Laboratory measurement of seismic wave dispersion and attenuation at high pressure and temperature

At sufficiently high temperatures and in the presence of fluids, the small-strain mechanical behavior of geological materials is inevitably frequency dependent. In order to understand the nature of viscoelastic relaxation manifest in the attenuation and dispersion of seismic waves, laboratory methods suited to the seismic-frequency interrogation of geological materials are required. Torsional forced-oscillation and related microcreep methods, providing for the measurement of shear modulus and the associated strain-energy dissipation under conditions of high pressure and temperature and independently controlled pore-fluid pressure, are reviewed. The unknown and an elastic standard of known modulus and negligible dissipation, connected mechanically in series, are subjected to a low-frequency sinusoidally time-varying torque. Although there are mechanisms capable of producing non-elastic dilatational strain particularly in phase-transforming or fluid-saturated media, the shear modulus is much more commonly subject to viscoelastic relaxation than is the bulk modulus. The chapter also illustrates the principle underpinning the study of viscoelastic behavior through torsional mode forced-oscillation measurements. The unknown and an elastic standard of known modulus and negligible dissipation, connected mechanically in series are subjected to a low-frequency sinusoidally time-varying torque. At sufficiently low frequency, the phase of the torque and of the supporting radial distribution of shear stress is essentially uniform throughout the assembly. Thus, the measurement of the relative amplitudes and phases of the torsional mode displacements in standard and unknown provides for the determination of the shear modulus of the unknown relative to the standard and of the strain energy dissipation Q^-1for the unknown. © 2005