Shear modulus and internal friction of calcite rocks at seismic frequencies

Abstract

The shear modulus G and associated internal friction Q-1 have been measured at seismic frequencies for a suite of four essentially monomineralic, calcite rocks with grain sizes ranging from 4 to 1000 μm. The measurements are based on a previously described technique for the observation of forced torsional oscillations of a series combination of a rock cylinder and a steel standard under high confining pressure at room temperature. The behaviour of each rock has been studied as a function of strain amplitude (10-8-10-6 maximum at surface), pressure (0 - 300 MPa) and oscillation period (1 - 300 s). No evidence is found of any departure from linearity in the range of strain amplitudes studied. Corrections, which are significant only at the shortest periods (1 - 3 s), have been applied to the raw data to account for the influence of torques exerted on the specimen assembly by the gas pressure medium. Up to about 100 MPa, increase in pressure results in a marked increase in G and decrease in Q-1, attributable to the closure of crack porosity-except in the fine-grained Solnhofen limestone in which both G and Q-1 are insensitive to change in pressure, indicative of the virtual absence of crack porosity. At higher pressures, G is much less pressure-sensitive and Q-1 is essentially pressure-independent, with values less than 0.001 in all four rocks. Since the rock specimens were jacketted under conditions of normal humidity, these relatively low values of Q-1 indicate a diminished role for adsorbed water in the anelasticity of rocks at high effective pressures. Within the pressure-independent regime, Q-1 is essentially independent of frequency for periods of 3-100 s and increases by only 30% as the grain size decreases from 1000 to 4 μm. Thus, within this regime, it is concluded that the observed value of Q-1 results to a large degree from an intracrystalline mechanism with a broad range of relaxation times. The possibility of significant dislocation damping will be discussed. Show more