Résumé
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.