Effect of pore pressure on compressional wave velocity in coals

A system has been developed for determining precisely the compressional wave velocities in coal specimens subjected to independent hydrostatic confining pressure and internal pore pressure. The velocity Vp of compressional waves has been measured, by the pulse transmission technique, on ten water saturated coal specimens in directions perpendicular and parallel to the bedding plane as functions of confining pressure (Pc) and pore pressure (Pp) to 25 MPa. Velocity changes due to changes in confining pressure were not exactly cancelled by equivalent changes in pore pressure. An effective pressure defined by Pe = Pc - nPp, n ≥ 1, rather than an effective pressure defined simply by the differential pressure Pd = Pc - Pp, is found to be the determining factor in the behaviour of Vp. As pore pressure and confining pressure, are increased at constant effective pressure, the value of n increases and approaches 1. But as effective pressure increases at constant pore pressure, the value of n decreases. Our experimental results are consistent with Biot's theory of the elastic wave propagation in a fluid-saturated porous solid.