Particle velocity measurements just beyond the region of pisstic deformation have been used in calculating the reduced displacement potential of the nuclear explosions-Rainier, 1.7 kt in tuff; Fisher, 13•5 kt in alluvium; Hardhat, 5 kt in granite; and Gnome, 3.5 kt in salt. After scaling to 5 kt, the potentials are convolved with the response of the Benioff instrument and with an attenuation operator appropriate for distances of approximately 500 km. The resultant amplitudes normalized to tuff are tuff 1, alluvium 0.25, granite 1.11, and salt 1.61. Models of the crust of the earth are developed for the four explosions, and the amplitudes of the arrivals from the Mohorovicic discontinuity are calculated using methods previously developed by the authors. The ratios then become tuff 1, alluvium 0.18, ganitc 226, and salt 2.11. These ratios are for the amplitude of the first half-cycle of motion. The theoretical amplitude-distance curves for explosions in tuff, alluvium, granite, and salt for the first 500 km are in agreement with the experimental values recorded by the Air Force Technical Applications Center for the nuclear explosions Antler, Logan, Blanca, Fisher, Hardhat, and Gnome.
Theoretical wave forms for the first cycle and a half are calculated for Romney's experimental recordings of underground nuclear explosions Blanca, Logan, and Tamalpais at distances of 96 to 714 km. Models of the crust are constructed from travel times. Zvolmskii s near‐front approximation is used to form the basis of amplitude calculations which include head coefficients, geometrical spreading, and corrections for superposed layers. The source function is scaled from measurements made of the Rainier shot. The effects of attenuation and instrument response are included. By convolving these factors, theoretical displacement amplitudes are calculated in millimicrons for the first half‐cycle which agree with the experimental measurements of Logan and Bianca from 300 to 600 km within +4 to −16 per cent. A single‐layer crustal model with a Q of about 400 is indicated by the amplitude calculations. The amplitudes of later half‐cycles are influenced by the reflection or interaction at the surface of the Rainier mesa. Additional data and calculations indicate that the surface reflection or interaction is nonlinear and has an amplitude about three times that expected on an elastic basis.
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.
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