Regional conductivity structure of Cascadia : preliminary results form 3D inversion of USArray transportable array magnetotelluric data

In conjunction with the USArray component of EarthScope, long period magnetotelluric (MT) data are being acquired in a series of arrays across the continental US. Initial deployments in 2006 and 2007 acquired data (10– 10,000 s) at 110 sites covering the US Pacific Northwest, distributed with the same nominal spacing as the USArray seismic transportable array (~75 km). The most striking and robust features revealed by initial three-dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern Oregon, and beneath the Cascade Mountains, contrasting with very resistive crust in Siletzia and the Columbia Embayment. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Washington backarc, and the most resistive corresponding to subducting oceanic mantle

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

We note anomalous phase (>90 degrees) 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

Dimensionality analysis of MT data using Mohr circle: A case study from Rewa–Shahdol region, India

Mohr circle in magnetotelluric (MT) is being used to represent dimensionality of subsurface structure. Mohr circle plot on individual axes for each frequency represents the dimensionality concerning frequency, whereas plotting of Mohr circle on common Cartesian coordinates for all frequencies displays the effects of noise on the impedance tensor. Here, we examine the Rewa-Shahdol region MT data with the Mohr circle approach to understand the sub-surface dimensionality structure, but the presence of noise in the signal has randomized the Mohr circle response. Hence, we made an effort to obtain the subsurface dimensionality using Mohr circle properties and derive two new invariants such as tan phi and sin theta. The two invariants represent the dimensionality and anisotropy nature of the subsurface structure. Results from the Mohr circle together with the new angles brought out the 1-D graben structure of the Gondwana and Vindhyan basins, 2-D/3-D nature of the underlying basement structure and the crustal structure below the Narmada-Son Lineament (NSL) zone. 2-D/3-D nature of the NSL zone represents the basement uplifted horst structure between Gondwana and Vindhyan basins. Further, the horst-graben structure of NSL zone is evident from the Mohr circle analysis suggesting of rifting and block movement

Effect of evaluation frequency separation on magnetotelluric resolution

Aiming at the resolution enhancement of the subsurface objects, an evaluation (target) frequency separation criterion is proposed for ma- gnetotelluric (MT) method. The proposed idea is based on the propa- gation geometry of diffused electromagnetic (EM) wave through the earth. Starting from a homogeneous medium and extended it to laye- red earth model, the present study proposed the frequency separation criterion in terms of apparent resistivity and the lowest frequency to be used. To apply the proposed idea in real situation the frequency separa- tion criterion is reduced to a convenient form which is expressed in ter- ms of the lowest frequency only. The effectiveness of the proposed con- dition is examined by applying it to synthetic data. The result shows that the new idea can improve the resolution of the subsurface objects.

Processing of noisy magnetotelluric time series from Koyna-Warna seismic region, India: a systematic approach

Rolling array pattern broad band magnetotelluric (MT) data was acquired in the Koyna-Warna (Maharashtra, India) seismic zone during 2012-14 field campaigns. The main objective of this study is to identify the thickness of the Deccan trap in and around the Koyna-Warna seismic zone and to delineate the electrical nature of the sub-basalt. The MT data at many places got contaminated with high tension power line noise due to Koyna hydroelectric power project. So, in the present study an attempt has been made to tackle this problem due to 50 Hz noise and their harmonics and other cultural noise using commercially available processing software MAPROS. Remote site was running during the entire field period to stand against the cultural noise problem. This study is based on Fast Fourier Transform (FFT) and mainly focuses on the behaviour of different processing parameters, their interrelations and the influences of different processing methods concerning improvement of the S/N ratio of noisy data. Our study suggests that no single processing approach can give desirable transfer functions, however combination of different processing approaches may be adopted while processing culturally affected noisy data

Some Insights into the Lithospheric Electrical Structure in the Western Ghat Region from Magnetotelluric Studies

Magnetotelluric (MT) studies along a few traverses, some cutting across the Western Ghats, during the last few years have provided basic insights into the shallow as well as the deeper electrical structure in the regions near and east of the Western Ghat belt. The MT models broadly show a two layered lithospheric electrical structure with an upper high resistive layer (several thousands of m) and a lower moderately conductive layer (a few tens to a few hundred m). The depth of the interface between the two layers is found to vary from about 120-160 km in the south in the SGT to around 80 km in the north in the northern DVP. Another impressive feature that could be noticed in these electrical models is the presence of well-defined major near vertical crustal conductive feature associated with the region of Western Ghat belt, presumably associated with the tectonic evolution of the Western Ghats. Further, these models also brought out several other well-defined conductors that might be linked to structural features like faults, shear zones, etc., in the region. These conductors pierce through the crustal column and some of these, particularly those oriented in NW-SE direction, i.e., oriented transversely with respect to the ambient compressive stress direction of the Indian shield, assume significance in understanding the seismicity of the region

Evolution of eastern segment of the Central India Tectonic Zone: an insight from a magnetotelluric study

The collision process between the South Indian Block (SIB) and North Indian Block (NIB) resulted in the development of the complex crustal nature of the Central India Tectonic Zone (CITZ). The evolutionary past of CITZ covers a long geological time (similar to 1000 Ma), which corresponds to the assembly and spreading of Columbia and Rodinia supercontinents. Despite several studies in the western and central parts of the CITZ, the location of the suture zone between the SIB and NIB is still under debate. In addition to that, the crustal structure in the eastern segment of CITZ is yet to be resolved. Therefore, for the first time, a dense station coverage magnetotelluric (MT) study is carried out along a 275 km transect in the eastern segment of CITZ from Pandaria to Rewa. The complexity of the Central Indian Shear (CIS) and Tan Shear Zone (TSZ) are reflected as anomalous phases (beyond 90 degrees) in the MT data. A deep crustal resistivity model derived from 2-D and 3-D inversion of the MT data brought out a high-to-moderate conductivity structure (10-100 omega-m) in the middle of the surface expressions of CIS and TSZ. The conductive structure could be related to a deformation zone formed by tectonic interaction of the CIS and TSZ or multiple tectonic boundaries in the middle of the CIS and TSZ. The conductive structure observed in the southern limit of the CITZ also may indicate the tectonic boundary between the SIB and NIB. The high conductivity in the deformation zone may be explained by the collision-related metallic rich sediments and/or mylonite associated with interconnected fluids. Moderately conductive vertical features delineated from the MT model correlate with the intrabasinal faults which might have acted as the pathways for Deccan volcanism. This study suggests that the CITZ could have been developed under the transition of oceanic subduction to continental collision processes at multiple geological times

Role of fluid on seismicity of an intra-plate earthquake zone in Western India: an electrical fingerprint from magnetotelluric study

Abstract The magnetotelluric (MT) investigation carried out in Koyna Seismogenic Zone (KSZ), an intra-plate earthquake region in Western India, along an E–W profile brings out moderately conductive (~ 700–1000 Ωm) near vertical features within the very high resistive (> 20,000 Ωm) granite/granite-gneiss basement. Occurrences of these anomalous moderate conductors are corroborated with sensitivity analysis. The alignment of earthquake hypocenters along the resistive–conductive boundary signifies the moderate conductor as basement fault. The conversion of resistivity values to the ratio of seismic P- to S-wave velocity (v p/v s) suggests that the moderate conductivity of the fault zone (as compared to the surrounding basement) appears due to the presence of fluid in the fault zone. Geophysical evidences reveal ~ 2.5–3.6 vol% fluid in the fault zone with ~ 1.8–2.6% interconnected porosity, which migrates along the structural boundary and invades the mechanically strong basement to nucleate the brittle failure within it. The present study proposes two mechanisms for the seismicity in the Koyna region. First: the meteoric water circulation due to the loading–unloading of nearby Koyna reservoir acts as potential fluid source for this triggered seismicity, which has also been suggested by previous studies. Second: the fluid circulation due to a deep-seated source. The present MT study brings out a conductive feature below 20 km depth which is thought to be emerged due to the dehydration of amphibole bearing rocks. The fluid generated from dehydration might act as a probable source to the triggered seismicity; since the conductive feature has a linkage to the upper crust. Graphical Abstrac

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (similar to 200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB