electrical imaging of karst terrene for managed aquifer

Electrical imaging of karst terrene for managed aquiferrecharge: A case study from Raipur, central India

TANVI ARORA1,* , TAUFIQUE WARSI

1, FAROOQ A DAR2 and SHAKEEL AHMED

3

1CSIR-National Geophysical Research Institute, Hyderabad 500 007, India.

2Department of Geology, Kashmir University, Leh Campus, Leh, Kashmir, India.

3Jamia Milia Islamia, New Delhi 110 008, India.*Corresponding author. e-mail: [email protected]

MS received 15 February 2020; revised 21 September 2020; accepted 29 September 2020

Supply of sustainable water is a priority for urbanization of the country. Managed aquifer recharge(MAR) is recommended to enhance the groundwater resources, but Bnding the favorable site for MAR isquite challenging, particularly, in urbanized karst terrains because of highly heterogeneous and aniso-tropic properties of carbonate aquifers. The expansion along Naya Raipur is posing a severe threat to itsgroundwater resources by altering the hydrological framework in the area. In this work, we characterizethe unsaturated zone for Bnding potential pathways for MAR in karst terrain of central India by usinghydro-geophysical techniques. Sub-surface geophysical investigations including electrical sounding andproBling, captured the shallow surface of the area to the depth of around 40 m. The Wenner–Schlum-berger and gradient methods could decipher strong lateral and vertical anomalies. The low resistivity inthe unsaturated zone, as compared to the high resistivity of limestone bedrock is the main path forinBltration. The low resistivity anomalies in the 2D inverted sections might be water Blled conduits orsolution channels with uncertain geometry. Hydrogeological heterogeneity based on geophysical datahelped to locate the favourable zones for construction of MAR structures. The Chokra Nala in theTelibandha area of Raipur is the favorable zone along which MAR could be more eAective in replenishingthe groundwater. The Telibandha lake area with the indication of the presence of hidden maBc dyke laterintruding the limestone after its deposition, was also inferred from the geophysical data.

Keywords. Electrical resistivity tomography; karst aquifer; managed aquifer recharge (MAR).

1. Introduction

MAR is a technique of intentional storage andtreatment of water in aquifers where naturalrecharge processes have been aAected (Dillon2005a, b; Dillon et al. 2009). MAR is a cost-effec-tive process that has solved groundwater quantityand quality problems in many stressed aquifersystems of the world. Though, MAR has gener-ated successful results in many karst areas, it isstill marginal in these aquifers due to high

heterogeneity and anisotropy of these systems. Forthe better solution of various problems, activemanagement is the most suitable approach forwater scarce karst aquifers but a lack of compre-hensive knowledge and improper aquifer charac-terization will have wrong implications on theMAR results (Dillon 2005a, b). ArtiBcial rechargein karst is also challenging because recharge isdependent on many factors like, presence of karstdepressions, nature of percolation, etc. Porosity isanother main property that aAects the function of

J. Earth Syst. Sci. (2021) 130:14 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-020-01514-w (0123456789().,-volV)( 0123456789().,-vol V)

MAR as it aAects the storage of water. Carbonateaquifers have a very low primary porosity in thematrix blocks and very high, near solutionallydeveloped zones. At sites where conduits areabsent or karstiBcation is very less, the perme-ability of the material is too low to allow any sig-nificant water to inBltrate. These are the siteswhere artiBcial recharge structures could be inef-fective in increasing volume of water. At the placeshighly modiBed by the solution activity (presenceof karren, epikarst, enlarged fractures, solutionchannels, conduits, etc.), the injected waterreaches and gets transmitted into the systemrapidly. The surface Cow in karstiBed areas is shortand enter the groundwater easily. After developingMAR sites in karst areas, the structures need post-MAR practices for their sustained operation over along time. One of the main and common problemsis the clogging of a MAR structure at some or otherstage of its development due to rapidly transmittedsediments with inBltrating water (Martin 2013).Therefore, the results of MAR program will be fullof ambiguities unless these technical and practicalissues are considered properly.

Many integrated geophysical studies have beencarried out to better understand hydrogeology ofkarst aquifers. However, a suitable method shouldbe carefully selected while developing the method-ology for identifying suitable zones of managedaquifer recharge. Various geophysical methods havebeen used that are not limited to the following, e.g.,audio frequency telluric methods (Chen 1988; ChinaGeological Survey (CGS) 2005; Gan et al. 2011),induced polarization (Li and Wang 2009), very lowfrequency methods (Turberg and Barker 1996;Bosch and M€uller 2001, 2005), VLF integrated withERT (Alexopoulos et al. 2011; Vargemezis et al.2012) microgravimetric and gravity gradient(Blizkovsky 1979; Butler 1984), seismic method,electromagnetic method (Kaspar and Pecen 1975;Vogelsang 1987), combination of seismic refraction,ERT and microgravity (Moore and Stewart 1983;Steeples et al. 1986) and combination of methods(Jacob et al. 2009; Jardani et al. 2006, 2007; Mey-erhoA et al. 2012). Electrical resistivity tomography(ERT) has evolved as a potential geophysical tech-nology to monitor the natural processes in shallowsubsurface (e.g., Dutta et al. 2006;Dahlin et al. 2010;Arora and Ahmed 2010, 2011; Carriere et al. 2013;Arora et al. 2016) and also has application to karst

studies (e.g., Gan et al. 2011; Vlahovic and Munda

2011; Metwaly et al. 2012). The method can help to

identify porous and water saturated zones in thisstudy (e.g., P�anek et al. 2010).Since the karst aquifers are known to be a very

complex aquifer system, it is always challengingand poses difBculties for the carbonate aquifers inorder to implement any kind of construction,recharge structure and economic development.Imaging plays a vital role to understand the sub-surface scenario in terms of resistivity, becauseamong all the physical properties of rocks, electri-cal resistivity varies mostly. While talking aboutthe understanding of system, it always demandsinclusive study and a comprehensive knowledgeabout the system, which is very complex in termsof geological and hydrological scenario, wellexplained by Ghasemizadeh et al. (2012). Karstsystem poses high degree of anisotropy andheterogeneity, which make this kind of aquifer tostand unique from others. Rock matrix in thisterrain impart slow seepage, whereas conduits,fractures and solution channels provide a clearpassage for water to Cow faster.In the present work, integration of geological

understanding with the geophysical imagingimpart a complete subsurface scenario to imple-ment proper managed aquifer recharge (MAR) inthe study area. This integrated approach andunderstanding has implications towards othercarbonate aquifers to implement eAective rechargetechniques. To our knowledge, no geophysicalstudy was carried out in the Raipur area with anaim to study the feasibility of artiBcial recharge.Here, we present a case study for identifyingsuitable recharge sites/zones and to assesstheir feasibility using geophysical data in the studyarea.

2. Study area

Raipur, the capital city and major business andeducational hub of Chhattisgarh, India (Bgure 1)has shown rapid urbanization during the last 15years. The city spreads over *13,083 km2 and hashuman population of *605,747 persons (2001census). Topography is generally Cat with surfaceelevation of 225–335 m asl. Average slope is \5�towards north. The excess runoA reaches surfacetanks and ponds of the area particularly duringrainy season.Average annual rainfall is 1258 mm and occurs

mainly in monsoon months (mid June–September).Daily minimum temperature is 20�C and the

14 of 14 J. Earth Syst. Sci. (2021) 130:14

maximum temperature reaches[40�C (IMD 2012).The area has hot summers and high annualevapotranspiration (average 4.9 mm/day).Soils are of two types: those derived from lime-

stone weathering are black, while other ones arered soil or Terra Rossa. The average thickness ofsoils is *10 m (Bodhankar and Chatterjee 1994;Dar et al. 2017). About 42% of the surroundingarea of the Raipur is cultivated mainly for rain-fedcrops (CGWB 2012). The study area shows theexposures of Chandi Formation of the RaipurGroup that overlies the basement granites andgranitoids. The rocks of the Chandi Formationinclude the lower limestone and the upper shaleand sandstone. The upper part of the limestone isquarried to 15–20 m depth for building stones.The rocks were intruded by maBc doleritic dykes

around 66 Ma before (Murti 1987; Das et al.1992, 2009; Patranabis-Deb and Chaudhuri 2007;Subba Rao et al. 2007; French et al. 2008; Chala-pathi Rao et al. 2011).Deodonger shale and sandsone member is

generally 0–30 m thick near Raipur (Mukherjeeand Khan 1996). The limestone aquifer provideswater for domestic, irrigation and industrial uses in

the Raipur. Nearly 34% of water requirement ofthe city comes from these aquifers. Groundwateroccurs in bedding planes, solution enlarged joints/fractures and cavities (CGWB 2012). Surfacekarst features are exposed in the area and tecton-ically developed fractures reach to about 130 mdepth.Water level in the aquifers are decreasing at a

decadal rate of 20 cm/year (CGWB 2012) due towhich most of the dug wells have dried up andpeople drill bore wells to 80 m deep. Chandi lime-stones has transmissivity ranging from 4 to 450m2/day and storativity values of 0.012 to 2.2910�6

(Mishra and Mohapatra 2002). General ground-water Cow direction is towards west. Rainfallrecharge is largely governed by solution cavitiesand fractures and joints (Guhey and Wadhwa1995; Guhey et al. 2009).A number of impractical activities aAect the

groundwater resources of the area which haveserious implications on its quality and quantityparticularly in karst aquifers (e.g., Schmidt et al.2013). The groundwater is becoming unsafe for useby humans. RunoA and recharge are aAected byclimate change, urbanization and anthropogenic

Figure 1. Location map of the study area with geology as background. Local drainage, fractures, and watershed boundary ofChokra Nala is also given.

J. Earth Syst. Sci. (2021) 130:14 of 14 14

activities. Groundwater draft exceeds the rate ofreplenishment due to low recharge conditions.Groundwater quality is constantly deteriorating.Therefore, management of the aquifers is of great

interest for future sustainability which could beachieved by enhancing inBltration through pur-poseful and managed recharge (Dillon et al. 2009).For this purpose, geophysics plays an importantrole in locating the suitable sites where managedrecharge could be successful.

3. Methodology

The electrical resistivity surveys were carried outkeeping in view the existing geological and hydro-geological information, already proposed rechargesites, lithology variation and viable depth ofinvestigation. It is a priori information that con-siderable resistivity contrasts occur between hardlimestone, intercalated cavities and other weaksubsurface zones.Initially dense vertical electrical soundings, VES

(12 soundings) and few ERT sections were rein-terpreted (from Dar et al. 2017) to delineate thearea of hydrogeological interest. The distancebetween current electrodes was from 100 to 500 m.Layered resistivity values were interpreted man-ually and using 1X-1D software. After reconnais-sance survey, a high resolution ERT was carriedout in order to image the subsurface karst for-mation. Based on the results, drilling was carriedat three sites where logging was also done tocompare the Bndings. Time domain electromag-netic soundings were also done but that could notproduce good resolution data due to less signalstrength.Resistivity measurements were carried out

using Syscal R1 Junior (IRIS Instruments,Orl�eans, France) equipped with 48 electrodes andwith ABEM Terrameter using 121 electrodes.The 1200 lengthy electrodes were placed 2–10 mapart in small hole, 20 cm below the groundcover, to ensure good ground contact for theduration of the experiment. Electrode separationvaries from 2 to 10 m depending upon the accessto surface area for laying the proBle. This leads tonon-uniform ERT section in terms of proBlelength and depth. In this study, the Wen-ner–Schlumberger and gradient conBgurationshave been utilized because it provides quite deeppenetration, reliable stability and ability to detectboth horizontal and vertical subsurface features

(Dahlin and Zhou 2004; Candansayar 2008). The2-D ERT data was acquired along 10 proBles nearthe Chokra Nala in Telibanda region of Raipur(Bgure 1).For locating the suitable pathways of recharge, a

dense proBling was done to cover maximum areaalong the drainage pattern. The data acquisitionstrategy was (i) to select at least one proBle veryclose to the drainage targeting to obtain typicalresistivity evidence for the suitable lithology and(ii) to situate additional possible proBles throughsites with similar lithological expression butunknown features.The ERT data was presented as inverted

models, which gave an approximate picture of thesubsurface resistivity distribution. The noise andspiky values were edited and then robust inver-sion (using L1 norm) procedure was utilized usingRES2DINV software, which is based on the reg-ularized least-squares optimization method(Sasaki 1989; deGroot-Heldin and Constable 1990;Loke et al. 2003).The experimental set up along the proBle E1, E3,

E4 and E5 included a row of 48 electrodes, whichwere separated by 4 and 10 m. A total number of529 datum points were measured at 23 data levels.The ERT proBles E2, E6, E7, E8, E9 and E10,

were acquired using the gradient protocol. A totalof 121 electrodes were used. We used ERT in thisstudy for the detection of aquifers or high porosityzones (e.g., fractures or solution channels), delin-eation of inBltration zones, and determination ofoverburden thickness.

4. Results and discussion

4.1 Vertical electrical soundings

The VES data and interpretation (Dar et al. 2017)show resistivity values ranging from 1–3000 Xmthereby indicating six contrasting subsurface layersup to 50 m depth. The Brst three layers up to 9 mdepth showed less contrast in values rangingbetween 1 and 200 Xm corresponding to the deeperlayers. This average thickness of layer 1 is less than1 m and the maximum thickness is 21.5 m for layer6. The 9–20 m deep zone showed up to 2000 Xmindicating a highly resistive structure (near ERTproBle E5, see Bgure 1) that shows a sharp transi-tion trending NE–SW. The highest resistivity upto 3000 Xm corresponds to dry and compactlimestone.


4.2 Electrical resistivity tomography (ERT)and lithologs

The ERT data obtained along 10 proBles (Bgure 1)was interpreted to infer a clearer depiction ofsubsurface hydrogeological variation in the lime-stone aquifer.

4.2.1 ERT proBles E1 and E2

The proBle E1 along NNE–SSW direction at Dev-puri with 10 m electrode spacing provided dataalong 480 m length, the pseudosection (Bgure 2a)of which indicates a weathered limestone or shalylimestone of nearly 50 Xm resistivity. This layerextends to 10 m depth below the surface. After thislayer, the ERT signature corresponds to hard/drylimestone up to a depth of 90 m with high resis-tivity of[150 Xm. A highly resistive cone in thecentre of the proBle is expected to be an intrusivebody because the resistivity values are too highthat fall in the range of dry limestone or a dryconduit.In Bgure 2(b), the proBle is in NNE–SSW

direction with Brst electrode towards NE and 121stelectrode in SW directions, respectively. The pro-Ble is 360 m long with electrode separation of only 3m and this resistivity data was able to pick smallervariation in lithological character at shallowerdepth. The low resistivity values at the two loca-tions (as marked in Bgure 2b) were believed to betwo sinkholes/depressions formed on the limestoneby dissolution. It was observed in the Beld that thediameter of the sinkholes was ranging from 1.5 to 2m and were approximately 60 m away from eachother. This proBle also indicated a sharp contrastin lithology at around 175 m distance below 15 mdepth. The high resistivity values on its left side

correspond to the dry limestone and the rightvalues represent the weathered or fractured lime-stone. Thus, this sharp contact is expected to be adeep fracture which has been widened by dissolu-tion activity.A very clear and sharp interface is observed

between ERT proBle E1 and E3, which indicates aconductive zone towards the SE of the study areaand a resistive zone to the NW of the area. FurtherERT proBle was carried out to identify this struc-ture. This shift also gives an indication of someexisting pathway or some weak structure betweenthis zone.The two ERT proBles (E1 and E2, Bgure 2)

intersected at around 198 m electrode of E3 wherea bore well (DW01) was drilled as shown by blackdashed line in Bgure 2(b). After observing thesurface geological features, the resistivity datawas compared with litholog and borehole loggingdata.The litholog of DW01 shows top iron rich sticky

clay up to 1.5 m depth below which lies a zone ofsticky yellow clay with limestone chips up to 11 mdepth. The bottom layer is the limestone with sub-angular black chips. In this well water striked atvery shallow depth of about 4–5 m and the staticwater level was also very shallow (0.9 m bgl). Theresistivity logging was performed to a depth of41.14 m at an interval of 0.5 m within the watercolumn (Bgure 3). The results of resistivity logshowed interesting features at different depths.Three major litho-units within the limestone andtheir depth ranges were demarcated based ondrilling results as highly-weathered Brst layer up to16 m with 35–100 Xm resistivity, semi-weatheredsecond layer from 16 to 25 m with 100–150 Xmand the bottom layer of fresh limestone (25–41 mdepth) with above 150 Xm resistivity. The kinks in

Figure 2. The 2D-electrical resistivity tomography inverted model along the proBle E1 and E2.


the logged resistivity data indicate small fracturezones at 24 and 34 m depths with low potential ofgroundwater yield.

4.2.2 ERT ProBle E3

This proBle was laid down in NW–SE directionfor the ERT data acquisition (Bgure 1). Themodel shown in Bgure 4 brings out the layeredstructure of the subsurface limestone. The lime-stone material from rock quarries has beendumped in the area and few locations were foundalong the line of this proBle as shown in Bgure 4.From depth of 5 m, 15–20 m thick of dampedlimestone is present with resistivity varyingbetween 50–150 Xm. Below this zone is drylimestone up to 90 m which possess high resis-tivity values of more than 300 Xm. These results

show a close analogy with the 1-D VES results inDar et al. (2017).

4.2.3 ERT proBle E4

The proBle is in NE–SW direction (Bgure 5) and isalmost parallel to the drainage ofChokraNalawhichis Cowing in SW–NE direction. The Brst electrodewas approximately 1–2 m away, central electrode30–35mawayand48th electrode 10–12maway fromdrainage line. The model shows that the top layerwith thickness of 8–10 m is conducting as it hasresistivity values of\80–100 Xm. The structure isnot so clearly layered as in previousmodel as there isa kink observed at a distance of 80–90 m from theBrst electrode position. Though the resistivity val-ues vary from 80 to 100 Xm at this location, belowwhich there is conductive layer with chunks of

Figure 3. Depth-wise qualitative comparison of 2D electrical resistivity tomography inverted model with the litholog of thedrilled well (DW01).

Figure 4. The 2D-electrical resistivity tomography inverted model along the proBle E3.


limestone. This conductive structure also suggeststhat the only top layer up to 10 m is conducting andthe possibility of recharge is not feasible as there areno pathways encountered here.

4.2.4 ERT proBle E5

The top layer of 6–8 m in this proBle is veryconducting with resistivity \15 Xm. The SadduNalah, northern part of the local drainage, isparallel to the proBle and Cows SW to NE.Weathered limestone/shale is interpreted from 8m up to 15 m depth below which occurs drylimestone with high resistivity more than 400 Xm.At this location, two cross-proBles were acquired.Figure 6 shows the cross proBles which intersectseach other at the tie-point marked in respectivemodel where the resistivity values exactly corre-lates. The overall resistivity variations give thesame lithology of damp limestone up to a depth of10 m on eastern Cank and 15 m on the westernside of the proBle. The resistivity varies\100 Xmup to 20 m depth on the western end. Belowwhich hard limestone occurs up to 45 m depthwith high resistivity up to 400 Xm.


There is Cowing nalah in SW–NE directionapproximately 25 m away from 60th electrodetowards the west of the proBle (Bgure 7). Theresistivity varies\100 Xm up to 8 m depth towardsnorth below which dry and hard limestone isencountered up to 45 m depth (resistivity morethan 1000 Xm). Another extension proBle E7 waslaid to the north of Chokra Nala, divided bynational highway. It also shows the hard and com-pact limestone after the depth of 10 m that con-tinues up to 50 m below. At these two sites, alreadythe check dam structures were proposed and erec-ted. But comparing with the resistivity structure ofsubsurface (Bgure 7), it is quite evident that theeDciency of these constructed MAR structures arenot suitable for artiBcial recharge as the hardlimestone below will not allow inBltration to takeplace. To conBrm, a borewell was drilled and wasquasi-correlated with the resistivity variations.The well (DW02) along Chokra Nala at E6

proBle was drilled at electrode position 160 m(Bgure 8). The litholog shows that top layer isblack cotton soil followed by yellow and brown soilwith dolomite chips. Underlying this layer is the


Figure 6. The 2D-electrical resistivity tomography inverted model along the proBle E5 with another cross proBle performed atthe same location.


fresh limestone containing water-yielding fracturesat 30 m depth. In DW02 well water striked at 9 mdepth at contact between soil and limestone and itsyield gradually increased. The yield was compara-ble to other two drilled wells. The highly weatherediron-oxide nodules were seen in top weathered zoneand cavern (dissolution) activity was also observedin this well. The collapse of this well after drillingand casing was done also indicates this dissolved

nature of the limestone below. Due to high pres-sure, resistivity logging could not be performed atthis location.

4.2.6 ERT proBle E8

The ERT proBle acquired along NE–SW direction(Bgure 9) showed very interesting features of verylow resistivity as compared to those corresponding

Figure 7. The 2D-electrical resistivity tomography inverted model along the proBle E6 and E7. The dotted line shows thelocation of drilled borewell.

Figure 8. Depth-wise qualitative comparison of 2D-electrical resistivity tomography inverted model with the lithology of thedrilled well at the site DW02.


to the hard limestone. Since it is very near to theCowing nalah and suspecting the possibility offracture zone, a cross proBle in NNE–SSW direc-tion was also carried out. Seeing the resistivityvariation in both (E8 and E9, Bgures 9 and 11) theproBles, the same low resistivity zone was con-Brmed, giving a future possibility of a hidden weakfracture zone in the area.A bore well (DW03) was drilled around 320 m

electrode (as marked in Bgures 9 and 10) of which alitholog was prepared by collecting crushed lime-stone samples with depth. The litholog showedhighly weathered limestone or shale unit with ironoxide nodules.

In all the wells, the observed lithologicaltrend is top thin black cotton soil followed bybrown to yellowish sticky clay (mud) withunderlying limestone (Bgure 10). The drillingresults have given different litho-units from topbottom. The top-most layer is covered with thinblack cotton soil up to 1.5 m followed by yel-lowish-brown soil up to 6 m depth. Theobserved bottom-most layer is limestone up tothe full drilled depth. In this well, water strikedat the depth of 30 m, i.e., from semi-fracturedzone. Weathering proBle is also observed show-ing solution channels and significant presence ofiron oxide as nodules.


Figure 10. Depth-wise qualitative comparison of 2D electrical resistivity tomography inverted model with the lithology of thedrilled well at the site DW03.



E9 and E10 are parallel proBles to the proBle E8(Kachna). E9 is at a distance of 2.5 km awayfrom E8, whereas E10 is at 5 km away from E8.The high resistivity feature of dry and hardlimestone encountered in E8 (Bgure 11) towardsSW of the proBle continues towards the westernpart of the study area. It is obvious that thesetwo parallel proBles reveal the thick, dry andcompact limestone as a layered formation. Thetop conducting layer conBrms that the traditionalpond/lakes exist on the massive part of dry and

hard limestone and not on the karst aquiferzones.

5. Proposing a scenario of subsurfaceaquifer for artiBcial recharge

From the detailed VES and ERT data combinedwith the topographic and geological information, amap (Bgure 12) is developed which shows theconductive and resistive zones in the aquifer. Basedon the results of all ERT proBles, it is found theresistivity ranges from 1 to 1500 Xm and forms

Figure 11. The 2D-electrical resistivity tomography inverted model along the proBle E9 and E10.

Figure 12. Surface electrical imaging around Chokra Nala and Telibandha lake demarcating the geological boundaries, leadingto potential recharge zones.


different zones. These proBles are independentlycorrelated with VES results in the same area andlithology of the drilled wells. A hydrogeologicalsection representing proBle E6 (in Bgure 7) repre-sent the geology of the area in Bgure 13. Thesezones correspond to the weak and/or fractured andfresh limestone area, respectively. The weak zonesare expected to be more suitable for artiBcialrecharge structures as the groundwater inBltrationwill be more as compared to the fresh, hard lime-stone of the resistive zones. In Bgure 2, the con-ductive zone exists in the weathered part inisolated patches. It is identiBed as probable sink-holes (as per the Beld conditions), but similar fea-tures can also be a part of parched water table in

the aquiclude of the area. In Bgure 9, the identiBedconduit can help in the delayed recharge to theChokra nala and then to the limestone aquiferbeneath. Normally these conduits have low storagecapacity and a moderate transmissivity of thelimestone. This may be an important aspect for thesustainability of the karst aquifers in the zone.Different zones are demarcated in the aquifer

which corresponds to the highly weathered, frac-tured and/or dissolved limestone. These weakzones are also quasi-correlated with the ERT andVES results located in this zone which gave verylow resistive values (Bgure 13).Similarly, the orientation of the possible under-

ground dyke is also proposed which is alreadyidentiBed and mapped dykes in the nearby areas(D1, D2, D3) (Chalapathi Rao and Lehmann2011). The southern part has more number ofsurface water bodies which is mainly due to thepossibility of more weathering/dissolution in thispart. The dyke also makes an underground barrierbetween the northern and southern parts of thestudy area where the drainage collected fromsouthern streams has more possibility to inBltratein the southern zone only.

6. Conclusion

Already existing artiBcial recharge structure (alongthe Chokra nala, near the nursery, shown inBgure 12) is located in the zone of resistive geologywhere the possibility of inBltration is low. If any voidis created due to deep groundwater table, the con-nected aquifer increments the storage and therebysupports the delayed recharge. In turn the saturated

Figure 13. Hydrogeological representation of resistivity variation of ERT proBle E6 shown in Bgure 7 in consideration of thedrilled lithology.

Figure 14. Lucid representation of 3D fracture system inlimestone from 2D ERT proBles for the establishment of MARalong the solution channels, conduits and fracture network.


zone thickness becomes more. Hence, this work alsoshows the necessity of adopting this methodology ofusing geophysical sub-surface methods over theother methods developed by other agencies inimplementing theMARprojects in different parts ofthe country. However, for MAR, these conductivezones with low resistivity are the most evident pla-ces for eAectively managing the aquifer recharge.The present carbonate aquifer model made on

this integration as shown in Bgure 14 is a lucidrepresentation of speciBc characteristics and pro-cesses of the karst region which represents thepassage of water through solution channels, con-nected with surface features (karst, sinkholes andsinking streams), which act as a source and carrierof water to feed the present aquifers.The present model is based on geophysical

investigation and geological logs to get a clearerpicture of the area. The earlier model presented inDar et al. (2017) has not explained the properchannelling system from the surface to the aquifer.However, the model (Bgure 14) in the presentmanuscript has the obvious view of the system andwell visualized the solution channel connectivitywith the Bssures and conduits. The major charac-teristics which differentiate the karst aquifers fromother kind of aquifers (mainly hard rock aquifers) isthe solution channels and conduits which providesas a better carrier for water transportation into theaquifers. In this region, the primary rechargesources are surface water which contribute as amajor source to recharge the aquifers throughsinkholes and solution channels.Geological modelling in the karst aquifers are

quite challenging due to the highly heterogeneousnature of the terrain. However, a good networkingof geophysical data and geological observation canBll the gap in order to visualize the real scenario ofthe karst system. Herein we are presenting the realsubsurface scenario of the carbonate terrain in theparticular area through the integrated approachbased on geophysics and geology.

Acknowledgements

Director CSIR-NGRI is acknowledged for thesupport during the work. The research was funded(grant agreement No. 282911) by the EuropeanCommission (EC) under the 7th framework projecttitle ‘SAPH PANI’ (http://www.saphpani.eu/).Authors are grateful to Prof. T Dahlin for criticallyviewing the manuscript and helping in improving

this publication. The Beld support from TarunGaur and Deepak Kumar is recognized in collectinggeophysical data. Editor and Reviewers are highlyacknowledged for bringing out this manuscript asper the expectation of the readers.

Author statement

The Brst author TA is responsible for designing thesurvey strategy, data processing and data inter-pretation. TA is the lead from inception of thiswork towards the writing of this publication. TWhelped in translating geophysical models into geo-logical cross sections. FAD provided the geologicalinputs for interpretation of the acquired dataset.SA contributed in providing his guidance duringthe whole process of bringing out this publication.

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electrical imaging of karst terrene for managed aquifer

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