Electrical resistivity and induced polarization as tools for mapping near surface hydrocarbon contamination

Two-dimensional electrical resistivity (ER) and induced polarization (IP) surveys were conducted on three sites in central and southern Alberta to investigate the effectiveness of ER and IP as tools for mapping near surface hydrocarbon contamination. Elevated normalized chargeability (MN) values were hypothesised to exist near the margins of the hydrocarbon bodies due to increased surface polarization at these locations. Near surface weathered hydrocarbons were imaged as elevated resistivity bodies which correlated with the surface expression of the contamination. No change in electrical resistivity was seen in the area containing free phase hydrocarbons which could be distinguished from the effects of geology. Elevated values of MN were present at the approximate boundary of the hydrocarbon bodies at all three sites. The location of the elevated values is consistent with the working hypothesis although the results are not definitive. The exact relationship between the location of elevated MN and the margins of the hydrocarbon bodies is complicated by use of 2-dimensional surveys over a 3-dimensional target. Based on results from the study the most effective survey protocol is a gradient survey type with an a-spacing of 2 m, a n-spacing of less than 4 and 4 second pulse duration.