Simulated Annealing 2-D Nonlinear Inversion of Geomagnetic Deep Sounding Data Near Ujjain-Guna, India

--An attempt has been made to use the global optimization technique of Simulated Annealing (S.A.) inversion to interpret the conductivity structure derived from the geomagnetic deep sounding data of N-W India. The present results supersede the earlier result obtained by 2-D conventional linearized inversion. The earlier linearized inversion, following an iterative gradient search technique on the same data set, has been re-evaluated and further constrained through an exhaustive search of the parameter space. The conductive response of an hypothesized conductor located between Ujjain and Guna, India, has been modelled by this random search tool. The location of the proposed model is now in agreement with the theory, since the conductive bodies are centered exactly below the center of the response function. In an earlier attempt by linearized inversion, this was not feasible. The central body is located at a depth of 19 km from the surface, suggesting a thickness of 15 km and resistivity of 14 ohm.m. The resistivity contrast of this ensemble of conductive bodies with the background varies by a factor of 100 to 385. This low conductivity contrast with respect to the background is in conformity to the low temperature as inferred from the available heat-flow data in the region. A marginally different estimate of the conductivity (normally mid-crustal conductors are assigned conductivities of the order of 50 to 200 ohm.m) for the mid-crustal conductor has been found. Existence of a mid-crustal conductor is clearly indicated which was also not detected in the earlier study. Low geo-temperature gradients existing in the region rule out the possibility of a thermal origin for this mid-crustal conductor. A likely explanation could be due to the presence of graphitic carbon at lower crustal depths. However, the role of electrolytic fluid present in the interconnected pore--spaces of rocks may be another tangible explanation.