Electrical Geophysics at the Bemidji Research Site

By Robert J. Bisdorf

ABSTRACT

Self-potential (SP), induced polarization (IP), and direct-current (DC) resistivity electrical geophysical methods were used to study the effect of crude oil on the resistivity of a sand and gravel aquifer near Bemidji, Minnesota. The SP measurements do not show any trends that correspond with the oil spill. The IP data show resistivities consistent with the DC measurements, but only a slight polarization effect. Direct-current resistivity measurements made into maps and cross sections, detect lower resistivities associated with the oil spill, especially in areas where the oil soaked into the sand and gravel. The observed resistivity measurements show a larger resistivity decrease than can be explained by laboratory measurements made on selected samples of the sand and oil.