Abstract
Change in shear-wave velocity for four dry sedimentary rocks has been studied as a function of the variation of both external hydrostatic pressure and internal pore pressure in the range 0 to 2,500 psi. The experimental method employs a beam of ultrasonic energy passing through a liquid in which a copper-jacketed parallel-sided slab of rock is rotated. The shear-wave velocity is calculated from the laws of refraction and reflection of waves at a liquid-solid boundary applied to the angle at which minimum energy is transmitted.The variation of shear-wave velocity with pressure has been found to be a function of net overburden pressure, P e -nP i , where P e = external hydrostatic pressure on the jacketed sample, P i = internal pore pressure and n = a pressure-dependent factor less than unity. The values of n at several differential pressures were chosen to yield a smooth curve passing through the displaced data points when the shear-wave velocities were plotted as a function of net overburden pressure. Using the n values so obtained, the matrix compressibility C r for two of the sandstones has been calculated from the relation n = [C B -C r ]/C B . The bulk compressibility C B for these two rocks had previously been obtained experimentally as a function of differential pressure. The values obtained for the matrix compressibility are in the range expected from a knowledge of the grain and cementing materials for these sandstones.
