Improved Imaging Of Dense Non-Aqueous Phase Liquids With Three-Dimensional Surface-To-Horizontal Borehole Electrical Resistivity Tomography (S2HB ERT)
Abstract
Clean-up of sites polluted with dense non-aqueous phase liquids (DNAPLs) remain a highly challenging problem. Numerous technologies are available for remediating DNAPL-contaminated sites, but their performance relies on accurate characterization and monitoring of the subsurface. Electrical resistivity tomography (ERT) is a well-established and widely used geophysical method that has been used effectively for mapping subsurface features and processes of interest. ERT can gather large volumes of continuous subsurface information in a non-destructive, cost-effective, and time-efficient manner, and exhibits highly desirable characteristics for application to DNAPL sites. However, the most traditional configuration for employing ERT is from the surface, and this suffers from poorer imaging quality with increasing depth from the surface. To overcome this issue, which is particularly problematic at DNAPL sites, ERT may take advantage of horizontal borehole technology to enhance image quality at depth.
The objective of this thesis is to evaluate the performance of novel three-dimensional ERT that utilizes both the ground surface and horizontal boreholes to improve characterization and monitoring capabilities of ERT at DNAPL sites. A range of numerical and laboratory tank experiments were conducted on various DNAPL targets in different environments (e.g., water and plastic, sand and NAPL). Results demonstrate the high potential of 3D S2HB ERT for characterizing DNAPL targets, especially when compared to surface ERT. Furthermore, implementation of a single borehole with a 2D surface array (i.e., S2HB-1BH) provided adequate resolving ability compared to a S2HB configuration that utilized a 2D horizontal borehole array matching the surface array (e.g., 11 surface lines and 11 borehole lines) (i.e., S2HB-FULL). This enhanced our understanding of ideal borehole electrode implementation. A full 2D array of boreholes would be highly impractical at DNAPL sites, and the adequate performance by a more practical single borehole is highly encouraging.