Efficacy of Electromagnetic Measurements for Exploring Aquifers at Greater Depths in Lateritic Terrains, West Bengal, India

Electrical and electromagnetic methods of geophysical prospecting are the most suitable and cost-effective techniques for the delineation of aquifers in different geological settings. However, due to rapid depletion of groundwater level in different parts of the world, exploration of aquifers located at large depths has become challenging for these geophysical methods. The present study deals with a classic practical problem where DC resistivity method failed to depict aquifer layers at a greater depth. Even though suitable aquifers could be present at deeper depths in the study area, apparent resistivity data obtained by the Schlumberger sounding technique fail to reveal the presence of aquifer layers. In the present study, detection of such aquifer layers has been investigated using various frequency and time-domain electromagnetic methods. The study reveals that controlled source frequency domain EM method is also unable to yield significant change in the responses to establish the presence of aquifer layers at a larger depth. However, it has been observed that presence and absence of aquifer layers at larger depths can be depicted well by the Audio frequency MT (AMT) and Time-domain EM methods. Further, the study demonstrates that maximum variation in the responses (with and without aquifers) is observed from the AMT measurements. Therefore, AMT data together with DC resistivity measurement could be able to solve such a problem. It is important to highlight that practically it will also be challenging for both the methods as AMT data will have small signal to noise ratio (due to dead band region of MT measurement) and time domain measurement will require very sensitive device to measure transient signal which will be in μV (microvolt) to nV (nanovolt) range at later times or transient will become too noisy at later times.