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A Model Study of The Effect of Salination on Groundwater Resistivity

by
Kamal Khair and Catherine Skokan

 

ABSTRACT

The excessive exploitation of groundwater aquifers leads to water table drawdown, and subsequently to the contamination of these aquifers by the intrusion of sea water or other hazardous sources. This worldwide environmental problem is becoming increasingly critical in coastal agricultural areas, where the fine grained materials develop thick fringe zone. By evapotranspiration the moisture of this zone pumps up the salt in the dry season, which cannot be efficiently washed away in the wet season. Over the years the salt will accumulate at different rates for different terrains and climates, which will ultimately deteriorate the land.

The electrical resistivity methods proved to be one of the most efficient geophysical tools in detecting and delineating salt water intrusion. The current study investigates the possibility of an early detection, through systematic observation of electrical resistivity in selected positions with fixed electrode arrays. The study observations used direct current electrical profiling system of Wenner configuration. They were carried out in a physical model of wood and plastic filled by partially saturated sand, with constant water flow of 1.6 I/mn. The model size is 148 x 85 cm for lateral dimensions and 25 cm of sand thickness, with a total porosity of 36%. The study concentrated on the indirect relationship between salinity and electrical resistivity upon salination, to about 32 g/l, and desalination back to 0.25 g/l. The results show that: the relationship is characterized by linear logarithmic function; the slow successive lateral change in resistivity does not reflect the velocity of the flow in the tank (0.1 ml/hr); the resistivity values for low salinity upon desalination are much different (smaller) than those upon salination of equivalent salt concentrations; the relative change of resistivity upon salination and desalination involves almost equally all features of the tank, which have distinctive resistivity values.

 

Introduction

The problem of the salination of groundwater aquifers arises in coastal areas, where the excessive pumping of unconfined coastal aquifers by water wells leads to the intrusion of sea water. This negative effect of human activity has been recorded in many areas of the world. Hence, this problem is likely to arise in areas of poor water resources (low precipitation and high evapotranspiration); from mismanagement of water resources; or in densely populated areas with a high consumption rate (e.g. Khair et al., 1994).

Because of the seasonal character of sea water intrusion, there should be constant observation to assess and delineate sea water intrusion, especially with aggravated demand during the dry season. This observation cannot be done only through systematic chemical analyses, as these do not provide integrated three dimensional cover of the terrains, whereas electrical resistivity methods have proved to be the best and most cost-effective tools to assess groundwater salinity. These methods have been applied in the investigation of coastal aquifers in different parts of the world, such as in the Netherlands by Van Dam and Meulenkamp (1967); in Belgium by De Breuk and De Moor (1969); in Israel by Ginsburg and Levanton (1976); and in New England by Urish and Frohlich (1990).

In arid climates, capillary action and evaporation lead to rapid transport of water to the surface, with the consequence that dissolved salts are left behind to hinder plant growth (McNeill, 1986). It is extremely vital to observe the extent of salination in agricultural areas; this is because even if the saline or brackish water is completely swept back to the sea during the wet season, there will be an appreciable amount of salt in the fringe zone drawn up by evapotranspiration. Neither the percolating (laterally) groundwater in winter reaches the top of the fringe zone to wash away the salt, nor the infiltrating (vertically) meteoric water efficiently dilutes salinity through the capillary zone. This salt is accumulated over the years, causing a major deterioration of agricultural lands.

The effect of the quality (mineralization) of saturating water on the apparent resistivity has been studied by many investigators. Resistivity measurements conducted by Sharapanov et al. (1974), showed indirect, two-segment, linear logarithmic relationship between apparent resistivity and mineralization. For sands, the low gradient segment corresponds to mineralizations of up to about 2500 mg/l, whereas higher mineralizations correspond to the higher-gradient segment. Other studies (e.g., Mares, 1984; Palacky, 1988; Kui, 1990; McNeill, 1990) although implying the direct relationship between salinity and conductivity (or indirect for resistivity), however, the nature of this relationship has not been discussed thoroughly. Moreover, Barker (I 990) showed that the relationship between chalk water conductivity and salinity (experimentally determined) constructed on a bilogarithmic scale is not characterized by a straight line, but rather by a parabola.

The current study aims to observe the quantitative influence of salinity on the apparent resistivity conducted on a physical model of sand. The depth to the water table in the model varies from one end to the other, with a vertically fixed plastic plate in the middle signifying in reality an impermeable barrier in its upper part (see fig. 1). There is a constant water flow through the sand of approximately 2.3 m3 /day (or 1.6 liters per minute). The respective drop and increase of the apparent resistivity upon salination and desalination observed for the model, could be applied in reality, where systematic measurements of the apparent resistivity in selected positions with fixed current and potential electrodes, might detect salt water intrusion at its lower extent, and sometimes before it reaches the pumping wells.

Figure 1. A schematic diagram of the tank: a-first water reservior; b- second water reservoir; c- middle sand reservoir; l- sand leel in the tank, m- middle plastic plate; p- pump with hose; s- slotted plastic plate; arrow indicates the direction of coordinates from left to right; solid dots- the locatio of observation points (center between potential electrodes) along the central profile; the numbers 51 and 95 indicate the coordinates (oigin at the left edge of the tank) of observation point; other numbers are the dimensions of the tank in cm.

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Methodology

The physical model

The model utilized is a wooden tank, 148 x 85 cm, covered inside with waterproof plastic plates and sheets. By fixing two vertically slotted plastic plates at 16 and 19 cm from the short sides, the tank is subdivided into three parts; two small parts, used as water reservoirs, separated by a large one in the middle filled with sand (see fig. 1). The slotted plates hold the sand back preventing it from seeping into the water reservoirs and secure continuous free flow of water. The large middle reservoir was subdivided in the middle by a vertically fixed plastic plate but not extending down to the lowest 10 cm. This plate, in reality, signifies an impermeable barrier in the upper part.

A moderate water pump is submerged in the first water reservoir (16 cm wide) continuously pumping water to the second water reservoir (19 cm). This creates a difference in water levels (hydrostatic head) in the reservoirs, which leads water to flow back to the first reservoir, percolating through the sand. The amount of percolating water is proportional to the hydrostatic head, that is, in turn, controlled by the position of the pump hose end. The higher the end the weaker the flow and the lower the end the stronger the flow. It is worth noting that during all observation measurements on salination and desalination the height of water was, respectively, 10.5 cm and 21 cm in the first and second reservoirs, whereas the water flow stabilized at about 2.3 ml/day or 1.6 I/mn. The lost amount due to evaporation was compensated on a daily basis by adding from one to three liters of water to the tank, to keep the water levels and the flow constant.

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Figure 2. Three-dimensional illustration of apparent resistivity along profiles, I, II, III, IV and V (Wenner configuration a =4cm), for: normal water levels 21 and 10.5cm (a); and shallow water levels 24.5 and 13.5cm (b). Numbers ar the lower left side indicatie the distance (coordinates) of observation points from the left edge of the tank. (a) top (b) bottom.
The preliminary measurements

To assess the boundary conditions, the Wenner configurations for resistivity profiling and vertical electrical sounding were applied. This was because the electrodes were inserted and fixed through equally spaced openings in a thin plastic bar. Each electrode was separately connected to an electrode board where the positions are numbered successively to be easily recognized and connected to the instrument.

The vertical electrical sounding was carried out in 6 positions along a profile midway in the sand tank, with the electrode configurations AB and MN spread perpendicular to it. The vertical sounding curves showed, in all stations and for two different water levels, moderate resistivity values at small distances ( AB = 6, 18 and 30 cm) and a significant increase at large distances (AB more than 30 cm). This fact implies that the large spread sounding values are affected by the tank bottom, and probably its side boundaries. Therefore, large spreads cannot be applied in the detailed study.

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Figure 3. Apparent resistivity along the central profile for different water levels (for further details see the caption of fig. 2)
As the sand has a high degree of homogeneity with limited thickness, the study concentrated on the lateral rather than the vertical variations of resistivity, which requires the application of resistivity profiling. The applied size of the electrode configuration, was AB = 12 cm or a = 4 cm as this proved to be more detailed than that of a = 8 cm. The profiling measurements were taken along five profiles set midway along the sand reservoir for different water levels. Figure 2 shows that the apparent resistivity jumps sporadically in the deep water level (downflow) side along profiles IV and V for shallow water level, and anomalous values along profiles I and V compared to profiles II, III and IV for normal water level. Figure 2 also shows that the resistivity values along the central profile III are the most consistent for different water levels. Furthermore, the abrupt jump in the middle (coordinate 71 cm) is common along all profiles for different water levels, this is due to the artificial barrier (plastic plate) in the middle of the sand tank. Hence, having the least boundary condition constraints the detailed measurements are taken only along the central profile III, as it is the best for the observation of resistivity variations.

The preliminary measurements were carried out at different water levels in the small reservoirs. Figure 3 shows the resistivity values along the central profile with two different water levels having a thickness of 13.5 and 10.5 cm in the first reservoir, and 24.5 and 21 cm in the second. The resistivity values for the shallow level have little contrast compared to the normal level. So, the last level is more optimal for detailed study of the influence of salinity on the resistivity. Hence, during the whole period of measurements, the electrode array (plastic bar) was set and fixed along the central profile with almost constant water level and steady flow without disturbing or compressing the sand, which would significantly affect the resistivity values.

There were several reasons (mostly technical) that excluded the use of conductive material for the boundaries, which would have helped to reduce the influence of the boundary effects and to simulate Dirichlet boundary condition (Mufti, 1976). However, conducting the measurements of the experiment along the central profile of the model with short electrode spacing made the boundary effects almost negligible.

Observation measurements

The resistivity profiling measurements were taken for the middle 12 instead of the total 18 positions along the central profile, to cut down the time of measurement to an acceptable minimum (about 360 sec). The first observation was done immediately before the disturbance in the salinity, the following second, third, fourth and fifth observations were done in the next 0.5 hr, the sixth, seventh and eighth in the next hour, the ninth and tenth in the next two hours, the eleventh after five hours and the last observation (1211) after 10 hours, which will be the first observation immediately before the following disturbance in the salinity. This procedure was applied upon all desalination and most of the salination operations.

Salination was performed in seven stages, one day for each, to observe the influence of salinity on the resistivity. In the first stage only 25 g of food salt were dissolved and added to the second water reservoir. This implies a salt concentration of 0.25 g/l, as the amount of water in the reservoirs and sand tank, for normal water level, is about 0. 1 ml (or 100 liters). In the following days: 50, 100, 200, 400, 800, 1600 g were successively added, to attain a total salt concentration of 31.75 g/l. This concentration was rounded, two days later, to 32 g/l. In the following week the resistivity was observed once a day until the beginning of desalination.

As salination was performed by doubling the amount of salt, desalination was done by reducing salt concentration by one half, through dilution carried out also in seven stages. This was performed by pumping out from the first reservoir 0.05 ml (half the total amount) and simultaneously replacing these by 0.05 ml of fresh water in the second water reservoir. So the theoretical salinity was decreased from 32 to, 16, 8, 4, 2, 1, 0.5 and finally to 0.25 g/l.

Results

Salination

It is not surprising that an increase in the salinity of saturating water reduces the apparent resistivity of the sand or any other porous material. However, the salination in the first stage (0.25 g/l) had reduced the resistivity for specific electrode positions from 164, 87, 134 and 73 Ohm-m to 121, 61, 94 and 46 Ohm-m, respectively; whereas the increase of

salinity from 0.75 g/l to 1.75 g/l reduced the apparent resistivity in the same electrode positions from 79, 31, 49 and 31 to 34, 16, 35 and 20, respectively. This implies that in the first stages of salination, the apparent resistivity was significantly reduced in spite of the low salinity.

Figure 4 shows a decrease in the apparent resistivity along the central profile during the third stage. This smooth decrease is clearly noticed for all other stages. The figure also shows that the high resistivity values of the observation point (51) at the extreme left and in the middle of the profile (71) are due to deeper water level for the first, and the presence of the plastic plate (barrier) for the second. These values, except for the last salination stage, remain consistently high relative to low values to the right side (75 and 95). This implies that upon salination the apparent resistivity of the whole model (3-D) will be affected, with almost the same ratio.

Figure 7 shows that the most significant change in apparent resistivity is in the first 5 hours after salination and desalination. Later observations show very slight change, except for one electrode position (51 cm) in the resistivity longer than 8 hours following the last stage of salination. Hence the 24 hr period for each stage was satisfactory for the observations. All other stages have similar patterns of decrease (see fig. 4 in Khair and Skokan, 1995).

Tables I and 2 show the values of apparent resistivity for all observation points in the first three hours upon the second and fourth stages of salination. A significant relative decrease exceeding 10% (shown in bold) of the apparent resistivity with respect to the first observation, would signify that the high salinity water parcels have reached the position concerned. The abrupt reductions in the apparent resistivity value "events" for successive observations in the first 3-4 hours indicate the following: 1- The mean

wpe21.jpg (21541 bytes)Figure 4. Three-dimensional illustrations of the apparent resistivity along the central profile for the third stage of salination from 0.75 to 1.75 g/l. Numbers at the lower right side of each diagram indicate the time, in hours, after disturbance (salination).

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Figure 5. Change of apparent resistivity (central profile) relative tot the first observation (before disturbance), in the first five hours after salination: a) from 3.75 to 7.75 g/l (5th stage)(top); and b) from 7.75 to 15.75 g/l (6th stage)(bottom).

velocity of water percolation through the sand, calculated by the above mentioned events is in the order of a few decimeters per hour, the time during which the water in the tank completes a whole cycle; 2- The resistivity reduction velocity ranges from 8-12 cm/hr across the barrier, to about 24 cm/hr in the upstream side and to about 30 cm/hr on the downstream side; 3- The low velocity in the middle of the profile is due to the plastic barrier, whereas the relatively high velocity in the downstream side, is due to the higher hydrostatic head between the two sides caused again by the barrier.

Figure 5 shows the change of apparent resistivity relative to the first observation in the first five hours after salination for 5 th and 6th stages. This figure shows higher relative change (decrease) of the apparent resistivity in the right (upstream) side as it is closer to the high salinity source than the left side.

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Figure 6. Three-dimensional illustration (central profile) of aaparent resistivity stabilized values for all stages upon salination (a), and desalination (b). Numbers at the lower right side indicate salt concentration in g/l. (a) top, (b) bottom.
Table 1. Values of apparent resistivity upon salination from 0.25 to 0.75 g/l, (italics -observation times, in hours, after salination).

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Table 2. Calues of apparent resistivity upon salination from 1.75 to 3.75 g/l, (italics - observation times, in hours after salination).
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The figure also indicates that the high relative decreases for the first 5 hours of observations are propagating from right to left or parallel to the water flow. All other stages have similar patterns of resistivity decrease upon salination.

Figures 6a and 9a confirm the indirect proportional relationship between the salinity and apparent resistivity, characterized by almost linear logarithmic function. On the other hand, the anomalous decrease of apparent resistivity in the electrode position to the extreme left (downstream) is ambiguous and could be due to some complications related to measurements and/or the stability of the electrode plastic bar over the sand, particularly this phenomenon is not observed upon desalination (see fig. 6a).


Desalination

The influence of salinity on the resistivity of the sand is also evident upon desalination (dilution) stages. Figure 8 shows the direct relationship between the dilution and resistivity, which increases smoothly throughout each stage and between successive stages in a similar, but reverse, way to salination (see fig. 4). The remarkable difference is that the resistivity upon desalination does not attain high values comparable to those upon salination, especially for low salinity stages. For example, in the last stage of desalination, with a drop in the theoretical salinity from 0.5 to 0.25 g/l, the resistivity does not attain the high values of the first or second stages of salination, and its values are roughly comparable to those of the third stage with a salinity increasing from 0.75 to 1.75 g/l. This difference is clearly shown in fig. 7.

This phenomenon is explained by the fact that the salt pumped to the fringe zone was not diluted upon desalination, as only the saturated zone is involved in this process. Hence, the fringe zone was pumping up the moisture from the saturated zone with its respective salt content. Thus, because of evaporation, the fringe zone moisture is always streaming upward, pumping up the salt, vaporizing and leaving it behind. Indeed, after the completion of all measurements, the water was totally pumped out of the tank, and as the uppermost sand particles dried out a thin white cover of salt appeared on top of them.

Tables 3 and 4 show the values of apparent resistivity for all observation points in the first three hours upon the second and fourth stages of desalination. The appearance of jumps (shown in bold) in resistivity "events" of more than 10% upon desalination, support the remarks concluded upon salination concerning the decrease in the velocity of water flow in the middle of the profile because of the plastic barrier. However, in contrast to the salination process the "events" velocity in the left (downstream) side, here, is less than that of the right side. It is worth noting that the process of desalination required 15-20 mn to dilute, by half, the water of the tank, whereas salination was impulsive and only required a few seconds to add salt solution to the tank.

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Figure 7. The change of apparent resistivity over time (central profile) after disturbance for four electrode positions, with coordinates 51, 71, 75 and 95 cm, upon the first stage of salination (a) (top); and last stage of desalination (b) (bottom).

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Figure 8. Three dimensional illustrations of the apparent resistivity along the central profile for the third stage of desalination. Numbers at the lower right side indicate time, in hours, after distubance (desalination).
Figure 10 shows the change of apparent resistivity relative to the first observation in the first five hours after desalination for 5th and 6th stages. This figure shows higher relative change (increase) of the resistivity in the right (dilution) side than in the left side. It also shows that the high relative increases for the first 5 hours of observations are propagating from right to left parallel to the water flow. All other stages have similar patterns of resistivity increase upon desalination. Finally, figs. 6b and 9b show almost a linear logarithmic function of the indirect relationship between salinity and apparent resistivity upon desalination.
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Figure 9. Stabilized values of apparent resistivity (central profile) for all stages upon salination (a) and desalination, top (b) for four electrode posistions, bottom.
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Figure 10. Change of apparent resistivity (central profile) relative to the first observation (before disturbance) in the first five hours after desalination: a) from 2 to 1 g/l (5th stage)(top); and b) from 1 to 0.5 g/l (6th stage)(bottom).
 

Discussion

Figure 11 shows that during 8 days of quiescence (no salinity disturbance) separating between salination and desalination, the resistivity in all electrode positions remained almost constant. This indicates the stability of the system, keeping constant water level and flow and stable electrode array. Furthermore, fig. 7 indicates some stability 5 hours after the disturbance in all stages of salination and desalination. This means that 5 hours are needed to attain a high degree of water co-mingling. On the other hand, the pump capacity at the measured level was in the order of 100 1/hr and as the total amount of water is about 100 1, hence the water flow cycle is one hour. This means that 5 circulations of tank water are needed to attain the above mentioned co-mingling. This fact invites the suggestion that upon salination or dilution the intruding water parcels of different salinity do not displace the existing ones, but are mixed with them, creating a gradual change of salinity across the intruded water front. Consequently, salt water intrusion can be detected before an appreciable increase in salinity appears in the pumped water if systematic measurements of resistivity are carried out in the area surrounding the producing wells, especially towards the sea.

The early stage detection of salt water intrusion is of great importance even in areas of moderate seasonal ground water flushing. Because during the dry season the evapotranspiration occurring in the fringe zone draws the high salinity water upwards and accumulates the salt in its upper part near the surface, this will not be diluted or flushed away in the wet season, because it is higher than the water table. This problem is severely aggravated in coastal areas of low precipitation as here the salt would not be sufficiently washed down to the water table for further lateral dilution and flushing. The thickness of the fringe zone, which is indirectly proportional to the particle size, is also critical. Hence, fine grained material such as silt and clay would develop thick fringe zones, which could extend from the surface to the water table. Thus, the continuous drawing up of salt by evapotranspiration in the dry season, can not be simply compensated for by discrete downward flow from heavy rain storms in the wet season, due to the low permeability. According to the above discussed mechanism, over a long period of time, the salinity will ultimately increase in the soil to the extent that it loses its value for most of agricultural uses.

The accumulation of salt in the soil profile seems to appear only in arid and semi arid areas. However, in the future this phenomenon is likely to extend to other coastal areas, where the problem of sea water intrusion is aggravated and not seriously dealt with. This is because the infiltration during wet season is not able to significantly dilute salinity downward through the capillary zone. Finally, a systematic observation of the resistivity using fixed electrode arrays, the well casing for instance, would detect any increase in the salinity and provide reliable tools to control the well pumping activities.

Table 3. Values of apparent resistivity upon desalination from 16 to 8 g/l, italics - observation times, in hours, after desalination.
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Table 4. Values of apparent resistivity upon desalination from 4 to 2 g/l, italics - observation times, in hours, after desalination.

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Conclusions

Maintaining constant water levels and steady flow with a fixed electrode array, resistivity observations in the sand tank indicate the following:

- Low resistivity values upon desalination compared to equivalent salt concentrations upon salination, and the appearance of salt on top of dried sand particles, indicate that the evaporation of fringe zone moisture draws up the salt and accumulates it in the upper part of the sand.

- The relative change of apparent resistivity upon salination and desalination over the deep and shallow water zones and over the barrier, indicates the potential capability of this method to detect any change in salinity in the underlying material.

- The velocity of water flow depicted on resistivity variations in the sand tank are in the range of a few decimeters per hour. These values disagree with the fact, that the water in the tank completes a whole cycle in only one hour.

- The apparent resistivity measurements upon salination or dilution, show that the intruding water parcels of different salinity, are mixed with the existing ones rather than displacing them, and create a gradual change of salinity across the intruded water front.

Finally, in view of the serious problem of groundwater salination in most coastal areas and the need to apply the most efficient preventive measures, the first of which would be early detection, the sand tank model could be considered as an analog to this problem, as it indicates the capability of resistivity methods in the delineation and timely detection of sea water encroachment into the groundwater aquifers.

Acknowledgment

This study was made possible by a Fulbright Grant to K. Khair to visit the Department of Geophysics at the Colorado School of Mines. Special thanks go to Dr. A.W. Ibrahim and Dr. J. Skokan for valuable suggestions. We also thank Mr. H. Schneider for his help.

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Figure 11. Three-dimensional illustration of the apparent resistivyt (central profile) over the period between salination and desalination. Numbers at the lower right side indicate the time, in days, after the last tage of salination.

References

Barker, R.D., 1990, "Investigation of groundwater salinity by geophysical methods", in, "Geotechnical and Environmental Geophysics", Nabighian, M.N., Ed, Vol. II, Tulsa, Society of Exploration Geophysicists, 201-21 1.

De Breuk, W., and De Moor, G., 1969, "The water table aquifer in the eastern coastal area of Belgium," Bull. Int. Assoc. Sci. Hydrol., 14, 137-155.

Ginsburg, A., and Levanton, A., 1976, "Determination of a salt water interface by electrical resistivity soundings," Hydrol. Sci. Bull., 21, 561-568.

Khair, K., Aker, A., Haddad, F., Jurdi, M. and Hachach, A., 1994,"The Environmental Impacts of Humans on Groundwater in Lebanon," Air Water and Soil Pollution, 78, 37-49.

Khair, K., and Skokan, C., 1995, "The Quantitative Influence of Salinity on the Apparent Resistivity on a Physical- Model upon Salination", Proceedings of the XXVI International Congress of IAH, Edmonton, Alberta, Canada.

Kui, F.L., and Huisheng, D., 1990, "Application of geophysical methods to surveys for groundwater in the Huang-Huai-Hai Plains, China", in, "Geotechnical and Environmental Geophysics", Nabighian, M.N., Ed, Vol. 11, Tulsa, Society of Exploration Geophysicists, 133-143.

Mares, S., 1984, "Introduction to applied geophysics", Kluwer Academic Publishers, Dordrecht, Holland.

McNeill, J.D., 1986, "Rapid, accurate mapping of soil salinity using electromagnetic ground conductivity meters, Geonics Limited Technical Note TN-20.

McNeill, J.D., 1990, "Use of electromagnetic methods for groundwater studies", in, "Geotechnical and Environmental Geophysics", Nabighian, M.N., Ed, Vol. 1, Tulsa, Society of Exploration Geophysicists, 191-218.

Mufti, I.R., 1976, "Finite-difference resistivity modeling for arbitrarily shaped two-dimensional structures", Geophysics, 41, 62-78.

Palacky, G.J., 1988, "Resistivity characteristics of geologic targets", in, Nabighian, M.N. (ed.) "Electromagnetic Methods in Applied Geophysics", Nabighian, M.N., Ed,Vol. 1, Tulsa, Society of Exploration Geophysicists, 53-129. Urish, D.W., and Frohlich, R.K., 1990, "Surface electrical resistivity in coastal groundwater exploration," Geoexploration, 26, 267-289.

Van Dam, J.C., and Meulenkamp, J.J., 1967, "Some results of the geoelectrical resistivity method in groundwater investigations in the Netherlands," Geophysical Prospecting, 15, 92115.

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