A resistivity survey near Waimakariri River, Canterbury Plains, to improve understanding of local groundwater flow and of the capabilities of the survey method

Abstract

Abstract Sixty‐four Schlumberger resistivity soundings have been conducted along three traverses across the alluvial bed and recent floodplain of the Waimakariri River west of Christchurch, New Zealand. An interpretation of the measurements, involving two major layers beneath the water table, has been developed. Estimated average properties of the shallower layer, which exhibits matrix conduction, are: resistivity, 720 O.m; apparent formation factor, 5.2; specific yield, 0.29; hydraulic conductivity, 5 × 10 ‐3 m/s. Estimated apparent formation factors range from 2.7 to 7.3. Resistivities in the deeper layer, between 620 and 310 O.m, decrease as the depth to the layer increases. Measured resistivites of groundwater, at 12.7°C, range from 52 to 218 O.m. The difference between the resistivities of the two layers is probably primarily attributable to differences in groundwater resistivities. The upper layer probably represents where recent water from Waimakariri River is most predominant. Maximum values of the product of the resistivity and thickness of the upper layer may indicate where subflows of former river water are greatest. Local shallow groundwaters predominantly flow through subsurface gravels along discrete channels. The boundaries of local aquifers cannot be reliably located, nor can reliable hydrogeological parameters of the thick alluvium be obtained from Schlumberger soundings alone. However, large scale hydrological deductions made using the soundings are reasonably consistent with independent findings.