Use of large cavities to reduce seismic waves from underground explosions

An analysis is given of an experiment designed to test the theory of seismic decoupling of underground explosions proposed by Latter, LeLevier, Martinelli, and McMillan [1961]. The amplitude of the seismic signal from a 1.7-kiloton nuclear explosion in a hole in salt was calculated and compared with the measured value from the 1.7-kt Rainier shot in tuff at the same distance. A decoupling factor of about 300 resulted. The experiment, called Cowboy,1 was designed to test the decoupling principle by carrying out a series of eight high-explosive shots in two spheres made in a salt dome, and nine tamped shots for comparison. The seismic data reported here were obtained primarily at ranges of 14,000 and 22,000 feet and at frequencies of 10 to 30 cps. A salt-to-salt decoupling factor of 100 was obtained which is consistent with the predicted tuff-to-salt factor of 300. When the sphere was overdriven so that the walls did not move elastically (which violates a condition of the theory for full decoupling), decoupling factors of 10 and 30 were measured. The seismic data are interpreted to give the dependence of decoupling on the various parameters of the experiment. The decoupling deduced from measurements made 80 feet from the shot points is found to be consistent with that deduced from the measurements at 14,000 and 22,000 feet. Project Cowboy was sponsored by the Atomic Energy Commission and carried out under the technical direction of the Lawrence Radiation Laboratory, University of California.