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.

Project Pre-Gondola I was a series of chemical explosive, single-charge cratering experiments in weak, wet clay shale conducted by the U.S. Army Engineer Nuclear Cratering Group as a part of the Joint Atomic Energy Commission-Corps of Engineers nuclear excavation research program. The four 20-ton (nominal) charges were detonated during the period 25 October to 4 November 1966, near the edge of Fort Peck Reservoir approximately 18 miles south of Glasgow, Montana. Motions of the ground surface were measured by high-speed photography of surface targets which had been positioned as far as 96 feet from each surface ground zero (SGZ). The results indicated that the maximum SGZ velocities recorded for these detonations were larger than those for cratering detonations in other media at comparable scaled burial depths. The four Pre-Gondola I detonations produced maximum SGZ velocities of 255, 200, 167, and 139 ft!sec for scaled burial depths of 152, 167, 187, and 202 ft/kt1/3, respectively. Maximum resultant surface velocities for the three shallowest detonations varied approximately as the -2.2 power of the radial distance from the charge. The maximum resultant surface velocities for the deepest detonation varied approximately as the -2.9 power of the radial distance from the charge.

Ground motion records from seven high explosive cratering events in northeastern Montana were analyzed for peak velocity, power spectral density, and velocity spectra. The events included four 20-ton single charges at depths of burst which varied from 42 to 57 ft, a 140-ton row charge consisting of three 20-ton charges and two 40-ton charges at optimum depths of burst, and a fully coupled charge of 0.5 tons and a decoupled charge of 0.5 tons at optimum depths of burst. It was found that at these depths and charge weights an increase in depth of burst resulted in an increase in peak velocities and power spectral densities as measured at distant points (>5 km), while no significant frequency shifts were noted. Power spectral density was found to be approximately proportional to the first power of yield. For this region it was determined that power spectral densities varied inversely as radius to the 3.55 power, and peak velocities varied inversely as radius to the 1.6 power. An increase in both velocities and power spectral densities for small decoupling factors was found to occur for a certain explosive-cavity configuration. Three analysis techniques, peak velocity, velocity spectra, and power spectral density, are compared and it is shown that power spectral density is the most consistent method when comparing records from different measuring stations.