Weiqun Shi

Induced-polarization detection and mapping of contaminant plumes

Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical properties of exposed soils/rocks and therefore produce measurable induced polarization IP signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A2D time-domain IP method is developed for mapping the FS12 contaminant plume at the Massachusetts Military Reservation MMR located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately 265 m 3 of fuel that erupted from a broken underground pipeline in the early 1970s. Benzene and ethylene dibromide EDB are the primary contaminants at FS-12, with concentrations exceeding the allowed maximum concentration levels MCL, while other constituents of the plume did not exceed their MCL. Therefore, the contaminants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concentrations 2400 and 1000 g/L, respectively among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resistivity and chargeability plots to 100 m depth and a horizontal extent of 400 m. By separately inverting IP data derived from time windows located at short and long decay times, a timedomain gross spectral chargeability difference is produced. Both the chargeability and gross spectral chargeability difference show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot otherwise be inferred from the sparse borehole distribution.

Application of electrical resistivity tomography to image Harrison Caves in Barbados, West Indies

In this presentation we demonstrate a new electrical resistivity tomography technique to image an underground cavern system. The success of this technique not only depends on the inversion procedure but also on the data acquisition methodology. We compare inversion results which use two such methodologies as inputs, e.g. standard pseudo-section data acquisition and data acquisition with a spatially varying source dipole. We illustrate that the variable source dipole tomographic data acquisition geometry can lead to better resolution and placement of sub-surface structure, especially for structures which contain extended lateral anomalies. The technique is tested on a numerically generated synthetic model and successfully applied to map a limestone cave system in Barbados, West Indies.

3-D Spectral Induced Polarization (IP) Imaging: Non-Invasive Characterization Of Contaminant Plumes

The overall objective of this project is to develop the scientific basis for characterizing contaminant plumes in the earths subsurface using field measurements of induced polarization (IP) effects. Three specific objectives towards this end are 1. 2. 3. Understanding IP at the laboratory level through measurements of complex resistivity as a function of frequency in rock and soil samples with varying pore geometries, pore fluid conductivities and saturations, and contaminant chemistries and concentrations. Developing effective data acquisition techniques for measuring the critical IP responses (time domain or frequency domain) in the field. Developing modeling and inversion algorithms that permit the interpretation of field IP data in terms of subsurface geology and contaminant plume properties.