Dispersions and attenuations in a fully saturated sandstone: Experimental evidence for fluid flows at different scales

Abstract

Dispersion and attenuation of elastic waves have been observed in crustal rocks. Existing theories and experimental measurements evidenced the existence of different energy-loss mechanisms. In rocks fully saturated by a Newtonian fluid, one major candidate is fluid flow at different scales, separating three regimes: the drained, undrained, and unrelaxed regimes. Here, two elastic transitions, between these three regimes, are investigated. Both the cause and the consequence of the drained/undrained transition are evidenced. Because the second transition measured occurs at higher frequency, where no global fluid flow occurs, it in turn is associated to the undrained/unrelaxed transition (or squirt flow). For both transitions, the amount of open microcracks seems to be the major contributor to the magnitudes of attenuation.