Electrical and Airborne Geophysics

The electrical geophysical methods are used to determine the resistivity distribution of the subsurface can be interpreted in terms of soil characteristics and/or rock type and geological structure. The controlled source methods like Induced polarization (IP) and complex resistivity (CR) techniques, the CSMT method, Ground probing radar (GPR) and Electrical resistivity tomography (ERT) are most commonly used for shallow investigations, from characterizing surficial materials to investigating resistivity distribution down to depths as great as 1 to 2 km, although greater depths of investigation are possible with some techniques and under some conditions. Natural source methods take advantage of naturally occurring electrical potentials viz. the self-potential (SP) method examines the slowly varying surface potential field caused by electrochemical and electrokinetic actions in near-surface materials. The method has been applied successfully in geothermal and mineral exploration and in the delineation of certain groundwater contaminants.

A resistivity modeling lab was one of the early facilities established at the NGRI during 1965-66 leading to important basic contributions by way of new technique development such as new electrode arrays and their efficacy in the detection of deeper bodies, resolution and enhancement of response. Two-electrode array was a landmark finding in this direction. The present resistivity modeling facilities embrace most modern concepts including pseudo-section and imaging. The resistivity measurements based on different electrode arrays were interpreted using type curves prepared theoretically or using laboratory set-ups. A major theoretical difficulty in evaluating a complicated integral involving Bessel functions was very efficiently resolved using digital linear filtering method for a variety of layered earth models and electrode arrangements. Theory and type curves with inhomogeneous variation of resistivity with depth were developed, applicable for the Indian region where ore bodies are covered with weathered layers. Electrical logs with different electrode systems were developed theoretically and also by laboratory measurements. A new concept of volume contribution was introduced for comparing various electrode arrangements for deciphering their effectiveness in probing earth.  These studies earned wide recognition to the Institute worldwide.

A whole gamut of work is being carried out for groundwater exploration using these techniques. Groundwater investigations were carried out at two Shanta Biotechnic plant sites of the company at RR district, Telangana state namely Medchal and Muppireddipalli to create additional groundwater resources to meet the water supply demand and strategies for water conservation measures. The investigations such as hydrological monitoring, resistivity surveys, hydrogeological tests, tracer studies were carried out to assess recharge and groundwater potential zones. Mapping subsurface hard and soft rock with complex geological terrain for groundwater prospecting in Tadipatri Mandal, Anantapur District, Andhra Pradesh, using ERT studies has delineated the water bearing formation to depths of 230 m. Hydraulic linkage studies in deep aquifers hard rock formations is another thrust area, currently being investigated near Neredmet, Hyderabad. Assessment of Contaminant Migration from disposal of Hussainsagar Lake Bed Sediments in the proposed Granite Quarry Pits, Gajularamaram, RR District, Telangana  has been conducted using ERT technique, outlining an appropriate location for disposing the pollutants. CSIR-NGRI, has conducted several Ground Penetrating Radar (GPR) profiles and several Electrical Resistivity Tomography (ERT) survey for Assessment of origin of seepage in and around the M/s. Sri Chaitanya Chlorides Pvt. Ltd., IDA Pashamylaram, Patancheru (M), Medak District, Telangana. Sub-surface conditions in and around the Peenya Industrial Area (PIA), Bangalore were assessed through deployment of Electrical Resistivity Tomography (ERT) imaging system and analyses of water samples. From the results of water quality analyses, it is found that the groundwater is contaminated with Total Chromium and Hexavalent Chromium and other toxic elements. Vertical Electrical Soundings carried out at 20 locations in Giddalur and Cumbum mandals in Prakasam district indicate moderate to higher potential aquifer zones within the depth zone of 200 m in the shale-phyllite and shale-quartzite contact zone formations.

The group, recently, has developed core competency on heliborne hydrogeophysics using integrated approach of airborne, ground and borehole geophysics as well as hydrogeological studies for aquifer mapping and management that was carried out in six pilot area in India representing major hydrogeological settings such as desert, Middle Ganga Plains (MGP), alluvium covered hard rock, crystalline hard rock, flood basalt, coastal terrains. The group is currently involved in airborne electromagnetic (AEM) surveys for smart city establishment in Surat and paleo channel mapping in interfluve region of Ganga-Yamuna Doab at Allahabad.  The group has developed capability of handling large data set and its translation in hydro geophysical models. Moreover estimation of groundwater fluxes, aquifer parameter estimation and characterization such as natural recharge, hydraulic conductivity and transmissivity, pollution studies, natural treatment of wastewater using wetland have been developed viz., Lithologically Constrained Rainfall (LCR) methods for spatio-temporal distribution of natural recharge in hard rocks; Cokrigged estimation of aquifer transmissivity; hardrock hydraulic conductivity estimation from geoelectrical parameters; Decision Support Tool (DST) with variable agro-climatic scenarios for sustainable Groundwater Management in semi-arid hard rock areas, etc. The group has been carrying long term study on pilot project since 1991 in Maheshwaram watershed under Indo-French Centre for Groundwater Research to understand the complexity of hardrock hydrodynamic and established experimental hydrological park at NGR campus Choutuppal for controlled and new experiments. The development at the pilot studies are further up-scaled and applied at larger areas in similar geological settings. The group has contributed to the society under Rajiv Gandhi Drinking Water Mission where large number borehole sites were provided using geophysical techniques and drilled towards supplying of water for drinking and domestic needs.