THICKNESS OF THE SIWALIK SEDIMENTS IN THE MOHAND-RAMNAGAR REGION USING MAGNETOTELLURIC STUDIES

Magnetotelluric studies were conducted over a 150 km long linear profile between Mohand and Ramnagar, covering the Siwalik sediments in the foothills of the Himalayas. Data were collected at 13 stations with an interstation spacing of 8-12 km in the frequency range 0.01-100 Hz. The response functions indicated a predominantly one-dimensional substructure. However, two-dimensional behaviour was observed at four stations in the low frequency range (< 0.1 Hz). The rotation of the impedance tensor indicated major and minor resistivities along N 80-degrees-W and N 10-degrees-E, respectively. The geoelectric substructure was predominantly four layered and was explained as being due to three layers of sediments overlying a resistive basement. The top layer (50 m thick) was due to the alluvial and postSiwalik sediments, deposited over a more resistive Upper Siwalik layer with a thickness of 700-1000 m. The third layer was about 3-4 km thick and was understood to be due to the Middle and Lower Siwaliks. The resistive basement was delineated below the Middle and Lower Siwaliks at depths in the range of 5-8 km from the surface. The observed two-dimensional behaviour at some of the stations and the absence of the signature of resistive basement in their vicinity supports the presence of a deep-seated conductor identified by the magnetovariational (MV) studies.