Near field CSAMT resistivity and model parameter determination

摘要

Cagniard resistivity approach in controlled source audiofrequency magnetotellurics/magnetotellurics (CSAMT/MT) data handling assumes a plane wave ER I source. Hence, near field Cagniard's resistivity does not permit the determination of model parameters. This paper presents the study of near/transition field apparent resistivity over multi-layered media by resorting to near field apparent resistivity and examines the possibility of extracting model parameters from near field CSAMT data, which hitherto has been ignored or corrected, using the well-known Marquardt algorithm. The per cent deviation (Delta alpha%) between the true and fitted geoelectric parameters for a two-layer case increases with increase in rho(2)/rho(1) when (rho(2) < rho(1)) for all values of r/h(1), h(1) being the thickness of the first-layer and r the distance between the transmitter and receiver. When r/h(1) 1, Delta alpha% is well below 15%. In case rho(2) > rho(1), the per cent deviation is within 5% when rho(2)/rho(1) > 20 and r/h(1) greater than or equal to 1. Over a three-layer geoelectric section the per cent deviation is within the 'acceptable' limits for the geoelectric parameters studied. Thus, this paper shows a way to invert the near zone CSAMT data which is a necessity to probe deeper layers. However, the scheme has poorer resolvability in the case of three-layer A-type geoelectric section. [References: 35]