Effects of Source Depth on Near-Source Seismograms

摘要

Source depth effects are examined for five 115-kg explosions buried at depths ranging from the optimum cratering depth of 1.8 m to the fully contained depth of 11.5 m. Data were recovered at near source ranges from 17 to 228 m. The waveforms are dominated by P and SV-Rayleigh energy. Depth effects are evident in the increase of P to SV-Rayleigh amplitude ratios and in the twofold increase of high-frequency energy for the deeper sources. Theoretical propagation depth effects are modeled by Green's functions calculated for a velocity gradient which approximates the velocity structure of the experimental site. The effects of depth on the explosion source function are predicted using the scaling laws of Mueller and Murphy (1971). These models did not reproduce the observed twofold increase of high-frequency energy. The discrepancy between the model and observations is attributed to increased coupling of high frequency P wave energy for fully contained sources. Energy calculations confirm that the shallowest event coupled 40% and the fully contained event 80% of the total seismic energy into the P wave. Source coupling efficiencies ranged from 0.7-1.0% for the near surface source to 1.5-2.9% for the fully contained explosion. Reprints. (EDC)