Radar detection of thin layers of hydrocarbon contamination

Computer modeling, laboratory experiments and a field program were used to evaluate the ability of GPR to detect thin layers of hydrocarbon contamination on or near the top of the water table. The Sihvola mixing formula, complex dielectric constant of the constitutive components, and calculations of the complex impedance as a function of frequency, porosity and liquid fraction were used in the model. Radiated wavelets, convolved with the reflection coefficient were used to predict GPR reflected wavelets when the thickness of hydrocarbon varied from zero to two pulse lengths. Results suggest for thin layers with distinct electromagnetic boundaries, the reflection coefficient can provide an indication of the presence of hydrocarbon. Using TOR with a coaxial sample container holding a column of soil with a water table, we propagated an electromagnetic pulse through the column of soil while a thin layer of diesel fuel was introduced at the water table. The oil tension saturated zone created a gradual transition between moist soil and oil-saturated soil. This gradual transition zone in the region of the oil saturated sand did not generate the sharp electromagnetic discontinuity. Subsequently, there was not a strong return of the electromagnetic pulse. In the field, GPR data was gathered along lines in areas with known contamination. Monitoring wells, soil borings and total petroleum hydrocarbon data provided ground truth. While GPR imaged a number of factors related to site assessment, we did not see the effect of the thin layers of contamination.