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INTERNATIONAL JOURNAL OF RESEARCHES IN BIOSCIENCES, AGRICULTURE AND TECHNOLOGY © VISHWASHANTI MULTIPURPOSE SOCIETY (Global Peace Multipurpose Society) R. No. MH-659/13(N)

www.vmsindia.org GEOELECTRICAL INVESTIGATIONS FOR POTENTIAL GROUNDWATER IN PARTS OF AND DISTRICTS,

Mahendra R. Gaikwad and Gautam Gupta 1* Department of Geology, G.B. Tatha Tatyasaheb Khare Commerce, Parvatibai Gurupad Dhere Art’s and Shri. Mahesh Janardan Bhosale Science College, Guhagar Dist. Ratnagiri - 415703 (M.S.), () 2 Indian Institute of Geomagnetism, New Panvel (W), Navi Mumbai - 410 218 [email protected]

Abstract Electrical resistivity is one of the most effective ge ophysical methods for investigating the presence of groundwater. In a hard rock terrain like the Kolhapur, Ratnagiri and adjoining regions in the Deccan Volcanic Province of Maharashtra, the nature and extent of we athering may vary significantly, depending mostly on the presence of fractures and lineaments at depth and the geomorphological features at the surface. Hence , electrical resistivity studies are vital in a hard rock terrain for identification and analysis of concealed lineaments and fractures zones. An attempt is made here to understand the tectonic framework of over Devrukh-Ganapatipule region and Malkapur- Sakarpa-Ratnagiri region and its relation to the movement of groundwater. A total of 43 vertical electrical soundings we re conducted in Devrukh-Ganapatipule and Malkapur-Sakarpa-Ratnagiri region. The profile over the Malkapur- Ratnagiri area was divided into two profiles. It is observed from the two dimensional geoelectric cross-section that this region has more potential groundwater aquifers than the previous profile. The resistivity ranges are also less than the Devrukh-Ganapatipule profile. A couple of lineaments were also identified over this profile. Keywords: Deccan volcanic province, electrical resistivity, geoelectrical sections, groundwater potential zones.

1 Introduction current pene trates is changed. However, the The electrical resistivity me thod is widely distance between the potential measuring used tool for mine ral and groundwater e lectrodes and the ir positions with respect to exploration, for mapping of basement current electrodes also affect the depth range configuration and for e ngineering and military from which information on the formation purposes. Se lf potential was the first parame ter resistivity is obtained. In this method, most to be observed some time in 1836 by Fox. commonly used are the four electrodes positioned Howeve r, Schlumbe rger and Wenne r developed on a straight line , the two current electrodes on their schemes during 1912 and 1915 the outside and the two potential measuring respectively. The resistivity sounding method was e lectrodes on the inside. first applied by Conrad Schlumbe rger in 1912. During a resistivity survey, current is The electrical resistivity me thod involves injected into the earth through a pair of current measuring the apparent resistivity of soils and e lectrodes, and the potential difference is rock as a function of depth or position (Alfano, me asured between a pair of potential electrodes. 1959). The resistivity of soil is a complicated The current and potential e lectrodes are function of porosity, pe rmeability, ionic content generally arranged in a linear array. Common of the pore fluids and clay mineralization. The arrays include the Schlumberger array, Wenner most common e lectrical methods used in array, dipole-dipole array and pole-pole array. hydrogeological and environmental The apparent resistivity is the bulk average investigations are vertical electrical soundings resistivity of all soil and rock influencing the (VES) and resistivity profiling (Baride et. al., current. It is calculated by dividing the measured 2017; Golekar et. al., 2014)). potential difference by the input current and The idea of resistivity sounding is to multiplying by a geometric factor spe cific to the explore the presence of anomalous conductivity array used and electrode spacing. in various forms, such as lumped (three In a resistivity sounding, the distance dimensional) bodies, dykes, faults and vertical or between the current electrodes and the potential horizontal contacts between two strata. To e lectrodes is systematically increased, thereby accomplish this purpose it is necessary to yielding information on subsurface resistivity arrange the measurements in such a manne r from successive ly greater depths (Mc Neil, 1990). that at different measurement point, the value of The variation of resistivity with depth is modeled the measured potential difference is affected by using forward and inve rse modeling computer the formation resistivities at differing depth software. ranges (Telford, 1990). This may be accomplished Direct current (DC) resistivity me thods by changing the distance between the current use artificial source of current to produce an electrodes, so that the depth range to which the e lectrical potential fie ld in the ground (Seidel and

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Lange 2007). In almost all resistivity me thods, a and 8326 sq. km. The district lies in the survey current is introduced into the ground through of India degree sheets 47 F, 47 G and 47H. The point electrodes (C1, C2) and the potential field is district is located between north latitudes 16°30' measured using two othe r electrodes (the and 18°04' and east longitude 73°20' and 73°52'. potential electrodes P1, P2), as shown in Fig. 1. The district is bounded in the north by Raigad The source current can be direct current or low Distric t, we st by Arabian Sea in the east by frequency (0.1 – 30 Hz) alternating current. The Satara, Sangali, Kolhapur district and in the aim of ge nerating and measuring the electrical south by Sindhudurg District. The geological potential fie ld is to determine the spatial formations in the area (Deshpande 1998) which resistivity distribution (or its reciprocal in desce nding order of the ir antiquity a re as conductivity) in the ground. As the potential below. be tween P1 and P2, the current introduced Alluvium, beach sand- Rece nt and Sub-Recent through C1 and C2, and the electrode Pleisto cene- Laterite and Lateritic spread configuration are known, the resistivity of the Miocene- Shale with peat and Pyrite nodules ground can be determined; this is referred to as Cretaceous to Eocene- Deccan trap Basalt lava the “apparent resistivity”. flow 1.1 Objective of the present study Upper pre Cambrian - Kaladgi series - quartzite, The purpose of this study is to de monstrate Sandstone , shale and associated lime stone potential applicability of e lectrical resistivity 1.4 Hydrogeology of the Ratnagiri Region surveys, especially vertical electrical sounding Deccan Trap lava flows (upper (VES) to delineate the potential zones of fresh cretaceous to lower Eocene age) Kaladgi water aquifers in a hard rock terrain. sandstone (Precambrian), Laterite (Pleistocene) 1.2 The study area and Alluvial deposit (Recent) is the water bearing Ratnagiri is one of the coastal districts of formations obse rved in . Maharashtra and forms part of the However Kaladgi formation occurs in ve ry limited region. It is situated in betwee n the Western patches and does not form potential aquifer in Ghats and the Arabian Sea and lies between the district. The Alluviums also has limited areal north latitude s 16°30' and 18°04' and e ast extent found mainly along the coast. Central longitude 73°20' and 73°52'. The present s tudy ground water Board periodically monitors 48 region is located in Ratnagiri and adjoining National Hydrograph Network stations (NHNS) Kolhapur districts. Two regions have been stations in Ratnagiri district, four time s a ye ar surveyed in this area. The first region Devrukh to i.e. in January, May (Pre monsoon ) August and Ganapatipule lies in be tween 17.00 to 17.20 N November (Post monsoon). The depth to water and 73.30 to 73.70 E. The second region levels in the district during May 2007 ranges Malkapur to Ratnagiri lie s in be tween 16.90 to between 2.25 (Shringartali) and 19.05 (Jaigad) 17.04 N and 73.40 to 73.95 E. mbgl. The shallow water levels within 10 mbgl are 1.3 Geology of the study area seen in almost entire district. The deeper water Geology of Kolhapur district levels in the range of 10-20 mbgl are observed in Kolhapur District located in Western isolated patches in parts and Rajapur, , Maharashtra Region between latitude 16° 5' 29'' Guhagar and Taluka. and longitude 74°14' 23''. The district is bounded 2 DATA ACQUISITION AND ANALYSIS in north and east by Sangali district, west by 2.1 Survey instrumentation Ratnagiri and Sindhudurg district and south by Model SSR-MP-AT Karnataka state. In the Kolhapur District two The IGIS signal stacking based signal distinct trends in the hill ranges are seen, one enhancement resistivity meter model SSR-MP-AT runs north-south pointing wild and hill slopes is a state of art microprocessor based data and valleys and the other trend stretching e ast acquisition system (Fig. 2). The instrume nt ward and merging gradually into plains. The design incorporates several innovative features geological formations in the area in descending and advanced techniques of digital circuitry to orde r of the ir continuity are as below. make it a reliable geophysical tool providing high Soil and Laterite -Recent and sub Recent quality data useful for mineral and groundwater Deccan trap -Lower Eocene exploration and any other geophysical Lower Kaladgi se ries -Cuddapah applications. Granite gne iss Dharwars- Archaean The SSR-MP-AT sends the entire current Geology of Ratnagiri district into the ground without wasting power for Ratnagiri district is located in the constant current generation thus increasing the Konkan region and covers a geographical area signal strength to probe deeper layers. It achieves

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exce llent depth penetration with relative ly low IPI2WIN is designed for automated and power inputs. It utilizes the signal stacking up to interactive semi automated interpreting of 16 successive reading to achieve good signal vertical electrical sounding and induced enhancement. In the presence of random (non- polarization data obtained with any of a variety of coherent) earth noises, the signal to noise ratio of the most popular arrays used in the electrical the SSR-MP-AT measurement will be enhanced prospecting (Bobachev, 2003). It is presumed by N where N is the numbe r of stacks. Hence that a use r is an experie nced interpreter willing SSR-MP-AT Resistivity Meter can be used for to solve the geological problem posed as well as depths of up to 600 m unde r favorable geological to fit the sounding curves. Targeting at the fie ld. geological result is the specific feature 2.2 Survey design with Schlumberger distinguishing IPI2WIN among other popular configuration programs of automatic inversion. Due to handy Schlumberge r electrode configuration has been controls the interpreter is able to choose from a adopted in the present study. In the set of equivalent solutions the one best fitting Schlumberge r configuration the potential and both geophysical data (i.e. providing the least current e lectrodes are arranged in the straight fitting error) and geological data (i.e. geologically line, where potential electrode spacing is smalle r sensible resistivity cross section). Comparing than the current electrodes. various concepts of the geological structure along the surveyed observation line rathe r than independe nt formal sounding curves, inve rsion is the approach implemented in IPI2WIN. This approach provides the opportunity to use a priory geological data and extract information to the greatest possible extent in the complicated There is large difference in the length between AB geological situations. For viewing curves and and MN, it is AB >> MN but, for getting good models, the pseudo cross- section of a specified results the length difference must be AB ≥ 5MN value and resistivity cross section are displayed (Kearey and Brooks, 1988). (VES-IP mode : or chargeability cross section) in 2.3 The spacing factor the pseudo cross section and resistivity cross Spacing factor for four electrodes situated section window, which can be used for on the surface of the earth is briefly obtained as interpretation. follows. If at the surface of the ground, an electric 2.5 Interpretation techniques current I is introduced by means of two point The concept of apparent resistivity is the key to electrodes A and B, when the current flows from the interpretation of the resistivity field data. A to B; potential at any point P on the surface is While the interpretation of the profile data is given by often qualitative, the quantitative interpretation Vp = 1/2 πσ (1/r1 - 1/r2) of the sounding field data has been facilitated by Whe re r1 and r2 are distances of the point P from the publication of resistivity master curves CGG A and B, respectively. Potential difference (1955) for Schlumberger array and Orellena and be tween two points P and Q, which are at Mooney (1966) for Wenner configuration. The distances r1, r2 and R1, R2 respectively from the curve matching technique was successfully used electrodes A and B is given by till Koefoed (1979) revolutionized the VP-VQ = V = 1/ 2 πσ (1/r1 - 1/r2 - 1/ R1 + 1/ computation of resistivity master curves by the R2) application of line ar digital filter after which Hence the resistivity is computer-aided interpretation and inversion ρ = 1/ σ = 2 π V/I [1/ (1/ r1 -1/ r2 - 1/ R1 + 1/ technique came more and more into picture. R2)] The curve matching technique is still being used Thus the factor 2 π / [1/ r1 - 1/ r2 - 1/R1 + 1/R2] as the first step which provides values of layer represents the spacing factor. This factor holds parameters from computer aided interpretation. good for all dispositions of the current and In the interpretation of resistivity sounding data potential electrodes and does not change with an the geological formations are assumed to be interchange of current and potential electrodes. horizontal or near horizontal (dip up to 15 o). The Difference in disposition of current and potential ground could have two layers, three or multi electrodes gives rise to the various e xploration layers, with different resistivity and thickness. In techniques in the resistivity method of sedimentary basins the re are layers of sandstone, prospecting. shale, limestone etc., whe reas in hard rock 2.4 Data analysis terrains soil and weathered rock formations

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overlie fresh rocks approximating a three-laye r In Devrukh to Ganapatipule region, three profiles section. If the top soil lies directly over the fresh (AB, CD and EF) are presented. rock, there are dominantly two laye rs. Detailed A-B Profile description of type curves and their AB profiles covers the stations VES 19, 21, 20, interpretation techniques have been provided by 18, 17, 16, 22, 23 and 3 from west to east. Fig. 5 Bhattacharya and Patra (1968), CGG (1955). a showed that below stations 19 to 22 hard and They have also provided computed master curve s compact rocks are encountered from shallow to for two and three -layer earth sections for different depths of about 20 m. This layer is having resistivity and variable thickness. The field resistivities in the range of 400 to more than 2000 curves are matched to yie ld layer resistivity and Ω.m. Furthe r be low stations 23 and 3, an aquifer thickness values, which in turn are interpreted in body is deve loped from depths of 2m up to 100 m terms of geology of the survey area. Computer having resistivitie s of about 60-150 Ω.m. Also at aided and inversion techniques (Das and Ghosh, lower levels this feature is trending towards west 1974; Ghosh, 1971; Inman, 1975; Koefoed, 1979) and is seen at depths of 50 m below the entire for the interpretation of the resistivity sounding profile. It is also seen that below station 22 and data have made rapid strides during the past 30 below stations 21, 20 and 18, ve ry low ye ars. Often the assumption of horizontal layers resistivities (be low 50 Ω.m) is de marcated at is not valid. Two-dime nsional structures or even depths of 50 m. It is possible that the aquifers at times three dimensional structures need to be developed are due to basalts or lateritic conside red. In such cases numerical techniques formation. In basaltic terrain, the groundwater suggested by (Mufti, 1980) have to be resorted to. occurs in vesicular zone as well as interflow zones In the present study, the concept of the profile in weathered and fractured zones. Laterite has interpretation is carried out using IPI2Win. It better porosity due to intricate ne twork of means that data for a profile are treated as a sinuous conduits making it a porous formation. unity representing the geological structure of the C-D Profile survey area as a whole, rather than a set of The CD profile lies in the ce ntral part of the study independent objects dealt separately. The region encompassing the VES stations 15, 14, 13, concept is implemented mainly by using the 12, 11, 10 and 9 from we st to east (Fig. 5 b). It is interactive se mi-interactive mode rathe r than the observed that below stations 15 and 14, the top automated interpreting the model. layer is not so resistive (130-250 Ω.m) up to IPI2Win is capable of solving resistivity electrical depths of about 5 m. Below this laye r a prospecting 1 D forward and inverse proble ms for conductive zone is encountered having a variety of commonly used arrays for the cross resistivities of about 60-130 Ω.m extending up to section with resistivity contrast within the range depths of 100 m. This conductive zone extends of 0.001 to 10000 ohm-m (Fig. 3). towards east below all the VES points at depth of The forward problem is solved using the linear 50 m. Very low resistivity (less than 50 Ω.m) is filtering. These thoroughly tested filters and seen below stations 14 and 13 in the depth range filtering algorithm implementation provide fast 50-100 m. The top 10 m be low stations 13-9 is and accurate direct problem solution for a wide having very high resistivity (more than 400 Ω.m). range of models, covering all reasonable This region is highly infested with lateritic geological situations. formation. It is possible that the fractures and The inverse problem is solved using a variant of fissures present in the lateritic terrain have the Newton algorithm of the least number of enabled the formation of groundwater zone at laye rs or the regularized fitting minimizing depths of 50 m and below. This profile is at a algorithm using Tikhonov’s approach to solving lower elevation than the profile AB. Be low station incorrect problems. A-priory information on laye r 9, an incomplete aquifer body is seen at depths depths and resistivites can be used for of about 80 m. regularizing the process of the fitting error E-F Profile minimizing. The inverse problem is solved Profile EF is in continuation of profile CD. This separately for each sounding curve . profile is encompassed with VES stations 9, 8, 7, 3 RESULTS AND DISCUSSIONS 6, 5, 4, 1 and 2. The station 9 is ove rlapping in The 2-D geoe lectrical section has been generated both CD and EF profiles for the sake of over five se lected profile s (3 over Devrukh and 2 continuity. It can be seen from Fig. 5 c that the over Sakarpa) in the study region in order to low resistive zone seen in profile CD is continuing understand the geome try of the aquifers at depths of 40 m and below. Howeve r below developed in this region (Fig. 4). station 6, an aquifer body is developed at shallow 3.1 Devrukh area depths of 1 m extending up to more than 100 m.

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Else where in the profile, the top layer up to 20 m ide ntified using satellite data (CGWB, 2009). is encompassed with hard rock reflecting From the foregoing it can be seen that the re are laterites. Towards VES 2, an incomplete aquifer well developed aquifer bodies over this profile seems to develop from depths of 2 m and below. with good potential for groundwater. Further stations towards e ast of VES 2 would C-D Profile have brought out the complete picture of the The sounding stations over profile CD are 14, 15, aquifer. 16, 17, 18, 20 and 19 (Fig. 6 b). This profile is in 3.2 Sakarpa area continuation of profile AB. Howeve r, due to the A total of 17 VES points have been sounded in Sahyadri mountain range and thus non Sakarpa region from Malkapur (Kolhapur) to availability of sites, a large gap of about 20 km Ratnagiri. The entire profile is divided into two exists between VES stations 13 and 14. The parts (AB and CD) for the sake of discussion (Fig. features reflected below VES station 13 (Profile 6). AB) continues be low station 14 and 15 (Profile A-B Profile CD). It can be seen that an aquifer body is The profile AB contains the VES stations 1, 11, 5, developed below station 15 from depths of 2 m 6, 7, 8, 9, 10, 12 and 13 (Fig. 6 a). It can be seen and continues down up to 100 m. This low from this figure that be low VES stations 1 and resistive zone (20-50 Ω.m) is obse rved below 11, hard rocks having resistivities of about 100 stations 14, 16, 17, 18 and 20 at depths of 20 m to more than 400 Ω.m is encountered at de pths and below. A high resistive feature (more than extending from 1 m up to 50 m. At VES station 5, 150 Ω.m) is delineated below stations 17 and 18. a clear groundwater aquifer zone is delineated Furthe r east high resistivity (more than 170 Ω.m) from shallow depths up to about 30 m. This is obtained below station 19. In be tween stations feature is extending further down and trending 18 and 19, a low resistivity (60-120 Ω.m) patch is towards we stern side below stations 11 and 1. observed. Perhaps the lineaments obse rved in This feature is very conductive having this region divide the low resistive station 20 from resistivities less than 10 Ω.m. Towards east of the high resistive station 18 and 19. station 5, a high resistivity block is encountered be low VES stations 6, 7 and 8. This block with resistivities of 190-400 Ω.m is extending from shallow depth up to about 50 m. Anothe r aquifer zone to the east of this resistive block is observed be low VES stations 9, 10 and 12. This feature is oval in shape having resistivities in the range of 10-50 Ω.m with depth extent from 1-20 m. Further below VES station 13, a small aquifer seems to de velop at shallow and deeper leve ls.

From Fig. 6 a it can be proposed that the low conductivity zone sandwiched be tween two high Figure 1 . Resistivity measureme nts of four resistive zones could be due to the prese nce of e lectrodes some lineaments. Many north-south and east- we st lineaments in this region have been

Figure 2. SSR-MP-AT Resistivity me ter

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Figure 3. Typical geoelectric cross-section

Figure 4. Location of VES sounding points over Devrukh and Sakarpa

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Figure 5. Geoelectrical model in Devrukh-Ganapatipule region

Figure 6. Geoelectrical model in Malkapur-Ratnagiri profile

4 CONCLUSIONS ACKNOWLEDGEMENT The resistivity technique is used to study Authors are thankful to the Director, Indian the horizontal and vertical discontinuities in the Institute of Ge omagnetism, Ne w Panvel (W), Navi electrical prope rties of the ground. In the present Mumbai and Principal, Gopal Krishna Gokhale work Schlumbe rger array configuration was College Kolhapur for providing instrume ntal adopted to categorize potential groundwater facilities to carry out this work. zones in the western part of Maharashtra. Two REFERENCES linear profiles were sounded in the districts of Alfano, L. Introduction to the interpretation of Ratnagiri and Kolhapur. The first profile was resistivity me asurements for complicated from Devrukh to Ganapatipule and the se cond structural conditions. Geophys. Prosp, 7, 311- 66, 1959. profile was from Malkapur to Ratnagiri. Two dimensional modeling was carried Baride Mukund Vasantrao, Patil Jitendra out in this region. The Devrukh-Ganapatipule Bhaskarrao, Baride Aarti Mukund, Golekar profile was divided into three parts for the sake of Rushikesh Baburao, Patil Sanjaykumar Narayan. Comparative study of Wenner and clarity. It is observed that the western part of the Schlumberger electrical resistivity method for profile is dominated by very high resistivity top groundwater investigation: a case study from laye r up to depths of about 10 m. The eastern Dhule district (MS), India Applied Water Science part is conductive and is potential for pp 1-20, 2017). groundwater exploitation. The causative factor https://doi.org/10.1007/s13201-017-0576-7 for high resistivity is the presence of laterites in Bhattacharya, P.K. and Patra, H. P. Direct this region. However, seve ral lineaments are cris- current Geoelectric. Elsevier, Amsterdam, 135 p., crossing this region and hence these lineaments 1968. are also playing a significant role in the Bobachev, A. Re sistivity Sounding Interpretation. movement of groundwater. IPI2WIN: Version 3.0.1, a 7.01.03. Moscow State The profile over the Malkapur-Ratnagiri University, 2003. area was divided into two profiles. It is observed CGG, 1955. Abaquer de Sondage Electripue. from the two dimensional geoe lectric cross- Geophy. Prosp. 3 Suppliment 3, The Ne therland. section that this region has more potential Central Ground Water Board (CGWB) groundwater aquifers than the previous profile. Groundwater Informa tion, Ratnagiri district, The resistivity ranges are also less than the Maharashtra; Technical report No. Devrukh-Ganapatipule profile . A couple of 1616/DBR/2009. lineame nts were also identifie d ove r this profile. Das, U.C. and Ghosh, D.P. The determination of filter coefficie nts for computation of standard

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