Seismic-Acoustic Delineation and Mapping Technique for Dense Nonaqueous Phase  Liquids Contaminated Geological Materials, 15-9186

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Principal Investigators
James R. Keys
Jorge O. Parra
Simon Hsiung

Inclusive Dates: 04/01/2000 - Current

Background - A contaminant of interest to many government agencies and industry is dense nonaqueous phase liquids (DNAPLs). DNAPLs often sink through an unsaturated zone or settle at a lower aquifer boundary, leaving residual and persistent contaminant. DNAPLs can dissolve into groundwater and are carcinogenic, and some DNAPL may have a long life, all of which may lead to the degradation of groundwater resources if not remediated.

The research team is conducting a laboratory proof of concept of a field study by Temples et al. (1997) in which the researchers found an acoustical response associated with very low levels of DNAPL. Their findings are controversial in the environmental and geophysical community, but if the premise is correct, it could be the basis of an efficient, nondestructive technique to delineate and map DNAPL in the shallow subsurface. The ability to remotely sense a contaminant plume over time while it is being remediated to monitor efficiency is a capability SwRI would like to develop.

Approach - An area of increased funding by industry and the government is the delineation and mapping of contaminant plumes in the shallow subsurface not accessible by other nondestructive methods such as electromagnetics, i.e., verify the treatment method without destructive sampling. In this project, geologic core samples will be tested mechanically and acoustically in a triaxial test apparatus under a range of pressures and fluid compositions. The intrinsic attenuation and dispersion of acoustical energy when the sample is "dry" will be calibrated against the attenuation and dispersion when different fluids (water, DNAPL, brine) are in the void spaces in the rock. The data will be used in modeling efforts where core velocities are related with well log (field data) velocities. The research team can relate the sonic velocity with RVSP, crosswell, and surface seismic data. Core data are critical when relating velocities with lithology, in particular with permeability, porosity, and fluid properties. After these relationships are understood at the bench scale, it will be possible to secure funding for pilot- or field-scale projects.

Accomplishments - Members of the research team presented the proposed work to Duke Engineering and Services (DES) in Austin, Texas, in May 2000. Arrangements were made to work with DES staff to acquire samples (geological samples, surfactants, and site DNAPL) after initial triaxial test runs. During initial testing, the team determined that the triaxial test cell had software and hardware problems. The software problems were corrected by the software provider. The team resumed testing but found the resulting waveforms to be inconclusive and inconsistent with expected results. The team then reconfigured the hardware and used a new power supply, which provided the expected results, as shown in the illustration.

Waveform with sample in triaxial test. Igneous tuff sample is 50.19 mm in diameter, 125.01 mm in length, and has a mass of 625.1 grams. Estimated velocity is approximately 3000 ms-1, which is within the expected range. The small peak denoted by the left arrow indicates an input pulse signal due to a small electrical coupling, while the arrow on the right denotes the first arrival of the compressional or p-wave.

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