Joint geomorphological and geophysical (electrical resistivity) investigation for the configuration of soil pipe

Soil piping is a complex mechanism of subsurface soil erosion, which results underground conduits (cave/tunnel) of varying dimensions. Soil piping associates with severe consequences, such as land subsidence and land slide. Therefore, the investigation of soil pipe is crucial. However, the study of soil pipe is challenging unless characteristic surficial evidences of the pipe are available. Based on the surficial evidences, soil pipe can be configured with geophysical techniques which in-turn aid in designing precursory measures. Therefore, in the present study, we carried out a combined geomorphological and geophysical investigation to configure the soil pipe at Kinanoor village, Kasaragod, Kerala, India. Based on the vital geomorphological information, we carried out resistivity survey and configured an underground soil pipe of diameter ∼6.5 to 7 m that is seated ∼3 m beneath the surface. This hollow pipe is underlain by the only accessible road of that locality which makes the road vulnerable for transportation. Therefore, a bridge like structure is recommended to construct at the pipe location to stabilize the risk factor. Since the study area is situated on a fringe-slope, the geomorphological investigation points out that the disturbance in natural course of the drainage system and the accumulation of water in the up-slope area due to the man-made activities might act as potential causes for the piping in the area. Therefore, it is suggested not to disturb the natural course of the drainage which may lead to subsidence of the area in future

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

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