Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the microscopic distribution of water and air in the pore space of a sandstone. Compressional wave velocity Vp was measured at a frequency of 1 MHz shear wave velocity was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with Sw during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically from 3 km/s to 4 knils. When Sw was decreased, pronounced saturation hysteresis was observed in the region 0.3 <Sw < 1.0, with Vp measured during drainage greater than Vp measured during imbibition. Similar results were obtained for Vs versus Sw, with Vs D D during drainage greater that Vs during imbibition in the saturation range Sw > 0.4. As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i. e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air-filled or water-filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, "crack-like" pores, these variations in fluid distribution cause related variations in velocity.

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