Ground Penetrating Radar Research at the Bemidji, Minnesota, Crude-Oil Spill Site

By Jeffrey E. Lucius

ABSTRACT

At the Bemidji, Minnesota, crude-oil spill site, the USGS collected ground penetrating radar (GPR) data to determine the distribution of oil concentrated in two subsurface pools, which remained after cleanup efforts. Physical property information from analysis of mixtures of sand and crude oil assisted in the interpretation of the GPR data. Laboratory measurements show that the crude oil is still very electrically resistive (greater than 10 6 ohm-m). Mixing clean sand with crude oil does not significantly change the relative dielectric permittivity (RDP) or electrical conductivity of the mixture. Four GPR lines were selected, from the large number of radar lines collected at the spill site since 1984, as typical examples of 80 MHz and 300 MHz data collected over the oil pools. At the spill site, GPR is sensitive to changes in electrical conductivity and RDP related to variations in water content, due to grain size and porosity, rather than variations in the oil saturation distribution (fraction of pore space occupied by oil). The oil pools at the Bemidji site are not easily detected using GPR. Nonetheless, GPR can detect those geologic features, such as silt or gravel layers in sand, that may affect the transport and fate of petroleum in the subsurface.