Crosshole resistivity imaging of aquifer properties

Abstract

Crosshole resistivity tomography has been trialled at the Bolivar site north of Adelaide to delineate aquifer properties and preferential flow paths associated with artificial recharge and recovery operations. To date only the pre-storage electrical measurements have been carried out, but follow-up monitoring and background subtraction will be conducted at various stages of pumping to delineate storm-water injection and harvesting from the aquifer. This paper presents the results of the resistivity imaging experiments before water injection at the Bolivar site. The resistivity imaging experiment has entailed crosshole profiling and scanning with a specified bipole-bipole electrode array between four pairs of boreholes located on a circle of a radius of 75 metres from the water injection well, and the data were inverted for the resistivity structure of the aquifer using a new imaging scheme. The four resistivity images obtained between each pair of boreholes show a somewhat similar pattern, with alternative zones of high and low resistivity within the aquifer. There are at least five zones which can be recognised. They correlate with the major subdivisions found from geophysical logging of the boreholes and correspond with the stratigraphy of the T2-aquifer, which consists of variably cemented fine carbonate and sandstone layers.