Effects of heterogeneities and anisotropy on sonic and ultrasonic attenuation in rocks

Abstract

We compare the attenuation obtained by the resonant bar technique in the sonic frequency range (3–10 kHz) with that obtained by the pulse transmission technique in the ultrasonic frequency range (500 kHz) on a set of 30 rocks, including sandstones and limestones. The sonic and ultrasonic measurements were performed under high effective pressure on adjacent water‐saturated samples from each rock block. Compressional ultrasonic attenuation was typically larger than extensional sonic attenuation, particularly in limestones. This is attributed to the presence of density heterogeneities, that can be seen on X‐ray scanner imagery. Heterogeneities at the scale of the ultrasonic wavelength produce diffraction of the waves, and as a consequence, high attenuation values. At ultrasonic frequencies, large apparent shear attenuation values, compared to compressional attenuation values, are explained by shear wave splitting.