Effects Of Borehole Fill On Resistivity Investigations

Drilled boreholes generally are the only feasible means to access the subsurface for the emplacement of downhole electrodes for most hole-hole and hole-surface resistivity experiments. However, the very existence of the borehole itself creates the potential for significant noise due to the inevitable conductivity contrast that develops between the borehole walls and the formation. Borehole effects develop whenever a current source is placed in a drilled borehole. Borehole geometries may range from nearly perfect cylinders to highly, irregular, rugose holes in consolidated rock, to relatively minor, collapsed, disturbed zones in caving sediments. Boreholes in non-caving formations generally are filled with artificial materials to afford crucial, electrical continuity between downhole electrodes and the borehole walls. Filled boreholes form cylindrically shaped heterogeneities that create significant noise due to preferential current flow up and down the conductive columns. Selected conditions are simulated with a finite difference model to illustrate the significance of borehole effects on holehole and hole-surface mise-à-la-masse electrical potentials near a current electrode. Preliminary modeled results of borehole effects for electrical resistance tomography (ERT) suggest the existence of an inhomogeneity between the two boreholes which may cause misinterpretation of the data, depending on the resistivity contrast of the borehole fill and earth material.