Modelling of subsurface geological structure related to anomalous magnetotelluric phase data exceeding 90° in Central India

Abstract

SUMMARY We note anomalous phase (>90°) in magnetotelluric (MT) measurements at several stations along the Narmada river course normal to the Tan Shear Zone (TSZ) in central India. We made efforts to derive a possible cause of the anomalous phase by comparing the results of numerical modelling based on the tectonics of the TSZ with the measured impedance tensor data. A zone of multiple reactivations of the TSZ leads to the formation of a damage zone and conjugate Riedel shear faults parallel and normal to the TSZ, respectively. The conjugate Riedel shear faults act as a pathway for the Narmada River course. The multiple tectonic activities associated with the TSZ induce surficial heterogeneity that results in distortion in the MT data. The Mohr circle and phase tensor analysis establish the effect of distortion in the data. To understand the cause of distortion as well as anomalous phase in the present data, we carried out synthetic 3-D modelling. We modelled the damage zone and conjugate Riedel shear faults by a cross shape. From the results, we noted that the near-surface 3-D heterogeneity across the TSZ critically distorted the YX component of the MT impedance tensor due to the strong current channelling. When the current flows across the conjugate Riedel shear zone (Narmada river course) charges are accumulated along the boundaries of near-surface heterogeneity. The charges associated with the current channelling brought out the reversion of the electric field direction that is reflected in the form of an anomalous phase in the YX component of the impedance tensor in measured data.