Electromagnetic Measurements (MaxMin)

5.3.2 Electromagnetic Measurements (MaxMin) 5.3.2.1 Introduction

The main objective was to test the applicability of electromagnetic techniques in order to locate a fresh-/saltwater interface in the alluvial sediments of the lower Jordan Valley. However, during the course of this study it was discovered, that unlike in coastal areas, no fresh-/ saltwater interface exists in the study area. Due to the high salt content within the lacustrine sediments groundwater becomes more salty during its natural flow course from east to west by the dissolution of these soluble salts.

Below, the principles of electromagnetic geophysical techniques are presented as well as the results of the field survey. This sub chapter commences with remarks on the applicability of this method in the study area.

5.3.2.2 Electromagnetic techniques for subsurface exploration

Electromagnetic (EM) methods can be subdivided into active and passive methods. Active EM methods use natural ground signals (Magnetotellurics) while passive EM methods employ an artificial transmitter to induce either a far- field (utilizing a high- powered military transmitter; VLF method) or a near- field (utilizing ground conductivity meters). The apparatus used in this study belongs to the latter group. The passive near- field methods can further be subdivided into a time- dependant- electromagnetic method (TDEM) or, as in the case of the MaxMin apparatus, into a frequency- dependent- electromagnetic method (FDEM).

MaxMin I-8 apparatus

The MaxMin I-8 apparatus of the company APEX (Ontario, Canada), purchased within the context of the project, belongs to the so called two coil small loop systems. One coil serves as a transmitter and the other as a receiver (where the primary and secondary field is recorded, Fig. 5.3-1). During measurements the inter- coil separation is maintained at a fixed distance while moving along a survey transect in discrete measuring intervals. The two coils are interconnected by a reference cable delivering the primary field to the receiver (Fig. 5.3-2).

5. Groundwater quality and salinization

Fig. 5.3-1: Sketch of a EM survey (after Grand and West 1965).

The point of reference is the mid point of the transmitter- receiver distance. Different coil separations (i.e. 25, 50, 100, 200, 300 m) and up to eight different frequencies (111, 222, 444, 888, 1,777, 3,555, 7,111, and 14,080 Hz) can be measured and used. The depth penetration of the eight different frequencies deliver also information about certain depths. The depth penetration of the different frequencies is limited by the so called skin effect. The deepest penetration is reached by the lowest frequency. Best sensitivity of the system is found for the depth of 40% of the inter coil separation.

Technically, the best resolution of resistivity contrasts can be found in these depths. Since the different frequencies, at a given separation, deliver information about different depth intervals, they can be used to perform depth soundings, similar to Schlumberger depth soundings (DC geoelectrics).

Fig. 5.3-2: Sketch of the MaxMin apparatus (Knoedel et al. 1997).

Layer models can be generated by using inverse or forward modeling programs. It should be noted, that the technical layer resolution of MaxMin measurements is somewhat lower than in Schlumberger soundings (commonly used for depth soundings), because it integrates over larger depths. But lateral variations of layer thickness should be easier and more precisely determined by using the MaxMin method. Another advantage is the small inter-coil separation compared to the large electrode separations necessary in Schlumberger depth soundings. The time consuming placing and coupling of the electrodes to dry alluvial sediments of the Jordan Valley can also be saved since the coupling to the ground happens by electromagnetic fields.

5.3.2.3 Interpretation of the results

The measured profiles can be interpreted differently. To obtain a first idea of resistivity variations, the outphase and inphase can be plotted along the survey transect (Fig. 5.3-3). Another quick illustration of the measured data can be done by plotting the electric conductivity values calculated by the apparatus (Fig. 5.3-6 through Fig. 5.3-8). These calculated values underlie the assumption of a homogenous half-space. Because the penetration depth is frequency dependant first predictions about the resistivity distributions in the subsurface can be made by measuring different frequencies at the same inter-coil separation.

Fig. 5.3-3: In- and outphase plotted along a survey transect near the town of Kafrein in east- west direction.

A first approximate interpretation of the resistivity distribution and the quality of the data can be made by plotting the inphase against the quadrature, the so-called phasor plot (Fig. 5.3-4). Through the different points (one for each frequency) a curve can be drawn that can be compared to model curves.

Fig. 5.3-4: left: Phasor plot of inphase and quadrature (after Reynolds, 1997); right: MaxMin sounding curve of the inphase and quadrature for the different recorded frequencies.

For more precise interpretation of the measured results inversion or forward modeling programs can be used. A layered subsurface model can be obtained either by depth soundings at single locations or as cross sections along a survey transect.

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The first field campaign took place at the end of March until the end of June 2002 and was conducted in order to delineate a salt-/freshwater interface by using a frequency dependent electromagnetic method (FDEM) called MaxMin. For the measurements the APEX MaxMin I-8 apparatus was used. In addition to the measured profiles, intensive data and literature collection was done in order to calibrate and validate the calculated MaxMin interpretations.

Field measurements:

During the first measurement campaign, 61 profiles with various lengths and different inter coil separations were conducted. The length of the profiles varied from several hundred meters to three or four kilometres. Since different depths of a possible salt-/ freshwater interface were expected different inter coil separations were applied. The inter coil separation ranged from 50 over 100 to 200 m. It should be noted that only six frequencies were measured at the latter two separations. Because of their unstable behaviour, the frequencies 111 and 222 Hz were omitted. Fig. 5.3-5 shows the location of the conducted profiles. Since the EM measurements are very sensitive to electrical power sources as well as to metal devices, measurement locations in the eastern part of the study area are limited.

For most of the measurement results forward or inverse modelling proved to be impossible. The problem of ambiguity couldn’t be minimized. Therefore only qualitative results from the conducted profiles were used for interpretation purposes. Fig. 5.3-6 through Fig. 5.3-8 show interpolated plots for different apparent conductivities calculated by the MaxMin computer for different frequencies and the best fit conductivity. The different values were interpolated (universal kriging) (with the help of the ArcGIS 9.2 package (Spatial Analyst) developed by ESRI Inc. As described above, these results can only be seen as quantitative results, since the MaxMin computer calculates these values on the basis of a homogenous half space. However, because the penetration depth is frequency dependant, conclusion about the resistivity distribution with depth can be drawn by plotting the apparent conductivities for different frequencies.

The apparent conductivities calculated on the base of the 888Hz frequency (Fig. 5.3-6) shows lowest conductivities around the outlets of the major wadis. Higher values are found in the western part of the study area and in the area between the major alluvial fans. The apparent conductivity values calculated on the base of 3555Hz frequency shows a partial different picture. Here high conductivity values can also be found between the villages of Kafrein and Rama (Fig. 5.3-7). Since the frequency is higher than for the first plot, it can be assumed that higher salt content in surface soil might be the result of these higher values. Fig. 5.3-8 shows the best fit conductivity for the measured frequencies. The figure shows almost the same trend as Fig. 5.3-6. Lowest conductivities are found around the outlet of the major wadis and highest between the alluvial fans and in the west to southwest of the study area.

These findings coincide with distribution maps of chapter 5.3.3.1. Therefore it can be assumed, that the salt content either of the deposited sediments or of the groundwater are responsible for high or low conductivities measured in the different profiles.

5.3.2.5 Lessons learned

Since the EM method is very sensitive to magnetic fields and since the study area is used intensively for agriculture, it was not always very easy to identify possible measurement sites. Such sources of disturbances are power lines, fuel driven generators, or metal pipes. In addition to these difficulties the salinization of the soil in lower Jordan Valley is far higher than expected which therefore reduces the penetration depth significantly. Especially in the west and southwest of the study area the upper soil salinization proved to be far too high, which prevents a deeper penetration. Therefore other methods that allow a deeper penetration in higher conductive environments were applied. However, the soundings give a soil conductivity distribution of the area, where lowest conductivities can be found near the outlets of the major wadis and highest in the west and between the outlets of the major wadis.

Fig. 5.3-5: Location of the conducted geophysical surface profiles. The MaxMin profiles are shown as solid red lines. The Schlumberger depth sounding locations are shown as points of different color, depending on the

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principal investigators (brown = BGR 1963/64; blue = BGR 1984/85; red = JICA 1995; dark green = Dr. Abou Karaki 1998; light green = Prof. Salameh; yellow = Dr. Al-Zoubi 1999; and black = Toll in 2003).

Fig. 5.3-6: Interpolated (universal kriging) plot of the apparent conductivity for one (888 Hz) frequency calculated by the MaxMin apparatus. Violet points represent the conducted profiles.

Fig. 5.3-7: Interpolated (universal kriging) plot of the apparent conductivity for one (3,555 Hz) frequency calculated by the MaxMin apparatus. Violet points represent the conducted profiles.

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Fig. 5.3-8: Interpolated (universal kriging) plot of the apparent conductivity of the best fit calculated by the MaxMin apparatus. Violet points represent the conducted profiles.MaxMin best fit conductivity.

5.3.3 Vertical electric soundings (Schlumberger depth soundings)