Noise and target strength degradation accompanying shallow‐buried explosions

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Abstract
Ground shock from shallow‐buried explosions is commonly accompanied by acoustic ‘noise.’ Analysis of records from a series of 10‐ton chemical detonations in Gulf Coast sediments provides a basis for the first quantitative description of such noise during a cratering event. The test series included shots at depths of 3–12 m leading to craters about 20 m in radius. Most of the records studied were taken within the eventual crater; they include stress, velocity, and acceleration. Oscillations arising from reflections off the free surface or subsurface layers are evident in stress records. High‐frequency oscillations, attributed to reflections or scattering from smaller scale inhomogeneities, are particularly striking in the acceleration records, where peak accelerations were 10–100 km/s2. Oscillations with frequencies of 5–25 kHz are obvious. They exhibit exponential decay with the dominant frequency decreasing with time. Approximate values of Q estimated from these accelerograms are 30±20, agreeing with Q for similar materials derived from seismic techniques. Since seismic Q's include both internal dissipation and scattering losses, whereas explosive Q's include internal dissipation and radiative losses, the magnitude of scattering and radiative losses must be comparable. The latter alone correspond to an effective Q of about 300, so that internal dissipation dominates Q in these poorly consolidated sediments. Acoustic fluidization may have occurred for a couple of milliseconds in the top 1 or 2 m of soil within the eventual crater (by this, we mean that the material was liable to flow if the threshold shear stress were exceeded). This result is inferred from ground shock stress duration, acoustic amplitude decay, and initial overburden stress. Fluidization of such limited duration and extent would have no effect on crater growth, just as would be expected for such a low yield event.

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