A qualitative description of the formation of a crater is developed on the basis of a series of laboratory-scale cratering experiments. The results of 41 one-pound, single-charge cratering detonations in a concrete sand placed under controlled conditions are summarized. The effect of depth of burst on crater dimensions is illustrated. The crater formed by a one-pound charge buried at a depth of burst of 2 feet is selected for analysis because the ratio of depth of burst to depth of apparent crater is similar to that considered desirable for a prototype nuclear excavation. On the basis of these experiments and a series of gram-size, half-space cratering experiments conducted behind a Plexiglas plate, a description of the formation of a crater is hypothesized. Subsidence or slumping of the cavity walls during the formation of the crater is shown to play an important part in the formational process. The importance of this mechanism to the analysis of crater stability is discussed.-^T7

A qualitative description of the formation of a crater is developed on the basis of a series of laboratory-scale cratering experiments. The results of 41 one-pound, single-charge cratering detonations in a concrete sand placed under controlled conditions are summarized. The effect of depth of burst on crater dimensions is illustrated. The crater formed by a one-pound charge buried at a depth of burst of 2 feet is selected for analysis because the ratio of depth of burst to depth of apparent crater is similar to that considered desirable for a prototype nuclear excavation. On the basis of these experiments and a series of gram-size, half-space cratering experiments conducted behind a Plexiglas plate, a description of the formation of a crater is hypothesized. Subsidence or slumping of the cavity walls during the formation of the crater is shown to play an important part in the formational process. The importance of this mechanism to the analysis of crater stability is discussed.-^T7

Measurements of intermediate range ground motions and of structural response were made during the Pre-Gondola high explosive cratering experiments at Fort Peck, Montana. Liquid nitromethane charges (1000-lb and 20-ton), emplaced at various depths of burst, and a 140-ton row charge were detonated in the Bearpaw shale, which is a weak, wet clay-shale medium. An additional experiment to validate a charge emplacement concept designed to decouple seismic energy proved inconclusive. All seismic measurements were of particle velocity. Using an inverse power law equation to describe the attenuation of seismic amplitudes with distance, it is found that the amplitudes -2 4 from the single charges decayed as approximately R ' , and amplitudes from the row--1 7 charge decayed as R " . A dependence of amplitudes on depth of burst exists, and a yield scaling exponent near 0.8 appears to be appropriate.-A -kfR It is suggested that a variation of particle velocity with distance as R e , where f is the signal frequency, is a more physically realistic description of attenuation than is the inverse power law. The preliminary data from the 140-ton row charge appear to fit this type of attenuation law, and indicate that A = 0.5 and k = 0.015 sec/km. An estimate is made of the near-source variation of peak seismic amplitude with frequency for the row charge, and predictions are made for possible future row-charge cratering experiments at the site.

Measurements of intermediate range ground motions and of structural response were made during the Pre-Gondola high explosive cratering experiments at Fort Peck, Montana. Liquid nitromethane charges (1000-lb and 20-ton), emplaced at various depths of burst, and a 140-ton row charge were detonated in the Bearpaw shale, which is a weak, wet clay-shale medium. An additional experiment to validate a charge emplacement concept designed to decouple seismic energy proved inconclusive. All seismic measurements were of particle velocity. Using an inverse power law equation to describe the attenuation of seismic amplitudes with distance, it is found that the amplitudes -2 4 from the single charges decayed as approximately R ' , and amplitudes from the row--1 7 charge decayed as R " . A dependence of amplitudes on depth of burst exists, and a yield scaling exponent near 0.8 appears to be appropriate.-A -kfR It is suggested that a variation of particle velocity with distance as R e , where f is the signal frequency, is a more physically realistic description of attenuation than is the inverse power law. The preliminary data from the 140-ton row charge appear to fit this type of attenuation law, and indicate that A = 0.5 and k = 0.015 sec/km. An estimate is made of the near-source variation of peak seismic amplitude with frequency for the row charge, and predictions are made for possible future row-charge cratering experiments at the site.

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