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GIS BASED GEOPHYSIAL STUDIES OF A WATERSHED
Dr. P. Porchelvan, Professor, SMBS, VIT University
Dr C Bala Murali Krishna, Asst. Professor, SMBS, VIT University
Ms. R Poornima, Lecturer, SMBS, VIT University
Introduction
The geophysical survey submit to the scientific dimensions of physical properties of the mother earth and which may be interpreted in terms of rock type, geological formation, weathering thickness, porosity and fracture zones. Among various geophysical techniques, the electrical resistivity methods have been widely used by the field hydrogeologist for groundwater studies. In recent years this method has become important in the scientific investigation of environmental problems. Due to this basis, electrical resistivity method has been used for interpreting the various geological formation and lithology of Mangadu mini water shed in the present study.
Study area
The study area viz., Mangadu mini watershed is one of the important watersheds of Palar river basin. It is one of the major 17 river basins of Tamilnadu state in India, covering one tenth of the state geographical area. The Mangadu watershed lies in Arcot and Walajah Blocks of Vellore District, Tamilnadu, India (Figure 1.1). It has an aerial extent of 112 km2. There are about 43 villages covered in this mini watershed. It lies in the Survey of India Topographic map No.57 P/5 and in between the coordinates of Latitudes 12° 48˘ 00˛ to 12° 57˘ 30˛ and Longitudes: 79° 19˘ 40˛ to 79° 25˘ 00˛ (Figure 1.2).
Geophysical investigation in the study area
Geophysical investigation using electrical resistivity method was carried out in 26 locations in the Mangadu mini watershed area. Schlumberger configuration with electrode spacing of 100 m maximum was employed by using a D.C resistivity meter. For evaluating the groundwater potential, lithology and quality zones in the study area, the results of the 26 Schlumberger soundings were analyzed.
The minimum and maximum apparent resistivity value ranged from 4 to 1930 W m. The table 1 summarizes the arithmetic mean of resistivity, the number of locations and the layer as observed in the study area.
TABLE 1 Arithmetic mean of resistivity thickness and locations & layers
All the 26 locations had three layers existing within 28m from the ground level. The arithmetic mean of resistivity 51 Wm, 136 Wm and 370 W m were obtained for 4, 12, 28m depth. It is observed only 22 locations had four layers. An arithmetic mean resistivity of 878 W m with a mean thickness for fourth layer as 32m was noted. The fifth layer was located in only one place called Manthangal. The mean resistivity values indicate that there was a steady increase in the resistivity value for most of the stations, where as in the fifth layer, there was a fall in resistivity which may be due to the development of fracture/fissures. The lithology of that location (Manthangal) indicate a weathered granitic gneiss exist up to 30m from the ground surface as followed by charnokite with joints
LITHOLOGICAL INTERPRETATION
Well cuttings and bore well details give visible evidence of the rocks and permit direct measurements and evaluation. These resistivities of different layers are correlated with the lithology information of the study area
There were wide variations in values of the resistivities of rock formation. The lowest resistivity value (10 W m) was observed at Paperi (location no: 18). This may be due to the presence of clay soil at the top. It is also noted that wherever the topsoil was clay, the resistivity of that place was low. At location16 and 20 the resistivity values were low which was attributed to the highly weathered formation having more thickness of weathered formation.
The resistivity values of 27 W m, 116 W m and 197 W m were recorded at 5m, 40m and 80m depth respectively in Manthangal (location no: 3). In this area the topsoil was black cotton soil existing up to a depth of 1m, followed by a highly weathered granitic gneiss with jointed and fracture gneiss was observed up to the depth of 30m. Because, of this presence of clay (black cotton soil) and highly weathered granitic gneiss a low resistivity was noted at 5m depth of Manthangal area. The turbidity of this water was very high. High TDS value was also observed in this area with high hardness. Manthangal is very close to the Ranipettai industrial zones. Due to the highly weathered granitic gneiss to the depth of 30m in Manthangal area the surface water along with industry wastewater can percolate easily to the subsurface and mixed in to the groundwater. Hence the low resistivity was recorded might be due to poor water quality also.
Generally in the study area a resistivity value greater than 200 W m corresponds to massive rock (Table 2), but it is charnokite with few fractures in Thirumalaicheri area at 40m depth. In general, the charnokite with few fractures will reduce the resistivity. But this being an exemption, the reason may be the lithology that is the gneiss with few joints were observed between 20m to 35m below the earth surface influence the resistivity value. The similar observations were made at Vannivedu (location no: 9) and Sarvanthangal (location no: 23).
In Thirumalaicheri area up to a depth 12m sandy alluvium deposits were identified. This alluvium consists of gravel, fine to coarse sand and clay followed by the weathered granitic gneiss observed below to the depth of 20m. Groundwater in this area occurs below the water table conditions in the weathered and jointed rocks of the crystalline basement and hence it is expected that the potential of water will be more in this location.
A massive charnokite was noted at 40m depth itself in Muppaduvetti (location no: 12) and Dasipuram (location no: 26) area. The resistivity values were 164 W m and 141 W m respectivily. These values were comparatively low for a charnokite, which may be due to the influence of the poor quality of water present in that formation.
Generally the igneous rocks that contain no water can have a very high resistivity. The minerals that form the matrix of a rock are generally poor conductors than groundwater, but conductivity of sediment increases with the amount of groundwater it contains. The conductivity of the groundwater, which is quite variable because it depends on the concentration and type of dissolved minerals and salts it, contains (Lowrie 1997). In general the resistivity ranges can give the lithological information of an area. From the above study, it is observed that the minimum resistivity value 10 W m at 5m depth and maximum 339 W m at 80m depth were recorded. These resistivity ranges and respective lithological nature are broadly grouped into four groups as given in the Table 2.
Table 2 Correlations of Resistivity Range and Lithology
Iso-resistivity map
The data of the geo-electric sounding was used for the preparation of the isoresistivity map of the Mangadu mini watershed area. Resistivity contour maps display the lateral variation in the subsurface geology of the area. The area with low resistivity value indicates the occurrence of relatively good conductors while those with high value indicate poor conductors. The resistivity value (Table 3) at the depth of 5, 10, 20, 40, and 80 meters depth are tabulated and given below. The equi-resistivity contours were prepared.
Table 3 Iso - Resistivity Values of the Layers
From the Table 3 it is observed that the resistivity value of Pinji, Thirumalaicheri, Gudimallur and Vannivedu area were 123 Wm, 180 Wm, 113 Wm, 160 Wm at 5 th metre and 118 Wm, 134 Wm, 87 Wm, 114 Wm at 10th metre respectively. The resistivities at 5m depth of these four areas were higher than at 10th metre readings. The remaining resistivity readings were increasing as like as the other 22 locations of the study area. At Pinji, Thirumalaicheri and Vannivedu the resistivity readings were low at 10th metre may be due to the heterogeneous layers of the areas. At Gudimallur area the low resistivity value at 10th metre depth may be due to quality of the water A case of chemical industrial pollution leading to salinization of and the presence ions in the groundwater has been investigated by geophysical resistivity technique. The resistivity methods for surveying pollution which is changing the conductivity depending on quality of water has also been used as a tool for monitoring groundwater pollution (Walraevens et.al 1999).
Iso-resistivity at 5 metre depth
The iso resistivity map (Figure 5.1A) details, the distribution of resistance at 5 metre depth. This represents the top layer resistivity soil zone. In general it is observed that a low resistivity value of < 30 Wm was recorded in the southern side of the watershed. The resistivity gradually increased towards the centre of the study area, where the Palar river flows from west to east direction. The banks of Palar river are covered by flood plains and contains full of sand, and hence the resistivity was more in the central part of the study area. In general the resistivity was relatively high at the central part of the study area and lower value at surrounding area of the study area. The lowest resistivity (10 Wm) was noted at Paperi location. This may be due to clayey formation. The topsoil with low resistivity (< 30 Wm) was found only in the southern part of the study area like Pudupadi, Kukkundi, Kirambadi, Paperi, and Velur. This is mainly due to the clayey soil overly to weathered granitic gneiss. The maximum resistivity value (180 Wm) was observed at Thirumalaicheri, which is very nearer to the river Palar, the lithology of this location indicates that up to 12m it contains sandy alluvium deposits which influence to resistivity value.
Iso-resistivity at 10 metre depth
Figure 5.1B shows the equiresistivity at the depth of 10 metre. The resistivity ranges from < 30-150 Wm. The south side of the watershed shows low resistivity compared to the north side. A low resistivity value of 17 Wm was observed at Paperi. The maximum resistivity (134 Wm) was observed at Thirumalaiheri. There was a steep rise in the resistivity in the centre part of the study area, which may be due to the existence of a poor conductive of sandy soil layer. But at Dasipuram a low resistivity of 44 Wm was registered which may be attributed to the existence of clay formation. At Thirumalaicheri a resistivity value was 134 Wm was observed and it diverged in all the directions, indicating the existence of poor conductive isolated alluvial sand formation.
Iso-resistivity at 20 metre depth
The figure 5.2A shows the equiresistivity at the depth of 20 m. The resistivity ranged from less than 30 to 180W. Thirumalaicheri area was only station which is having more than 150 Wm. The low resistivity was less than 30 Wm. are observed in Kukkundhi and Papperi areas. It may be due to the coarse grained jointed gneiss. Nearly 50 per cent of the study area was having resistivity value between 90-120 Wm indicating the nature of the substrate.
Iso-resistivity at 40 metre depth
Figure 5.2B shows the equiresistivity at the depth of 40 metre. The resistivity ranged from less than 30 to 210 Wm. Only in the south part of the study area Kukkundhi, Papperi locations, the resistivity value were observed less than 120 Wm. All other parts of the study area were covered with high resistivity 150-210 Wm. The high resistivity was observed in the central part of the watershed. The western boundary of the watershed showed higher resistivity. Near Sathur, Sarvanthangal in the western side of the watershed showed a high resistivity of 210 Wm. The resistivity decreased steeply towards south and at Paperi the resistivity was less than 61 Wm. The low resistivity zone in the south direction from Kukkundhi to Paperi, may be indicative of existence of highly fractured with few joints of charnokite.
Iso-resistivity at 80 metre depth
Figure 5.3 shows the equiresistivity of the layer at the depth of 80 metres. The resistivity ranged from 120 to more than 330 Wm. and the appearance of the contours was similar to those at 5m depth. In the 5m depth contour map (Figure 2) the high resistivity was located only the centre parts of the study area. In this map (Figure 7), it was noticed that the centre part of study area the resistivity was more than 279 Wm. It extends towards NE and SW direction. These maps were compared with lineament map (Figure 8). In the lineament map, the lineaments were occurred in the NE direction in Ananthalai area etc., and these were closely associated at SW direction at Sathur, Dasipuram, Kalar, Sarvanthangal areas. So, these locations had the comparably low resistivity. Lowrie (1997) stated that the conductivity depends on the fraction of the rock that consists of pore spaces and the fraction of this pore volume that is water filled. Hence, if there was pore space like lianment, fracture, joint, the resitivity will be low. The centre part is having high resistivity; this may be because the centre part of the study area doesn’t have any lianment.
INTERPRETATIONS BASED ON THE RESISTIVITY
Numerous geophysical investigations have been done in various locations over the globe to demarcate the potential zones of groundwater and thick and low resisitivity horizons are considered as favorable development. In igneous and crystalline rocks, the groundwater usually can be tapped from the weathered zones and these are generally found at comparatively shallow depths. Such zones and pockets have lower resistivity values when compared to the more compact and fresh rocks and can be easily located by resistivity surveys. Water tapped in joints and fissures of the undecomposed rocks may also be detected by relatively low resistivity values. Balakrishna and Ramanujachary (1979), Balakrishna et al., (1983), Verma et al., (1980), Sharma (1982), Sharma and Sastri (1986), Balasubramanian (1986) are few notable workers published in relation to such studies in hard rock terrain. Sathyamoorthy (1991) in his integrated studies on groundwater in and around Tiruchirapalli, Tamilnadu has presented a groundwater development map. Based on the variations, the formations were divided into three major types such as topsoil, weathered zone and massive rock. The top layer is normally represented by topsoil of low to moderate resistivity, the middle layer by an aquifers of moderate resistivity and the bottom layer by hard rock of very high resistivity.
CONCLUSION
The study is classified into three zones as Top soil, weathered zone and Massive rock.
TOPSOIL
The different types of soil found in the study area consist of mostly clay and clayey loamy soil. In the central part of the study area near Walajapet, Mangadu, Sennasamudram, Kadapanthangal covered with sandy clay and loamy sand. The northeastern part of the area is covered by the Red loamy soil. The dark red, fine sandy clay were covered at Ladavaram, Ananthalai. The yellowish red, loamy soils were at Gudimallur, Pundi area. The black cotton soils covered at Manthangal. The clay soil with a resistivity as low 10 to 40 Wm were covered most part of the study area. The alluvium consisting of dry sand and clay in varying proportion with a resistivity of as high as 180 Wm covered the eastern end of the study area where the Palar river flows.
WEATHERED ZONE
The weathered zone in the area is aquifer zone where the rocks are jointed and fractured. The thickness of this weathered zone ranged from 1m to 35m. The maximum weathered thickness of 35m was observed at Ammanthangal area. The resistivity value at Ammanthangal area was 119 Wm at 40m depth indicating potential zone for groundwater. In this study area these weathered zone are acted as water bearing formation.
MASSIVE ROCK
The depth at which the massive fresh rock lies varies from place to place. According to Davis and Dewiest (1966), who summarized the work of the previous workers, the resisitivity method has some success in locating the contact between alluvium and bedrock in river valleys and in locating layers of gravel or sand below clay and silt. In the present study, the depth to bed rock ranged from 24- 90m with a resistivity of more than 200 Wm. The depth of basement in the area was very high in the southern part of study area. In Paperi location the depth of basement rock was 90m. Most of the area, depth of basement is moderate
References
Balakrishna, S. and K.R. Ramanujachari (1979) Resistivity investigations in Deccan trap regions, Geophys, Res. Bull., 16, 1, pp. 31-40.
Balakrishna, S., K. Subrahmanyam, B.S. Gogte, S.V.S. Sarma, and Venkatanarayana, B (1983), Ground water investigations in the Union territory of Dadra and Nagar Haveli, Geophys, Res. Bull, 21, 4, pp. 347-359.
Balasubramanian, A. (1986), Hydrogeological investigations of Tambraparani River Basin, Tamil Nadu, Unpublished Ph.D. Thesis. University of Mysore, 349 pp.
Davis, S. N. and Dewiest R.J.M (1966), Hydrogeology, John Wiley and Sons Inc, New York, 463pp.
Sathyamoorthy, S. (1991), Integrated Ground water studies in and around Tiruchirapalli, Tamil Nadu, Unpublished Ph.D., Thesis (Bharathidasan University) , 258 pp.
Sharma, K.K. and Sastri, J.C.V., (1986), Electrical Resistivity surveys for ground water: A case study from Regional college of Education, Mysore Jour. A.E.G., Hyderabad, 7(1), pp. 37-42
Sharma, S.N. (1982), Hydrogeological investigations in Varada basin - A subtributary to Krishna river, Karnataka, unpublished Ph.D. Thesis (University of Mysore), 125 pp.
Verma, R.K., M.K. Rao, C.V. Rao, and Manoj K. Shukla (1980), Resistivity investigations for ground water in metamorphic area near Dhanbad, India. Ground water, 18(1), pp 46-55.
Walraevens, K., E. Beeuwsaert, and W. De Breuck (1999), Geophysical methods for prospecting industrial pollution: A case study, European Journal of environmental and engineering geophysical, 2, pp 95-108
ACKNOWLEDGMENT
Authors express their sincere thanks to Mr. G Viswanathan Chancellor. Mr. Sekar Viswanathan, Mr. G V Selvam Pro- Chancellors, VIT University, Dr D P Kothari Vice -Chancellor and Prof. S Narayanan, Dean SMBS for their support and encouragement
Dr C Bala Murali Krishna, Asst. Professor, SMBS, VIT University
Ms. R Poornima, Lecturer, SMBS, VIT University
Introduction
The geophysical survey submit to the scientific dimensions of physical properties of the mother earth and which may be interpreted in terms of rock type, geological formation, weathering thickness, porosity and fracture zones. Among various geophysical techniques, the electrical resistivity methods have been widely used by the field hydrogeologist for groundwater studies. In recent years this method has become important in the scientific investigation of environmental problems. Due to this basis, electrical resistivity method has been used for interpreting the various geological formation and lithology of Mangadu mini water shed in the present study.
Study area
The study area viz., Mangadu mini watershed is one of the important watersheds of Palar river basin. It is one of the major 17 river basins of Tamilnadu state in India, covering one tenth of the state geographical area. The Mangadu watershed lies in Arcot and Walajah Blocks of Vellore District, Tamilnadu, India (Figure 1.1). It has an aerial extent of 112 km2. There are about 43 villages covered in this mini watershed. It lies in the Survey of India Topographic map No.57 P/5 and in between the coordinates of Latitudes 12° 48˘ 00˛ to 12° 57˘ 30˛ and Longitudes: 79° 19˘ 40˛ to 79° 25˘ 00˛ (Figure 1.2).
Geophysical investigation in the study area
Geophysical investigation using electrical resistivity method was carried out in 26 locations in the Mangadu mini watershed area. Schlumberger configuration with electrode spacing of 100 m maximum was employed by using a D.C resistivity meter. For evaluating the groundwater potential, lithology and quality zones in the study area, the results of the 26 Schlumberger soundings were analyzed.
The minimum and maximum apparent resistivity value ranged from 4 to 1930 W m. The table 1 summarizes the arithmetic mean of resistivity, the number of locations and the layer as observed in the study area.
All the 26 locations had three layers existing within 28m from the ground level. The arithmetic mean of resistivity 51 Wm, 136 Wm and 370 W m were obtained for 4, 12, 28m depth. It is observed only 22 locations had four layers. An arithmetic mean resistivity of 878 W m with a mean thickness for fourth layer as 32m was noted. The fifth layer was located in only one place called Manthangal. The mean resistivity values indicate that there was a steady increase in the resistivity value for most of the stations, where as in the fifth layer, there was a fall in resistivity which may be due to the development of fracture/fissures. The lithology of that location (Manthangal) indicate a weathered granitic gneiss exist up to 30m from the ground surface as followed by charnokite with joints
LITHOLOGICAL INTERPRETATION
Well cuttings and bore well details give visible evidence of the rocks and permit direct measurements and evaluation. These resistivities of different layers are correlated with the lithology information of the study area
There were wide variations in values of the resistivities of rock formation. The lowest resistivity value (10 W m) was observed at Paperi (location no: 18). This may be due to the presence of clay soil at the top. It is also noted that wherever the topsoil was clay, the resistivity of that place was low. At location16 and 20 the resistivity values were low which was attributed to the highly weathered formation having more thickness of weathered formation.
The resistivity values of 27 W m, 116 W m and 197 W m were recorded at 5m, 40m and 80m depth respectively in Manthangal (location no: 3). In this area the topsoil was black cotton soil existing up to a depth of 1m, followed by a highly weathered granitic gneiss with jointed and fracture gneiss was observed up to the depth of 30m. Because, of this presence of clay (black cotton soil) and highly weathered granitic gneiss a low resistivity was noted at 5m depth of Manthangal area. The turbidity of this water was very high. High TDS value was also observed in this area with high hardness. Manthangal is very close to the Ranipettai industrial zones. Due to the highly weathered granitic gneiss to the depth of 30m in Manthangal area the surface water along with industry wastewater can percolate easily to the subsurface and mixed in to the groundwater. Hence the low resistivity was recorded might be due to poor water quality also.
Generally in the study area a resistivity value greater than 200 W m corresponds to massive rock (Table 2), but it is charnokite with few fractures in Thirumalaicheri area at 40m depth. In general, the charnokite with few fractures will reduce the resistivity. But this being an exemption, the reason may be the lithology that is the gneiss with few joints were observed between 20m to 35m below the earth surface influence the resistivity value. The similar observations were made at Vannivedu (location no: 9) and Sarvanthangal (location no: 23).
In Thirumalaicheri area up to a depth 12m sandy alluvium deposits were identified. This alluvium consists of gravel, fine to coarse sand and clay followed by the weathered granitic gneiss observed below to the depth of 20m. Groundwater in this area occurs below the water table conditions in the weathered and jointed rocks of the crystalline basement and hence it is expected that the potential of water will be more in this location.
A massive charnokite was noted at 40m depth itself in Muppaduvetti (location no: 12) and Dasipuram (location no: 26) area. The resistivity values were 164 W m and 141 W m respectivily. These values were comparatively low for a charnokite, which may be due to the influence of the poor quality of water present in that formation.
Generally the igneous rocks that contain no water can have a very high resistivity. The minerals that form the matrix of a rock are generally poor conductors than groundwater, but conductivity of sediment increases with the amount of groundwater it contains. The conductivity of the groundwater, which is quite variable because it depends on the concentration and type of dissolved minerals and salts it, contains (Lowrie 1997). In general the resistivity ranges can give the lithological information of an area. From the above study, it is observed that the minimum resistivity value 10 W m at 5m depth and maximum 339 W m at 80m depth were recorded. These resistivity ranges and respective lithological nature are broadly grouped into four groups as given in the Table 2.
Iso-resistivity map
The data of the geo-electric sounding was used for the preparation of the isoresistivity map of the Mangadu mini watershed area. Resistivity contour maps display the lateral variation in the subsurface geology of the area. The area with low resistivity value indicates the occurrence of relatively good conductors while those with high value indicate poor conductors. The resistivity value (Table 3) at the depth of 5, 10, 20, 40, and 80 meters depth are tabulated and given below. The equi-resistivity contours were prepared.
From the Table 3 it is observed that the resistivity value of Pinji, Thirumalaicheri, Gudimallur and Vannivedu area were 123 Wm, 180 Wm, 113 Wm, 160 Wm at 5 th metre and 118 Wm, 134 Wm, 87 Wm, 114 Wm at 10th metre respectively. The resistivities at 5m depth of these four areas were higher than at 10th metre readings. The remaining resistivity readings were increasing as like as the other 22 locations of the study area. At Pinji, Thirumalaicheri and Vannivedu the resistivity readings were low at 10th metre may be due to the heterogeneous layers of the areas. At Gudimallur area the low resistivity value at 10th metre depth may be due to quality of the water A case of chemical industrial pollution leading to salinization of and the presence ions in the groundwater has been investigated by geophysical resistivity technique. The resistivity methods for surveying pollution which is changing the conductivity depending on quality of water has also been used as a tool for monitoring groundwater pollution (Walraevens et.al 1999).
Iso-resistivity at 5 metre depth
The iso resistivity map (Figure 5.1A) details, the distribution of resistance at 5 metre depth. This represents the top layer resistivity soil zone. In general it is observed that a low resistivity value of < 30 Wm was recorded in the southern side of the watershed. The resistivity gradually increased towards the centre of the study area, where the Palar river flows from west to east direction. The banks of Palar river are covered by flood plains and contains full of sand, and hence the resistivity was more in the central part of the study area. In general the resistivity was relatively high at the central part of the study area and lower value at surrounding area of the study area. The lowest resistivity (10 Wm) was noted at Paperi location. This may be due to clayey formation. The topsoil with low resistivity (< 30 Wm) was found only in the southern part of the study area like Pudupadi, Kukkundi, Kirambadi, Paperi, and Velur. This is mainly due to the clayey soil overly to weathered granitic gneiss. The maximum resistivity value (180 Wm) was observed at Thirumalaicheri, which is very nearer to the river Palar, the lithology of this location indicates that up to 12m it contains sandy alluvium deposits which influence to resistivity value.
Iso-resistivity at 10 metre depth
Figure 5.1B shows the equiresistivity at the depth of 10 metre. The resistivity ranges from < 30-150 Wm. The south side of the watershed shows low resistivity compared to the north side. A low resistivity value of 17 Wm was observed at Paperi. The maximum resistivity (134 Wm) was observed at Thirumalaiheri. There was a steep rise in the resistivity in the centre part of the study area, which may be due to the existence of a poor conductive of sandy soil layer. But at Dasipuram a low resistivity of 44 Wm was registered which may be attributed to the existence of clay formation. At Thirumalaicheri a resistivity value was 134 Wm was observed and it diverged in all the directions, indicating the existence of poor conductive isolated alluvial sand formation.
Iso-resistivity at 20 metre depth
The figure 5.2A shows the equiresistivity at the depth of 20 m. The resistivity ranged from less than 30 to 180W. Thirumalaicheri area was only station which is having more than 150 Wm. The low resistivity was less than 30 Wm. are observed in Kukkundhi and Papperi areas. It may be due to the coarse grained jointed gneiss. Nearly 50 per cent of the study area was having resistivity value between 90-120 Wm indicating the nature of the substrate.
Iso-resistivity at 40 metre depth
Figure 5.2B shows the equiresistivity at the depth of 40 metre. The resistivity ranged from less than 30 to 210 Wm. Only in the south part of the study area Kukkundhi, Papperi locations, the resistivity value were observed less than 120 Wm. All other parts of the study area were covered with high resistivity 150-210 Wm. The high resistivity was observed in the central part of the watershed. The western boundary of the watershed showed higher resistivity. Near Sathur, Sarvanthangal in the western side of the watershed showed a high resistivity of 210 Wm. The resistivity decreased steeply towards south and at Paperi the resistivity was less than 61 Wm. The low resistivity zone in the south direction from Kukkundhi to Paperi, may be indicative of existence of highly fractured with few joints of charnokite.
Iso-resistivity at 80 metre depth
Figure 5.3 shows the equiresistivity of the layer at the depth of 80 metres. The resistivity ranged from 120 to more than 330 Wm. and the appearance of the contours was similar to those at 5m depth. In the 5m depth contour map (Figure 2) the high resistivity was located only the centre parts of the study area. In this map (Figure 7), it was noticed that the centre part of study area the resistivity was more than 279 Wm. It extends towards NE and SW direction. These maps were compared with lineament map (Figure 8). In the lineament map, the lineaments were occurred in the NE direction in Ananthalai area etc., and these were closely associated at SW direction at Sathur, Dasipuram, Kalar, Sarvanthangal areas. So, these locations had the comparably low resistivity. Lowrie (1997) stated that the conductivity depends on the fraction of the rock that consists of pore spaces and the fraction of this pore volume that is water filled. Hence, if there was pore space like lianment, fracture, joint, the resitivity will be low. The centre part is having high resistivity; this may be because the centre part of the study area doesn’t have any lianment.
INTERPRETATIONS BASED ON THE RESISTIVITY
Numerous geophysical investigations have been done in various locations over the globe to demarcate the potential zones of groundwater and thick and low resisitivity horizons are considered as favorable development. In igneous and crystalline rocks, the groundwater usually can be tapped from the weathered zones and these are generally found at comparatively shallow depths. Such zones and pockets have lower resistivity values when compared to the more compact and fresh rocks and can be easily located by resistivity surveys. Water tapped in joints and fissures of the undecomposed rocks may also be detected by relatively low resistivity values. Balakrishna and Ramanujachary (1979), Balakrishna et al., (1983), Verma et al., (1980), Sharma (1982), Sharma and Sastri (1986), Balasubramanian (1986) are few notable workers published in relation to such studies in hard rock terrain. Sathyamoorthy (1991) in his integrated studies on groundwater in and around Tiruchirapalli, Tamilnadu has presented a groundwater development map. Based on the variations, the formations were divided into three major types such as topsoil, weathered zone and massive rock. The top layer is normally represented by topsoil of low to moderate resistivity, the middle layer by an aquifers of moderate resistivity and the bottom layer by hard rock of very high resistivity.
CONCLUSION
The study is classified into three zones as Top soil, weathered zone and Massive rock.
TOPSOIL
The different types of soil found in the study area consist of mostly clay and clayey loamy soil. In the central part of the study area near Walajapet, Mangadu, Sennasamudram, Kadapanthangal covered with sandy clay and loamy sand. The northeastern part of the area is covered by the Red loamy soil. The dark red, fine sandy clay were covered at Ladavaram, Ananthalai. The yellowish red, loamy soils were at Gudimallur, Pundi area. The black cotton soils covered at Manthangal. The clay soil with a resistivity as low 10 to 40 Wm were covered most part of the study area. The alluvium consisting of dry sand and clay in varying proportion with a resistivity of as high as 180 Wm covered the eastern end of the study area where the Palar river flows.
WEATHERED ZONE
The weathered zone in the area is aquifer zone where the rocks are jointed and fractured. The thickness of this weathered zone ranged from 1m to 35m. The maximum weathered thickness of 35m was observed at Ammanthangal area. The resistivity value at Ammanthangal area was 119 Wm at 40m depth indicating potential zone for groundwater. In this study area these weathered zone are acted as water bearing formation.
MASSIVE ROCK
The depth at which the massive fresh rock lies varies from place to place. According to Davis and Dewiest (1966), who summarized the work of the previous workers, the resisitivity method has some success in locating the contact between alluvium and bedrock in river valleys and in locating layers of gravel or sand below clay and silt. In the present study, the depth to bed rock ranged from 24- 90m with a resistivity of more than 200 Wm. The depth of basement in the area was very high in the southern part of study area. In Paperi location the depth of basement rock was 90m. Most of the area, depth of basement is moderate
References
ACKNOWLEDGMENT
Authors express their sincere thanks to Mr. G Viswanathan Chancellor. Mr. Sekar Viswanathan, Mr. G V Selvam Pro- Chancellors, VIT University, Dr D P Kothari Vice -Chancellor and Prof. S Narayanan, Dean SMBS for their support and encouragement