calculations of partial decoupling showed that the seismic wave spectrum at an epicentral distance of 2.3 km is consistent with an amplitude decoupling factor of 20, which is consistent with the energy decoupling factor 30.
Ð During the period 1965 to 1988, the former Soviet Union (FSU) conducted over 120 peaceful nuclear explosions (PNE) at locations widely dispersed throughout the territories of the FSU. These explosions sample a much wider range of source conditions than do the historical explosions at the known nuclear test sites and, therefore, seismic data recorded from these PNE tests provide a unique resource for use in deriving improved quantitative bounds on the ranges of seismic signal characteristics which may require consideration in global monitoring of the Comprehensive Test-Ban Treaty (CTBT). In this paper we summarize the results of a detailed statistical analysis of broadband seismic data recorded at the Borovoye Geophysical Observatory from 21 of these PNE tests at regional distances extending from about 7 to 19 degrees, as well as the results of theoretical waveform simulation analyses of near-regional (D < 25 km) seismic data observed from a selected sample of nine of these PNE tests. The results of these analyses have been found to be consistent with those of previous teleseismic investigations in that they indicate that the seismic source coupling eciencies are very similar for explosions in a wide variety of hardrock and water-saturated media, while explosions in water-saturated clay are observed to have signi®cantly higher coupling eciencies. Moreover, the scaling of the seismic source function with explosion yield and depth of burial inferred from these analyses of the Soviet PNE data are shown to be generally consistent with the predictions of the Mueller/Murphy source model. These results suggest that the Mueller/Murphy source model can provide a reasonable basis for estimating the expected variation in regional phase spectral composition over a wide range of nuclear source conditions of potential interest in CTBT monitoring.
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