The authors present their results as if Berea sandstone were an elastic material; that is, velocities are given as functions of confining and pore pressure. In fact, most rocks are inelastic and velocities depend on the history of the confining and pore pressure, and not just on the present values.
Some measurements of hysteresis were reported by Gardner et al. (1965). The confining pressure was cycled between two pressures Pc(1) and Pc(2) for a fixed pore pressure Pp, following a fixed schedule of pressure changes, until repeatable values of velocity were obtained. (At any intermediate pressure the velocity measured for increasing pressure was different from the value for decreasing pressure, giving rise to a hysteresis cycle). When the same schedule of pressure changes for the differential pressure Pc − Pp was followed by holding Pc fixed and varying Pp, the measured velocities followed the same hysteresis curve within the limits of experimental accuracy. In brief, when hysteresis was taken into account, changes in pore and confining pressures were equally effective in changing velocity.
In their article, Christensen and Wang do not refer to hysteresis; perhaps they would like to comment on its relevance.