Transient Electromagnetic investigations for delineation of aquifer layer along Anjar-Rapar Corridor, eastern Kachchh basin, Gujarat

Abstract:

Kachchh basin is an intraplate region of rifted Precambrian craton in the north-western India. The basin is characterized by a thick succession of sedimentary rocks of middle Jurassic to Holocene that overlie the Precambrian basement. Kachchh district of Gujarat is a water scarce region and ground water has become one of the important sources to meet the water requirements of various sectors. However, in the district Tertiary and Mesozoic formations ingeneral do not form promising aquifer mainly because of the clayey nature and poor groundwater (salinity) quality. Additionaly, the areas close to the sea-coast suffer from the problem of sea water intrusion. The upper Bhuj series constitutes a relatively good aquifer because it has in general low salinity. We have carried out a transient electromagnetic (TEM) survey along the Anjar - Rapar (AR) corridor in the eastern part of the Kachchh basin to map potential aquifer layer in the region. Fifteen fixed in-loop TEM soundings with 100m sided transmitter loop were carried out. The measured transient decay curves were used to model the depth and subsurface resistivity of the undrelying structure. The modeled 1-D section reveals a 20 to 50m thick conductive layer of 10-15 ohm.m resistivitywith its top surface at 10-15 m depth. We infer this conductive layer as aprobable aquifer comprising of saturated sandstone with moderate to low salinity. At some places along the corridor, we have also obtaind a second aquifer layer at deeper levels(150-250 m) overlying a relatively low resistive layer probably comprising of clay and/or clayey sand. Apart from the first layer, subsequent subsurface levels of the area (upto 200-250m) appear to be heterogeneous in nature and are inferred to be composed of clay (>5 Ohm.m), Clayey sand/ sandstone with high content of saline water (5-10 ohm.m) and shale (20-50 Ohm.m). The results are integrated with a well-log data to better constrain the geophysical inferences.