Shear wave velocity and attenuation from pulse‐echo studies of Berea sandstone

Abstract

The pulse‐echo spectral‐ratio technique has been adapted to the determination of ultrasonic shear wave attenuation in sandstone at variable states of saturation and pressure. The method can measure shear attenuation coefficients in the range 0.5 dB/cm to 8 dB/cm to within ±0.5 dB/cm. For the Berea sandstone, this range corresponds to values of the shear quality factor (Qs) between 10 and 100. Spectra of Qs show that between 600 and 1100 kHz, Qs decreases with frequency, particularly at high pressures (up to 70 MPa). Ultrasonic shear wave attenuation in a 90% water‐saturated sample was intermediate between that for dry samples and the relatively high attenuation in fully saturated rock. Strong pressure dependence is seen in the shear attenuation for all saturation states, indicating a dominant role of dissipation mechanisms operating within open and compliant cracks. Substantial shear attenuation remains at the highest effective pressure applied to the saturated sample, which may be due to a more “global” fluid‐flow loss mechanism. Scattering losses, as described by weak scattering theories for compressional waves, do not appear to be dominant at these frequencies.