Shallow subsurface detection of buried weathered hydrocarbons using GPR and EMI

Abstract

Weathered hydrocarbons, commonly emulsified or in the form of tar balls, wash ashore along beaches due to natural oil seepages or offshore oil spills. They remain buried in the sand until a storm or erosion exposes them; therefore it is important to understand the progression and extent of these hydrocarbons. Elmer’s Island, Louisiana, a site known for having large amounts of oil washed ashore from the 2010 Deepwater Horizon oil spill, was selected for a geophysical survey in order to detect the presence of buried weathered oil. Two survey trips to Elmers Island were carried out using 200 MHz, 400 MHz, and 900 MHz ground penetrating radar (GPR) antennas. The 400 MHz data show two distinct anomalous layers with positive amplitudes and 900 MHz data show anomalous features that also display positive amplitudes. An electromagnetic induction (EMI) tool, used over the same traverses as GPR, provided insight into subsurface conductivity. The conductivity maps from the first survey trip display rows of anomalies and two large anomalous zones. These anomalous zones correspond with the 400 MHz GPR data. During the second survey trip, a three-dimensional GPR survey was conducted over a small grid where similarities between the two instruments were evident. Field observations confirmed the existence of contaminated sand (beach sand that enclosed small aggregates of weathered hydrocarbons) and tar balls buried at the survey site in distinct layers. These contaminated sand layers are most likely associated with the anomalies found on both the GPR and EMI data. Thus, a strong correlation with GPR and EMI anomalies co-locating buried weathered hydrocarbons suggests they can be used in future oil spill clean up efforts to map the extent of these hazardous materials. This integrated technique also has implications for the investigation of other buried features.