HR: 13:50h
AN: NS33A-02    [Abstracts]
TI: Time-Lapse Monitoring of Salt Transport Using 3-D Electrical Resistivity Imaging
AU: * Gharibi, M
EM: mgharibi@ucalgary.ca
AF: Department of Geology and Geophysics, University of Calgary, 2500 University Drive, Northwest, Calgary, AB T2N 1N4 Canada
AU: Bentley, L R
EM: lbentley@ucalgary.ca
AF: Department of Geology and Geophysics, University of Calgary, 2500 University Drive, Northwest, Calgary, AB T2N 1N4 Canada
AB: Remediation of salt contaminated soils is a priority for the Canadian upstream oil and gas industry due to historic pipeline breaks, flare pits and other releases of co-produced saline water and brine. Detailed mapping of the distribution of salt in contaminated soil and ground water is essential for planning a remediation program, evaluating remediation progress, and for conclusive risk-based contaminated site closure assessments. Time-lapse 3-D Electrical Resistivity Imaging (ERI) was used at a site with high concentrations of salt that was undergoing remediation. The site is underlain by glacial till with irregular thin sand units. Salt has penetrated in places 6 to 8 m below ground level (mbgl). The average water table was 3 to 4 mbgl and a tile drain system was installed at a depth of about 2 m. The installation is designed to remove saline waters and reduce the mass of salt in the soils. The first 3-D ERI surveys were conducted in July 2004 and repeated in November 2004. The late summer and early fall had unusually high precipitation and a significant mass of salt was removed by the tile drain system. Salt contaminated soils in the July electrical conductivity image are seen as elevated electrical conductivity (EC) anomalies as high as 400 mS/m. The thickness and the distribution of the high concentration areas are quite irregular highlighting the need for high resolution 3-D ERI to accurately characterize the distribution. The overall distribution EC in the November survey is similar to that of July. However, difference maps calculated by subtracting the EC of the July result from the November result shows an overall reduction in EC, up to 50 mS/m, from the surface to about 2 mbgl, the approximate depth of the tile drains. Below 2 m, EC increases by up to 50 mS/m and the depth to the bottom of the high EC zone has increased. The decrease in EC of the upper 2 m appears to reflect changes in site EC. However, imaging the bottom of a conductive anomaly can be problematic. Since the EC of the upper 2 m decreased, it may be that the bottom of the anomaly is better resolved in the November survey. Consequently, a question remains as to whether the increase in EC below 2 m reflects changing site conditions or a difference in image resolution. A series of numerical experiments is used to explore the reliability of the difference images below the 2 m.
DE: 0925 Magnetic and electrical methods
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly