Mass distribution on Earth is continuously changing due to various physical processes beneath the Earth's surface or on the surface. Some of the primary sources for these mass displacements are tidal forces, atmospheric and oceanic loading, and seasonal changes in continental water distribution. The development of relative cryogenic gravimeters, the Superconducting Gravimeters (SGs), has made it possible to characterize and monitor such mass variations at orders of magnitudes as small as a few nm/s2 (1 nm/s2–10–10 g where g is the mean gravity at the Earth's surface). Our study focuses on the hydrodynamics of the 900 m thick unsaturated zone of the low-noise underground research laboratory (Laboratoire Souterrain à Bas Bruit, LSBB) located in Rustrel (France) using a unique configuration of two SGs vertically arranged 520 m depth apart. The installation of an SG (iGrav31) at the site surface several years after installing the first (iOSG24) inside a tunnel has provided several new insights into the understanding of the hydrological processes occurring in the LSBB. By comparing differential and residual gravity time-series together with global hydrological loading models, we find that most water-storage changes occur in the unsaturated zone between both SGs. The misfit between the observed gravity time-series and the gravity effect corresponding to local hydrological contribution calculated from global hydrological models can be explained by large lateral fluxes and rapid runoff occurring in the LSBB site. Finally, we implement a rectangular prism method to compute forward gravity responses to water storage changes for a homogeneous water-layer following the site topography using a 5-m digital elevation model. In particular, we analyse the sensitivity of the differential record from both SGs to the extent and depth of the water storage changes by computing the corresponding 2D admittances. This gravity difference is sensitive to an extension up to about 2500 m laterally before tending towards an asymptotic value corresponding to the Bouguer plate approximation. We show that the zone of water-storage changes that best fits observed differential gravity signal is located at depths larger than 500 m (below iOSG24). This fitting is improving when the integration radius increases with depth. This is the first time that hydrological processes are investigated when the baseline configuration of two SGs is vertical.

This paper reviews the Magnetotelluric (MT) studies carried out in India during last five years. These MT field studies covered area, which includes Garhwal Himalaya, Sikkim Himalaya; Indo-Gangetic Plain (IGP); Gujrat, Dharwar craton (DC), Eastern Ghat Mobile Belt (EGMB), North Singhbhum Mobile Belt (NSMB), Dalma basalts, Dharwar region, Central Indian Shear (CIS) zone, Narmada-Son Lineament (NSL), Cuddapah basin, Banaganapalli quartzites, Narji limestone, Koyna-Warna and 3D MT inversion software- AP3DMT. The field investigations aimed to delineate deep crustal structure, the basement depth geometry, resistivity structure in the Bhuj earthquake region, electrical resistivity characterization of Fault zones, mapping of geothermal source zone, lithosphere architecture. Software, AP3DMT-MATLAB code for 3D inversion of MT data was developed and tested. This code is made versatile and user friendly by an efficient use of MATLAB's inbuilt functions.

ABSTRACT The impact of untreated sewage irrigation and waste disposal practices on groundwater is investigated by 3D joint inversion of radio magnetotelluric and electrical resistivity tomography data. In this case study, electrical resistivity tomography and radio magnetotelluric field measurements were carried out on several profiles near a waste disposal site which was irrigated with untreated sewage water for agriculture purpose. In addition, radio magnetotelluric and electrical resistivity tomography measurements were carried out, far away from the waste site, to derive the uncontaminated geology. The data were analysed earlier using 2D inversion techniques. However, for the 2D inversion of the electrical resistivity tomography and radio magnetotelluric data, assumptions about the strike direction are required. As no clear strike direction is evident for the contamination, we considered the problem as 3D and interpreted the present data set using the 3D inversion algorithm ‘AP3DMT‐DC’. The inverted 3D resistivity model shows an unconfined aquifer of low resistivity which is overlain by an unsaturated slightly resistive near surface formation. With increasing distance from the waste sites, an increase in the resistivity of the shallow unconfined aquifer is observed. Furthest away from the waste site undisturbed geology is expected. We derived consistent and meaningful 3D resistivity models. The uncontaminated reference site indicates an increased resistivity for the aquifer layer. A synthetic 3D study was carried out to demonstrate and validate algorithm performance as well as convergence capabilities. The study demonstrates that the two methods, electrical resistivity tomography and radio magnetotelluric, complement each other. Besides, a better resolved inverted model is obtained through a 3D joint inversion, in comparison to individual 2D and 3D inversions.

The effect of the dipping and the finiteness of a 2D prismatic body were investigated in Electrical Resistivity Tomography (ERT) measurements. For this purpose a free-air analogue modeling laboratory was designed. This enabled to control the parameters of the body and carry out measurements in field conditions as this is the most reliable method to verify the field applicability of an array. The imaging capacity of the best four-electrode traditional array, the Dipole-dipole (Dp) was compared to that of special configurations, three members of the series of the γ11n quasi-null arrays. This is the first attempt to study in field circumstances the imaging capacity of quasi-null arrays whose theoretical background is very different from that of the traditional arrays. In the lack of available processing codes that can invert the field data of these arrays a new routine, the Res2D-Hu code was used. Dp array proved to be robust, insensitive to both dipping and finiteness of the body, and very good in imaging shallow objects. The results of the γ11n arrays are less certain, but they may detect deeper bodies and resolve the model also vertically. Due to the complementary nature of the Dp and γ11n arrays, the joint interpretation of their images is recommended. In the present study - with a minimally increased time investment in relation to the individual Dp measurement - the joint interpretation enabled the following aspects: (1) separate clearly the real anomaly from possibly artificial ones; (2) make clear that the investigated model is not exactly 2D, it ends close to the first profile; (3) delineate the model better both horizontally and vertically and (4) detect the body from a larger depth than it would have been possible by the application of only the traditional array. In order to make the above points explicit, investigations had to be done besides the traditional Dp array by the γ116.

24 th EM Induction Workshop, Helsingør, Denmark, August 12-19, 2018 1 /4 MATLAB based code for 3D joint inversion of Magnetotelluric and Direct Current resistivity imaging data M. Israil 1 , A. Singh 1 Anita Devi 1 , and Pravin K.Gupta 1 1 Indian Institute of Technology, Roorkee, 247667, India, mohammad.israil@gmail.com

Geoelectrical characterization of subsurface soil has been done at a bridge foundation site on the banks of Bhagirathi River at Tehri reservoir site, Uttarakhand, India. For this purpose, the Electrical Resistivity Tomography (ERT) and the Standard Penetration Test (SPT) data, recorded at both banks of Bhagirathi River are analyzed. A total of six ERT profiles, recorded on both the West and East banks, were interpreted to determine an electrical resistivity image showing the resistivity variations with depth. The borehole data and geological inputs were used for lithological correlation and calibration of the resistivity values to the subsurface formation. Subsequently the electrical parameter (resistivity) for different subsurface lithological units has been inferred. Further, at selected points, the electrical resistivity sounding data, derived from the ERT, have been correlated with the Standard Penetration Test (SPT) data. This correlation results from the fact that in the subsurface soil both the electrical resistivity variations and the soil strength measured by SPT are controlled by the soil properties: grain size distribution, compactness, porosity and water saturation. It has been observed that the N-values smaller than 16 are unreliable and inconsistent. In the River Borne Material (RBM) on the West Bank it is due to the presence of coarse gravels while on the East Bank it is due to the boulders. The N-values greater than 16 mainly correspond to the weathered rock formation. For these values, there exists a linear relationship between N-values and resistivity with a correlation coefficient greater than 0.80. The coefficients of linear relationship at the two banks vary due to varying amount of clay content. Such a relationship is important for any site in tough Himalayan terrain because it can be used as an alternative to the SPT for determining soil strength parameters from ERT.

The development of a MATLAB based computer code, AP3DMT, for modeling and inversion of 3D Magnetotelluric (MT) data is presented. The code comprises two independent components: grid generator code and modeling/inversion code. The grid generator code performs model discretization and acts as an interface by generating various I/O files. The inversion code performs core computations in modular form – forward modeling, data functionals, sensitivity computations and regularization. These modules can be readily extended to other similar inverse problems like Controlled-Source EM (CSEM). The modular structure of the code provides a framework useful for implementation of new applications and inversion algorithms. The use of MATLAB and its libraries makes it more compact and user friendly. The code has been validated on several published models. To demonstrate its versatility and capabilities the results of inversion for two complex models are presented.

We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian–Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.

During the period 2011-2015, scientific investigations in the Himalaya incorporated various geological and geophysical aspects of evolution of this mountain, including sub-surface configuration, structure and metamorphism of the Lesser Himalaya and Himalayan Metamorphic Belt, geochemistry, magmatism, stratigraphy and paleontology of the Paleo-Mesozoic Tethyan sedimentary cover, the Himalayan foreland basins, exhumation, paleoseismology and GPS measurements of convergence rates.Certain remote areas of western Arunachal Pradesh in Kameng were covered for their metamorphism and exhumation.Sm-Nd isochron plot of garnet crystals from the Lesser Himalayan Jutogh Group metamorphics provided a mean regression age of 479.7±8.5 Ma as the timing of its crystallization during an Early Ordovician tectonometamorphic event.High-resolution work on metamorphism of the Lesser and Higher Himalayan belts of Sikkim incorporating P-T-t paths and geochronology of the imbricate zones of the Main Central Thrust provided better insight into their evolution.

We analyse magnetotelluric data recorded in the Garhwal Himalaya corridor, India, to study dimensionality and strike direction of the geoelectric structure. The geoelectric structure dimensionality in the corridor shows spatial variation with period. Geoelectric strike directions of the main Himalayan thrusts associated with the main arc - Main Frontal Thrust, Main Boundary Thrust and Main Central Thrust - are N72 degrees W +/- 8 degrees, N70 degrees W +/- 3 degrees and N71 degrees W +/- 6 degrees. respectively. Cross-strike directions oriented transverse and oblique with respect to the main arc geometry are also delineated. Transverse feature is constrained using 3D modelling.

We present the features and results of a newly developed code, based on Gauss-Newton optimization technique, for solving three-dimensional Controlled-Source Electromagnetic inverse problem. In this code a special emphasis has been put on representing the operations by block matrices for conjugate gradient iteration. We show how in the computation of Jacobian, the matrix formed by differentiation of system matrix can be made independent of frequency to optimize the operations at conjugate gradient step. The coarse level parallel computing, using OpenMP framework, is used primarily due to its simplicity in implementation and accessibility of shared memory multi-core computing machine to almost anyone. We demonstrate how the coarseness of modeling grid in comparison to source (comp`utational receivers) spacing can be exploited for efficient computing, without compromising the quality of the inverted model, by reducing the number of adjoint calls. It is also demonstrated that the adjoint field can even be computed on a grid coarser than the modeling grid without affecting the inversion outcome. These observations were reconfirmed using an experiment design where the deviation of source from straight tow line is considered. Finally, a real field data inversion experiment is presented to demonstrate robustness of the code.

Geoelectric strike and resistivity structure of the crust have been estimated from 37 magnetotelluric (MT) data sites along a profile from Roorkee to Gangotri in Uttarakhand Himalaya. Impedance decomposition schemes based on Bahr's, Groom Bailey and Phase tensor were implemented in a MATLAB code for the average strike estimation. Geoelectric strike direction varies with period as well as in different litho-tectonic units along the profile. In the period band from 1 to 100 s average geoelectric strike in the southern end of the profile (Indo-Gangetic Plains) is N79°W, which is slightly rotated to the north in the Lesser Himalayan region and becomes N68°W whereas it is N81°W in the Higher Himalayan region. However, average strike is stabilized to N77°W for the entire profile in the long period band (100–1000 s). Geoelectrical structure of the crust has been obtained along the profile by 2D inversion of MT data. Major features of 2D resistivity model are: (i) southern part of the model is a low resistivity (<50 Om) zone at shallow depth (5–7 km) representing the loose sediments of the Indo-Gangetic Plains (IGP), whose thickness increases in the south; (ii) highly resistive (>1000 Om) layer below the IGP sediments is the basement rock, representing the resistivity of the top of the subducting Indian Plate; (iii) the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT) zones can be seen in the electrical image. However, the Himalayan Frontal Thrust (HFT) could not be resolved and (iv) a low resistivity (<10 Om) feature in the MCT zone extending to the depth of 30 km is delineated. This low resistivity could be due to fluid-filled fractured rock matrix or partial melt zone. Hypocenters of many earthquakes are concentrated along the boundary of this low resistivity zone and relatively high resistivity blocks around it. The resulted model supports flat-ramp-flat geometry of the Main Himalayan Thrust along which the Indian Plate is subducting.

The study area is a sewage farm in Saliyar in the state of Uttarakhand, India, which is irrigated using untreated sewage from the nearby city of Roorkee. Previous studies showed that due to sewage irrigation the first unconfined aquifer at a depth of 5–15 m is contaminated. The aim of this study is to characterize the deeper aquifers to a depth of 100 m, underlying the first unconfined aquifer. Therefore 46 in‐loop TEM measurements were carried out in the farming area and an uncontami‐nated reference site with a transmitter loop of and receiver loops of and . To benefit from the geologically expected one‐dimensional subsurface we applied the spatially constrained inversion technique on the data resulting in a geologically reasonable model and allowing the identification of a conducting layer of 28 Ωm as the second aquifer. The resolution of model parameters was computed using singular value decomposition. The low resistivity of the underlying aquifer, however, is not locally restricted to the sewage farm but is also visible in the reference area; it is therefore unlikely caused by irrigation with untreated sewage.

The impact of sewage irrigation and groundwater contamination were investigated near Roorkee in north India using the Direct Current Resistivity (DCR) method and the Radiomagnetotelluric (RMT) method. Intensive field measurements were carried out in the vicinity of a waste disposal site, which was extensively irrigated with sewage water. For comparison a profile was investigated on a reference site, where no contamination was expected. In addition to conventional 1D and 2D inversion, the measured data sets were interpreted using a 2D joint inversion algorithm.The inversion results from the data obtained from the sewage irrigated site indicate a decrease of resistivity up to 75% in comparison with the reference site. The depth range from 5 to 15 m is identified as a shallow unconfined aquifer and the decreased resistivities are ascribed as the influence of contamination. Furthermore, a systematic increase in the resistivities of the shallow unconfined aquifer is detected as we move away from the waste disposal site.The advantages of both, the DCR and RMT methods, are quantitatively integrated by the 2D joint inversion of both data sets and lead to a joint model, which explains both data sets. (C) 2011 Elsevier B.V. All rights reserved.

Broadband magnetotelluric (MT) soundings have been applied to determine the deep electrical conductivity structure across Garhwal Himalaya corridor along the Deoband-Gangotri profile passing through major Himalayan thrusts: Himalayan Frontal Thrust (HFT), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The average Geoelectric Strike estimated for the 15 station MT data of Deoband-Gangotri profile along NE-SW was N75°W, and after that 2D smooth inversion was carried out for 15 station MT data. Where the near surface conductive feature in foothills and Siwalik Himalaya is relating to the molassic sediments transported from Higher Himalayan region. The strong lateral discontinuities along the profile are associated with the various thrust zones. The conducting zone near MCT is a typical example of presence of mid crustal conductor. Along this profile the mid crustal conducting zone near MCT coincides with the intense microseismic activity zone (Khattri, 1992). Then we have correlated this model with the geoelectrical model of RoorkeeGangotri profile of Himalaya Region (Tyagi et al, 2007) and also with geoelectrical model along the central Nepal – Himalaya profile (Lemonier et al., 1999).

We present a fresh approach to the mathematical computation of apparent resistivities in electrical prospecting. The method is based on an exponential approximation of the kernel function which reduces the integral equation for the potential over a layered earth to a simple algebraic equation. The coefficients in the approximation are obtained using a least‐squares inversion technique. A single, unified matrix equation allows computation of apparent resistivity values for arbitrary four‐electrode arrays over a layered earth. The unified G function automatically reduces to that of a symmetrical four‐electrode array and dipole array function with the proper interelectrode separation. Computations for some two‐, three‐, and four‐layer earth models (Schlumberger configuration), along with a few Wenner and radial dipole apparent resistivity values, demonstrate the versatility of this unified equation.