Delineate Subsurface and Groundwater Investigation of Ongur Watershed, South India

The electrical resistivity technique is extremely supportive to investigate the nature of subsurface lithology by understand the variations in their electrical properties. The Vertical Electrical Sounding (VES) technique by Schlumberger electrode array applied in 77 Locations at Ongur River Sub Basin in Tamil Nadu, India. The Signal stacking Resistivity Meter Model SSR-MP-ATS has been applied to gather the VES data by employed a Schlumberger array, one end current electrode (AB/2) ranging from 1 to 100 m, other side placing potential electrode (MN) from 0.5 to 10 m. The concept of the VES data interpreting is the foundation of IPI2Win. It means for a VES data are treated as a unity representing the geological structure of the Ongur River watershed. The output Geo-electrical layers, isoresistivities and thickness of this area were prepared in spatial maps by using ARCGIS software. Consequently, the following zones with different resistivity values are detected consequent to different formations: (1) identification of lithology Ongur River Sub Basin, (2) layer saturated with fresh groundwater, (3) determine saltwater horizon.


Introduction
Geophysical surveys employ advanced parameters of scientific measurement to study the physical properties of the earth in order to find out the differing patterns relating to geological formation, rock type, weathering thickness, porosity and fractured zones (Plummer et al., 1999, (Singh et al., 2006.This type of scientific investigation has acquired greater importance in recent times in studying environmental problems and for assessing sub-surface water potentials (Sarma et al., 2004, Arulprakasam et al., 2009, Senthilkumar et al 2017. The information acquired, when interpreted together with specific field observations lead to a better understanding of geophysical characteristics and considerably reduce ambiguities and uncertainty and help in narrowing down the range of possible solutions in a given specificity. Hydrogeophysical methods within the general ambit of geophysical surveys have become crucial in recent times. Estimation of ground water potential through geophysical prospecting has become universal owing to the fact that water has become one of the most precious resources of nature in many parts of the world (Olorunfemi and Fasoyi, 1993;Olasehinde, 1999;Alile et al., 2008). The ever increasing demand for water, the geophysical constraints which limit the supplies, over exploitation, unscientific management, pollution, changing world climatic pattern and such other contributory factors have made water a premium product. A combination of these factors has also led to massive shrinkage in ground water levels. Hence, the adoption of advanced methods for proper targeting, evaluation and management of ground water wealth have become significant. In recent days, mach consequence is attached for exploration of ground water in hard rock region has done pioneering work in hard rock terrain to estimate ground water resources of the Deccan area using the electrical resistivity methods.
Among the many options available in geophysical methods, electrical resistivity methods have been the widely used ones by field geologist in ground water exploration. Even though other methods like seismic, gravitational and magnetic methods are used, electrical resistivity is most widely used method in regional and local surveys because it's great resolving power, inexpensiveness, its extensive and wide range of field applicability. Irregular environment or in homogeneities within the subsurface, such as electrically rich or poor conducting formation are contingent from prevent current and distort the normal potential. The underlying fact is that a high-quality electrical resistivity compare exists between the water occurring formations and the sub-surface lithology (Zohdy et al., 1974). The initial fluids in rocks conduct current electrolytically and resistivity is inhibited by porosity, water content and quantity of dissolved ions (Baines et al. 2002).
In resistivity method, a current are introduce into the subsurface through two current electrodes and potential difference due to current electrode were measured by potential electrode in suitably location respect to current electrode. The potential variation with respect to rate current penetrating through the ground was measure in electrical resistance of the ground among the probes. The calculated resistance are a utility of the geometrical constitution of the electrodes and the electrical parameters of the ground. There are basically two types of resistivity measurements, likewise first is known as geoelectric profiling and geoelectric sounding. In Profiling, the electrodes and probes are shifted without their relative positions being changed. The second method is the geoelectric sounding which takes recourse to changing the position of the electrodes with reference to a fixed point. In present study, focused on the geolectric sounding has been built and delineated subsurface lithology conditions of this region.

Study Area
The Ongur sub basin are lies between 79°30' and 80°00' east longitudes and 12°15' and 12°30' north latitudes covered by Survey of India topographic sheets no. 57P/11, 52P/15, 57P/16 and 57P/12 (Fig. 1). The total areal extent of the Ongur sub basin covered to an area of 1480.08 Sq.km. The sub basin are enclosed major part of Kancheepuram district in north and remaining part from Tiruvannamalai and Villupuram districts in west and south of Tamilnadu, South India.

Geology and Climate:
An obvious separation of crystalline massive terrain in the western part and coastal sedimentary rock units occupied in the eastern part were experiential based on the lithological units. The oldest crystalline rocks of Archaean age are covering in major part comprise granites, gneisses, charnockites and associated basic and ultrabasic intrusive formation. Charnockites formation are well foliated and strike in NNE-SSW direction enclosed by thick weathered layer supporting good vegetation and land use practice of the study area. Geomorphologically following features have been deduced, Buried Pediment is covered in major part (63.34 %) of the study area. Low lying lands are seen along the central portion of the study area. Structural hills are exist in east of Acharapakkam town. Coastal areas are occupied by beach ridges, lagoon, marshy land, coastal high lands and coastal plains. Flood plain is seen along the river course of Ongur River. Lateritic upland observed in the eastern part extends of 127.06 sq. km (8.59%) and areal extent of coastal dune is 123.65 sq. km (8.35%). A lagoonal landforms are observed to near the confluence of River Ongur, covering an area of 80.6 sq. km and influence Kalveli Tank throughout high tide period of equally diurnal and seasonal.
Ground water level keeps variable within the aquifer. This is due to the natural twelvemonthly Hydrological cycle where ground water yielding aquifer is principally recharged through rain water. This recharging depends on a variety of factors like climate, geomorphology, topography, and soil and importantly sub surface geology (Senthilkumar et al., 2014). Due to the retreating monsoon, south India receives the rain fall of more than 50% during the month of October to December. One or more cyclones cross the study area during this period with heavy rain. For a better understanding of the hydrometeorological data becomes important while dealing with ground water studies and it is imperative to know the behaviour of different meteorological parameters of an area.
The study area covers the entire Parts of study area of Ongur sub river Basin. The tropical climate condition prevails in the study area and located in the tropographical climate zone. Ongur river sub basin experiences the tropical climate. It conveniently divided into four seasons, the post monsoon period from January to February, the summer from March to May, the south-west monsoon seasons from June to September and North-east monsoon from October to December. The Ongur sub basin is a familiar to hot climate with mean annual temperature of 32°C. The maximum temperature recorded in the summer season, rarely the temperature exceeds this maximum in the study area. The study area, maximum humidity reaches in the month of November and December and minimum humidity is 65 % and 80 % from the month of March and August. The northeast monsoon contributes a majority in total rainfall of the region (October to December) compared to the southwest monsoon (July to September). The average rainfall recorded value is 720 mm and nearly 60% of which is recorded in northeast monsoon.

Field Survey
Schlumberger electrode configuration has been carried out in 77 locations of the present study area. VES have been conducted to describe the sub-surface water potential as well as its quality, resistivity of a variety of sub-surface formations, existence of deeper fresh water aquifer and depth to basement configuration of this region. The maximum distance of spread of electrodes is 100 meters. The resistivity value (ρ) for fixed distance between the electrodes has been noted by passing current between them. The value 'ρ' corresponds to the true resistivity, if the ground is homogeneous and isotropic. The obtained 'ρa' is from the measurement over a layered heterogeneous.

Interpretation
The main aim of the resistivity technique is to understand the nature of the subsurface formations through the dimensions made over the surface of the earth. This is achieved in two steps: (a) determining parameters like resistivities and depths of various geoelectric layers from the field data and (b) translating this derived information into meaningful site-specific subsurface information. The first stage, i.e., the process of getting resistivities and thicknesses of the subsurface formations from the observed resistivity data is being termed as 'interpretation' in geophysical literature. But in real sense this process should be termed as 'analyses' and inferring the nature of subsurface formations from the results of analysis should be termed as 'interpretation'. The resistivities of formations by themselves do not carry any meaning unless they are translated in terms of lithological and geological formations. For this purpose, the background knowledge regarding the interrelation between the lithological units and their resistivities is necessary. This information can only be obtained through detailed correlative studies. The success of resistivity data interpretation depends on this background knowledge.

Inverse Slope Method
The facilities in fast computation of apparent resistivity curves for multilayered media has brought the concept of using iterative process of analyzing the resistivity sounding data. In this process an initial guess is made of the model parameters for resistivities and thicknesses of the layers. Based on these values apparent resistivity curve is generated using either recurrence formula or filter coefficients. This curve is compared with the field curve and the differences between the points of the observed and model curves are obtained and the model is automatically revised to minimize the differences. This process known as iteration is continued till the differences between the observed and theoretical curves are reduced to the minimum. The final model having least error represents the layer parameters of the sounding (Fig. 2).

Fig. 2. IPi2WIN computer inversion program output in Naravakkam Village
Zohdy (1989) suggested a method in which no initial guess is required. Zohdy starts assuming that the number of layers in the initial model as well as in the updated onesequals the number of digitized points (equally spaced on logarithmic scale) on the observed apparent resistivity curve. The resistivity of the first layer is taken to be the value of the first point; the second layer resistivity takes the second point value and so on along the curve. The depth of each layer is taken as the electrode spacing at which the resistivity was measured multiplied a constant which is determined by calculating the root mean square (RMS) % deviation among the observed and deliberate apparent resistivity values at the data points. The adjustment of depths by this procedure continues until the RMS% deviation is a minimum. The adjustment of the amplitude of apparent resistivity is done iteratively by varying the resistivities of the model layers although keeping the boundaries fixed. Each layer resistivity is adjusted by a factor equal to the ratio of the observed and calculated apparent resistivities.
The final interpretation has been made using the computer inversion program IPi2WIN (Gopinath et al 2015). The computed value has been compared with relevant field values (manual interpretation value). It is noted the error sandwiched between computed and manual is very meagre. The obtained resistivity and thickness of various layers, iso apparent resistivity for different depths and type of curves for the study are furnished in results.

Results of Analysis
The shapes of the field curves for different combinations of resistivity layers is obtained in electrical sounding are plotted on log-log scale against half current electrode separation AB/2in case of Schlumberger and electrode separation ain case of Wenner configuration.
Then the shape of the curve is critically observed to get an idea qualitatively about the number of layers and the order of resistivities.
If the subsurface is a single homogeneous layer of infinite thickness (thickness very large compared to electrical sounding spread) the apparent resistivity curve will be a straight line parallel to AB/2 or a axis and its ordinate value gives the resistivity of the formation.
(Example: Thick uniform clay deposit, uniform sandy layer saturated up to the surface etc).
If the subsurface formation is composed of two layers, a surface homogeneous layer of resistivity ρ1 overlying an infinitely thick homogeneous layer of resistivity ρ2 depending on the values of ρ1 and ρ2 two situations may arise. One of the situations is -resistivity of the second layer ρ2 is greater than the resistivity of the first (top) layer. In this case for very small current electrode separations compared to the thickness of the first layer (h), the apparent resistivity values will be equal to ρ1 and for very large electrode current separations compared to h the value will be nearly equal to ρ2 (asymptotically approaches the valueρ2 with increase of electrode separation). At intermediate values of AB/2 or "a" the curve raises from ρ1 to ρ2 smoothly. In case the second layer resistivity is infinite (very high), the curve rises continuously at an angle 45 o (example: Uniform saturated sandy formation overlying bedrock. If the bedrock has finite resistivity r2 the curve approaches the value of ρ2 for large values of AB/2. These curves are called ascending type curves. In the second situation when ρ2 <ρ1, the apparent resistivity curve starts with a value of ρ1 for small separations, (compared to the thickness of the first layer) and decreases with increasing separations finally reaching the value of ρ2 asymptotically for very large electrode separations. If the second layer has very low resistivity (near to zero) the curve goes on decreasing continuously, sandy formation overlying clay or sands saturated with saline water like seawater. If the bottom layer is clay the resistivity of the curve reaches the value of clay at large electrode spacing's and if it is with saline water (like seawater) the curve shows a continuous decrease. These curves are called descending type of curves.

HK-Type
Silty sand

Clay Clay
Silty sand Sand with fresh water Sand with fresh water Clay / Sand with saline water Clay / Sand with saline water

KH-Type
Clay

QH-Type
Sandy layer Sand with fresh water Silty sand Clay Clay Bedrock Bedrock

KQ-Type
Clay

QQ-Type
Sand Sand with fresh water Silty sand Silty sand Clay Clay / Sand with saline water Sand with saline water From the IPi2WIN output, it is inferred that 74 samples comprised of three layer curves and the remaining 3 are four layered curves. By studying these curves a typical geoelectrical section has been formulated for the study area and tabulated below.

Isoresistivity
On the basis of interpreted VES results, iso-resistivity contour maps are prepared for the four geoelectrical layers. It is possible to demarcate the area with different ground water quality from the geophysical data (Pal and Majumdar, 2001). The first layer (Fig.3) resistivity ranges from 1.1 ohm. m to 221 ohm.m. The low resistivity (less than 3 ohm.m) exists within major portion of the study area. It may be presence of sand with saline water.
The high resistivity (>200 ohm.m) in south eastern side of the area is leads to Coastal Alluvium pattern. The resistivity values from 12 -50 ohm.m which exists in most of the area, due to sand formation. The second layer (Fig. 4) resistivity various from 2.4 to 2747 ohm.m. The low resistivity (<3 ohm.m) exists in Grandipuram (Loc.No. 58) and Marakkanam (Loc.No.77) may be due to sand with saline water. The high resistivity (more than 1000 ohm.m) which exists in Eastern side may be due to Coastal alluvium Formation. The third layer (Fig. 5) resistivity ranges from 0.5 to 47040 ohm.m. The low resistivity layer (<3 ohm.m) existing around Eyipakkam is attributed to Shelly sand formation. The layer with resistivity ranging from 500 to 1000 ohm.m existing at the western side may be due to semi fractured rock. The resistivity >150 ohm.m which exist towards western side for compact hard rock with minor fractures while <150 ohm.m towards eastern side of the area which are occupied by sedimentary rock. The resistivity contour 150 ohm.m is the boundary between hard and sedimentary rocks of the region.
The fourth layer (Fig. 6) resistivity ranges from 1.00 to 1999 ohm.m. The low resistivity (<3 ohm.m) which exists in south of Kaluveli lake may be due to sand with saline water. The resistivity (>1000 ohm.m) which exists towards western and northern sides is attributed to the presence of massive rock. Here, the boundary of the hard rock is shifted towards the eastern side of the area. The thickness of first layer (Fig. 7)

Iso-apparent Resistivity
The Iso-apparent Resistivity maps at the depth of -10 m, -20 m, -30 m, -40 m and -50 m from the surface have been prepared and to identify high and low resistivity zones (Pal et al,2001) in the study area. These iso-apparent resistivity contours are very much helpful in delineating the lateral variation of the subsurface geology. Generally, high resistivities formations show poor conductivity values and a low resistivity value indicates good conductors, have suggested iso-apparent resistivities are useful in delineation of low apparent resistivity zone equals with that of thicker weathered formation. Iso-apparent resistivity at -10 m ranges from 3.76 Ω m -214.68 Ω m. The low resistivity has been observed at Timmapuram (Loc.No.8) and the highest resistivity of 521 Ω m has been identified in Sengenikuppam (Loc.No.36) (Fig. 10).
The high resistivity (above 50 ohm.m) at -10 m below ground surface are indicate, the presence of compact formation at shallow depth. These high resistivity zones are seen as pockets in the study area.
The Iso-apparent resistivity at -20 m details the distribution of resistance at the depth of 20 m from the surface. The low and high resistivity values range from 4.98 Ω m to 342.17 Ω m respectively (Fig 11). The minimum resistivity is observed in Timmapuram (Loc.No.8) and at the 4.98 Ω m and maximum resistivity value of 342.17 Ω m is observed at Sengenikuppam (Loc.No.36).
Low resistivity values (below 50 ohm.m) in this depth indicate presence of good weathered formation. Major part from central region to eastern coastal comprises abovementioned low resistivity values which are suitable groundwater exploration. Iso-apparent resistivity at the depth of -30 m show the resistivity ranges from 2.75 Ω m-428.84 Ω m (Fig. 12). The low resistivity is observed at Timmapuram (Loc.No.8) and maximum resistivity is at Sengenikuppam (Loc.No.36). Generally, in hard rock terrain, -30 m depth are occupied by the massive rock formation. In the -30 m depth, low resistivity indicates the presence of weathered / fractured zones. In the study area low resistivity (30 m depth) are found in Nemam (Loc. No. These low resistivity zones are good potential groundwater zones. The low resistivity found in the some places near coastal tract are may be due to saline intrusion. The resistivity values ranges from 1.98 Ω m-504.72 Ω m at -40 m depth (Fig.13). The low resistivity is observed in Naravakkam (Loc.No.69) and highest resistivity in Sengenikuppam (Loc.No.36). Presence of low resistivity at -40 m depth indicates existence of fracture zones. Sea water intrusion is indicated by low resistivity at few coastal regions at this depth. Isoapparent resistivity at -50 m depth is range from 2.33 Ω m to 5989.78 Ω m (Fig. 14). The low resistivity has been observed at Naravakkam (Loc.No.69) and the highest resistivity of 521 Ω m has been identified in Sengenikuppam (Loc.No.36).
The weathering and fractured zone are prominent at the south part, few pockets at central regions. Sea water intrusion is confirmed by the presence of low resistivity along the coastal locations at various depths.

Conclusions
The three layers comprised of top soil, weathered zone and massive formation whereas four layers characterized by top soil, weathered part, fractured zones and massive rocks. Compact formation show high resistivity value and low resistivity value indicate presence of the fractured formation in hard rock terrain and sea water intrusion along the coastal area. Iso-apparent resistivity at various depths such as -10, -20, -30, -40, and -50 m from the surface reveal that the depth to the basement increases towards the coast from western portion of the studied area to eastern region.