• Dewashish Kumar

      Articles written in Journal of Earth System Science

    • Hydrogeological and geophysical study for deeper groundwater resource in quartzitic hard rock ridge region from 2D resistivity data

      Dewashish Kumar V Ananda Rao V S Sarma

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      Electrical resistivity method is a versatile and economical technique for groundwater prospecting in different geological settings due to wide spectrum of resistivity compared to other geophysical parameters. Exploration and exploitation of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. In the present study, two-dimensional Electrical Resistivity Tomography (2D-ERT) technique using two different arrays, viz., pole–dipole and pole–pole, were deployed to look into high signal strength data in a tectonically disturbed hard rock ridge region for groundwater. Four selected sites were investigated. 2D subsurface resistivity tomography data were collected using Syscal Pro Switch-10 channel system and covered a 2 km long profile in a tough terrain. The hydrogeological interpretation based on resistivity models reveal the water horizons trap within the clayey sand and weathered/fractured quartzite formations. Aquifer resistivity lies between ∼3–35 and 100–200 𝛺 m. The results of the resistivity models decipher potential aquifer lying between 40 and 88 m depth, nevertheless, it corroborates with the static water level measurements in the area of study. The advantage of using pole–pole in conjunction with the pole–dipole array is well appreciated and proved worth which gives clear insight of the aquifer extent, variability and their dimension from shallow to deeper strata from the hydrogeological perspective in the present geological context.

    • Integration of geophysics and petrography for identifying the aquifer and the rock type: A case study from Giddalur, Andhra Pradesh, India


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      A comprehensive geophysical and petrological study was carried out at Giddalur area in Prakasam district, Andhra Pradesh, which is geologically a highly deformed area and is difficult to delineate the aquifer zone(s). The task was to find out the exact rock type in which aquifer is concealed as well as to delineate the aquifer zone, which can yield sufficient quantity of water. The resistivity models derived from geophysical dataset were interpreted in terms of hydrogeology and the results revealed substantial resistivity contrast of the geological formations within the study area. We have delineated two major groundwater potential zones based on this study. These zones were tapped at different depths in diverse rock types. Drilled hand specimens (rock cuttings) were not adequate, so these specimens were petrographically studied to reveal the exact contact zones of the rock type. On integration of the geophysical and the petrographic results, it was illustrated that two aquifer zones were struck at a depth of 92 and 122 m between shale-phyllite and phyllite-quartzite, respectively. These findings were correlated, which matched with the lithology of the drilled borehole. This integrated approach will be helpful in strategy for groundwater assessment as well as prospecting groundwater resources in different geological terrain.

    • Deep insight to the complex aquifer and its characteristics from high resolution electrical resistivity tomography and borehole studies for groundwater exploration and development


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      Discovering and locating the source and availability of groundwater in a plateau region of Chhotanagpur gneissic complex, where there is a varied hydrogeological characteristics, is a crucial task for earth scientists. One such region located at Garh Khatanga near Ranchi, Jharkhand, India was closely studied for groundwater assessment and exploration. High resolution electrical resistivity tomography 2D data were acquired to probe deep inside the earth up to a maximum depth of 220 m using state-of-the-art electrical resistivity tomography technique and mapped geoelectrical subsurface images at 16 sites in three different blocks along a 7.2 km line for prospecting and exploration of groundwater resources. The geophysical inversion of the 2D resistivity data revealed prospect groundwater scenario at six sites based on the hydrogeological interpretation and the significant resistivity contrast between the highly weathered/fractured and the massive rocks. The modelled resistivity sections revealed different degree of weathered, fractured and saturated weathered/fractured strata as well as clearly indicated the presence of a totally hard massive rock within the subsurface lying between $\sim$30 and 220 m depths. The geophysical anomalies were confirmed and validated by borehole drilling at four sites up to a maximum depth of 215 m with yields ranging from 2.0 to 4.25 inch, which is equivalent to 5632–63769 l/hr of groundwaterexploitation. These yields of groundwater resources are rated as good aquifer(s) in the plateau region of Chhotanagpur gneissic complex. The characteristics resistivity for fracture zone varies from 140 to 1300 $\Omega$m, while for saturated weathered/fractured it ranges from 10 to 1000 $\Omega$m. On joint interpretation of the 2D resistivity models and the borehole lithology data, it clearly shows the average resistivity of the aquifer zone lies in the range 50–500 $\Omega$m. The present study along with the conceptual geological models provided a sound knowledge of hard rock hydrogeology in the plateau region with complex geological settings and these helped to achieve significant results for groundwater exploration and development of the resources of the studied area as well as take up such challenging work in exploring the prospect groundwater resources in other similar geological setting of the country.

    • Groundwater exploration in limestone–shale–quartzite terrain through 2D electrical resistivity tomography in Tadipatri, Anantapur district, Andhra Pradesh


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      Two-Dimensional (2D) Electrical Resistivity Tomography (ERT) survey was carried out at 11 sites within an area of $10 \rm{km}^{2}$ to delineate deeper potential groundwater zones in a complex geological terrain underlain by quartzite, shale and limestone formations with varied resistivity characteristics. The area is in medium rainfall zone in Tadipatri mandal of Anantapur district, Andhra Pradesh state, India. The investigation was carried out to meet the growing demands of water supply. Interpretation of the highdensity 2D resistivity dataset results revealed potential zones at only three sites in Tummalapenta, Ayyavaripalle and Guruvanipalle villages within the depth zone of 24–124 m. A major fault zone orientedin EW direction is mapped at Tummalapenta site. Based on high resolution geophysical data interpretation and significant anomalies, four boreholes were drilled in complex, viz., limestone, shale and quartzite formations up to a maximum depth of 192 m in the area with the yield ranging from $300$ to $\sim 5000$ liter per hour (lph). These four anomalous drilled borehole sites corroborates with the aquifer zone delineated through ERT technique. The aquifer parameters estimated from pumping tests show that the transmissivity varies between $\sim 0.3$ and $\rm{179.5 m^{2}/day}$ while the storage coefficient ranges from 0.137 to 0.5 indicating large variation in aquifer characteristics of the system in a smaller area. Suitable water conservation measures were suggested for improving the groundwater condition and yield of the pumping wells.

    • Application of multi-criteria decision making (MCDM) and electrical resistivity tomography (ERT) techniques for identification of groundwater recharge zone(s) in granitic hard rock aquifer


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      Groundwater recharge is a hydrological process where water flows from sub-surface layers to the water table of the aquifer and is the backbone of the hydrogeological system. The present study is carried out in a granitic hard rock aquifer region within and surroundings of the CSIR-NGRI campus, Hyderabad. The aim of this study to identify the potential groundwater recharge zone(s) using GIS based multi-criteria decision making (MCDM) along with sub-surface mapping from Electrical Resistivity Tomography (ERT) technique. The assigned weight of the different thematic layers of surface and sub-surface parameters and their specific characteristics was determined based on their relative contribution to the groundwater recharge and thus the normalized weight was computed using MCDM technique. These thematic layers were integrated with the help of ArcGIS to accurately identify the recharge zones within the study region. The resulting recharge map has been categorized into five classes viz., very poor, poor, moderate, good and very good. Numerically 23.11% of the study area is in a moderate zone of recharge, 4.97% in good and very good zone, while 71.92% falls under the poor and very poor zone, i.e., unsuitable for groundwater recharge. The recharge zone map of the study area is found to be in agreement with 2D inverted resistivity models for two different time periods, which revealed distinct geological features and thus identified the near surface recharge property, where recharge zone resistivity values lie between ${\sim}$20 and 70 $\Omega$.m up to 11 m depth. Thus, the integrated results from the present study delineated groundwater recharge zone(s) for sustainable groundwater resources in the granitic hard rock system.

    • Electrical resistivity tomography of Mesoarchaean chromitite bands from Katpal in Sukinda Ultramafic Complex, Odisha


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      Katpal in Mesoarchaean Sukinda Ultramafic Complex (SUC) hosts chromitites within the ultramafic rocks emplaced in the Tomka–Daitari–Mahagiri greenstone belt, Singhbhum Craton. Chromite deposits occur as seams, lenses or pockets in ultramafic–mafic rocks mainly comprising serpentinised dunite, peridotite and gabbro. Chromite deposits at Kalrangi, Kaliapani and its precincts in SUC are currently being exploited by M/s. Odisha Mining Corporation (OMC), whereas their continuity is elusive in the subsurface in the Katpal area, Sukinda, Odisha. High-resolution electrical resistivity tomography (ERT) was carried out in quarry-G of M/s. OMC in Katpal on four profiles each with a profile length 400 m and each of them is separated at an interval of ${\sim}$100 m to capture and visualise a maximum anomaly variation within the host rock. The objectives of this study are to understand the behaviour of chromite seams vis-à-vis with a high-resolution electric potential signal with depths and to evolve a method to discriminate chromitites from the host rock both in horizontal and vertical directions with depths, which can be used to locate the extension of chromite seams around the study area. The interpretation of the ERT models revealed interesting resistivity anomalies, which is inferred as chromite associated with the ultramafic host rock in the area. Exploratory borehole data of the area confirmed the subsurface occurrence of chromitites within 22–40 m depths as lensoidal bodies within the mafic–ultramafic rocks. The chromite ore is mapped as low-resistive zones with a resistivity of ${\sim}$35–200 ${\Omega}$ m and extended up to a depth of 170 m within the high-resistive (${\ge}$500 ${\Omega}$ m) host rocks. Two distinct chromite bands were identified at different depths: 20–40 and 90–110 m based on the specific resistivity contrast and were validated using the existing shallow borehole data. The second band is highly folded whereas the first band resembles sill-like feature within the host rock. The electrical tomography technique can be aptly applied in an unknown area in SUC to establish the extension of the chromite ore body, its resistive signature and their variation with depths within the host rock for future resource prospecting and exploitation in the given geological setting of the area.


      $\bullet$ The chromite ore is mapped as a low-resistivity signature and its extension is delineated as a low-resistivity anomaly of ~35–200 Ω m.

      $\bullet$ The electrical resistivity models indicate low resistivity as chromite and high-resistivity as ultramafic rock (~500–10,000 Ω m) up to a depth of 170 m.

      $\bullet$ The light green up to yellow bands in the inverted resistivity models characterise various grades of mineralisation within the host rock setting.

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