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 from the single charges decayed as approximately R( -2.4), and amplitudes from the row-charge decayed as R( -1.7). A dependence of amplitudes on depth of burst exists, and a yield scaling exponent near 0.8 appears to be appropriate. It is suggested that a variation of particle velocity with distance as R( -A)e( -kfR), 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.

Four microbarograph stations recorded waves at a range of approximately 130 mi from a row charge of high explosives buried near optimum cratering depth. Comparison with propagations from three airburst calibration detonations showed that a source model derived from close-in data was appropriate for distant effects predictions. This model predicted that wave amplitudes from explosives at this scaled depth would be 20% of amplitudes expected for a free air burst. All amplitudes perpendicular to a row charge are proportional to the 0.7 power of the number of charges in the row.

Project Pre-Gondola III, Phase I, a continuation of chemical-explosive cratering experiments conducted in saturated clay-shale near Fort Peck, Montana, was designed to excavate a row crater with relatively flat side slopes. The technical programs conducted during the experiment were: (1) measurements of the apparent crater, (2) surface-motion analysis, (3) cloud-development studies, and (4) airblast measurements. (Author).