SEISMIC-WAVE ATTENUATION IN VOLCANIC-ROCKS FROM VSP EXPERIMENTS

Seismic wave attenuation in the Columbia Plateau basalts and the Snake River Plain volcanics was analyzed using vertical seismic profiling (VSP) data. The computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth. This approach does not require any assumptions on the dependence of Q on frequency. 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, in agreement with recent findings by other authors. The effective attenuation of seismic energy in basalts (about 1.1 x 10(-4) db s/m for the peak frequency) is comparable to the effective attenuation observed in sedimentary rocks (between 1.5 x 10(-4) and 4.3 x 10(-4) db s/m). Results from two VSPs recorded in the Snake River Plain volcanics using air gun and vibrator sources 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 air gun energy (with a peak frequency near 15 Hz) clearly detects a difference in the attenuating properties of the two types of rocks. The vibrator energy, on the other hand, also detects this difference, but only for the lower frequencies. For frequencies near the peak frequency (31 Hz), attenuation is almost the same in the two units. 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 both sources by analyzing the amplitude decay in the time domain. The peak-frequency attenuation coefficients for the lower section are 1.7 x 10(-4) and 2.6 x 10(-4) db s/m for the vibrator and air gun sources, respectively. For the upper section, the corresponding values are 1.9 x 10(-4) and 6.4 x 10(-4) db s/m. The difference in attenuation implied by the last two coefficients is probably not real, because the decay of energy in the time domain for the two sources is much closer to each other. The Columbia Plateau and Snake River Plain VSPs show that the poor quality of reflection data commonly associated with volcanic rocks cannot be explained by unusually high attenuation.