Delineation of kaolinised and aquifer formations using electrical resistivity tomography (ERT) and induced polarization (IP) techniques

The majority of well failures in the khondalitic terrain of northern parts of Eastern Ghats of India are due to the extent of the depth of kaolinisation (highly weathered material) of the khondalitic aquifer. Since One-Dimensional Vertical Electrical Sounding (VES) surveys show similar resistivity values for both kaolinised and weathered khondalitic aquifer, the Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) techniques are employed along the success and failed well pair in the khondalitic terrain. Very thin kaolinised formation followed by the aquifer for the success well and the complete kaolinisation up to deeper depths for the failed wells are the characteristic features of the difference in well yields. The ERT images found that the depth of kaolinised material with resistivity values less than 25 Ωm at the failed wells is extended about 21–50 m deeper than the success wells. The thickness of aquifer formation with a resistivity range of 25–65 Ωm is varied from nearly 15–31 m at success wells while the same is maintained only about 7–9 m at failed wells. IP images revealed that the layers with moderate chargeability values of 2.63–3.58 ms are indicated as weathered/fractured khondalitic aquifers and the layers with lower chargeability values of 0.23–1.0 ms are characterized as kaolinised formations. The study found that ERT and IP imaging are potential techniques for mapping of contrasts in kaolinised and non-kaolinised aquifer formations in the khondalitic suit of rocks.