Numerical simulation of coast effect on magnetotelluric measurements: Fid-Bau Portal

Numerical simulation of coast effect on magnetotelluric measurements

Malleswari, D. / Veeraswamy, K.

Abstract The coast effect on magnetotelluric (MT) data over South India has been studied using 2D Forward modelling. The peninsular shape of Indian shield is bounded by the Arabian Sea in the west, the Indian Ocean in the south and the Bay of Bengal in the east. The distance between Arabian Sea and Bay of Bengal varies from ‘0’ in the south to 1200 km near 19°N Latitude which passes through Mumbai. This region is close to sea, which can cause a distortion in the MT parameters of apparent resistivity and phase. MT covers a wide frequency range ($ 10^{+4} $ to $ 10^{−4} $ Hz), which enables us to image the resistivity of subsurface layers from few hundred meters to as deep as few hundreds of km. MT results in the TE and TM mode show significantly different behavior when the subsurface is non-one dimensional. Due to coast, in general apparent resistivity increases as period increases in TM-mode, while apparent reistivity decreases with increasing period in TE mode. Near the coastal sites (5, 30 km) magnetotelluric data gets distorted due to coast effect when the period is more than 0.1 s. Coast effect depends on Land distance, Land Resistivity and depth to the top of the conducting asthenosphere. If land distance is more coast effect is less, resistivity is more coast effect is more and if depth of conductor is small coast effect is more. Our numerical study on coast effect quantifies the effect on MT responses and discusses the necessity of including coast in the models while modeling with the field data. The field data obtained along 240 km long profile in Dharwar region has been inverted by considering TE, TM and TE & TM modes. On the basis of these results, it can be concluded that the resistivity model obtained from TE-mode is closer to the actual model as compared to TM-mode.