Abstract
Gardner suggests that velocity data in which confining pressure changes are not canceled by equal pore pressure changes may be due to hysteresis effects that prevent velocity from being a unique function of the differential pressure. This is supported by shear velocity measurements in dry Berea sandstone as a function of cyclic variations of differential pressure (Gardner et al., 1965). High porosity and clayey sandstones indeed can demonstrate this behavior. Cyclic pressure variations can lead to work hardening and other inelastic behavior. For our measurements, however, we believe that the effects of hysteresis were minimal because of (1) the pressure cycle followed to obtain velocity as a function of pore and confining pressures, and (2) the large elapsed time between our measurements.
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