Field Measurement of Penetrator Seismic Coupling in Sediments and Volcanic Rocks (Final Report)
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Field experiments were conducted to determine experimentally how well a seismometer installed using a penetrator would be coupled to the ground. A dry-lake bed and a lava bed were chosen as test sites to represent geo- logical environments of two widely different material properties. At each site, two half-scale penetrators were fired into the ground, a three- component geophone assembly was mounted to the aft end of each penetrator, and dummy penetrators were fired at various distances to generate seismic signals. These signals were detected by the penetrator-mounted geophone assembly and by a reference geophone assembly buried or anchored to surface rock about 1-m from the penetrator. The recorded signals were digitized, and cross-spectral analyses were performed to compare the observed signals in terms of power spectral density ratio, coherence, and phase difference. Most of the energy in the recorded signals was in the frequency range of 3 to 30 Hz. The analyses indicate that seismometers deployed by penetrators will be as well coupled to the ground as are seismometers installed by conventional methods for the frequency range of interest in earthquake seismology, although some minor differences were observed at frequencies near the upper limit of the frequency band.