Fractured rock geophysical studies in the Clare Valley, South Australia

Abstract

Groundwater in the Clare Valley is drawn from fractured rock aquifers. As such aquifers underlie approximately 40% of Australia, an understanding of sustainability and distribution of groundwater resources is important for agricultural and domestic purposes. The primary goal of the work described here was to assess the value of geophysical methods in determining the capture zones of irrigation bores in the Clare Valley aquifers. Azimuthal resistivity surveys utilising DC resistivity and EM induction were performed to evaluate the presence of electrical (and thus hydraulic) anisotropy and heterogeneity. A mise-à-la-masse survey was also carried out at one borehole. Both symmetric and asymmetric azimuthal DC resistivity and EM induction data were acquired. The asymmetric-array results indicate the presence of heterogeneities. The major axes of the DC-resistivity ellipses are approximately parallel to the direction of vertical bedding planes for electrode separations between 20 and 40 m. However, EM-resistivity ellipses at most sites are nearly perpendicular, with resistivity minima corresponding to observed bedding planes.