WAVE PROPAGATION IN POROUS ROCK AND MODELS FOR CRUSTAL STRUCTURE

The physics of wave propagation and attenuation in porous rock is a critical input to crustal exploration seismology. New experimental results are reported on velocity and attenuation in wet sandstone at elevated temperature and pressure. Both velocity and attenuation decrease with increasing temperature at all pressures. This proves that thermal relaxation is not operating under these conditions. Rather dissipation is controlled by a viscous fluid flow mechanism, in which a sharp frequency peak in attenuation is shifted from 2 kHz at room temperature to 8 kHz at 120(DEGREES)C as the pore fluid viscosity is decreased with increasing temperature. The frequency dependence is not spread or suppressed with increased pressure, but the modulus defect is decreased.;Some of these deep reflections are believed to be caused by ductile shear zones in the basement. Field studies of exhumed mylonite belts and laboratory measurement of physical properties of mylonites and their associated protoliths have shed light on the physical properties of dynamically metamorphosed rocks responsible for their seismic signature. In rock containing significant amounts of phyllosilicates and little feldspar, strong anisotropy may develop during ductile deformation. True amplitude seismic modeling incorporating anisotropy and the fine layered structure of the fault zone indicates an anisotropy of 7% may produce strong seismic reflections even though the mean velocity of the mylonite is unchanged. It is also shown that zones of elevated pore pressure may produce seismic reflections in crystalline and sedimentary rocks if permeability is sufficiently low.;A greater knowledge of Q in crustal rocks has aided interpretation of deep seismic reflection profiles of the Wind River mountains, Wyoming. Numerous reflections at long travel times are controversial because of the possible presence of multiples from the shallow sedimentary section. True amplitude synthetic seismograms are computed, using a Q structure controlled by spectral analysis of near surface reflections, and compared to true amplitude data records. It is found that multiples from sedimentary rocks do not arrive with sufficient amplitudes at long travel times to obscure true reflections from the crystalline basement of the lower crust. The deep reflection horizons observed at 6 to 15 seconds are primary events caused by a fine laminar structure of the crystalline section. Lamina thicknesses of 100 to 300 meters, each with reflection coefficient of 0.02 to 0.05 adequately model the data.