Seismic source characteristics of nuclear explosions in water-filled cavities

During the period from 1975 to 1979, the former Soviet Union conducted a se ries a six nuclear explosions in a water-filled cavity in salt which was cr eated 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 sei smic data recorded from these tests have been processed and analyzed in an attempt to characterize the seismic source characteristics of these explosi ons and assess their relevance to the cavity decoupling evasion scenario. T he results of these analyses indicate that the explosions in the water-fill ed cavity were not decoupled, but rather show evidence of enhanced seismic coupling with respect to that which would be expected from tamped explosion s of the same yields in salt. Theoretical finite difference simulations of these tests have been conducted in which the complex, nonlinear interaction s between the shock effects in both the water and surrounding salt medium h ave 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 wat er and the shock-wave reflections from the cavity wall. More specifically, it has been found that the shock-wave reflection from the cavity wall retar ds 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 r adius and a shorter bubble oscillation period.