Experimental study of the amplitude effect on wave velocity and attenuation in consolidated rocks under confining pressure

Abstract

A series of experiments were carried out to investigate the influence of strain amplitude on wave velocity and attenuation in dry sandstone and smoky quartz. Measurements were performed using the reflection method at a frequency of 1 MHz at strains ∼(0.3-2.0) × 10-6 under a confining pressure of 20 MPa. The dependence of the compressional velocity on the strain amplitude in sandstone is ambiguous. On one hand, the velocity calculated by means of measurement of the propagation time of the first pulse minimum (V minp) does not change with increasing amplitude. On the other hand, the velocity determined by the propagation time of the first pulse maximum (Vmaxp) slightly but surely increases with amplitude. In view of the small variation in velocity (∼0.7%) this result is regarded only as a tendency. The Q-1p in sandstone decreases with increasing amplitude. The decrease is 16% in all the amplitude range. Vp and Vs in smoky quartz do not depend on the strain amplitude, but Q-1p and Q-1s decrease with amplitude. The decrease in attenuation is 10% and 6.5% respectively. This result contradicts the existing concept. The unusual behaviour of the attenuation is presumably explained by the inelasticity of the rock (at least by microplasticity). Amplitude dependence can be used as an additional criterion in the geological interpretation of seismic data. © 2005 Nanjing Institute of Geophysical Prospecting.