Electrical resistivity surveys for groundwater between Waimakariri and Rakaia rivers, Canterbury

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Risk, G.F. 1982 Electrical resistivity surveys for groundwater between Waimakariri and Rakaia rivers, Canterbury. [s.l.]: [s.n.]. Report / Geophysics Division 191 25 p

Abstract: Electrical resistivity soundings using the Schlumberger electrode configuration have been made at 46 sites on the Canterbury Plains between the Rakaia and Waimakariri Rivers. The interpreted sections exhibit widely varying resistivities for the surface soil layer (140 to 9000 ohms x m) and the water unsaturated layer lying between the soil and the water table (1000 to 40,000 ohms x m). The interface separating these upper layers from a deeper layer of 500 to 700 ohms x m is related to the water table, but at 18 out of 20 sites where a comparison of the levels is possible, the resistivity interface was much shallower than the water table, demonstrating that the method is not reliable for measuring water table depths in this area. Two distinct resistivity layers were inferred to represent the water-saturated region along a traverse line from Rakaia township to McLeans Island. The upper of these extends down to about 20 m above sea level, and its resistivity varies between 500 and 700 ohms x m. The underlying layer has lower resistivities of between 250 and 450 ohms x m probably representing older, more weathered sediments. The resistivity of water in the Selwyn River, unlike that of waters in the Rakaia and Waimakariri Rivers, decreases by about 50 percent between foothills and sea due to the inflow of ions from shallow aquifers. Aquifer resistivities also decrease towards the sea. No simple relationship could be found between resistivity formation factor and aquifer permeability, because clays and fine materials appear to contribute significantly to the conduction process, making Archie's law invalid. Zones of higher than average resistivity seem to be associated with good permeability. In the Rakaia gorge the rhyolite basement was found to have a low resistivity of 20 ohms x m