groundwater exploration in basaltic terrains-problems and …aggs.in/issues/jgwr-2013204.pdf ·...

Journal of Groundwater Research, Vol.2/2, December 2013

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Groundwater exploration in basaltic terrains-problems and prospects

S.N. Rai*, S. Thiagarajan and Dewashish Kumar CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad-500 007, India

*Email:[email protected] Abstract

India has an area of 3, 287, 240 sq. km which accounts for about 2.4 per cent of the total surface area of the world. About two third of it’s territorial area is occupied by different types of hard rocks such as basalts, granites, granulites, gneisses, charnockites, quartzites etc. About half a million sq km area in central and western India is occupied by volcanic basaltic rocks popularly known as Deccan traps. Genesis of the Deccan traps is the volcanic eruptions which took place in-termittently about 65 million years ago. Stratigraphic sequence of Deccan traps consists of mul-tiple layers of solidified lava flows deposited over the paleo-ground surface. Two lava flows are separated by sedimentary beds called intertrappeans. These intertrappeans mark the interval be-tween two consecutive lava flows. Each lava flow consists of an upper vesicular unit and a lower compact unit. Vesicular unit together with overlying intertrappeans forms aquifers. Other sources of groundwater occurrence in Decaan traps are the geological structures like faults, fractures and joints within the traps and sedimentary formations below the traps. A sizable population lives in the Deccan volcanic province whose livelihood is largely depends on the agricultural income. In se-quel, the agricultural income is dependent on the availability of water supply for irrigation. Groundwater is the main source of water supply. Acute shortage of ground water in hard rock ter-rains such as Deccan traps is well known because of is occurrence in limited quantity in unevenly distributed aquifers and geological structures of finite areal extent. Because of complex hydrogeo-logical conditions, delineation of ground water potential zones poses a challenging task. This pa-per present an overview of the problems and future prospects of groundwater exploration based on the case studies pertaining to scientific investigations carried out for ground water exploration in Deccan traps terrains. 1. Introduction

Hydrogeological framework of the India is highly complex because of the occurrence of di-versified geological formations with different tectonic setup and prevailing climatological vagaries. The hard rock formations such as volcanic rocks, granites, granulites, compact metamorphic sedi-mentary rocks etc occupy almost two third territorial area of the country and host big chunk of In-dian population. In hard rock formations, groundwater occurs in secondary porosity developed due to weathering, fracturing, faulting etc. Deccan traps occupies almost half a million square kilome-ter areas spread over between 69o-79o East longitudes and 16o -22o North latitudes in parts of Ma-harashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and Karnataka as shown in figure 1. About 80% surface area of Maharashtra state extending from its west coast to the west of Nagpur city is occupied by Deccan traps. Climatic condition in Deccan traps region is mostly semi-arid.

Geological sequence of Deccan traps consists of multiple layers of lava flows underneath a composite layer of soil on the top and weathered /highly fractured mantle at the bottom. This com-posite layer forms unconfined aquifers which are the main source of groundwater available in the dug well. Thickness of the piles of lava flows varies from~ 2000 m near the western margin to few tens of meters in its eastern fringe such as in the west of Nagpur. Two consecutive layers of lava flows are separated by sedimentary deposits which are known as intertrappeans. Intertrappeans


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mark the interval between the two consecutive volcanic eruptions. Each lava flow consists of two units. The bottom unit is massive basalt which may or may not be fractured/faulted. The upper unit contains vesicles. These upper vesicular units together with the sedimentary intertrappeans form confined /semi-confined aquifers. These geological formations/structures are of limited areal ex-tent. Therefore, groundwater stored in these aquifers is of limited quantity. This leads to acute shortage of water supply in hard rock areas like Deccan trap regions. Hydrogeology of Deccan traps has been described by Singhal (1997), Ghosh et al (2006) and Mehata (1989). An overview of the development and management of ground water resources in Deccan traps province has been presented by Limaye (2010).

Fig. 1: Geological map of India (after GSI)

Because of the extensive use of groundwater mainly for drinking and irrigation purposes in rural area, majority of the dug wells dried up with the onset of summer season by the end of March. This situation continues till the onset of rainy season. Therefore, to overcome from this problem of water shortage, delineation of deeper sources of groundwater within and below the traps is essential on one hand and selection of suitable sites for aquifer recharging is on the other hand. Implementations of both measures are essential for the sustainable development and man-agement of groundwater resources.


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The presence of groundwater in geological formations/structures considerably reduces their resistivity in comparison to the host geological formations devoid of groundwater. This phenome-non makes the geophysical electrical methods more suitable than any other geophysical methods for delineation of potential groundwater resources. Results of geophysical electrical surveys are also useful in the selection of suitable sites for managing aquifer recharge. In this paper application of these two measures of sustainable development and management of groundwater resources will be discussed with the help of three case studies of geophysical electrical surveys carried out in Deccan traps region by authors. 2. Geophysical Electrical Surveys

Geophysical electrical surveys with four electrodes configurations such as Schlumberger, Wenner, Dipole-Dipole, Pole-Dipole are being widely used since more than five decades in India for delineation of groundwater resources in different geological provinces. In this context many electrical surveys have also been carried out in Deccan traps region. For example, Bose and Ram-krishna (1978) conducted electrical survey in parts of Sangli district for delineation of groundwater resources. Muralidharan et al (1994) have carried out deep electrical resistivity survey for delinea-tion of deeper sources of groundwater in Jam river basin falling under Katol taluk of Nagpur dis-tricts. Rao et al (1983) have carried out electrical survey in Deccan traps covered region of Auran-gabad district. Murthy et al (1986) have carried out electrical survey for the delineation of Gondwana formation below the traps in Umrer, Bander, Kamathi, and Katol troughs in Nagpur district. In all these studies mostly vertical electrical soundings (VES) technique of electrical sur-vey has been used. The greatest limitation of such surveys with four electrodes configuration is that it provides only 1-D model of resistivity variation below the centre of the survey profile and does not take into account lateral changes in the resistivity value due to presence of geological forma-tion/structure such as faults, fractures, joint etc which are the major sources of groundwater in hard rock formations. Therefore, because of sporadic distribution of groundwater bearing geologi-cal structures like faults, fractures, joints etc of finite areal extent in hard rock terrains, their delin-eation by 1-D model is not possible unless these structures coincidently lies below the center of the profile and are of bigger dimension. Therefore, a more accurate model of the subsurface is a 2-D model which provides information about the resistivity variations in the vertical as well as lateral direction along the survey profile. Development of 2-D resistivity models becomes possible with the development of Electrical Resistivity Tomography (ERT) technique. The main advantages of ERT are: (1) Automated acquisition of large amount of data in less time and cost, and (2) presenta-tion of images of sub-surface litho units along the entire survey line with high resolution due to mapping of same location a number of times for different electrodes spacing. The following section describes the methodology of ERT surveys.

2.1 Electrical Resistivity Tomography

Electrical Resistivity Tomography is carried out by using multi-electrode resistivity imag-ing system and effective data processing software based on inversion techniques. For ERT, strings of multi core cables with many electrodes take outs are connected together to form a multi-electrode setup where selection of any four (two for current injection and two for potential mea-surements) of those electrodes is possible. The number of electrodes differs from system to system. Some systems carry 64 electrodes, some carry 72 electrodes like that. Figure 2 shows typical field arrangements of an ERT based electrical resistivity survey with four multi-core cables. In each multi-core cable, 16 electrodes are placed at equal spacing. Selection of spacing between elec-


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trodes can be made based on the nature of survey. For subsurface images of high resolution, small-er spacing is used. Multi-core cables are connected to an electronic switching unit. The switching unit is connected to an ERT system and the ERT system is connected to a laptop. Laptop based software together with the electronic switching unit is used to select automatically four relevant electrodes (two current electrodes and two potential electrodes) for each measurement. Provision is made for resistivity survey using different electrode configurations such as Wenner, Schlumberger, Dipole-Dipole, Pole-dipole, Pole-Pole etc (Loke, 2000). Equipments and field techniques to carry out ERT have been developed by a number of international companies. In the present study the ABEM made Terrameter LUND Imaging System, SAS 4000 as a Resistivity meter is used. Elec-trical Resistivity Tomography is carried out using four multi-core cables each having 16 elec-trodes. Spacing between two electrodes is 10 m. Spread length for this ERT unit is 630 m.

Fig.2: Field arrangements of ERT survey with 4 multi-core cables

2.2 Data Acquisition Information regarding the sequence of measurements to take, the type of array to be used

and other survey parameters such as the intensity of current to be used is entered into a text file which can be read by a computer program uploaded in a laptop. After reading the control file, the computer program then automatically selects the appropriate electrodes (two current electrodes and two potential electrodes) for each measurement. After that, the measurements are taken automati-cally and stored in the laptop. For demonstration purposes we consider an example of ERT survey with a multi- core cables fitted with 16 electrodes. In this example, the spacing between adjacent electrodes is “a”. The first step is to make all the possible measurements for the Wenner array with an electrode spacing of “1a”. For the first measurement, electrodes numbers 1, 2, 3 and 4 are used. Electrode 1 is used as the first current electrode C1, electrode 2 as the first potential electrode P1, electrode 3 as the second potential electrode P2 and electrode 4 as the second current electrode C2. For the second measurement, electrodes number 2, 3, 4 and 5 are used for C1, P1, P2 and C2, respectively. This is repeated down the line of electrodes until electrodes 13, 14, 15 and 16 are used for the last measurement with “1a” spacing. The total number of mea-surements for 1a spacing will be 13.

After completing the sequence of measurements with “1a” spacing, the next sequence of measurements with “2a” spacing is made. First electrodes 1, 3, 5 and 7 are used for the first mea-


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surement. The electrodes are chosen so that the spacing between adjacent electrodes is “2a”. For the second measurement, electrodes 2, 4, 6 and 8 are used. This process is repeated down the line until electrodes 10, 12, 14 and 16 are used for the last measurement with spacing “2a”. The same process is repeated for measurements with “3a”, “4a”, and “5a” spacing. For “5a” spacing is only one measurement. Thus the total number of measurement is 35 for one time laying of the electrodes instead of only one measurement for conventional survey with four electrodes. As the electrode spacing increases, the number of measurements decreases. The number of measurements for ‘na’ spacing is calculated by using the relation ‘N-nx3’ in which N is the total number of elec-trodes and n is the multiplication coefficient of the spacing. For example for 16 numbers of elec-trodes with 2 as a multiplication coefficient of spacing, the total number of measurement is given by 16-2x3, i.e. 10. The number of measurements that can be obtained for each electrodes spacing for a given number of electrodes along the survey line depends on the type of array used. The Wenner array gives the smallest number of possible measurements compared to the Schlumberg-er and other common arrays that are used in 2-D ERT surveys. It is evident from figure 3 that the depth coverage by ERT is more below the central part of the profile and it decrease with distance away from the centre of the profile. To extend the depth coverage laterally, Roll- along method is used. In this method, after completing the sequence of measurement for one field setup, the cable is moved along the survey profile past one of its end by several unit electrode spacing in such a way that in the second sequence of measurements, the depth coverage left over in the previous sequence of measurement is completed. By this way it is possible to achieve the complete depth coverage of the resistivity measurements for a desired segment of the survey line.

Fig.3: Sequence of measurements used to build up a pseudo section with a multi-core cable having 16 electrodes

The next step followed is converting the surface measured apparent resistivity values in to a 2-D sub-surface true resistivity model which can be used for geological interpretation in order to identify water bearing geological formations and structures such as fractures, faults, joints etc. Practically all commercial multi-electrode systems come with the computer software to carry out this conversion. This task is accomplished by using inverse modelling in which the ap-parent resistivity values are used to create a layered sub-surface resistivity model.


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2.3 Electrical Resistivity Tomography Inverse modeling of the measured apparent resistivity data is carried out using

RES2DINV program (Loke, 1997) to create subsurface resistivity model. This program automati-cally creates a 2-D model by dividing the subsurface into rectangular blocks. To initiate modelling work, some resistivity values will be assigned to the model blocks. Thereafter, program calculates the apparent resistivity values of the model blocks and compares these calculated apparent resis-tivity values to measured apparent resistivity values. The resistivity value of the model block is adjusted iteratively until the calculated apparent resistivity values of the model are in close agree-ment with the measured apparent resistivity values. The final output is the 2-D inverse model of sub-surface resistivity variation below the survey line.

Geological interpretation of this resistivity model is used for identification of groundwater potential zones. Resistivity images are also helpful in suitable site selection for the development of recharge structures. ERT is also used to prepare 3 D model of subsurface. More details about the procedure of survey and data interpretation using ERT technique is given by Loke (2000). This technique is now a day widely used for ground water exploration (Kumar et al, 2010, Ratnakuma-ri et al, 2012, Perez-Bielsa et al, 2012; Rai et al, 2012). Other than groundwater exploration, the ERT technique is also used for many other purposes such as geothermal exploration (Dewashish Kumar et al, 2011: Haile and Abiye, 2012), mineral exploration, Engineering geology for bed rock investigation, mapping of groundwater pollution, prevention of land slide in mines (Rai et al, 2011), selection of suitable artificial recharge sites, delineation of sub-surface geological struc-tures affecting water level in surface water body (Roningen and Burbey, 2012) etc.

Fig. 4: Chandrabhaga River basin and sites of VES and ERT represented by Sn and Pn respectively

3. Study Areas

Acute shortage of groundwater has been reported from different parts of Deccan traps such as Vidarbha region and Ratnagiri district of Maharashtra. Vidarbha region consists of eleven dis-tricts, namely Nagpur, Wardha, Washim, Yavatmal, Amaravati, Akola, Bhandara, Buldana, Chandrapur, Gadchiroli and Gondia. Groundwater is the main source of water supply for agricul-ture which is the main source of income to the rural population like any other rural areas of the


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country. This region is infamous for maximum numbers of farmer’s suicides. This tragic pheno-menon is attributed to the economic losses incurred by farming community due to acute shortage of water supply for irrigation. The other region of Deccan traps facing acute shortage of water supply is the Ratnagiri district in the western Maharashtra. In order to verify the reporting of non-availability of groundwater in these two different parts of Deccan traps, authors have selected two areas for delineation of groundwater resources using geophysical electrical surveys in the 11th five year plan of the CSIR-National Geophysical Research Institute. One area is the Chandrabhaga riv-er basin under Nagpur district and the other one belongs to the Chiplun taluk of Ratnagiri district.

Fig.5: Map of Chiplun taluk showing ERT Profiles represented by Ln

3.1 Chandrabhaga River Basin Chandrabhaga river basin lies on the eastern fringe of Deccan traps where trap thickness

reduces to few tens of meters. Based on the drainage pattern shown in the Survey of India topo-sheet nos 55K/11, 55K/12, 55K/15 and 55K/16, Chandrabhaga river basin lies between 78o42’-79o East longitudes and 21o11’-21o20’ North latitudes. Map of Chandrabhaga river basin is shown in figures 4. The western part of the basin falls under Katol taluk, central part under Kal-meshwar taluk and a small segment towards it’s eastern boundary under Nagpur rural. Most of the Chandrabhaga river basin is occupied by Deccan traps. Gondwana sedimentary formation is found toward north-eastern part of the basin. The basin is spread over in about 400 km2 area en-compassing 45 villages with an average population of 1000 per village. The basin is traversed by five major creeks out of which four flows from west to east direction and one creek in Nagpur ru-ral area flows from south to north. All creeks are tributaries to Chandrabhaga river. These creeks are responsible for the draining of groundwater as base flow from top aquifers. As a result many dug wells of this region go dry with the onset of summer. In this basin Vertical Electrical Sound-ings (VES) have been carried out at 45 sites and Electrical Resistivity Tomography (ERT) at 54 sites. Some of the VES and ERT surveys results are published by Rai et al (2011) and Ratnaku-mari et al (2012), respectively. Interpretation of VES and ERT results indicate presence of nu-merous potential sources of groundwater in the form of intertrappeans and fractured zones within the traps and Gondwana sedimentary formation below the traps at most of the investigated sites. These resources were not known to the local populace earlier. Six exploratory wells are drilled to


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confirm the occurrence of delineated source of groundwater. Three bore well are drilled at VES sites, namely S10 (Kotwalbirdi), S38 (Raulgaon) and S41 (Ubali) and the remaining three wells are drilled at ERT sites, namely at P1 (Ghogali), P20 (Sonegaon), P21 (Khari) and P30 (Budha-la). Locations of these sites are given in figure 4. For demonstration purpose two case studies are considered. Case-1 deals with the delineation of groundwater potential zone at one site of VES near Kotwalbardi village while Case -2 deals with the delineation of groundwater potential zones at a site of ERT near Ghorad village.

3.2 Chiplun Taluk

The other investigated area under Chiplun taluk in Ratnagiri district lies at the western fringe of the Deccan traps where trap thickness attains its maximum value of ~ 2000m. It is spread over between 73o19’-73o34’ east longitudes and 17o12’37”-17o47’50” north latitudes en-compassing 4 villages, namely Unhavare, Aravali, Tural and Rajwadi as shown in figure 5. In this region ERT surveys have been carried out at eight locations with the purpose of delineation of groundwater resources and potential geothermal fields. Case-3 deals with the results of ERT sur-vey carried out at Tural village. Detailed discussion about the results of the above mentioned three cases are presented below.

Fig. 6: 1-D resistivity model and comparison of computed litho units with the litho units of bore well at S38 site 3.3 Case 1

Figure 6 presents 1-D model of resistivity variation with depth at a S38 site (Raulgaon) and comparison of computed thickness of litho units with the thickness of the corresponding unit obtained from bore wells. Interpretation of the resistivity model is done on the basis of the resistiv-ity values suggested by Central Groundwater Board (CGWB website) for different litho units of Deccan trap regions. These values are: Alluvial and black cotton: 5-10 Ohm m; wea-thered/fractured/vesicular basalt saturated with water: 20-40 Ohm m; moderately wea-thered/fractured/vesicular basalt saturated with water: 40-70 Ohm m; massive basalt; > 70 ohm


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m; water saturated Lameta bed: < 10 Ohm m; Water saturated Gondwana sedimentary formation: 15-30 Ohm; and dry Gondwana formations: > 50 Ohm m. Based on these values of litho units, the interpretation of 1-D resistivity model suggests two water bearing formations of 2 m and 4.4 m thicknesses between two successive pairs of massive basalt layers of 6.4 m, 23.7 m and 8.6 m thicknesses. The deepest layer of the massive basalt is underlain by water saturated Gondwana formation characterized with 22.7 m thicknesses. Yield of the bore well is shown in figure 7. This bore well is in use since last 4 years. Here, it is worth to mention that because of the limitation of VES using four electrode array, rsisitivity variation with depth is measured only below the centre of the survey profile of 600 m length. If there would have been any water bearing geological for-mations/structures occurring on either side of the centre, their delineation is not possible. But this is not the case with ERT. This will be demonstrated in Case -2 which deals with the sub-surface imaging using ERT survey.

Fig.7: Showing yield of bore well at site S38

3.4 Case- 2

Figure 8 presents 2-D subsurface resistivity image together with resistivity index at bot-tom along a 460 m long profile for an ERT survey conducted at P1 site near Ghogali village. Cen-ter of the profile is located at 230 m. Depth of investigation is maximum (73.8 m) in the central part of the profile and gradually decrease towards both ends of the profile. Interpretation of the 2-D resistivity model suggests 10-12 m thick layer of alluvium/ weathered formation which consti-tutes the unconfined aquifer. This layer is underlain by moderately fractured basalt (40-70 Ohm m) followed by two units of massive basalts (>70 Ohm m). These massive basalt units are sepa-rated by a fractured zone at 300m distance, below which lies a potential groundwater zone (20 Ohm m) at 35 m depth. This water bearing zone is widening with depth and has extended beyond 73.7 m depth of investigation at this site. Depth distribution of interpreted litho units are verified by comparison with the depth distribution of litho units of an exploratory well drilled at the same distance of 300m. Both sets of litho units are found in good agreement. As per the expectation


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groundwater was struck at 35 m depth. This bore well is in operation since last three year. Similar scenario of presence of groundwater is found at most of the ERT sites suggesting presence of suf-ficient groundwater to meet the local requirements of irrigation and drinking. It is worth to men-tion here that if VES would have been carried out for the same spread length and centre at 230 m, it would have not possible to delineate such a potential groundwater zone. Below the centre lies massive basalt formation. This demonstrates the merit of multi-electrode ERT survey over elec-trical survey with four electrode configuration.

Fig.8: Inverse resistivity model for case 2 (after Rai et al 2011)

S N

F

F F

FF

F

Fig.9: Inverse resistivity model for case 3 (after kumar et al 2011). 3.5 Case- 3

Figure 9 presents 2-D inverse resistivity model obtained by the ERT survey conducted along a 460 m long profile near Tural hot spring. Location of this profile is shown in figure 5 as L3. A clear fault plane almost in E-W direction is identified at 205 m distance separating a con-ductive zone from a highly resistivity zone (>140 Ohm m). The formation on the southern side of the fault with <13 Ohm m resistivity in the central part appears to be the hot spring reservoir. This formation is at a depth of ~25 m and is trending vertically downward beyond 77 m depth which is the maximum depth of investigation for this site. It suggests that the reservoir is connected to a deeper heat source which is emerging the low resistivity zone. This zone is overlain by massive basalt (112-266 Ohm m) and is exposed to the ground surface between 95 m to 125 m. Towards northern side of the massive basalt unit, another source of geothermal reservoir with <13 Ohm m resistivity value can be seen beyond 315 m distance . This water body appears to be separated by a fault plain dipping NEE-SWW direction from the massive basalt unit and is laterally and verti-cally extending northward.


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4. Conclusion

There has been extensive reporting about the acute shortage of water supply mainly for drinking and irrigation. This is attributed mainly to the depletion of groundwater resources due to over exploitation of groundwater in excess to its replenishment by natural/artificial means. This fact may be true to some extent for shallower aquifers. But based on the results of geophysical survey carried out in two draught prone regions, namely Chandrabhaga river basin located on the eastern fringe of Deccan traps in Vidarbha region and Chiplun taluk located on the western fringe of the Deccan traps, we came across the total different scenarios. At most of the investigated sites which include 54 ERT sites and 45 VES sites alone in Chandrabhaga river basin and 8 ERT sites in Chiplun taluk, we have been able to delineate potential sources of groundwater in the depth range of 15-70 meters within the Traps and < 90 m in Gondwana sedimentary formation (only in Chandrabhaga basin) below the traps which are sufficient to meet the local water supply demand. Based on these field results we arrived at a conclusion that the projected acute shortage of water supply is not because of the depletion of groundwater resources but due to lack of systematic scientific investigations using appropriate methods like ERT for delineation of potential ground-water resources and lack of coordination mechanism for their sustainable development and man-agement. Knowledge of sub-surface images obtained from ERT survey is very useful in selection of suitable sites for aquifer recharging which is very much essential to maintain the productivity of an aquifer. Acknowledgment

Authors are grateful to Prof. Mrinal K. Sen, Director, CSIR-NGRI for according permis-sion to publish this work. Thanks are due to CSIR for granting financial support.

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