Seismic wave tomography is a potentially powerful tool for detecting and delineating nonaqueous phase liquid (NAPL) contaminants in the shallow subsurface. To develop this application, we are conducting laboratory and numerical studies to understand the mechanisms of P-wave transmission through NAPL-water-sand systems. P-wave measurements of traveltime and amplitude were taken in the 100-900 kHz frequency range through saturated sand with variable NAPL content. To simulate the stress conditions of the shallow surface, a low confining and axial pressure of 60 and 140 kPa, respectively, was applied. The measurements show a significant change in the traveltime and amplitude of the primary arrival as a function of NAPL saturation. To simulate the laboratory measurements, we performed numerical calculations of P-wave propagation through a 1-D medium. The results show that the main behavior of traveltime and amplitude variation can be explained by P-wave scattering. This represents an alternative explanation to the theories that describe local fluid flow as the dominant mechanism for seismic wave attenuation and velocity dispersion.

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