Ultrasonic attenuation measurements in unconsolidated sand with pore fluids ranging in viscosity between 0.001 and 1 Pa•s were compared with the predictions of fluid flow and scattering theories. Laboratory experiments were performed for P waves propagating through sand samples saturated with water, castor oil and two different silicone oils. The attenuation shows a frequency squared dependence for all measurements, regardless of viscosity, in the range between 100 and 1000 kHz. The results show that for unconsolidated sand, fluid flow models which imply significant effects of the viscous pore fluids on ultrasonic waves cannot explain the laboratory measurements. The main attenuation effects observed in the laboratory can be simulated with a three-dimensional generalized dynamic composite elastic medium model, which includes scattering from the pores and grains as well as intrinsic attenuation caused by the viscous pore fluids. For the studied sand samples, scattering is the main attenuation mechanism for ultrasonic P waves.
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.
Abstract. Laboratory cross-well P-wave transmission at 90 kHz was measured in a 61 cm diameter by 76 cm tall watersaturated sand pack, before and after introducing a nonaqueous phase organic liquid (NAPL) (n-dodecane). In one experiment, NAPL was introduced to form a lens trapped by a low permeability layer; a second experiment considered NAPL residual trapped behind the front of flowing NAPL. The NAPL caused significant changes in the travel time and amplitude of first arrivals, as well as the generation of diffracted waves arriving after the direct wave. The spatial variations in NAPL saturation obtained from excavation at the end of the experiment correlated well with the observed variations in the P-wave amplitudes and travel times. NAPL residual saturation changes of 4% were detectable and the 40 to 80% NAPL saturation in the NAPL lens was clearly visible at acoustic frequencies. The results indicate that small NAPL saturations may be more easily detected with amplitude rather than travel time data, but that the relationships between the amplitude changes and NAPL saturation may be more complex than those for velocity.
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DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document. The last study concentrates on the wettability of the grains and its effect on 3 elastic wave propagation and electrical resistivity. Experiments have been performed in both initally water-saturated and initally 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.
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