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Strain and frequency-dependent attenuation estimates in salt based on SALMON and STERLING near-field recordings

TELEDYNE GEOTECH, 314 MONTGOMERY STREET, ALEXANDRIA, VIRGINIA 22314

Abstract

The amplitude and spectra of teleseismic and regional seismic P waves depend on the material properties in the source region. The effects of strain and frequency-dependent attenuation are therefore important for understanding the source characteristics of coupled versus decoupled explosions. Near-field velocity and acceleration data from the SALMON and STERLING explosions in salt were analyzed for strain and frequency-dependent attenuation. Attenuation, parameterized as 1/Q, was estimated for various source-receiver distance ranges and frequency bandwidths using two methods: mean spectral ratio slope and average amplitude spectral ratio. Results from SALMON indicate that (1) in the frequency range of 1 to 25 Hz, Q is about 5 to 10; (2) Q increases with source-receiver distance, suggesting lower Q for larger strain levels; and (3) Q in the frequency range of 25 to 50 Hz is substantially higher than in the frequency range of 1 to 25 Hz, or Q increases with frequency. Analysis of data from STERLING indicates that Q is frequency independent and about one order of magnitude larger than for SALMON. It appears therefore that the near-source attenuation was strain and frequency dependent and hence nonlinear for SALMON out to a range of at least 600 m.