The interpretation of ERT data requires inversion, which is challenging due to its nonlinear and ... more The interpretation of ERT data requires inversion, which is challenging due to its nonlinear and illposed nature. We aim to solve the inverse problem using a machine learning technique that approximates the complex non-linearity using the data provided without specifying any geophysical or mathematical equations. Speci¦cally, we make use of CNN with U-Net architecture to estimate the true subsurface resistivity model from observed apparent resistivity data (pseudosection). U-Net is selected because it works well when both input and output data are in the form of images and in the present work, input (pseudosection) and output (subsurface resistivity model) have spatial correspondence and existing local patterns. For ERT inversion, U-Net learns by minimizing pixel mean squared error between the true and predicted subsurface resistivity images. The ERT dataset was generated by simulating a dipoledipole survey over a 400m x 100m earth containing two anomalous blocks with varying resi...
Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution elect... more Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution electrical resistivity tomography (HERT) have been jointly carried out on an experimental basis in the field. The motive is to study shallow subsurface features (i.e. faults traces, cavities and palaeo-channels) in the foothill zone of Northwest Himalaya. These techniques have shown their potentiality in successfully identifying shallow (0-24 m) fault traces and dissolution features/palaeo-channels. Depending on the sensitivity of the MASW and HERT techniques, geophysical signatures of the subsurface features were recorded and further resolved with the help of synthetic simulation. The synthetic simulation of 2D electrical response has been carried out over the initial model for subsurface fault traces as well as palaeo-channels. The initial model has been refined iteratively to bring the synthetic response close to the field response and hence the final refined model is considered to be the ...
We have investigated a network of fractures forming the flow paths within the crystalline graniti... more We have investigated a network of fractures forming the flow paths within the crystalline granitic rocks of an Experimental Hydrogeological Park (EHP) with the help of electrical resistivity surveys. The experimental study located at the managed aquifer recharge (MAR) site of EHP has measured a distinct variation in the apparent resistivity for deeper electrical signals that localize the presence of interconnected water-saturated fractures. Usually, profiles close to the MAR tank depict low apparent resistivity values from deep signals across in situ fractures and resistivity amplitude increases away from the tank. We modeled and simulated the presence of water-saturated fractures by a simple three-layered model having embedded shallow heterogeneities in the saprolite layer, vertically interconnected multiple thin conductive horizontal layers in the fissured zone, and an underlain unweathered crystalline granitic basement. These fractures produce a distinct variation in the resistiv...
... Aadil Nabi Bhat 179 15. Reconstruction of Water Level Time Series in an Aquifer Using Geostat... more ... Aadil Nabi Bhat 179 15. Reconstruction of Water Level Time Series in an Aquifer Using Geostatistical Technique Dewashish Kumar and Shakeel Ahmed 191 16. Governing Equations of Groundwater Flow and Aquifer Modelling Using Finite Difference Method Shazrah Owais, S ...
... Further approaches were developed on the basis of relationships between geophysical and hydra... more ... Further approaches were developed on the basis of relationships between geophysical and hydraulic properties (eg [Jones and Buford, 1951] , [Kelley, 1977] , [Urish, 1981] , [Frohlich et al., 1996] , Chandra et al., 2008 and [Vouillamoz et al., 2005] ), which have led to the ...
With the growing dependence on groundwater, escalating demand, and increasing depletion of aquife... more With the growing dependence on groundwater, escalating demand, and increasing depletion of aquifers, the Government of India has accorded special emphasis to management of groundwater resources through precise aquifer mapping. We have carried out hydrogeophysical surveys, including airborne electromagnetic (AEM), in six hydrogeologically divergent areas comprising sedimentary basins, basalts, weathered and fractured granite gneisses and schists, desert, and costal alluvium with the objective to establish suitability of AEM for a countrywide aquifer mapping in India. Efficacy of the dual-moment AEM surveys in mapping the shallow and deep aquifers is evaluated in conjunction with geologic, geophysical, and borehole data. It is found that the AEM surveys provide reliable images of the subsurface resistivity distribution defining the 3D geometrical and electrical attributes of aquifers in different areas. The surveys helped identify the suitable zones for managed artificial recharge (MA...
Knowledge of the spatial distribution of the hydrogeological parameters such as hydraulic conduct... more Knowledge of the spatial distribution of the hydrogeological parameters such as hydraulic conductivity (K) and transmissivity (T) are essential for management and development of groundwater resources. In general, the hydrogeological parameters are estimated by the classical techniques like pumping tests, which are uneconomical and time consuming. This technique has strong application in alluvial region, a well known formation with homogeneous and stratified layers set up. However, in crystalline rocks such as granites, gneisses and schists, the groundwater reservoir is mainly formed by the zone of weathering and to a lesser extent by the fractures and fissures in the fresh rock, that have high spatial variability. Therefore, in hard rock terrain a dense measurement is required for reliable mapping of spatial distribution of aquifer parameters. Geophysics can play a major role in defining these parameters. The work carried out here deals with a mathematical tool developed to estimate...
An integrated hydrogeophysical studies in the arsenic affected region of Balia-Patna sector in Mi... more An integrated hydrogeophysical studies in the arsenic affected region of Balia-Patna sector in Middle Ganga Plain of the states of Uttar Pradesh and Bihar in India led to an important finding that the lithological set up plays a major role in understanding the arsenic contamination in groundwater. Although alluvial deposits are known for their hydrogeological homogeneity and stratification, the heterogeneities found in stratification close to the river and are the probable reason of arsenic contaminations in patches. The thickness of clay pockets varies pinching out at some places. The high resolution of electrical resistivity tomography has provided subsurface lithological discontinuities from the surface to even deeper level (say 90 m). The discontinuity of the layer may create leakage from upper aquifer to lower and contaminate the safe zones. This paper also presents possible scenarios for optimal withdrawal of Arsenic free groundwater from deeper and/or from twin aquifers in a ...
Oozing of water and rise in water level were reported by villagers and media during the third wee... more Oozing of water and rise in water level were reported by villagers and media during the third week of January 2005, in parts of Rangareddy, Nalgonda and Mah-boobnagar districts of Andhra Pradesh. The event also coincided with the aftermath of the 9.3 magnitude earthquake in Sumatra, Indonesia and the tsunami in the Indian Ocean. The villagers and State and Central agencies were baffled by the unusual phenomenon. After a general reconnaissance, an integrated study com-prising water-level monitoring, water quality analyses, topography survey, isotopic analyses, mise-à-la-masse survey and electrical resistivity imaging (ERI) was carried out at two sites near Mukindapur and Devanur of Yalal mandal in Ranga Reddy district, where the phenomenon of water-oozing was observed. The integrated study revealed that the oozing of water at the studied location is purely hydrological/hydro-geological and a shallow phenomenon. Scientific data could not attribute any strong connection to the deeper t...
In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady source... more In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady sources for groundwater due to inadequate understanding of the fracture networks. A comprehensive knowledge of fracture distribution at the regional scale is necessary to delineate sustainable aquifers and manage them efficiently. The resistivity maps derived from the airborne electromagnetic (AEM) survey over the Ankasandra watershed in Karnataka, India, reveal sharp and deep zones of low formation resistivity, which indicate groundwater-bearing zones. It is found that some of these zones are hydrogeologically connected through fracture networks resulting in augmented yield. AEM results in combination with an in-depth understanding of the geological structures successfully map these groundwater-saturated fracture networks (or hydrogeological lineaments) that we term as ‘Hydrolins’. As groundwater occurrence is generally associated with lineaments, we analyzed the drilling and geophysical logs from 21 wells within a 380 sq.km area to study the relationships of various lineaments with ‘Hydrolins’, particularly in respect of their groundwater potential. AEM results, though calibrated and correlated with a limited number of well data, revealed a threshold groundwater horizon (TGWH), found to be at 80 m depth for Ankasandra watershed, beyond which a strong correlation exists between the depth of a well and its yield. While the TGWH may differ for different watersheds, the approach presented here can be readily adopted to map sustainable groundwater sources in hardrocks worldwide
The interpretation of ERT data requires inversion, which is challenging due to its nonlinear and ... more The interpretation of ERT data requires inversion, which is challenging due to its nonlinear and illposed nature. We aim to solve the inverse problem using a machine learning technique that approximates the complex non-linearity using the data provided without specifying any geophysical or mathematical equations. Speci¦cally, we make use of CNN with U-Net architecture to estimate the true subsurface resistivity model from observed apparent resistivity data (pseudosection). U-Net is selected because it works well when both input and output data are in the form of images and in the present work, input (pseudosection) and output (subsurface resistivity model) have spatial correspondence and existing local patterns. For ERT inversion, U-Net learns by minimizing pixel mean squared error between the true and predicted subsurface resistivity images. The ERT dataset was generated by simulating a dipoledipole survey over a 400m x 100m earth containing two anomalous blocks with varying resi...
Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution elect... more Geophysical studies using multichannel analysis of surface waves (MASW) and high-resolution electrical resistivity tomography (HERT) have been jointly carried out on an experimental basis in the field. The motive is to study shallow subsurface features (i.e. faults traces, cavities and palaeo-channels) in the foothill zone of Northwest Himalaya. These techniques have shown their potentiality in successfully identifying shallow (0-24 m) fault traces and dissolution features/palaeo-channels. Depending on the sensitivity of the MASW and HERT techniques, geophysical signatures of the subsurface features were recorded and further resolved with the help of synthetic simulation. The synthetic simulation of 2D electrical response has been carried out over the initial model for subsurface fault traces as well as palaeo-channels. The initial model has been refined iteratively to bring the synthetic response close to the field response and hence the final refined model is considered to be the ...
We have investigated a network of fractures forming the flow paths within the crystalline graniti... more We have investigated a network of fractures forming the flow paths within the crystalline granitic rocks of an Experimental Hydrogeological Park (EHP) with the help of electrical resistivity surveys. The experimental study located at the managed aquifer recharge (MAR) site of EHP has measured a distinct variation in the apparent resistivity for deeper electrical signals that localize the presence of interconnected water-saturated fractures. Usually, profiles close to the MAR tank depict low apparent resistivity values from deep signals across in situ fractures and resistivity amplitude increases away from the tank. We modeled and simulated the presence of water-saturated fractures by a simple three-layered model having embedded shallow heterogeneities in the saprolite layer, vertically interconnected multiple thin conductive horizontal layers in the fissured zone, and an underlain unweathered crystalline granitic basement. These fractures produce a distinct variation in the resistiv...
... Aadil Nabi Bhat 179 15. Reconstruction of Water Level Time Series in an Aquifer Using Geostat... more ... Aadil Nabi Bhat 179 15. Reconstruction of Water Level Time Series in an Aquifer Using Geostatistical Technique Dewashish Kumar and Shakeel Ahmed 191 16. Governing Equations of Groundwater Flow and Aquifer Modelling Using Finite Difference Method Shazrah Owais, S ...
... Further approaches were developed on the basis of relationships between geophysical and hydra... more ... Further approaches were developed on the basis of relationships between geophysical and hydraulic properties (eg [Jones and Buford, 1951] , [Kelley, 1977] , [Urish, 1981] , [Frohlich et al., 1996] , Chandra et al., 2008 and [Vouillamoz et al., 2005] ), which have led to the ...
With the growing dependence on groundwater, escalating demand, and increasing depletion of aquife... more With the growing dependence on groundwater, escalating demand, and increasing depletion of aquifers, the Government of India has accorded special emphasis to management of groundwater resources through precise aquifer mapping. We have carried out hydrogeophysical surveys, including airborne electromagnetic (AEM), in six hydrogeologically divergent areas comprising sedimentary basins, basalts, weathered and fractured granite gneisses and schists, desert, and costal alluvium with the objective to establish suitability of AEM for a countrywide aquifer mapping in India. Efficacy of the dual-moment AEM surveys in mapping the shallow and deep aquifers is evaluated in conjunction with geologic, geophysical, and borehole data. It is found that the AEM surveys provide reliable images of the subsurface resistivity distribution defining the 3D geometrical and electrical attributes of aquifers in different areas. The surveys helped identify the suitable zones for managed artificial recharge (MA...
Knowledge of the spatial distribution of the hydrogeological parameters such as hydraulic conduct... more Knowledge of the spatial distribution of the hydrogeological parameters such as hydraulic conductivity (K) and transmissivity (T) are essential for management and development of groundwater resources. In general, the hydrogeological parameters are estimated by the classical techniques like pumping tests, which are uneconomical and time consuming. This technique has strong application in alluvial region, a well known formation with homogeneous and stratified layers set up. However, in crystalline rocks such as granites, gneisses and schists, the groundwater reservoir is mainly formed by the zone of weathering and to a lesser extent by the fractures and fissures in the fresh rock, that have high spatial variability. Therefore, in hard rock terrain a dense measurement is required for reliable mapping of spatial distribution of aquifer parameters. Geophysics can play a major role in defining these parameters. The work carried out here deals with a mathematical tool developed to estimate...
An integrated hydrogeophysical studies in the arsenic affected region of Balia-Patna sector in Mi... more An integrated hydrogeophysical studies in the arsenic affected region of Balia-Patna sector in Middle Ganga Plain of the states of Uttar Pradesh and Bihar in India led to an important finding that the lithological set up plays a major role in understanding the arsenic contamination in groundwater. Although alluvial deposits are known for their hydrogeological homogeneity and stratification, the heterogeneities found in stratification close to the river and are the probable reason of arsenic contaminations in patches. The thickness of clay pockets varies pinching out at some places. The high resolution of electrical resistivity tomography has provided subsurface lithological discontinuities from the surface to even deeper level (say 90 m). The discontinuity of the layer may create leakage from upper aquifer to lower and contaminate the safe zones. This paper also presents possible scenarios for optimal withdrawal of Arsenic free groundwater from deeper and/or from twin aquifers in a ...
Oozing of water and rise in water level were reported by villagers and media during the third wee... more Oozing of water and rise in water level were reported by villagers and media during the third week of January 2005, in parts of Rangareddy, Nalgonda and Mah-boobnagar districts of Andhra Pradesh. The event also coincided with the aftermath of the 9.3 magnitude earthquake in Sumatra, Indonesia and the tsunami in the Indian Ocean. The villagers and State and Central agencies were baffled by the unusual phenomenon. After a general reconnaissance, an integrated study com-prising water-level monitoring, water quality analyses, topography survey, isotopic analyses, mise-à-la-masse survey and electrical resistivity imaging (ERI) was carried out at two sites near Mukindapur and Devanur of Yalal mandal in Ranga Reddy district, where the phenomenon of water-oozing was observed. The integrated study revealed that the oozing of water at the studied location is purely hydrological/hydro-geological and a shallow phenomenon. Scientific data could not attribute any strong connection to the deeper t...
In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady source... more In hardrocks that cover about 20% of the Earth’s surface, it is difficult to locate steady sources for groundwater due to inadequate understanding of the fracture networks. A comprehensive knowledge of fracture distribution at the regional scale is necessary to delineate sustainable aquifers and manage them efficiently. The resistivity maps derived from the airborne electromagnetic (AEM) survey over the Ankasandra watershed in Karnataka, India, reveal sharp and deep zones of low formation resistivity, which indicate groundwater-bearing zones. It is found that some of these zones are hydrogeologically connected through fracture networks resulting in augmented yield. AEM results in combination with an in-depth understanding of the geological structures successfully map these groundwater-saturated fracture networks (or hydrogeological lineaments) that we term as ‘Hydrolins’. As groundwater occurrence is generally associated with lineaments, we analyzed the drilling and geophysical logs from 21 wells within a 380 sq.km area to study the relationships of various lineaments with ‘Hydrolins’, particularly in respect of their groundwater potential. AEM results, though calibrated and correlated with a limited number of well data, revealed a threshold groundwater horizon (TGWH), found to be at 80 m depth for Ankasandra watershed, beyond which a strong correlation exists between the depth of a well and its yield. While the TGWH may differ for different watersheds, the approach presented here can be readily adopted to map sustainable groundwater sources in hardrocks worldwide
Uploads
Papers by Subash Chandra