Velocity and attenuation of seismic waves in imperfectly elastic rock

Abstract

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.