DETERMINATION OF A SALT-WATER INTERFACE BY ELECTRIC RESISTIVITY DEPTH SOUNDINGS

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1 Hydrological SciencesBulletin des Sciences Hydrologiques, XXI, 4 2/976 DETERMINATION OF A SALTWATER INTERFACE BY ELECTRIC RESISTIVITY DEPTH SOUNDINGS A. GINZBURG Department of Environmental Sciences, Tel Aviv University, Ramat Aviv, Israel and A. LEV ANON The Institute for Petroleum Research and Geophysics, Holon, Israel Received 2 January 976, revised 27 July 976 Abstract. Vertical electrical resistivity soundings were measured near hydrological observation wells in order to ascertain whether geophysical means could be used to map saline water intrusion into a fresh water aquifer in Israel. The soundings showed that the low resistivity layers associated with the salt water are readily discernible. The technique was applied to the entire coastal belt and resulted in a detailed study of the saline water body and its extent. Measurements were repeated six years later and good agreement between the two sets of measurements was noted. This technique is therefore judged to be an accurate tool for the mapping of salt water intrusion in freshwater aquifers. Détermination d'un interface d'eau salée à l'aide des mesures verticales de résistivité électrique Résumé. Nous avons procédé à des sondages électriques verticaux en vue de constater si les moyens géophysiques sont suffisants pour dessiner une carte de l'invasion de l'eau salée sur les nappes d'eau douce en Israel. Ces sondages ont montré l'apparition immédiate des couches de faible résistivité qui sont associées avec l'invasion de l'eau salée. On a opéré avec cette technique tout le long du littoral et comme suite à ces efforts on a fait une étude approfondie de la masse d'eau salée et son étendue. On a répété ces mesures six ans plus tard et elles ont montré un accord complet avec les deux expériences initiales. Nous pensons donc que cette technique peut devenir un outil précis pour dessiner les cartes de l'invasion de l'eau salée sur les nappes d'eau douce. INTRODUCTION One of the main sources of water supply to the centres of urban population in Israel is the Quaternary aquifer which exists at shallow depths below Israel's coastal plain. Due to overpumping in recent years, salt water intrusion into the aquifer is progressing from the west thus polluting water supplies. The rate of movement and the extent of the salt water were monitored by means of direct water salinity measurements in a network of observation wells. These wells were drilled in a series of eastwest profiles which cover the entire coastal plain. The network of observation wells is not dense enough for close control of the salt water invasion. Therefore following the successful application of surface geophysical measurements to the solution of this problem in several countries (Flathe, 967, 968; Flathe and Pfeiffer, 964; van Dam and Meulenkamp, 967; Zohdy, 969) it was decided to try to augment the direct observations by surface geophysics, thus avoiding the expense of drilling additional wells. The experimental survey was conducted in cooperation with the Hydrological Service, Ministry of Agriculture. 56

2 GEOHYDROLOGICAL BACKGROUND The coastal aquifer is a sandstone aquifer of PlioPleistocene age. This formation extends over a belt of varying width extending over the entire length of the coastal plain from the Judean foothills to the shore (Fig. ). The formation is composed mainly of calcareous sandstone and sands interbedded with loams and clays and it overlies a very thick sequence of Pliocene and Miocene marls. < / IT / >" LOCATION MAP SCALE 0 BEERSHEVA Fig. Location map. 562

3 The Quaternary aquifer varies in thickness from about 50 m in the east to about 50 m near the coast. In the west the sand and sandstones interdigitate with loams and clays which cause a subdivision of the single aquifer into a number of aquifers. The coastal aquifer is recharged in the east, where it is phreatic, by rainfall, by flood water flowing west from the hilly areas in the east and also by inflow from deeper aquifers. In the west the subaquifers are semiconfined and confined because of the interbedded clay and loam lenses. The fresh water of the aquifer is in contact in the west with saline water which percolates into the freshwater aquifer from the Mediterranean. The interface between the fresh water in the aquifer and the saltwater front is sharp. GEOPHYSICAL BACKGROUND The resistivity of a formation depends, assuming equal temperature, on its mineralogical composition, its porosity and the shape of the pores, the fluid saturation and the electrical conductivity of the formation fluid. Thus for a given formation of fairly constant composition and porosity one can expect a constant resistivity provided the chemical composition of the formation fluid remains constant. Increasing concentrations of ions in the fluid would increase the conductivity of the fluid and therefore lower the overall resistivity of the rock. The resistivity unit is the ohm metre. Typical resistivities of rocks saturated with fresh water in the coastal plain of Israel are: limestone m, sandstone m, clay 050 Q, m. The direct current resistivity technique of geophysical exploration is based upon the fact that if an electrical current is introduced into the ground through a pair of electrodes it will penetrate to a depth governed by the current, the electrode separation and the resistivity of the subsurface formation. It will cause a potential difference at points between these electrodes which will be governed by the same factors. Thus, if we increase the distance between the current electrodes we will cause increasing penetration into the ground which will be expressed at the surface by changes in the potential difference as measured at the potential electrodes. In the case of a layered bedrock the apparent resistivity (p a ) obtained at the surface for a given electrode configuration is not the true resistivity of the formation but bears some relationship to it. We may think of it as a weighted average of the resistivities of all the rock layers to the maximum penetration of the current. Knowing the distribution of apparent resistivity versus the electrode separation, we can compute the distribution of true resistivities versus depth. Fig. 2 The Schlumberger electrode configuration. The electrode configuration used in this experiment was the Schlumberger array (Fig. 2) which has two fixed potential electrodes M, N at the centre and two moving current electrodes A and B. The distance AB is much larger than the distance MN. For this configuration, 563

4 the apparent resistivity of the centre of the array is Pa7r ( b "") where A F is the potential drop, a = yl.b/2 and b = MV. The inversion of the apparent resistivity curve was effected by the use of master curves (Orellana and Mooney, 966) and the results checked by using a computer program to calculate the apparent resistivity curve of the calculated earth model and comparison to the field curve. RESULTS The experiment was initiated by the measurement of vertical electrical soundings using the Schlumberger electrode configuration near observation wells where the waters were sampled and their salinities measured in the laboratory. The correlation of the calculated resistivity 5 Am. Fig. 3 Correlation depth sounding: 969 apparent resistivity curve, 975 curve. log with formation boundaries at wells where the entire aquifer was fresh water yielded the typical formation resistivity for this aquifer. Sounding 4 in Fig. 6 shows such a correlation. As is both evident and expected, relatively thin interbedded clays within the major sand 564

5 stone body are not detected by the surface resistivity measurement. As can be seen, fresh water bearing sands have a resistivity of 3000 Om; clays 20 2m. Next, having established the resistivities of the fresh water aquifer, vertical electrical soundings were made near wells where saltwater invasion was recorded in water analyses. As can be seen in Fig. 5, the salinity of the water caused the lowering of the resistivity of sandstone from 74 to 5 2m at the depth recorded by sampling and analysis. This of course means that the saltwater/freshwater interface is mappable by resistivity surveying. J Am ' ' ' 3 ' ' " ^ V T" I Ï i 6 n r j 3.8 ~! rrrr. : SAND ^ ^ 460 5, X > X X N. L. SS., I ~"""""** ~ tu 3 5 i,,, 2 t ^ ^ 5 ii L i t i " _ ". I " j i i x,q 000 AS in m Fig. 4 A correlation depth sounding showing a comparison between the 969 data ( the 975 data( ). ) and Figures 3 and 4 show sets of comparisons of resistivities and salinity in the calcareous sandstone aquifer. The drop of resistivity with the rise in salinity is very clear. Based on the clear cut results of the correlation measurements series of vertical electrical soundings were measured along the profiles of observation wells. Measurements were made at well sites in each profile and at points between wells in order to determine accurately the location of the saltwater front. Examples of such profiles with a geological and salinity interpretation are shown in Figs. 5 and 6. Here the saltwater front is clearly seen to extend from the shoreline to a point inland. The low resistivity layer at the base of the sector represents a marly layer. 565

6 Using the combined borehole and depth soundings profiles the eastern edge of the saltwater front was mapped over the entire area investigated. The initial measurements were made in 969. The measurements were repeated in 975 and a good agreement with the previous set of measurements was found (Figs. 3 and 4). The observed depth to the saltwater interface in those observation wells that were still functioning changed by only a few metres. Although such small changes are almost beyond the resolution of the DC resistivity method, the trend of change in the depth of the interface can be followed. The extent of the intrusion could again be mapped with certainty thus proving the efficiency of this technique. WATER ZONE I W»TER ZONE ^' as looorr Fig. 5 A combined geological resistivity profile showing the saline water invasion. CONCLUSIONS The experiments reported above clearly show that saline water in a fairly uniform aquifer is mappable by surface resistivity measurements. In areas of saltwater intrusion into freshwater aquifers the extent of the invasion can be accurately mapped with minimal well control by the DC resistivity method. 566

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8 REFERENCES Flathe, H, (967) Interpretation of geoelectrical resistivity measurements for solving hydrogeological problems. Mining and Groundwater Geophysics/967, Geol. Survey of Canada, Econ. Geol. Report 26, Flathe, H. (968) Geoelektrische Untersuchung der Grundwassersalzung in sudlichen Jordantal. Geologisches Jahrbuch 85, Flathe, H. and Pfeiffer, D. (964) Outlines on the hydrology of the isle of Madura (Indonesia). In Subterranean Water (Proceedings of the General Assembly of Berkeley 963), pp : IAHS Publ. no. 64. Orellana, E. and Mooney, H.M. (966) Master Tables and Curves for Vertical Electrical Soundings over Layered Structures: Interciencia, Madrid. van Dam, J.C. and Meulenkamp, J.J. (967) Some results of the geoelectrical resistivity method in groundwater investigations in the Netherlands. Geophys. Prosp. 3, Zohdy, A.A.R. (969) The use of Schlumberger and equatorial soundings in groundwater investigations near El Paso, Texas. Geophysics 34,

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