Estimates of seismic attenuation using vibrational resonance and pulse transmission in four large blocks of rock

Abstract

SUMMARY In many laboratory studies it is traditional to use small cylindrical core specimens in order to measure the material attenuation (intrinsic and scattering losses) of travelling waves in rock. Unfortunately, the geometric spreading within typical core specimens can dominate the material attenuation, especially for specimens having a mean grain size comparable to the core size. In this case the elastic scattering losses cannot be separated from the geometric losses, since the specimen surfaces themselves promote increased scattering via boundary reflections. However, it is possible to isolate the separate attenuation components by using large blocks of rock. In the present study, seismic attenuation was measured in large blocks of norite (a fine-grained gabbro in which the pyroxene has been altered by heat and pressure), medium-grained granite, coarse-grained granite, and sandstone, with each material type having significantly different mean grain sizes. The geometry was identical for each rectangular block, being large enough to permit the installation of a 0.7 m linear seismic array yet small enough to permit whole-body resonance. It was thus possible, in the same volume of rock, to use vibrational resonance and seismic pulse transmission to separate the intrinsic and scattering losses. A reasonable agreement was found between the measured attenuation and the predictions based upon models of the intrinsic and scattering loss mechanisms for seismic waves in rock. In all blocks the transmitted waveforms showed direct evidence of non-negligible elastic scattering, and it was found that the scattering Q(Qs) decreases with increasing mean grain diameter. Furthermore, the treatment of the scattering losses at the present ultrasonic frequencies is scale-independent and is thus applicable to scattering at the lower frequencies of relevance to crustal and exploration seismology.