5. GROUNDWATER QUALITY AND SALINIZATION
5.3 SPATIAL DISTRIBUTION OF SALINE GROUNDWATER FROM RESISTIVITY
5.3.3 Vertical Electric Soundings (Schlumberger depth soundings)
Surface geoelectric resistivity soundings are a well established method in the investigation of fresh- saltwater environments. Notable contributions to this subject are among others Flathe and Pfeiffer (1963), Flathe (1967, 1968), van Dam and Meulenkamp (1967), Zohdy (1969), Ginzburg and Levanon (1976), Urish and Frohlich (1990), and Frohlich et al. (1994). Apart from Flathe (1967) all other researches focused on phreatic coastal aquifers. Within the framework of different geoelectric resisitivity sounding campaigns, Flathe (1967) addressed the interpretation of geoelectrical measurements for solving hydrogeological problems. Examples from inland basins and coastal basins are given, whereby one case study deals with the resistivity distribution in the lower Jordan Valley.
This subchapter deals with the applicability of geoelectric resistivity soundings for the characterisation of the unconsolidated aquifer in the study area. First, the principles and interpretation of the method will be discussed. Second, on the basis of sounding results undertaken during the course of this study, the information and limitations of this method will be addressed.
5.3.3.2 DC Geoelectric Resistivity Method
The basis of DC geoelectrics forms the induction of direct current into the subsurface. Fig. 5.3-9 shows the basic principle of a geoelectric resistivity measurement using a general four- electrode configuration. This configuration consists of a pair of current electrodes (A and B) that induce direct current I to the subsurface and a pair of potential electrodes (M and N) that measure the potential difference ΔU. The induction of direct current into the subsurface results in a stationary electrical field. This potential field, which depends on the distribution of the specific electrical resistivities in the subsurface, can be measured. Information regarding the electrical properties of a certain area of the subsurface can be attained from the potential difference between two points on the surface (M and N).
Fig. 5.3-9: The basic Principle of a geoelectric resistivity measurement using a general four- electrode configuration, that consists of a pair of current electrodes (A and B) and a pair of potential electrodes (M and N) (Flathe and Leibold 1976).
The electrodes (usually stainless steel pegs) are connected to the ground in different configurations to each other. By increasing the distance between the current electrodes A and B a deeper penetration of the subsurface by the electrical field can be reached. The penetration depth depends also on the geological set up, which are the electrical properties of the subsurface.
From the induced current I and the measured voltage U the resistance R can be calculated using Ohm’s law:
5. Groundwater quality and salinization
R = U/I [Ω] (1)
Taking the arrangement of the four electrodes towards each other into account results in a geometric factor K, which is characteristic for a specific electrode arrangement. Using this factor K, the aerial distribution of the electrical field around a point source either in a homogenous half- space (electrode on the ground) or full- space (electrode noticeable deep in the subsurface) is accounted for (Militzer 1985). The so-called Schlumberger configuration is predestined for vertical electric soundings (VES).
The goal is to observe resistivity variations with depth. The mid-point of the Schlumberger array is kept fixed while the distance between the current electrodes is progressively increased. This results in an increasing depth penetration of the current lines, where the penetration depth depends on the resistivity of the subsurface. The geometric factor K for of the Schlumberger array along with its derivation is shown in Fig. 5.3-10.
Fig. 5.3-10: Geometry of current and potential electrodes of the Schlumberger configuration (Lowrie 1997).
Under natural conditions the half- or full- space is not homogenous. A subsurface, that consists of layers with different electrical properties (different electrical resistivity) for example, influences the current field between electrode A and B (Fig. 5.3-9). Therefore, the resistivity measured is not the resistivity of a homogene halfspace, which was assumed above, but an apparent specific electric resistivity ρa of the subsurface, since the measured resistivity does not represent the true resistivity of any part of the ground (Militzer 1985):
ρa = K * UMN/I [Ωm] (2)
By taking measurements with different electrode spacings (but maintaining the specific electrode configuration) predictions of electric resistivity for different depths of the subsurface are possible. It should be noted, that a simple direct current can cause charges to accumulate on the potential electrodes, which results in spurious signals. A common practise is to commutate the direct current so that its direction is reversed every few seconds. The little manual written by Flathe and Leibold (1976) proved to be very helpful for the field work. It should be mentioned, that, in order to achieve a electrode coupling to the ground, the electrodes should be watered in order to maintain the transition resistance between the electrodes and the ground low.
5.3.3.3 Interpretation
A first look at the quality of the data and the resistivity depth distribution can be made by plotting the effective electrode spacing (L/2 or AB/2 [m]) against the measured apparent resisistivity [Ωm]. A smooth sounding graph should be the result of the measurement (Fig. 5.3-11 left). From the graph it is possible to pick first information about the number of layers and the range of their respective resistivity. A key assumption in depth sounding interpretations is, that material with different electric resistivities is horizontally layered. Should vertical boundaries between electric resistivities also be present, they would be recognized in abnormal and unexpected VES- curves. However, in the case of the study area no vertical boundaries are expected. The information about the number of layers and their respective resistivity range can be used to start one- dimensional inversion of the field data by using iterative procedures. This method assumes equations for theoretical responses of a multi- layered ground. Since only a one- dimensional approach (depth) is used each layer is characterized by its
thickness and resistivity. Through iterative procedures the theoretical calculated curves are matched to the field data. The procedure is repeated as long as either the theoretical and the field data match to certain small mean error or until a stop criterion is reached. In this case the iterative procedure has to be repeated with different start values. This one dimensional inversion of the field data was undertaken by using the commercial software package RESIX by Interpex Ltd. at the Department of Geophysics at the University of Tübingen.
5.3.3.4 Results
In order to complete a geoelectric picture of the study area, the results of the VES, conducted within the framework of this study, should be seen as complementary soundings to soundings undertaken by previous investigators. The locations of all VES soundings are displayed in Fig. 5.3-5. The results of the VES, their significance, implications, and their limits will be illustrated by using two example VES. The remaining sounding graphs can be seen in Fig. 5.3-18.
Fig. 5.3-11 left shows the sounding graph of a VES at shot point No.2. On the right side possible interpretations of the shot point can be seen. The shortest spacings of AB/2 show an already high conductivity of around 10 Ωm. At bigger spacings the apparent resistivity decreases significantly (down to 4 Ωm), before it reaches its maximum at the highest AB/2 spacing. For the sounding graph it can be concluded, that a thin surface layer of a resistivity of around 10 ΩM is followed by a zone of a very low resistivity which is followed by a higher resistivity zone.
Fig. 5.3-11: Left: Smooth sounding graph of VES No. 2 undertaken within the course of this study (the location can be found in Fig. 5.3-5 (numbering of the VES goes from south to north). Right: Possible interpretations of a four layered model together with equivalence models (up to a mean error of 4%).
On the right side of Fig. 5.3-11 an interpretation (solid line) of the shot point as a four- layered model together with a possible equivalent model (dashed lines) can be seen (up to an error of 4%). This illustrates one of the problems one faces in interpreting surface geoelectric depth soundings and accounts for most of the surface geophysical methods: the problem of ambiguity. The interpretation difficulty multiplies when the number of layers is not known. This is shown in Fig. 5.3-12. For the sounding graph (E) interpretations as a 4-, 5-, 6-, and 7-layered model (solid line) along with their respective equivalent models (dashed lines, up to an error of 4%) were undertaken. A purely physical interpretation of the geoelectric depth soundings cannot be made. However, this problem can be minimized or even eliminated by undertaking reasonable assumptions about the subsurface. These assumptions could be upper and lower resistivities ranges of the different layers and/or their respective thicknesses. These assumptions can be based on the knowledge of the geological underground, and/ or
5. Groundwater quality and salinization
calibrations with other available data, like lithological logs of nearby wells or, like in the case of the study area, information about soil salinization and groundwater quality.Fig. 5.3-12: Problem of ambiguity illustrated for one measured Schlumberger sounding curve. A: interpreted as a four layer case (including equivalent interpretations); B: interpreted as a five layer case (including equivalent interpretations); C: interpreted as a six layer case (including equivalent interpretations); D: interpreted as a seven layers case (including equivalent interpretations); E: sounding curve that formed the basis for the interpretations.
All equivalence models for each layered model are in a range of up to 4% error tolerance.
The hydrogeological assumptions for the study area are difficult to estimate. The following factors play a role in the electrical conductivity of the subsurface (after Dietrich 1999):
- the geometric properties of the material (porosity, shape and size of the pores, number, shape, and size of the connected pore- necks, tortuosity)
- the degree of fluid saturation - the concentration of dissolved salts - temperature
- the amount, nature, and distribution of electric conductive bulk components - electro-chemical interaction on the matrix surface
Tab. 5.3-1 shows the specific electric resistivity of some selected material, that is present in the study area. These values can be seen as an upper resistivity boundary for VES interpretations. Temperature and the presence of conductive bulk components is minimal. As stated in chapter 5.2, the highest
variations are expected in the degree of fluid saturation and the concentration of dissolved salt within the different rock material. Therefore, major resistivity change in the subsurface does not depend on the nature of the rock material, but rather on the water saturation and on the concentration of the dissolved salt in the saturated and unsaturated zone. In subchapter 5.3.5.2 the relationship between the dissolved salt content of soil and subsurface conductivity is addressed and discussed on the base of soil analysis from the study area.
Tab. 5.3-1: Specific electric resistivity of some selected material (average values: (a) after Parchomenko 1965 and Dortman 1976, (b) after Nosske 1977; taken from Schön 1983).
(a) R [Ωm] (b) R [Ωm]
Although VES has proved to be a tool for detecting a possible salt/ freshwater interface (Flathe 1967, Van Dam and Meulenkamp 1967, Ginzburg and Levanon 1976), it is difficult to detect exact depths due to large electrode separations, that are needed to penetrate the highly conductive underground of the study area. The processed data of these large separations can only be regarded as average values between these large electrode separations. Like all surface geophysical explorations they suffer from the problem of ambiguity. Therefore, the interpretations of VES proved to be a difficult task.
Moreover the resistivity of the subsurface depends mostly on the dissolved salt concentration in the subsurface. Consequently lowering the problem of ambiguity by the means of hydrogeological assumptions is very limited since high variations in soil salinity exist in the study area (= high resistivity variations). It is believed that water flows in the coarse alluvial sediments which deposited during single events and are over- and underlain by clayey to marly lacustrine sediments (chapter 4.2).
In order to investigate this geological complicated picture more selective information would be desirable. Hence direct measurements of resistivity with depth and/or of the chemical soil composition are strongly recommended. These pieces of information can be obtained by the employment of the Geoprobe direct push technique for larger depths, since they allow an almost undisturbed quick determination of different parameters, such as resistivity, adaptive determination of sampling points, gamma-ray, geotechnical parameters and are, compared to conventional drilling, very quick and reasonably priced. Since the different parameters are directly measured at the tip of the probe and different parameters can be determined at the same time, the problem of equivalence and or ambiguity can be minimized or even eliminated.
5.3.4 Geoprobe direct-push geoelectric Measurements