An inversion of geomagnetic deep sounding data using simulated annealing

A maiden attempt on the use of the global optimization technique of Simulated Annealing (S.A.) inversion to model the conductivity structure derived from the geomagnetic deep sounding data of NW India is reported here. 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 which was not feasible by the earlier linearized inversion. The central body is located at a depth of 19 km from the surface, suggesting thickness of 15 km and resistivity of 14 Ω m. The resistivity contrast of this ensemble of conductive bodies with the background is varying by a factor of 100 to 385. Existence of mid-crustal conductor (the conductivity estimate is marginally different) is clearly indicated which was also not detected in the earlier study. 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 explanation.