Effects of pore fluids in the subsurface on ultrasonic wave propagation

This thesis investigates ultrasonic wave propagation in unconsolidated sands in the presence of different pore fluids. Laboratory experiments have been conducted in the sub-MHz range using quartz sand fully saturated with one or two liquids. Elastic wave propagation in unconsolidated granular material is computed with different numerical models: In one-dimension a scattering model based on an analytical propagator solution, in two dimensions a numerical approach using the boundary integral equation method, in three dimensions the local flow model (LFM), the combined Biot and squirt flow theory (BISQ) and the dynamic composite elastic medium theory (DYCEM).;The combination of theoretical and experimental analysis yields a better understanding of how wave propagation in unconsolidated sand is affected by (a) homogeneous phase distribution; (b) inhomogeneous phase distribution, (fingering, gas inclusions); (c) pore fluids of different viscosity; (d) wettabilities of a porous medium.;The first study reveals that the main ultrasonic P-wave signatures, as a function of the fraction of nonaqueous-phase liquids (NAPL) in initially water-saturated sand samples, can be explained by a one-dimensional scattering model. Wave attenuation and velocity dispersion increase with increasing NAPL saturation. Velocity is sensitive to NAPL fraction only, whereas, the amplitude behavior is more complicated. A two-dimensional model shows that inhomogeneous fluid distribution, e.g., fingering and gas inclusions, can cause additional attenuation. Hence, amplitudes reflect the fraction of NAPL present in the sample as well as its distribution.;The next study investigates effects of pore fluid viscosity on elastic wave propagation, in laboratory experiments conducted with sand samples saturated with fluids of different viscosities. P-wave attenuation always shows a frequency squared dependence, independent of the pore fluid viscosity. The LFM and BISQ theory can not explain the laboratory observations for all the viscosities. Only the DYCEM model gives the correct frequency dependence and the calculations fit the measurements within the uncertainty.;The last study concentrates on the wettability of the grains and its effect on elastic wave propagation and electrical resistivity. Experiments have been performed in both initially water-saturated and initially n-dodecane-saturated media, for water-wetting and n-dodecane-wetting sand, as a function of n-dodecane saturation. Changes in the wettability of a n-dodecane-water-mineral system affect the effective amplitude of the P-wave, the capability of measuring S-waves and the electrical resistivity.