Abstract

MT is a passive geophysical method, utilizing naturally occurring electromagnetic energy. The physical property of the subsurface measured is the electrical resistivity, familiar from the common electric logs. The measurement of time-varying electric and magnetic fields and the computation of apparent resistivity from these fields are precise, analytical steps and should be repeatable by any practitioner. The transition to depth from frequency is an interpretive step involving assumptions, the validity of which are case-dependent. The interpretation of geology based on vertical and lateral resistivity variations is, of course, an interpretive step. While not as inexpensive as gravity or magnetics, MT has been found by many operators to be a cost-effective way to enhance an exploration program. Typically, in the "pathfinder" role in overthrust exploration, MT is used first to determine if a prospective subthrust section is present, and then to define the extent of the subthrust section to place realistic limits on the amount of seismic data required. Then, as an exploration program matures, MT is used to augment seismic exploration as described above and by full integration with seismic data to obtain an MT-derived or constrained velocity model. MT survey has been conducted over the region of Narmada Son lineament, central India, Siwalik Himalayas, Iapetus Suture Zone in central Ireland, Central India and Kota-Kekri and from these studies, the geo-electric structure has been explored. Comparing with the gravity and seismic method, it is given a satisfied result. MT is the best tool for deep crustal study.