INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 3, No 1, 2012

INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 3, No 1, 2012 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN 0976 4380 Groundwater potential zone identification using geoelectrical survey: A case study from Medak district, Selvam.S, Sivasubramanian. P Department of Geology, V.O.Chidambaram College, Thoothukudi, Tamilnadu, India geoselvam10@gmail.com ABSTRACT A Geoelectrical resistivity survey using Vertical Electrical Soundings (VES) was carried out in Medak District,, in order to assess the subsurface geology and groundwater potential zones. Twenty six vertical electrical soundings were recorded with Schlumberger electrode configuration with current electrode spacing (AB/2) half ranging from 1 to 150m. The field data has been primarily interpreted using curve matching and electrical imaging computer software IPI2WIN. The curves are prominently of A and H type indicating the presence of three layers followed by combination of curves AK, HA, KH, HK, AH indicating the four layer followed by combination of curves HAA, HAK, AKH, QHA, AAA indicating the presence of five layer sub-surface layers. Interpretations revels the number of subsurface layers, their thickness and their water bearing capacity within the study area. The best layer which acts as the good aquifer of Medak districts is the second layer which consist the fracture/weathered rock formations at the depth between 5 to 15 m. Keywords: VES, Schlumberger electrode, curve matching, subsurface layer, aquifer 1. Introduction The rapid urban development in Medak district, and the associated increases in population demands excesses utilization of groundwater. Because of the over exploitation of groundwater, the groundwater level has been declined in recent times warrants groundwater assessment for sustainable utilization within the study area. Geophysical techniques are the highly useful one for the identification groundwater potential zones and groundwater contamination studies. Among various geophysical studies, we have selected Vertical Electrical Sounding (VES) to carryout groundwater potential zone identification in the present study area. Vertical Electrical Sounding method was chosen because, the instrumentation is simple, field logistics are easy and straight forward while the analysis of data is less tedious and economical (Zhody et al 1974, Ekine and Osobonye 1996, and Ako and Olorunfemi 1989). Using this method, depth and thickness of various subsurface layers and their water yielding capabilities can be inferred. The main objective of the present study is to apply electrical resistivity method (VES) to identify the groundwater condition and the nature of subsurface layers within the study area. 2. Study area Medak district is located 20 km north of Hyderabad (Figure 1). In 2006, the Indian government named Medak one of the country s 250 most backward districts. Medak district occupies an area of approximately, 9519 Square Kilometers. It forms a part of Deccan Plateau under Godavari basin and lies between North Latitudes 17 0 27 and 18 0 18 and East longitudes 77 0 28 and 79 0 10 falling in topographical sheet nos. 56 F, G, J and K of Survey of India. Manjira, a perennial tributary of River Godavari with its tributaries of Haldi Submitted on May 2012 published on July 2012 55

(Pasupuyeru) and Kundlair drains through district. Though the Perennial River Manjira is flowing through the district, there are no major irrigation projects worth mentioning in the districts. So the agriculture of the district mainly depends up on groundwater sources. The district receives an average rainfall of 873 mm, which increases from South to North. The mean maximum and minimum temperature varies from 40 0 C to 26 0 C. The Pre-monsoon groundwater level depth varies from a minimum of 4m to a maximum of 20m below ground level. Most of the area has water levels below 5m. In the Eastern part of the district, groundwater level ranges from 5 to10m. In the Post-monsoon, depth of water level ranges from a minimum of 1.8 to maximum of 14.5 m. Groundwater levels with less than 2m were reported at Northern and North western part of Sangareddy town. The areas having water levels of less than 5m during pre-monsoon have come up to 2-5 meters in post-monsoon. Figure 1: Location map of the study 2.1 Geology of the study area The study area is an elongated plateau trending NNE-SSW and sloping west and east. The important rock types are Peninsular Gneissic complex, Dharwar supergroup associated with younger intrusive of Archaean age separated by unconformable with overlying Basaltic flows of late Cretaceous to early Eocene age with sub-recent to Recent alluvium along with stream courses. Geologically Deccan basalt whose age is about 65m.y occupies most part of the study area. The isolated hill in the northern region is also capped by basalt. The bedrock is granite and a thin layer of silicified and fossiliferoused sediment (Lameta Formation). The geological succession is as follow, 56

3. Materials and method Black Soil Decan Basalt Lameta Formation Granite and Granitic Gneiss Electrical prospecting makes use of a variety of principles, each based on some electrical properties or characteristics of the materials within the earth (Olorunfemi et.al 1995 and Singh et.al 2002). In this study, Vertical Electrical Sounding (VES) method has been applied. VES survey was carried out in 26 locations (Figure 1) using schlumberger electrode configuration. The Schlumberger method was adopted for this study because of the fieldwork is faster, easier and economically save the money and software s are readily available for its interpretation (Todd 1980, Fetter 1994, Patra and Nath 1999, Selvam et al 2010). The resistivity values of the layers were measured using the ABEM SAS 300B Terrameter. The Schlumberger soundings were carried out with current electrode spacing (AB) ranging from 2 to 300m (AB/2=1m to 150m).The distance used for potential electrode spacing (MN) ranged from 0.3m to 10m (MN/2=0.15m to 5m). At each VES station electrodes were placed in a straight line and the inter-electrode spreads were gradually increased about a fixed center. The current was sent into the ground and the potential difference (V) due to this current was measured and recorded against the electrode spacing. With these values of currents (I) and potential (V) of the electrode configuration adopted one can get the apparent resistivities (ρa). The apparent resistivity values were plotted against AB/2 on double log graph sheets. The manner in which apparent resistivity values increase or decrease with electrode separation forms the basis for choosing the shape of the field curve that can perform quantitative interpretation of the sub surface resistivity distribution (Guana Sundar and Elango 1999, Singh et al, 2002 and Muthuraj et.al, 2010). The field data are interpreted by a 1D inversion technique followed by forward modeling by IPI2WIN software was used. 4. Result and discussion The field data were interpreted and processed qualitatively and quantitatively by using partial curve matching techniques and computer to obtain the resistivity values of different subsurface layers and their corresponding thickness (Table 1). From the interpretation of VES curves, 3 to 5 subsurface layer where identified within the study area. The curves are prominently of A and H type indicating the presence of three layers followed by combination of curves AK, HA, KH, HK,AH indicating the four layers followed by a combination of curves HAA, HAK, AKH, QHA, AAA indicating the presence of five layer sub-surface layers (Figure 2). These curves were interpreted using the partial curve matching technique using two and three layer master curves (Rijkwaterstata 1966, Orellana, and Mooney 1966) and corresponding auxiliary curves to obtain the resistivity and thickness of each of the layers delineated. Locations S7, S10, S14, S15, S22 and S25 can be explained by a three layer model (ρ1>ρ2< ρ3 and ρ1<ρ2< ρ3 ) with the presence of H and A type curves (Figure 2a). The first layer is the top soil with resistivity ranging from 14-30 ohm-m and a thickness range from 0.5m - 6.2 m. Their Second layer is a fracture zone with resistivity ranging from 3.8-38 ohm-m and 57

thickness ranges from 0.33m to 6.7m. The basement resistivity is 300-1191 ohm-m respectively. The basement resistivity data indicates the rock type is hard and massive. Table 1: Summary of VES data interpretations with positions VES No 1st Layer Resistivity ρ1 (Ohm meter) 2nd Layer Resistivity ρ2 (Ohm meter) 3rd Layer Resistivity ρ3 (Ohm meter) 4th Layer Resistivity ρ4 (Ohm meter) 5th Layer Resistivity ρ5 (Ohm meter) h1 (meter) h2 (meter) h3 (meter) h4 (meter) Depth to basement in meters 1 21.4 8.49 4.09 32.8 6168 0.48 3.87 4.18 28.9 37.4 2 18.1 41.1 8.37 7691 0.91 0.72 19.7 21.4 3 50.5 4.49 18 99.6 5576 0.52 0.55 4.46 66 71.5 4 36.7 8.48 141.8 3783 0.53 3.81 60.97 65.32 5 8.9 2.9 3.8 678 0.78 2.5 4.2 7.5 6 14 1.7 4.7 15 1430 0.46 0.55 2.3 8.1 11 7 26 4.3 301 0.57 6.5 7.1 8 14 5.8 18 285 0.89 2.7 1.9 5.5 9 31.9 14.8 107 1216 6.6 1.07 1.22 18.8 17.3 38.5 10 17 14 291 6.2 0.33 6.5 11 22 8.5 900 54 0.83 4.6 3.3 8.7 12 65 18 22 187 0.97 4.9 0.98 6.9 13 5.8 6.4 288 0.002 3 2.2 552 557 14 14 38 50 1.1 6.6 7.7 15 34 3.8 1191 0.51 6.7 7.2 16 20.3 7.25 3.32 31.2 5645 0.45 2.86 4.11 29.35 36.77 17 17.2 42.2 3.39 8526 0.99 0.65 18.6 20.24 18 52.3 4.4 15 100.1 5555 0.62 0.96 6.03 75 82.61 19 33.3 7.62 163.2 3666 0.52 2.99 55.66 59.17 20 6.2 2.3 3.2 556 0.66 2.3 3.2 6.16 21 1.3 1.6 4.4 10 1111 0.41 0.65 2.1 8.0 11.16 22 27.8 5.5 362 0.51 6.5 7.01 23 13 5.6 15 14.5 0.88 2.1 1.8 4.78 24 30.2 16.3 102 1211 3.6 1.02 1.01 17.8 14.6 34.43 25 16 14 261 5.6 0.32 5.92 26 23 27 533 52 0.76 3.2 3.2 7.16 Locations S2, S4, S5, S8, S11, S12, S13, S17, S19, S20, S23 and S26 can be explained by a four layer model (ρ1< ρ2< ρ3> ρ4) (ρ1> ρ2< ρ3< ρ4) (ρ1> ρ2< ρ3> ρ4) (ρ1<ρ2> ρ3< ρ4)) with AK, HA, KH, and HK type curves. The resistivity of the first layer (top soil) ranges from 5.8 to 65 ohm-m and a thickness is between 0.5m - 3 m. In the second layer, hard and compact sand was inferred with the high resistivity values ranging from 23-27 ohm-m and 1.6m - 9.6m. The Third layer has resistivity ranging from 4.1-18 ohm-m and a thickness 8.4 m. This layer acts as the shallow aquifer in these places because this layer consists of fracture or weathered zone which constitutes an aquifer of very good quality of groundwater. The fourth layer is the hard rock without many fractures (Figure 3). 58

Figure: 2 Interpretations of Vertical Electrical Sounding Field Curves Figure: 2a Interpretations of Vertical Electrical Sounding Field Curves 59

Locations S1, S3, S6, S9, S10, S18 and S21 can be explained by a five layer model (Table 2) ((ρ1> ρ2> ρ3< ρ4< ρ5) (ρ1> ρ2< ρ3< ρ4< ρ5) (ρ1> ρ2< ρ3< ρ4> ρ5) (ρ1< ρ2< ρ3< ρ4< ρ5)) with QHA, HAA, HAK and AAA type curves (Figure 4). Here, we have two aquifers. The shallow aquifer is within the second layer at a depth of about 27m with resistivity value of 1.3-8.1 ohm-m. The second aquifer in the fourth layer at a depth of 66m and resistivity value of 15-100 ohm-m is not a confined layer. Basement and hard rock resistivity value is 173-1430 ohm-m. Figure 3: Resistivity pseudo section and 2D inverse model (VES S4, S2, S3, S18 and S17) Figure 4: Map showing resistivity curve type distribution within the study area. 60

Table 2: Qualitative analysis of curve types where ρ represents resistivity of the layer VES Curve type Curve Characteristics No. of Geo-electric layer S1, S16 QHA ρ1> ρ2> ρ3< ρ4< ρ5 5 S2, S17 KH ρ1<ρ2> ρ3< ρ4 4 S3, S6, S18 HAA ρ1> ρ2< ρ3< ρ4< ρ5 5 S4, S5, S8, S12, S19, S20 S7, S10, S15, S22, S25 5. Conclusion HA ρ1> ρ2< ρ3< ρ4 4 H ρ1>ρ2< ρ3 3 S9, S24 HAK ρ1> ρ2< ρ3< ρ4> ρ5 5 S11, S23 HK ρ1> ρ2< ρ3> ρ4 4 S13, S26 AK ρ1< ρ2< ρ3> ρ4 4 S14 A ρ1< ρ2< ρ3 3 S21 AAA ρ1< ρ2< ρ3< ρ4< ρ5 5 Present work has shown that in a hard rock environment, Vertical Electrical Sounding (VES) has proved to be very reliable for underground water studies and therefore the method can excellently be used for shallow and deep underground water geophysical Resistivity investigation. The most part of the study area consists of good quality of groundwater because the study area is dominated by the H type curve. The top layer is the black cotton soil and it is followed by a weathered zone which is underlain by basement rock. The best layer which acts as the good aquifer of Medak districts is the second layer which consists of the fracture/weathered rock formations at the depth between 5 to 15m. Figure 4 revels the distribution of resistivity curve types within the study area. Using this Figure 3 can easily understand the water bearing capacity of a particular location within the study area. Acknowledgement The first author is thankful for Indian Academy of Science, Bangalore, Indian National Science Academy, New Delhi and The National Academy of Science, Allahabad for providing financial assistance through summer research fellowship to carryout this work. The first author is also thankful to Dr.T.Seshunarayana, Head, Eng. Geophysical Division, NGRI, Hyderabad for his valuable guidance during my Summer Research Fellowship Camp at NGRI. 6. References 1. Ako, A O., M O Olorunfemi., (1989), Geoelectric survey for groundwater in the Newer Basalts of Vom Plateau State. Nigeria journal of mining and geology, 25, pp 247-450. 61

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