f Integrated geophysical survey for the detection of hydrocarbon pollution

Geophysical methods provide useful tools to detect and map large areas potentially subjected to hydrocarbon pollution; many technical papers on the use of different geophysical techniques are reported in literature (Atenkwana, 1998, Godio and Morelli, 1998). Electromagnetic mapping, GPR investigation and electrical methods have proven to be effective for detecting polluted areas. An extensive survey has been undertaken on contaminated sites in Northern Italy over the last two years. The site assessment using the hydro-geological approach and destructive test (drillings) was completed with integrated geophysical investigations. The survey involved the characterisation of the subsoil assessment in some test sites, where oil recovering wells, tanks and pipelines were located. The main goal of the combined investigations was to find the presence of hydrocarbon pollution due to the leakage from the old tanks and pipelines. Because of the geological setting of the region, the main aim was to localise the free hydrocarbon phase above the shallow aquifer (2-3 meters in depth) and the presence of contamination in the vadose zone. Due to the long permanence of hydrocarbons in the soil (probably more than 20 years), chemical and biological reactions could have occurred in the polluted zones; these phenomena could complicate the interpretation of the geophysical response. A short description of the results of the geophysical tests is here reported: the selected examples relates to the survey performed on an area of about 50 x 100 m, previously employed to collect gas and oil. Most of the cultural features, such as tanks and pipelines, were removed from the site. The soil and water sampling confirmed the suspicion of the existence of a high contamination. The main goal of the geophysical survey was to verify the extension of the polluted zone, in order to plan a subsequent recovering of the area. Different methods were employed: a preliminary fast mapping was carried out using the low induction technique (LIN); the CM031 (Geophyzica) instrument in a dipole vertical mode was used. Ground probing radar (GPR) acquisition, which operates in the range between 100 MHz up to 500 MHz, was performed. Finally, electrical resistivity tomography (ERT) was carried out to confirm the response of the former geophysical investigations. Electrical measurements were performed employing the geo-electric intelligent nodes scanning system Syscal R2 (IRIS instrument). An electrode spacing of 2 meters was adopted in the dipole-dipole and Wenner array configuration for a total of 32 electrodes. The data processing involved the mapping of the in-phase and conductivity (quadrature component) of the electromagnetic response of the LIN acquisition; the GPR data were processed in order to obtain the time-slices of the reflection amplitude. ERT data were processed using a least square inversion (2D) with smoothness constrains, according to the procedure described by Morelli and Labrecque (1996).