2010 2nd International Conference on Chemical, Biological and Environmental Engineering (ICBEE 2010)

Geophysical Applications in Exploration of Groundwater in the Hard Crystalline

Terrains - An Example from Sri Lanka

N.D. Subasinghe Institute of Fundamental Studies,

Kandy, Sri Lanka e-mail: deepal@ifs.ac.ik

Abstract-Resistivity imaging and geomagnetic techniques were employed in combination to investigate the ground water accumulations in hard crystalline metamorphic terrains in Sri Lanka. This paper discusses the results of first such survey conducted in Sri Lanka. First, magnetic survey was conducted in selected areas to identify magnetic anomalies suspected as ground water accumulations. Resistivity imaging was carried out on the locations where magnetic anomalies were observed. Integration of different geophysical techniques complements each other and provides a better picture in groundwater investigations.

Keywords-geophysics, ground water, resistivity imaging, geo­magnetic survey

I. INTRODUCTION

The amount of surface water available for domestic, industrial and agricultural use is becoming insufficient to fulfill the current demand in most parts of the world, especially in areas with growing populations. In the dry zone of Sri Lanka, where the land is mostly flat and precipitation is less, water is fast becoming scarce. This is made worse by the underlying hard metamorphic crystalline terrains in the country. Therefore, exploration for groundwater in crystalline terrains (possibly with latest technology) is vital to fulfill the current needs. A few studies on hard metamorphic rocks have been carried out to estimate the recharging in the dry zone [1,2]. Some other studies have been conducted on agro wells in the dry zone which have become quite popular among the farmer community in Sri Lanka [3], and on the hard crystalline rocks in Kandy area [4].

Geophysical methods such as direct current resistivity soundings, very low frequency, self-potential and magnetic are the key techniques for groundwater investigations in hard terrains. However, direct current resistivity sounding is insufficient to get reliable results in deep water investigations (more than 100m) in hard rock terrain because polarization hinders the penetration efficiency of the electricity through electrodes. This may hinder the efficiency of groundwater prospecting in dry metamorphic terrains. Very low frequency surveys are more useful in locating groundwater in deep fractures than direct-current resistivity method [5].

The variation of lateral magnetization is very important in identification of significance geological features. The

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mapping of lateral magnetization changes provides useful regional geological information. The magnetic contacts always correspond to the rock boundaries as well as the shape and current deformation of the structures [6].

Regional magnetic anomalies are good indicators to identify the regional geological structures such as fractures, fissures, and faults. These structures are well known potential sites for groundwater accumulations in hard crystalline terrains. Geomagnetic surveys, which were used in detection of submarines during the world wars, can be used to identify the magnetic anomalies on the earth's crust. In applied geophysics, magnetic anomalies are thought to be associated with local mineralisation or subsurface structures. In the global geophysics magnetic anomaly surveys over the mid-oceanic ridges provided a lot of information that led to the development of plate tectonics theory and to reveal the history of the movements of earth's magnetic field [7].

Small-scale spatial variations of the geomagnetic field are due to local variations in the magnetic properties of the crustal rocks. The small-scale changes are superimposed on the larger scale geomagnetic field. The magnetism of the local rock is responsible for the anomalous field.

With advancement of technology, hydrogeologists and geophysicists paid their attention to the geophysical techniques such as magnetic, resistivity imaging, self potential, very low frequency, direct- current resistivity sounding, resistivity imaging, and electro kinetic in locating groundwater. Those techniques are very important in exploration geophysics and directly address to finding out prospective ground water accumulations such as faults, fractures, fissures, and bedding planes. Though very low frequency and resistivity techniques are common and well­known techniques, magnetic, resistivity imaging, self­potential and electro kinetic methods are still in experimental stages in exploration geophysical investigations in Sri Lanka.

The overall objectives of the investigation was to integrating the magnetic, resistivity imaging and self potential techniques for applications in groundwater prospecting thus leading to an enhanced efficiency of exploration of groundwater accumulations in hard crystalline rocks.

II. GEOLOGY OF THE STUDY AREA

More than ninety percent of outer crust of Sri Lanka represent Precambrian metamorphic rocks, some of which

2010 2nd International Conference on Chemical, Biological and Environmental Engineering (ICBEE 2010)

are dating back to several billion years. Sri Lanka consist of three major crustal units namely Wanni, Highland, and Vijayan complexes. The Wanni complex is laying the northern surface of the country while Vijayan is in southern tip and Highland complex in between other two crustal units rising as highland of the country. The study area (Monaragala) crosses both Vijayan and the Highland Complexes covering the both litho tectonic boundaries [8].

Most of the regional structural features in the study area are parallel to the northwest (NW)-southeast (SE) and northsouth (NS) to southwest (SW) trend lines. The mega lineament which runs along the major river is aligned to northwest - south east direction. Most of the locations in the study area are covered by the ridge and valley type topographic features. Those are aligned to the strike direction of the structural features. Most of drainages are run along the geologically weak zones and they are mainly controlled by the regional geology of the study area.

The major rock types in the study area are metamorphic gneisses. The commonest are biotite gneiss, hornblende­biotite gneiss, migmatites, granites and calc gneisses. The almost whole area is covered by the top soil which formed from weathered rocks except few rock exposures. Some narrow areas are covered by unconsolidated sediments along the rivers and stream beds.

III. THEORY

A. MagneticSurvey There are two different methods to calculate geomagnetic

anomalies. Firstly, the anomalous field Aano may be determined by

Where,

Aano = (Frp,t- Fdv) - Fm•in (1)

Aano Frp,t Fdv Fm•in

= Anomalous field = Large scale spatial field = Diurnal variation = Observed magnetic field

Alternatively the magnetic anomaly is calculated with respect to base anomaly field by subtracting the base station reading (Fba,t) at the time t from the field reading (Frp,t)

Aano = (Frp,c Fha,t) (2)

Where, Aano = Magnetic anomaly Frp,t = Field magnetic reading Fba,t = Base station reading

Equation 2 was used to calculate regional magnetic anomalies in this study.

B. Resistivity Survey Resistivity technique involves passing an electrical

current through the ground and measuring the resulting potential. Through knowing the potential and the current within the ground, the resistance of the ground can be calculated (equation 3). Resistivity is then calculated from the resistance (the equation for calculating resistivity in

19 1

figure is that used for the Wenner array, equation 4). The inverse of resistivity is the conductivity (equation 5), which represents the ability of the ground to pass electricity, so a highly conducting body has a low resistivity.

V R=- (3)

I

P = 2n:aR (4)

1 (Y=- (5)

P

(R = Resistance, V = voltage, I = current, p = resistivity,

a = electrode spacing, (Y = conductivity) Resistivity surveys are generally carried out using one of

two set-ups: the Dipole-Dipole array or the Wenner array. The Wenner array gives different advantages than the other arrays. The good signal to noise is given by Wenner is important to get large offset measurements. Also, it shows high sensitivity to detect vertical displacement of geological features [9]. The Wenner array is better adapted for lateral profiling and when used in a sounding profile, gives a good image of the subsurface, and so was used in Monaragala, Sri Lanka. The Wenner array uses four equally spaced electrodes for each measurement. A current is passed through the outer two electrodes, and the potential difference is measured across the centre two electrodes.

The contrast in the resistivity method is given by the different lithological units. The better contrast between rock units may cause for better achievements of the results. Also, the good contrast in electrical resistivity between different rock units represent the electrical current flows through the units [10].

IV. MATERIALS AND METHODS

The location chosen for this study was Monaragala located (Latitude 779500 N to 785500 N and Longitude 8969500 E to 8973500 E, average altitude 157 m) in the southeast of Sri Lanka as shown in Fig. 1. The severe shortage of water, not availability of any major rivers or river diversion schemes (at least in the planning stage) led to the decision to carry out this study at Monaragala, which is a major town in Sri Lanka trying to raise it's head both socially and agro-economically though severely curtailed by lack of water. The possible groundwater accumulations in this area are to be in secondary geological formations.

The regional geology and geomorphology are key indexes to locating groundwater reserves in a given area. Therefore, geological maps, topographic maps and aerial photographs were studied prior to investigations to identifY the most favourable sites for groundwater. The probable fracture zones, faults, drainages and lineaments were identified by interpretation of above maps and images.

2010 2nd International Conference on Chemical, Biological and Environmental Engineering (ICBEE 2010)

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A11anmulla

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Kill

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Figure 1. Study area and resistivity sample locations. The coordinates are given as kilometres east of the Greenwich meridian and north of the equator.

192

2010 2nd International Conforence on Chemical, Biological and Environmental Engineering (ICBEE 2010)

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Figure 2. The map represents the regional magnetic anomalies of the study area. Red dotted lines represent the magnetic survey traverses.

The geographic coordinates of magnetic survey was taken in each sample points using NA V 500 Global positioning system. Also, geographic positioning of resistivity imager and self potential were taken at the starting and end points during the both survey.

A. Magnetic survey Two proton precession magnetometers (PPM's) were

used for the magnetic survey at Monaragala. One was used for field data acquisition and other was used as the base station magnetometer. The base magnetometer took the readings automatically at regular intervals and stored the data in the internal memory of the magnetometer. The base magnetometer data was used to correct the diurnal variations in the earth's magnetic field which is an important parameter in regional scale magnetic studies.

The field magnetometer was operated manually and magnetic field readings were recorded with the time of measurements. In generally three readings were taken at each location and the average value was recorded. The survey included a total number of 62 traverses and more than two thousand sample data with a 30 m sample interval within each line. The total lengths of the survey lines were --65km and the total area surveyed is � 70 km2•

B. Resistivity Survey Resistivity imager system uses a 2D electrical imaging!

topography surveys to map areas with complex geology [11]. This kind of surveys normally uses large number of electrodes. The number of electrodes may be 25 or more, connected to a multi-core cable. The electrode spacing is

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normally a constant [12]. In this study we have used 5 m electrode spacing instead of 10 m due to the instrumental malfunctions during the survey. A laptop computer together with electronic switching is used to automatically select the relevant four electrodes for each measurement. Finally, data are presented as resistivity 2D images with different colour combinations representing different resistivities.

In this study, resistivity imager system was used along with the self potential method to compare the two methods. Both methods were applied in parallel at selected locations (Fig. 1). Although total number of fifteen resistivity imager profiles was carried out with eight parallel SP profiles for the transects shown in Fig lin the study area, only the results of two sites are presented here in order to keep the paper short.

This was the first time application of both methods together for groundwater explorations have been carried out in Sri Lanka. However, limited work has been conducted using resistivity imager system on thermal springs, archaeological sites and groundwater exploration investigations in Sri Lanka [13].

V. RESULTS AND DISCUSSION

The regional magnetic anomalies in the study area are represented in figure 2. Magnetic survey results were used to identifY the possible fractures and faults in the study area. Resistivity imager and self potential survey sites were decided upon the regional magnetic anomalies because regional magnetic anomalies are always known to be the possible groundwater accumulations. Some locations were decided according to regional structural features, although magnetic anomalies were not detected. However, not all the structures identified with magnetic anomalies are confirmed by resistivity measurements. Therefore, self-potential surveys were also conducted parallel to imager profile to get a better comparison of the two methods.

! ____ _ _ ___ _ CJ _____ _ I lSO S2S 110 231 ISIi 10X1 21«2 U1J � �lI'CJl'loIwnlll

Distance (m)

Figure 3. Resistivity and self-potential (SP) profiles at W4W2 in Fig I. The peak positive SP value indicates possible occurrence of fluid within the fractured rocks. This could be a potential site for groundwater exploitation.

2010 2nd International Conforence on Chemical, Biological and Environmental Engineering (ICBEE 2010)

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Figure 4. Resistivity and self-potential profiles at T5Wl in Fig 1 indicating a possible fracture zone in the centre.

The magnetic survey results are described as follows. The significant magnetic anomalies in magnetic anomaly map (Fig. 2) are marked as A, B, C, D, E, F, G, H, I, and J.

The major significance magnetic anomaly could be observed at the site of point A. There is a well defined magnetic anomaly which aligned to North West south east direction. In topographic map same geological structure shows along the Mandappan oya (river). The magnetic anomaly of this area is about 420 nano Tesla (nT). This area was selected to test for resistivity imager and self potential surveys because the area is structurally suitable for groundwater occurrences. The resistivity survey crossing this linear structural feature indicated low-resistive profile (Fig 3).

Other negative magnetic anomalies could be observed at site D and E. The magnetic anomaly at point D is aligned to almost parallel to north 60 0 west and south 60 0 east directions. The magnetic anomaly at point E is shown north east southwest directions. The magnetic anomalies varies at D is about -140 to -220 nT and E is about -20 to -60 nT. Researchers have been suspected those regional magnetic anomalies are generated due to the geological structures aligned to those directions. Resistivity imager profiles conducted in this area indicated a possible fracture zone confirming the results from magnetic survey (Fig 4).

Two positive anomalies could be observed at point F and G respectively. The magnetic anomaly at F is parallel to northwest southeast direction while G is almost parallel to east west direction.

The site marked as H on the magnetic map is another site tested using resistivity imager system. The results indicate that there is a magnetic anomaly running along northwest­southeast direction. The magnetic anomaly in this area varies from 220 to 320 nT. The aerial photo interpretation and field investigation indicated a directional fracture zone in this area.

SUMMARY

Magnetic surveys help indentifY potential ground water occurrences in hard rock terrains, since ground water is

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always found in weak geological areas such as fracture zones. Resistivity surveys help narrowing down the areas and improving ground water location. Identification of faults and other geological features are comparable in both methods though resistivity imaging gives a much clearer picture of the subsurface strata and structures.

ACKNOWLEDGMENT

Dr Bruce Hobbs, Dr. G.M. Fonseka, and Dr. R.P. de Silva are gratefully acknowledged for their support in numerous ways. Department of Physics of The Open University of Sri Lanka and Department of Geology and Geophysics, University of Edinburgh, UK are thanked for providing geophysical instruments and assistance.

REFERENCES

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[2] R. P de Silva, "Estimating groundwater recharge in the dry zone of Sri Lanka with a soil water budget model. II. Application of the model to estimate recharge in different locations in the dry zone," J. App. Hydrology XIV (2&3), 2001. 22-35.

[3] C. S. de Silva and E.K. Weatherhead, "Optimising the dimensions of agro wells in hard-rock aquifers in Sri Lanka," Agric. Water Management 33 (3), 1997, 1 17- 126.

[4] U. de S. Jayawardena, "Sources of groundwater in crystalline hard rocks of Kandy area, Sri Lanka," Asian J. Water & Env. Pollution 1 ( 1&2), 2004, pp. 1 19- 122.

[5] C.J. Power, K. Singha, and F.P. Haeni, "Integration of surface geophysical methods for fracture detection in bedrock of Mirror Lake," Chaleston proceedings, NewHampshere 3, 1999, 757-768.

[6] M. Pilkington, "Locating geologic contacts with magnitude transforms of magnetic data," J. App. Geophysics 63, 2007, 80-89.

[7] W. Lowrie, Fundamentals of geophysics, Cambridge University press, 1997, 354 pp.

[8] P.G. Cooray, The geology of Sri Lanka (second edition), Dept. Govt. Printing, Colombo, Sri Lanka, 1984, 324 pp.

[9] F. Nguyen, S. Garabois, D. Chardon, D. Hermitte, O. Bellier and D. Jongmans, "Subsurface electrical imaging of anisotropic formations affected by a slow active reverse faults, Provence, France," J. App. Geophysics. 62, 2007, 338-353.

[10] S. Yilmaz, "Investigation of Giirbulak landslide using 2D electrical resistivity image profiling method (Trabzon, Northeastern Turkey)," J. Env. & Eng. Geophysics. 12, 2007, 199-205.

[ I I] D.H. Griffiths and R. D. Barker, "Two dimensional resistivity imaging and modelling in areas of complex geology," J. App. Geophysics 29, 1993, 2 1 1-226.

[12] M.H. Loke, Electrical imaging surveys for environmental and engineering studies, A practical guide to 2-D and 3-D surveys, 2000, 63 pp.

[13] G.M. Fonseka and S.u.P. Jinadasa, "Multi-electrode geo­electric soundings - some applications in Sri Lanka" (abstract), Annual Tech. Sessions, Geol. Soc Sri Lanka, 1999, 6 1-62.