Seismic wave attenuation in volcanic rocks from VSP experiments

Abstract

Seismic wave attenuation in the Columbia Plateau basalts and the snake River Plain volcanics was analyzed using vertical seismic profiling data. For the Columbia Plateau basalts, the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic VSP generated using the sonic and density logs, indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt. Therefore, it is concluded that only a lower bound for Q can be established. Results from two VSPs recorded in the Snake River Plain volcanics show some frequency-dependent effects. The depth range analyzed covers two different lithologic units (rhyolitic rocks with interbedded volcanic sediments above more homogeneous rhyodacitic rocks). The difference in attenuation for the two types of rocks is real and cannot be explained as processing artifacts, because it can be observed for two sources by analyzing the amplitude decay in the time domain. -from Authors