EFFECT OF P-WAVE SCATTERING ON VELOCITY AND ATTENUATION IN UNCONSOLIDATED SAND SATURATED WITH IMMISCIBLE LIQUIDS

Seismic wave tomography is a potentially powerful tool for detecting a nd delineating nonaqueous phase liquid (NAPL) contaminants in the shal low subsurface. To develop this application, we are conducting laborat ory and numerical studies to understand the mechanisms of P-wave trans mission through NAPL-water-sand systems. P-wave measurements of travel time and amplitude were taken in the 100-900 kHz frequency range throu gh saturated sand with variable NAPL content. To simulate the stress c onditions of the shallow surface, a low confining and axial pressure o f 60 and 140 kPa, respectively, was applied. The measurements show a s ignificant change in the traveltime and amplitude of the primary arriv al as a function of NAPL saturation. To simulate the laboratory measur ements, we performed numerical calculations of P-wave propagation thro ugh a 1-D medium. The results show that the main behavior of traveltim e and amplitude variation can be explained by P-wave scattering. This represents an alternative explanation to the theories that describe lo cal fluid flow as the dominant mechanism for seismic wave attenuation and velocity dispersion.