An exact method of computing volume changes under high pressure from acoustic-wave-velocity measurements is developed. It is applicable to large as well as small compressions. To illustrate the application of the method, precision ultrasonic-velocity measurements made in mercury at pressures up to 13 kbar for three temperatures have been used to compute V as a function of T and P. The volume is determined to an accuracy of better than 0.01% at each pressure and temperature. An analytical expression for the pressure dependence of the volume in which all coefficients are expressed in terms of the bulk modulus and its derivatives is developed and shown to give a better representation of the P—V data than many of the equations now in use.

The mechanical properties of crystalline rock have been studied in the laboratory as a function of temperature and pressure at the low frequencies and the strain amplitudes characteristic of seismic waves. The inelasticity observed arises at interfaces in the rock structure and is insensitive to temperature but highly pressure dependent. It cannot be described in terms of viscoelasticity but causes the rock to display static hysteresis. The internal friction ϕ of the rock is accompanied by a large modulus defect, δM/M; both ϕ and δM/M are shown to be independent of frequency and strain amplitude throughout a very wide range. A model based on a network of cracks in an elastic medium accounts for these properties. The presence of a fluid phase in the rock does not in itself significantly increase the internal friction in the range of frequencies of seismic waves, but the presence of a small amount of intergranular fluid can make interface inelasticity persist in the presence of a large confining pressure. Interface inelasticity can occur at substantial depths in the earth, resulting in low seismic velocities and low Q. The low velocity zones in certain areas are interpreted in this way as arising from the presence of a fluid phase.

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