Resistivity meter was used to conduct vertical electrical sounding (VES) using the Schlumberger electrical resistivity in the study region for a depth of 150 m. Geophysical study indicates four layers such as top soil, weathered first fractured zone and second fractured zones. 68.52% of study area is dominated by AA, AK and HA type curve-indicating low to moderate groundwater prospective zones. The survey result indicates that Calc-granulite and limestone rock types are better aquifers than the other rock types. GIS overlay indicate that good groundwater potential zone constitutes 93.37 sq.km (36.32%), in the study area located in Northeast, East and South edges of the study region. High groundwater potential zones fall in an area of 9.37 sq.km (3.65%).
Introduction Groundwater is one of the main sources of
freshwater supply in the municipal corporation area of Coimbatore city due to the intermittent piped water supply. Geophysical resistivity techniques are the successful and cost-effective method for groundwater investigation. Quantitative interpretation of resistivity sounding data, involves the study of the types of sounding curves obtained (ie. gives the numbers of layers present in the subsurface), such curves are A, K, H and Q or a combination of two or three curves type, e.g., AK and AKH Suresh et al., Yaramanci et al., De Domenico et al., Wattanasen and Elming, Ewusi et al., Soupios et al., Gnaneshwar et al., Perttu et al., Khalil et al.,1-9 Weathered zone and fractured zone groundwater conditions through geoelectrical resistivity method was carried out by Majumdar et al.,10 in West Bengal, India. Elangovan Balasubramanian11 explained the procedure to conduct resistivity survey and interpret the results.
Geoelectrical resistivity using Schlumberger configuration in an unconfined aquifer was carried out by Chandra et al.,12 in Maheswaram, watershed, India. The thickness of vertical layers and resistivity of the aquifer were identified using geoelectrical resistivity studies in Nigeria by Ezeh and Ugwu, Ezeh13&14. Coker15 conducted VES studies to demarcate the groundwater possible zone in Akobo area, Ibadam,
Nigeria. This study shows that high thickness of weathered zone and fractured zone area have low resistivity and act as a good water-bearing zones. Selvarani et al.,16 used electrical resistivity survey to delineate groundwater potential zones in Noyyal river basin. Maheswaranet al.,17 has delineated groundwater potential zones in Salem district using resistivity methods. Also Samson and Elangovan18 have used the geoelectrical survey to identify groundwater potential zones in Namakkal district. These hard rocks zones are similar in nature with the study area Coimbatore city. Study Area
Coimbatore is a one of the largest cities (Second position) in Tamil Nadu. It is also the most important commercial and industrial centre in Tamil Nadu. Ever since 1932 when power from Pykara was made available to Coimbatore Corporation, the population of the city is growing. Coimbatore Corporation covers an area of 257.06 sq.km, out of which 7575.20 sq.km ie. 17.70% is the developed area. The gross density of the local planning area is work out as 37.26 persons/sq.km. The study area is located in part of Coimbatore district of Tamil Nadu, India. It lies between the latitudes N 10°54′45” and 11°6’12” and longitudes E 76°52′14” and 77°3′52”. Figure 1 shows the maps of the study area. It is situated in western
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corner of Tamil Nadu and is bounded by Erode on the east, Dindigul on the southeast direction, Teni and Idukki on the south direction, Eranakulam (Kerala) on the southwest direction, Palakkad (Kerala) on the west direction and Nilgiri district on the North side of the study area.
It is surrounded by mountains on the west and with reserve forests and river basin on the northern side, whereas the eastern part of the district starting from the city is predominantly dry. Due to the presence of the mountain pass, most parts of the district benefit for rainfall from the southwest monsoon season. After a pleasant September, regular monsoon starts from October lasting till early November. These rainfalls are not enough for the entire year. Coimbatore district, called as Manchester of South India is situated on the banks of River Noyyal, a part of Cauvery basin and various tanks fed by Noyyal river, Singanallur tank is one of the biggest and most polluted tanks.
A geological study shows that the city is comprised of fissured zones with the age group of Pre-Cambrian period. The lithological characteristics include gneiss, charnockite and other basic intrusive. The ground water is restricted to a depth of around up to 20 m in
the study area. The aquifer of the area is discontinuous with secondary inter-granular porosity and fractures (Central Ground Water Board).
District resource maps were collected from Geological Survey of India (GSI). The collected maps were registered in Geographic Information System (GIS) with help of georeferencing tool and digitized.
Geoelectrical investigation is one of the electrical methods of surface geophysics methods. Resistivity survey has been carried out using Geosensors digital signal stacking resistiyity meter, adopting Schlumberger configuration. 54 VES soundings were carried out to signify complete lithology of the Coimbatore corporation. The maximum electrode separation AB/2 is 150 m. The VES curves have been analysed in depth and the qualitative and quantitative interpretation have been done by the IPI2 WIN software program and the geoelectrical parameters have been obtained (Table-1).
Geographic Information System (GIS) was employed in this present study. To prepare the spatial distribution, thematical maps were prepared using geoelectrical resistivity interpretation results such as topsoil, weathered zone, first fractured zone and second fractured zone resistivity and thickness. The resistivity and thickness were classified based on the quartile statistical method. Quartile statistical method is representing the whole data set is dividing into four classes. We must sub-divide into two data sets based on the average. Less than average values are a first group. More than values are a second group. Next, we have to calculate the average for each group and whole data set. Now, it is divide the quartiles (Four classes) with representing the average values.
Results and Discussion The district resources map was obtained from
Geological Survey of India (GSI). The map was scanned, digitized and then taken to GIS. Geology of the study area is mainly controlled by tectonic and magmatic activities that have occurred in Archaean to Cainozoic period. The study area lies mainly over the Archaeancrystallines rocks (Figure. 2), and the groundwater occur under pharetic conditions of the hard-rock aquifers. The study area is made up of high-grade metamorphic rocks Hornblende biotite gneiss, Fissile Hornblende biotite gneiss and Fluvial (Black cotton soil with Gypsum) deposits. The Fissile hornblende biotite gneiss occupied in north-eastern part of the study area. The rocks of Fluvial (Black
Fig. 1—Location of the Coimbatore Corporation map withGeophysical resistivity survey locations marked in points and studyarea details.
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Table 1 — Lithology wise Minimum and Maximum values of Resistivity and Thickness of the various layers. These result of the various location of resistivity survey locations.
Minimum 3.30 0.75 15.09 17.45 0.39 1.63 9.29 80.00 KH-1, AA and
QH – 2,2
Maximum 155.26 608.93 3462.79 1054.52 3.30 14.90 57.50 122.00 AK – 7 Average 50.70 160.02 704.95 270.75 1.84 6.39 26.96 104.54 HA - 4
(Contd.)
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Table 1 — Lithology wise Minimum and Maximum values of Resistivity and Thickness of the various layers. These result of the various location of resistivity survey locations. (Contd.)
cotton soil with Gypsum) occur in north-western part of the study area.
Majority of the lower portion are occupied in Hornblende biotite gneiss with trending towards southeast direction. The other types of rocks are present in very small portion of the study area. The Fissile hornblende biotite gneiss (90.22 sq.km) occupies in more or less one third out of the entire portion. Next dominated group of rocks followed by hornblende biotite gneiss (59.95 sq.km) and Fluvial (Black cotton soil with Gypsum-85.90 sq.km) deposits. The development of drainage networks
mainly depends on the underlying geology, precipitation, exogenic and endogenic processes of the area.
The resistivity data was analysed by curve matching techniques for 54 VES locations. From the data obtained, 68.52% of study area is dominated by AA, AK and HA type curve indicating low to moderate groundwater prospective zones (Table 1). 24.07% of the area underline by moderate to good (HK and KH) groundwater potential zones and rest of the 7.41% of the study area is classified as Good to very good (QH) groundwater portions.
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Four major zones were delineated as topsoil, weathered zone, first fracture zone and second fracture zone. Each and every location sub-surface lithological sequence and maximum, minimum, average of resistivity and thickness are obtained from each layer (Table 1).
17 Schlumberger’s VES survey was conducted in Hornblende-biotite gneissic rock. Interpretation results are AA-5, HA-4 and AK-2 curve types out of 17 VES locations. This zone is a low to moderate groundwater potential zone. Rest of the curve types such as HK-4 and KH-2 indicate a moderate to good groundwater potential zones.
In Hornblende-biotite gneiss a minimum, maximum and average of 22.25 ohm-m to 132 ohm-m and 51.03 ohm-m of resistivity was observed in topsoil, overall semi-wet condition of soil. Weathered zone resistivity 4 ohm-m to 316.51 ohm-m and average 65.23 ohm-m was observed. It is overall nature is almost semi-arid to arid condition. First fracture zone resistivity 18.84 ohm-m to 2051.68 ohm-m and average 408.25 ohm-m was observed. Second fracture zone resistivity 20.28 ohm-m to 4501 ohm-m and average 882.45 ohm-m was observed. It is overall observation reveals that some of the locations have groundwater, but most of the locations do not possess water.
In Fissile hornblende-biotite gneiss, resistivity was ranging from 3.30 ohm-m to 155.26 ohm-m and an average 50.70 ohm-m of resistivity was observed in topsoil, overall semi-arid condition of soil. Weathered zone resistivity 0.75 ohm-m to 608.93 ohm-m and average 160.02 ohm-m was observed. It is overall
nature is almost wet condition. First fracture zone resistivity 15.09 ohm-m to 3462.79 ohm-m and average 704.95 ohm-m was observed. Second fracture zone resistivity 17.45 ohm-m to 1054.52 ohm-m and average 270.75 ohm-m was observed. The overall observation reveals that the first fracture zone is in dry condition. But the second fracture zones possess groundwater.
In Fluvial (Block cotton soil with Gypsum) region, a minimum, maximum and average of 6.69 ohm-m to 132.94 ohm-m and an average 49.25 ohm-m of resistivity was observed in topsoil, overall wet condition of soil. Weathered zone resistivity 2.47 ohm-m to 304.64 ohm-m and average 124.85 ohm-m was observed. It is almost wet condition. First fracture zone resistivity 27.76 ohm-m to 3410.04 ohm-m and average 517.61 ohm-m was observed. Second fracture zone resistivity 23.31 ohm-m to 5651.82 ohm-m and average 1004.77 ohm-m was observed. It is overall observation reveals that the first fracture zone is dry condition. But the second fracture zones possess meagre amount of groundwater.
In Calc-granulite and limestone, a minimum, maximum and an average of 5.89 ohm-m to 66.40 ohm-m and an average 39.62 ohm-m of resistivity was observed in topsoil, overall wet condition of soil. Weathered zone resistivity 6.92 ohm-m to 141 ohm-m and average 52.88 ohm-m was observed. It is almost wet condition and some places shallow water was occurred. First fracture zone resistivity 16.49 ohm-m to 330 ohm-m and average 123.71 ohm-m was observed. Second fracture zone resistivity 87.40 ohm-m to 324.47 ohm-m and average 187.49 ohm-m was observed. It is overall observation reveals that the first and second fracture zones have water. It is a good semi-confined aquifer.
Spatial variation for resistivity of weathered layer (Figure. 3) which has 4 major zones of resistivity such as “High Resistivity” “Moderate Resistivity” “Low Resistivity” and “Very low Resistivity”. The northwest part of the Coimbatore corporation region has higher resistivity zone. Therefore, that portion is dry groundwater condition only weathered zone. South zone and East zone were observed in lower resistivity ranging from 50 ohm-m to 100 ohm-m. These areas are classified as good groundwater possible zones. Geology play a major role along with vertical movement of groundwater, in governing resistivity of weathered layer in Hornblende-biotite gneiss and Calc-granulite and limestone regions but of
Fig. 2 — Geology of the study area with spatially represented andland marks. Detailed rock types.
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lesser importance in Fissile hornblende-biotite gneiss and Fluvial (Block cotton soil with Gypsum) region.
Weathered layer thickness in major part of the study area was ranging from 4m - 11m (Figure. 4). Higher thickness was noted in norther part and southern part of the study area due to the intensive weathering of Fissile hornblende-biotite gneiss, Fluvial (Black colon soil with Gypsum) and Calc-granulite and limestone.
The first fractured layer spatial diagram (Figure. 5) for resistivity indicates that, major part of the study area is covered by low resistivity ranging from 100 ohm-m to 500 ohm-m. There are two major factors influencing resistivity of the regions namely structure and lithology of which structure plays a dominant role. It was noted that resistivity was lower in places
of regions having higher intersection with lineaments. Higher resistivities are noted in northwest corner, northeast corner and central part of the Coimbatore corporation region.
The first fractured layer thickness ranges from 30m to more than 45m zones followed by 20m to 30m in majority of study area (Figure. 6). Lithology plays a dominant role for determining thickness for first fracture layer along with structures. It is also significant to note that thickness of first fracture layer is greater in gneissic rocks. Fracture pattern study on similar and adjacent hard rock region also indicates lithology plays a significant control over fracture zone of rocks and groundwater potential.
The second fractured layer spatial diagram (Figure. 7) for resistivity indicates that, major part of
Fig. 3 — Weathered Zone Resistivity Spatial represented and landmarks map.
Fig. 4 — Weathered Zone Thickness Spatial represented andlandmarks map.
Fig. 5 — First Fracture Zone Resistivity Spatial represented andlandmarks map.
Fig. 6 — First Fracture Zone Thickness Spatial represented andlandmarks map
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the study area is covered by low resistivity ranging from 100 ohm-m to 700 ohm-m. There are two major factors influencing resistivity of the regions namely drainages and slope. It was noted that resistivity was lower in places of regions having higher intersection with drainages and intersection of geology. Most of the study regions fall under low resistivities. The low resistivity is indicated that groundwater potential zones.
The second fractured layer thickness ranges from 90m to more than 110m zones followed by 80m to 90m in majority of study area (Figure. 8). Lithology plays a dominant role for determining thickness for first fracture layer along with structures. It is also significant to note that thickness of first fracture layer
is greater in gneissic rocks. Fracture pattern study on similar and adjacent hard rock region (Anbazhagan19) also indicates lithology plays a significant control over fracture zone of rocks and groundwater potential.
The various geophysical resistivity spatial maps (Weathered zone Resistivity, First Fracture Zone Resistivity and Second Fracture Zone Resistivity Spatial Maps) was integrated in the GIS environment to generate groundwater potential zone map. Integrated groundwater potential zone spatial map (Figure. 9) shows that 80.20 sq.km (31.20%) fell in “Poor groundwater potential zone”. It is located that Northwest and Southwest part of the study region. Moderate groundwater possible zone occurred in an area about 74.12 sq.km (28.83%) spread all over the study area. Good groundwater potential zone 93.37 sq.km (36.32%), that area is located in Northeast, East and South edges of the study region. Very Good groundwater potential zones fall in very small area of 9.37 sq.km (3.65%). Conclusion
The Geophysical studies indicate four layers such as Topsoil, weathered, first fractured zone and second fractured zone. 68.52% of study area is dominated by AA, AK and HA type curve indicating low to moderate groundwater prospective zones. Rest of the 7.41% of the study area is classified as Good to very good (QH) groundwater portions. Geology wise geophysical resistivity and thickness of weathered zone, fracture zone high groundwater potential zones fell in Calc-granulite and limestone (good unconfined aquifer).
Fig.7 — Second Fracture Zone Resistivity Spatial represented andlandmarks map.
Fig. 8 — Second Fracture Zone Thickness Spatial represented andlandmarks map.
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Spatial analysis reveals that the parameters such as structure, lithology, geology, drainages and slope play the major role along with vertical movement of groundwater, in governing resistivity of weathered layer in Hornblende-biotite gneiss and Calc-granulite and limestone regions but of lesser importance in Fissile hornblende-biotite gneiss and Fluvial (Block cotton soil with Gypsum) region. Higher resistivities are noted in northwest corner, northeast corner and central part of the Coimbatore corporation region.
Integrated study show that Good groundwater potential zone covers 93.37 sq.km (36.32%), in the area Northeast, East and South edges of the study region. High groundwater potential zones fall in very small area 9.37 sq.km (3.65%). Acknowledgements
The Authors are thankful to Mahendra Engineering College, Namakkal District, Tamil Nadu and PSG College of Technology, Coimbatore, Tamil Nadufor extending the available facilities and support to carry out the research work. References 1 Suresh, M., Gurugnanam, B., Vasudevan, S., Dharanirajan,K.
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