Modelling of the terrain effect in magnetotelluric data from the Garhwal Himalaya region

Abstract. The magnetotelluric (MT) method is a passive geophysical technique based on using time variations in the geoelectric and geomagnetic field to measure the electrical resistivity of the surface layer. It is one of the most effective geophysical techniques to study the deep structure of the Earth's crust, particularly in steep terrain like the Garhwal Himalaya region. MT responses are distorted as a result of undulating/rugged terrain. Such responses, if not corrected, can lead to the misinterpretation of MT data with respect to geoelectrical structures. In this study, two different correction procedures were used to compute the topography distortion for a synthetic model of the Garhwal Himalaya region from the Roorkee to the Gangotri section. A finite-difference algorithm was used to compute the MT responses (apparent resistivity and phase) for irregular terrain. The accuracy of the terrain correction procedures was checked using the results of different topography models for various periods from the literature. The relative errors between two terrain correction procedures were calculated with respect to the flat earth surface and were almost equal to zero for most of the sites along the Roorkee–Gangotri profile except at the foothill, where the error was high for shorter periods. The similar topography procedures of two terrain-corrected responses (TCR1 and TCR2) showed that there is no need for topography correction along the Roorkee–Gangotri profile because the slope angle is less than 1°.