Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

G. V. S. Poornachandra Rao ¹, S. B. Singh ¹, K. J. Prasanna Lakshmi ¹

Affiliations
1 Palaeomagnetism Laboratory, National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

The Jodhpur Sandstone (Marwar Supergroup) is investigated from three sites in western Rajasthan, India, using thermal and AF demagnetization techniques. The Characteristic Remanent Magnetic (ChRM) direction isolated is having a normal polarity of Dm=19.59°, Im=+46.27° (K=206.19, α₉₅= 5.63°, N=3) and yielded a Virtual Geomagnetic Pole (VGP) at λp=32.4° S, Lp = 51.3° E (δp=4.63°, δm=7.22°). The palaeomagnetic data of the Jodhpur Sandstone is in good agreement with that of the Upper Rewa Sandstone of the Vindhyan Supergroup rocks that are correlated with the Jodhpur Sandstone on lithological similarities. The Malani Rhyolite overlain by the Jodhpur Sandstone with well constrained radiometric and palaeomagnetic results thus assign a maximum age of < 745±10 Ma to the Jodhpur Sandstone and in turn to the Rewa Sandstone of the Vindhyan Supergroup which were correlated with them. This correlation helps in constraining the age of the Rewa Group rocks in the Upper Vindhyans, which are devoid of any fossil evidence and suitable radiometric material to date them.

Keywords

Palaeomagnetism, Neoproterozoic, AF Demagnetization, Rodinia, Jodhpur Sandstone, Marwar Supergroup, Rajasthan.

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U. K. Singh ¹, R. K. Tiwari ², S. B. Singh ², S. Rajan ¹

Affiliations
1 National Centre for Antarctic & Ocean Research, Vasco Goa - 403 804, IN
2 National Geophysical Research Institute, Hyderabad 500 007, IN

Abstract

The artificial neural network (ANN) technique is at present most efficient and modern tool for parameter estimation and inversion of geophysical data. This paper deals with the application of ANN technique for the inversion of vertical electrical resistivity sounding(VES) data obtained from the NNW SSE part of Barmer district, Rajasthan. The efficiency of ANN technique is tested first on synthetic resistivity data generated from the numerical model and then trained on the actual VES field data. The analyses predict sediment thickness of the order of 172 m at Rawtra (S 15, and indicate that there is possibility of fresh aquifers at all sounding locations along the profile except at Sonadi (S 1). These results match with the depth-Resistivity structure obtained by the conventional method. However, the high accuracy and faster ANN imaging system seems to have highly correlated with that of conventional method for mapping the complex subsurface resistivity structures with less ambiguity. These finding also correlate remarkably well with known drilling results and geologic boundaries.

Keywords

Artificial Neural Network, Backpropagation Algorithm, VES Data, Resistivity Layer Parameters, Fresh Aquifer, Saline Aquifer, Barmer District, Rajasthan.

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Shashank Shekhar ¹, S. B. Singh ¹, Saleem Romani ¹

Affiliations
1 Central Ground Water Board, 18/11 Jamnagar House, Mansingh Road, New Delhi-11, IN

Abstract

The hydrogeology of the southwest district of National Capital Territory (NCT) Delhi is challenging on the account of the fact that the quality of groundwater in the district shows horizontal and vertical variation with respect to salinity. The present paper tries to study the variation in the depth to fresh (electrical conductivity equal to or below 1500-2000 micro siemens per cm)/saline (electrical conductivity above 1500-2000 micro siemens per cm) water interface of the district and locate the factors controlling the variation in the depth to fresidsaline interface in the groundwater of the district. The map showing variation in the depth to fresh/saline interface In the groundwater of the district was studied Vis-a-Vis geology and cultural practices In the district, in order to identify the factors controlling the variation in the depth to fresh/saline water interface in the groundwater of the district.

Keywords

Hydrogeology, Groundwater, Salinity Zone, Delhi Area.

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S. B. Singh ¹, Jimmy Stephen ¹, Y. Srinivas ¹, U. K. Singh ¹, K. P. Singh ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Delineation of groundwater resources in hard rock terrains is one of the important topics to be treated with a more advanced approach than simple one-dimensional attitude, particularly when looking for deeper water saturated horizons. The present study introduces an integrated geophysical approach to identify structural features such as fractures, weak zones and intrusive dyke bodies, which can control and host potable groundwater at greater depths. The study is conducted in an area of about 8 km² along east coast of Tamil Nadu, which has been identified as a major rainshadow zone. Geophysical measurements were carried out in very closely spaced grids making use of resistivity, magnetic and electromagnetic techniques. The sensitivity of each technique to different rock properties in varying directions has been considered to resolve the problem. A major weak zone running in SW-NE direction is demarcated which is characterised by many intrusive dyke bodies. The conductivity distributions at different depths throw light on many structural features. The varying thickness of weathered rock calculated from the Horizontal Loop Electromagnetic (HLEM) and Deep Resistivity Sounding (DRS) studies for the entire area reflects the extension of weathering associated with dyke emplacements. Geo-electrical sections obtained through DRS studies, concentrated mainly along the weak zone have shown the occurrence of potable groundwater resource controlled by structural features.

Keywords

Integrated geophysics, Groundwater, Hardrock Terrain, Deep Resistivity sounding, Horizontal Loop Electromagnetics (HLEM), Magnetic Method, Tamil Nadu.

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B. S. Paliwal ¹, G. V. S. Poornachandra Rao ², S. B. Singh ², K. J . Prasanna Lakshmi ²

Affiliations
1 Department of Geology, Jai Narain Vyas University, Jodhpur - 342 005, IN
2 NGRI, Hyderabad, IN

Abstract

No Abstract.

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S. B. Singh ¹, G. Ashok Babu ¹, B. Veeraiah ¹, O. P. Pandey ¹

Affiliations
1 National Geophysical Research Institute, (Council of Scientific & Industrial Research), Uppal Road, Hyderabad - 500 007, IN

Abstract

Hyderabad granitic region (HGR) forms one of the most unusual geotectonic segment of the south Indian shield. Analysis of multiparametric geological, geophysical and IGS-GPS studies have earlier suggested that this region is neotectonically uplifting at a rapid rate. We propose that consequent to such uplift, only a thin veneer of surface granitic layer now remains. In order to quantitatively examine the thickness of highly resistive granitic-gneissic crust below HGR, a audio-magnetotelluric (AMT)/controlled source AMT (CSAMT) experiment was conducted at three separate locations, situated about 50 km east of Hyderabad. The study reveals a maximum thickness of 5.5 to 6.0 km for the granitic - gneissic crust beneath HGR, which is underlain by thick low resistive exhumed intermediate (granulitic ?) crust. This finding is in sharp contrast to that of a thick granitic-gneissic crust (15-20 km) usually found in comparable late Archaean terrains elsewhere.

Keywords

Hyderabad Granitic Region, Audio-Magnetotellurics, Uplift, Resistive Crust, Granitic-Gneiss, Granulites, Crustal Velocity, Dharwar Craton.

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