Elastic properties of sand-peat moss mixtures from ultrasonic measurements

Abstract

Effective remediation of an environmental site requires extensive knowledge of the geologic setting, as well as the amount and distribution of contaminants. Seismic investigations provide a means to examine the subsurface with minimum disturbance, Laboratory measurements are needed to interpret field data.In this experiment, laboratory tests were performed to characterize manufactured soil samples in terms of their elastic properties. The soil samples consisted of small (mass) percentages (1 to 20 percent) of peat moss mixed with pure quartz sand. Sand was chosen as the major component because its elastic properties are well known except at the lowest pressures. The ultrasonic pulse transmission technique was used to collect elastic wave velocity data. These data were analyzed and mathematically processed to calculate the other elastic properties such as the modulus of elasticity.This experiment demonstrates that seismic data are affected by the amount~of peat moss added to pure sand samples. Elastic wave velocities, velocity gradients, and elastic moduli vary with pressure and peat moss amounts. In particular, ultrasonic response changes dramatically when pore space fills with peat. With some further investigation, the information gathered in this experiment could be applied to seismic field research.

Laboratory Velocity Measurements Used for Recovering Soil Distributions from Field Seismic Data

“…These were ultrasonic velocity measurements of compressional and shear wave velocities for sand-peat mixtures at low pressures (Trombino, 1998;Berge et al, 1999;Bonner et al, 1999). Although other laboratory data sets are available in the exploration geophysics, marine geophysics, and soil mechanics literature (e.g., Rao, 1966;Domaschuk and Wade, 1969;Domenico, 1976;Hamilton and Bachman, 1982), few studies include both compressional and shear velocity measurements as a function of pressure at the extremely low pressures representing the shallow subsurface.…”

“…Although other laboratory data sets are available in the exploration geophysics, marine geophysics, and soil mechanics literature (e.g., Rao, 1966;Domaschuk and Wade, 1969;Domenico, 1976;Hamilton and Bachman, 1982), few studies include both compressional and shear velocity measurements as a function of pressure at the extremely low pressures representing the shallow subsurface. The laboratory measurements described in Trombino (1998) were made at pressures between 0 and about 16 psi (about 0.1 MPa) in pressure increments of 1.5 psi, and represent the top few meters of the subsurface. Both compressional and shear velocities were measured for a set of samples containing various proportions of Ottawa sand and commercially available peat moss (Trombino, 1998).…”

“…Such samples may be representative of shallow soils having a high organic content. Sample construction and laboratory measurement techniques are described in detail in Trombino (1998) and will not be repeated here. Resulting velocities determined from the measurements are described in detail in Berge et al (1999) and Bonner et al (1999).…”

“…Recent advances in highresolution seismic field methods (e.g., Carr et al, 1998;Steeples et al, 1998) and laboratory measurements of ultrasonic velocities at low pressures in unconsolidated materials (Bonner et al, 1997Trombino, 1998) provide incentive and data needed to improve seismic imaging of the shallow subsurface.…”

“…The code minimizes the misfit between the observed Vp, Vs pairs of data in the region of interest and sets of soil velocities measured in the laboratory. The soil distribution is determined by empirical correlation between lithology and velocities that is based on recent laboratory ultrasonic measurements for soils (Trombino, 1998;Berge et al, 1999;Bonner et al, 1999). Our intention was to demonstrate how the addition of the S velocity will constrain the inversion in realistic models when used in conjunction with the P velocity.…”