K. Ravi Shankar (7219202)

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G   WATER RESOURCES: RAVI SHANKAR22520th WEDC Conference Colombo, Sri Lanka, 1994AFFORDABLE WATER SUPPLY AND SANITATIONGroundwater explorationWATER IS THE basic need for any life to exist in this world.Prehistoric man was leading a nomadic life on the banksof rivers. With natural calamities such as floods, earth-quakes etc. he was disturbed and up rooted from hisdwelling place. With the advent of civilization the use ofwater increased by leaps and bounds, first for his drink-ing needs, then for supplementing agriculture, irrigationmunicipal requirements and later for industrial growth.Naturally, when surface water is in short supply one hasto depend partly or wholly on ground water.Groundwater is more desirable than surface water forat least six following reasons:-• It is commonly free of pathogenic organisms and needno elaborate purification for domestic and industrialuses.• Temperature is nearly constant which is a great ad-vantage if the water is used for heat exchange.• Turbidity and colour are generally negligible.• Chemical composition is generally constant.• Ground water storage is always greater than surfacewater storage, so that ground water supplies are notseriously affected by short duration droughts.• Biological contamination in ground water is seldomnoticed.Aim of investigationThis paper is a result of a study done in the southern partof India.The aim of the investigation was to assess the groundwater potential in the unconfined aquifer, method oftapping ground water without any adverse effects, therate and extent of salt water intrusion and then to suggestprecautionary measures to prevent salt water intrusion.Electrical resistivity surveys were carried out in the fieldfor locating the availability of water and its quality.Electrical resistivity methodResistivity, the inverse of electrical conductivity, is theresistance of the geologic medium to current flow when apotential difference is applied.R=V/I, where R is the resistance, V is the voltage and Iis the current. For a given material being measured andinversely proportional to its cross-sectional area R = ϑL/A or ϑ = RA/L, where P is the characteristic resistivity ofthe geologic medium, A is the unit cross-sectional areaand L is its length. Unit of resistivity are usually given inohm - meters.K. Ravi Shankar, Colombo, Sri Lanka.In the resistivity surveys, a direct current or low fre-quency alternating current is sent through the groundbetween the two metal electrodes. Because earth materi-als offer resistance to the passage of a current somevoltage loss will occur as the current flows from oneelectrode to another. The voltage loss that occurs as thecurrent moves through the ground is measured at otherelectrodes placed between the current electrodes.Resistivity values are obtained by two different surfaceexploration methods. The first of these, called electricalsounding, involves vertical exploration. In this proce-dure, a series of stations are established and careful depthsounding are taken. These soundings are later transferredto a vertical cross-section. By evaluating the resistivityvalues, an understanding of subsurface materials can bedeveloped. This exploration method is especially usefulfor estimating the depth of sand, gravel bedrock or waterbearing strata, or for estimating the thickness of selectedformations.The second resistivity exploration technique is calledelectrical profiling. As in the case of sounding, numerousstations are selected. Resistivity measurements are thenmade, this time for the same depth, at each station.These values once plotted, produce a numerical pictureof the subsurface material at the chosen depth across ahorizontal plane. Electrical profiling is  most often used insearching for ore bodies, faults or fault zones, for evaluat-ing sand and ground deposits, for delineating bounda-ries, and for finding dipping contacts of different earthmaterials.Field procedureResistivity methods are employed for both lateral andvertical exploration.• Lateral exploration (or horizontal profiling) - A seriesof measurements of resistivity are made with a con-stant electrode spacing, moving the whole of theelectrode arrangements consecutively to a number ofpoints. Thus the lateral variations of resistivity of theground can be measured. The resistivities so detainedare plotted on the control points along a profile.This method of exploration is termed “constant depthtraversing”.• Vertical exploration (or depth sounding) - A series ofmeasurements of resistivity are made by increasingthe electrode spacing in successive steps about a fixedpoint. This method of vertical exploration is known asG   WATER RESOURCES: RAVI SHANKAR226the “expanding electrode method”, “resistivity sound-ing” or “depth probing”.It is known as vertical electrical sounding (VES). Thebasis for making an electrical sounding, irrespective ofthe electrode array used is that the further away from thecurrent source the measurement of the potential, or thepotential difference of the electric field is made, thedeeper the probing will be. It has been stated in manyreferences on geophysical prospecting that the depth ofprobing depends on how far apart two current electrodesare placed. Furthermore, when sounding with a Wenneror Schlumberger array, when the distance between thecurrent electrodes are increased, the distance between thecurrent and potential electrodes, at the centre of the arrayis also increased. It is this latter increase that actuallymatters. In electrical sounding with the Wenner,Schlumberger arrays, the respective electrode spacing asAB/2 is increased at successive intervals and the value ofthe appropriate apparent resistivity W,s is plotted as afunction of the electrode spacing on logarithmic co-ordi-nate paper. The curve of ϑ = f(a) AB/2 is called anelectrical sounding curve.Only vertical electrical sounding (VES) or depth sound-ing was carried out in the study area.Interpretation of curvesThe ability of a rock unit to conduct an electrical currentdepends primarily on three factors.• The amount of open space between particles (Poros-ity).• The degree of interconnection between those openspace.• The volume and conductivity of the water in thepores.The presence of water and its chemical character are theprincipal controls on the flow of the electric currentbecause most rock particles offer high resistance to elec-trical flow. Thus, resistivity decreases as porosity, hy-draulic conductivity, water content and water salinityincrease. Clay and Shale have low resistivities than dosaturated sand and gravel.There are two approaches in interpreting the resistivitydata;• Qualitative• QuantitativeIn qualitative interpretation the potential zone is foundout by studying the nature of the field curve. In quantita-tive interpretation there are again two approaches• Indirect method• Direct methodIn indirect method, the theoretically computed mastercurves are used to find out layer parameters by curvematching technique where as in direct method, comput-ers are employed to find out the layer parameters.Discussions and conclusionsDue to economic considerations and restrictions for thefinancial support, exploration geophysics has not playedan important role in ground water investigations as muchas it has in petroleum and mineral exploration. It is theprimary concern of societies and voluntary organisationsto develop the water resources especially in arid andsemi-arid regions. Due to population increase and indus-Table 1: Resistivities and thickness of different layersG   WATER RESOURCES: RAVI SHANKAR227Figure 1(a): Electrical circuit for resistivity determinationFigure 1(b): Common arrangements for resistivity determinationG   WATER RESOURCES: RAVI SHANKAR228trialisation and also due to pollution of major resources,water has become more acute. The problem is basicallytechnical, economic and political consideration for themanagement of adequate resources of fresh water.Vertical electrical sounding have been carried out atmany selected points in villages Bellairu, Padupanamburand Tokur in Mangalore-Karnataka, South India, usingresistivity meter, employing Schlumberger electrode ar-rangements. During the field survey, the apparentresistivities obtained for different electrode spacing arerecorded. These apparent resistivities are plotted againstcurrent electrode spacing as double log graph sheetshaving 62.5 modulus to get the field curves. The fieldcurves are interpreted from curve matching technique.The resistivities of the first layer varies from 100-400ohm-m. Where are the resistivities of the second layervaries from 10-100 ohm-m. But the resistivity increases inthe deeper zones and is sometimes above 1000 ohm-m.Therefore, the second layer is interpreted as the saturatedzone from which sufficient water may be tapped.References1 Dathu Rao, D., and Basavana Gowd, R.M., (1981) Soiland water testing manual, Soil Chemistry DivisionGovernment of Karnataka (KSDA).2 Fleming, P.M., and Smiles, D.E., (1975) Infiltration ofwater into soil, in Proc, National symposium on Hy-drology prediction in catchment hydrology, Australianacademy of sciences, Canberra, November PP. 83-110.3 Gajendragad M.R., Ranganna G., and Gurappa K.M.,(1982) Adverse effect of improper irrigational practicesand polluted ground water on farm production incoastal regions - A case study, Internal symposium,IAH, Prague Czechoslovakia.4 Horton, R.E., (1940) The role of infiltration in the hydro-logical cycle, am geophy. Union trans., 14, pp 446-460.5 Horton, R.E., (1940) Proc. of Amer. Soc. of soil science,5 pp. 399.6 Karanth, K.R., (1978) Studies of hydrological param-eters of groundwater recharge in water balance compu-tations in Andhra Pradesh, Hydrology review, acontribtion to IHP CSIR, New Delhi, 4(1-4) pp. 65.7 Karanth K.R. (1988) Groundwater assessment develop-ment and management, Tata McGraw-Hill PublishingCompany Ltd. pp.26.8 Kuchibhatla K.R. (1988) Role of geophysics for ground-water resources, Groundwater New Oct-Nov.9 Lokesh K.N., Naganna C., Srinivas G., (1983) Resistiv-ity surveys for groundwater location in Bangalore Uni-versity Campus Area, Jour. Assoc. Expl. Geophys., Vol.III, 4: pp35-39.10 Panchanathan P.V., Soman K., (1982) on the resistivitycharacteristics of selected literate profiles in parts ofSouth Kerala, Jour. Assoc. Exp. Geophys. Vol. III, No. 2,pp. 7-11.11 Raju T.S. (1986) Infiltration process and techniques ofmeasurement. Bhu-Jal News, Vol. No. 3 Quarterly Jour-nal of Central Groundwater Board, pp. 8-11.12 Ramachandra Rao, M.B., (1975) outlines of geophysicalprospecting, A manual for geologists, Mysore UniveristyPub.13 Rijkswatersaat, The Netherlands (1975) Standardgraphys for resistivity prospecting, Pub. by EuropeanAssocn. of Exploration geophysicists.14 Rae, K.N., (1988) Resistivity Survey - Advantages andlimitations - a review, groundwater news, Oct-Nov.15 Walton Williams C., (1970) Groundwater resourcesevaluation, McGraw Hill Book Co., New York.

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