Effect ofP-wave scattering on velocity and attenuation in unconsolidated sand saturated with immiscible liquids

Abstract

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.

Monitoring microbe-induced physical property changes using high-frequency acoustic waveform data: Toward the development of a microbial megascope

“…The apparatus and methodology used here were similar to those described in Seifert et al [1998] computer for subsequent analysis. An initial set of measurements was made down the length of two of the columns (inoculated and control) in 1cm intervals immediately after the addition of the growth solution.…”

Effects of pore fluids in the subsurface on ultrasonic wave propagation

“…This hypothesis will be scrutinized by looking at the stability of the displacement process and the uniformity of the resulting saturation distribution. Measured and calculated velocities agree well for all three NAPLs and are, therefore, not 37 further considered in this study (Seifert et al, 1998).…”

“…Seifert et al (1998) showed that if the amplitude decreases monotonically with increasing NAPL saturation, and if a stable propagation of the NAPL-water front can be assumed, the amplitude and velocity change can be explained with a one-dimensional scattering model. However, if the fluid distribution is heterogeneous due to unstable displacement mechanisms or the pore fluid is very volatile and small changes in pressure or temperature produce gas inclusions, the one-dimensional scattering model still gives good velocity estimates but underestimates the magnitude of attenuation.…”

“…The data show that the seismic wave is strongly affected by the NAPL saturation and the physical properties OE the pore-filling fluids. Figure 3.1 also shows synthetic data (Seifert et al, 1998), which were computed with a one-dimensional model based on the propagator method (Kennett, 1983). The significant differences between the calculated and measured amplitudes for iso-octane and freon experiments may be caused by the model assumption of a sharp NAPL-water front during displacement.…”

Extensional wave attenuation and velocity in partially‐saturated sand in the sonic frequency range

“…The acoustic properties of poorly-consolidated sands is a topic of importance in a number of fields, including geotechnical investigation of soil properties for stability (Stoll, 1989;Ishihara, 1996), environmental monitoring of contaminants in the shallow subsurface (Geller and Myer, 1995;Seifert et al, 1998), and geophysical characterization of sand reservoirs and aquifers for petroleum production (Gardner et al, 1964) and CO 2 sequestration (Eiken et al, 2000).…”