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Seismic source characteristics of cavity decoupled explosions in salt and tuff

S-CUBED DIVISION OF MAXWELL LABORATORIES, INC., P.O. BOX 1620, LA JOLLA, CALIFORNIA 92038-1620

Abstract

Nonlinear finite-difference simulations are performed to model the seismic source function for decoupled, partially coupled, and overdecoupled explosions in air-filled cavities in salt and dry, porous tuff emplacement media. A principal conclusion resulting from these simulations is that the low-frequency decoupling level decreases slowly as yield is increased above the yield required for complete decoupling. In both the tuff and salt models used in this study, low-frequency decoupling is reduced by only about a factor of 2 at an explosion yield eight times the Latter decoupling criterion. These results suggest that it should be possible to detonate a 20-kt explosion in a cavity sufficient to fully decouple only a 5-kt explosion and still achieve a decoupling factor sufficient to reduce m_b to less than the m_b 3.5 identification threshold cited in the 1988 OTA report on seismic monitoring.

In tuff cavities, the dominant nonlinear mechanism operating at low yields is pore crushing, which reduces the low-frequency coupling efficiency and therefore increases decoupling relative to a linear model. Pore crushing affects the seismic source function even for overdecoupled explosions and increases the decoupling factor by a factor of 2 at the Latter decoupling threshold. Above the Latter threshold, the decoupling factor decreases as plastic flow begins to become the dominant mechanism. In salt cavities, partial coupling begins at about the Latter threshold and reduces the decoupling factor at all higher yields.

The source functions for all of the cavity explosions are enhanced at high frequencies relative to the source function for a simple step pressure source because of the initial air shock and cavity reverberations. This reduces the decoupling factor at high frequencies. High-frequency decoupling factors are larger for overdecoupled explosions than for partially coupled explosions. The decoupling factors are reduced to less than 1 at sufficiently high frequencies.

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