An electromagnetic modelling tool for the detection of hydrocarbons in thesubsoil

Electromagnetic geophysical methods, such as ground-penetrating radar (GPR) , have proved to be optimal tools for detecting and mapping near-surface co ntaminants. GPR has the capability of mapping the location of hydrocarbon p ools on the basis of contrasts in the effective permittivity and conductivi ty of the subsoil. At radar frequencies (50 MHz to 1 GHz), hydrocarbons hav e a relative permittivity ranging from 2 to 30, compared with a permittivit y for water of 80. Moreover, their conductivity ranges from zero to 10 mS/m , against values of 200 mS/m and more for salt water. These differences ind icate that water/hydrocarbon interfaces in a porous medium are electromagne tically 'visible'. In order to quantify the hydrocarbon saturation we devel oped a model for the electromagnetic properties of a subsoil composed of sa nd and clay/silt, and partially saturated with air, water and hydrocarbon. A self-similar theory is used for the sandy component and a transversely is otropic constitutive equation for the shaly component, which is assumed to possess a laminated structure. The model is first verified with experimenta l data and then used to obtain the properties of soils partially saturated with methanol and aviation gasoline. Finally, a GPR forward-modelling metho d computes the radargrams of a typical hydrocarbon spill, illustrating the sensitivity of the technique to the type of pore-fluid. The model and the s imulation algorithm provide an interpretation methodology to distinguish di fferent pore-fluids and to quantify their degree of saturation.