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The application of geophysical methods for the successful exploration of groundwater in sedimentary terrain requires a proper understanding of its hydrogeological characteristic, especially in arid and semi-arid areas in order to increase the possibility of successful drilling of water wells. Additionally, there is an increasing demand for geophysical surveying in areas of limited information regarding existing wells or hydrogeological data. However, new approaches were necessary to adopt geophysical techniques for subsurface investigations and to deal with special problems inherent in shallow geophysical surveying. In this context, increasing overall efficiency of the geophysical data, model quality, and their sensitivity and resolution of near-surface geological investigations comes into account first.
The study area (El- Salloum Basin) extends for about 37 km along the Northwestern coast between Buqbuq at the West of Sidi Barrani and El -Salloum City at the western border of Egypt with a southward extension of about 22 km. It lies between longitudes 25.15º & 25.55º E and latitudes 31.35º & 31.50º N. The study area is characterized by very low relief and mild topography. A flat coastal strip 2-4 km wide is found behind the coastal ridges, located about 10 km east of El- Salloum town. The area between the coastal strip and the plateau has a rather pronounced relief and there are several terraces and a few large depressions to the scarp of the plateau. The northern scarp of the tableland (plateau) is dissected by a series of drainage lines (about 83 wadis). Surface water in El -Salloum basin is very limited most of the year due to unstable patterns of seasonal rainfall. The Middle Miocene rocks (Marmarica Formation) represent the oldest exposures in the investigated area with variable thickness, being controlled by the buried structures on which they have been deposited. The Marmarica Formation is exposed at the surface, forming the Diffa plateau and the bedrock of El-Salloum depression. This formation is considered as the watershed area that drains all the streams and rainfall to a common outlet (e.g. outflow of Wadi Arkeet). This succession is overlain by Pliocene and Pleistocene calcareous rocks, with Holocene and recently deposited white sands forming the coastal ridge. Borehole geological information is limited across the basin. Data of only three water wells are available in the study area (Desert Research Center (DRC), Abu Zreba (AZ) and Ogereen (OG) wells). These wells were drilled by the Desert Research Center. The Abu Zreba well is located towards the north of the study area and the depth to water is 17 m in Oolitic limestone. The DRC well is located in the NE portion of the coastal plain. The depth to water is 21 m and the water bearing formation is fractured chalky limestone interbedded with marl. Ogereen well is located in the southwest of the study area. The depth to water is 49 m in sandy limestone interbedded with dolomitic limestone.
The area is affected by numerous NE and NW trending faults with well-developed joints and fractures on the scarp boundaries of El Salloum basin (Selim, 1969). The application of three geophysical techniques sensitive to the subsurface electrical conductivity at different scales proved to be an effective approach to groundwater exploration in El-Salloum basin.
The present study aims at determining the vertical and horizontal extensions of the lithologic succession, determining the structural elements that affect the area of study (major and minor faults, fracture system…etc.), identifying the potential water bearing layer/layers, identifying the fracture zones which likely play an important role in determining groundwater occurrence and locating the most suitable sites for drilling groundwater productive wells.
Depending on the lithologic succession and the depth to the targeted water bearing formation, geoelectrical methods, including 1D Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Imaging (ERI) proved to be useful techniques in locating groundwater in both porous and fissured media because of their sensitivity to the rock conductivity and fluid flow. On the other hand, the VLF-EM method is considered a quick reconnaissance tool best suited for the identification of conductive zones with steep boundaries and can be particularly useful in locating water bearing fractures and other structures that control groundwater flow and characterized by resistivity contrast at the boundaries of fractured zones.
Applying such integrated geophysical techniques (1D-VES, 2D-ERI and VLF-EM) is considered as a promising approach for exploring the water bearing fractured zones in the Middle Miocene Limestone (Marmarica Formation) in El Salloum Basin.
The objectives of the present study had been achieved by conducting 18 1D-VES distributed all over the area as a grid of, approximately, 5 km spacing between VES locations. The geoelectrical resistivity field data were acquired using the earth resistivity meter SAS 1000 Terrameter. The Schlumberger array with current electrode spacing up to 300 m in the northern part of the basin and up to 1400 m in the southern part was used. The software IX1D (Interpex Ltd., USA), v. 3.39 was used for the quantitative interpretation of the 1D-VES data. It is a software package for Windows 9x which allows for forward and inverse modeling of 1-D Direct Current (DC) resistivity, Induced Polarization (IP), Magnetotelluric (MT) and Transient electromagnetic (TEM) sounding data. The quantitative interpretation was carried out depending on an initial model created from the available geological and hydrogeological information of the study area.
The 2D- ERI, was subsequently applied along specific transects to image the distribution of the electrical resistivity from which complex geologic features such as fractured zones can be outlined. The Wenner array was used to acquire the 2D ERI data due to its high signal to noise ratio and the relatively good sensitivity to structures at depth when compared to other possible configurations (e.g. dipole-dipole). ERI datasets were acquired using a Syscal Junior resistivity meter (IRIS Instruments, France) connected to a linear array of 72 electrodes with electrode spacing varying between 2 to 20 m at the northern and southern parts of the basin, respectively. Sixteen profiles varying in length from 142 m to 3500 m were acquired in roll–along fashion where needed. ERI lines were located between pairs of VES points to identify the extension of the water bearing formation, saline water zones and the extent of the clay layer/layers in two dimensions and at specific points of interest within the basin.
ERI datasets were inverted using two software, Res2Dinv (Loke, 2015) and R2 (Binley A., Lancaster university, UK). The depth of investigation (DOI) was estimated for the 2D-ERI profiles to identify the possible artifacts. In the present study, the method which is developed by (Deceuster et.al, 2014) was applied. This method is based on a statistical estimation to calculate the scaled DOI index. This method is performed by carrying out two inversions with homogeneous reference models of differing resistivity (0.1 and 10 times the mean of the logarithm of the apparent resistivity values). The cumulative distribution of computed DOI index values is fitted using the sum of two normal distributions allowing an interpretability index to be computed for every cell of the model. A new inversion is then performed with a third homogenous reference model (mean of apparent resistivity). The interpretability index is an ‘alpha blending’ step so that the resistivity model is plotted in a way to discriminate between well- and poorly-constrained cells.
On the other hand, twenty eight VLF-EM profiles were measured across the basin on a similar grid to that used for VES acquisition. ABEM-WADI instrument was used for the data acquisition at 10 m intervals along profiles varying in length between 100-560 m. The VLF profiles were oriented perpendicular to the direction of anticipated E -W faults and fractures. Three remote radio-transmitters located in Russia (18.2 kHz), Italy (20.3 kHz) and France (21.7 kHz) was detected at the sites. These three transmitters were well separated by azimuth between study site and transmitter location. The measured dip angle response needs to be filtered to reduce random noise and long spatial wavelengths. Firstly, it passes by Fraser Filter which is a simple numerical filter converting crossovers of current polarity into peaks by differencing successive values of the real component along the profile or shifts the measured dip angle data by 90o. Secondly, the real component of the vertical magnetic field of the VLF-EM data was filtered using the developed Karous and Hjelt filter that is considered to produce the secondary currents in the ground.
This filter provides an apparent cross-section of the equivalent current density. This qualitative current density cross-section provides information on the relative distribution and/or relative depth of geological features, such as faults and shear zones as well as information on discontinuities filled with water or clay. The KHFFILT software version 1.1a, (Pirttijärvi, 2004) was used to filter the data and obtain both the Fraser and Karous-Hjelt responses (equivalent current density cross-section).
A consistent picture of the shallow resistivity structure emerges from the three methods. Indeed, the shallow resistivity structure shows fair resemblance, at least in the upper 50 m, when comparing 1D and 2D electrical resistivity models. However, the reliability of the models may be compromised by the presence of thick conductive layers (clay layers or saline water) which limit the depth of current penetration. The DOI estimated for each 2D-ERI survey line helps to prevent over or misinterpretation of the resistivity models. In the present study the application of electrical tomography method to image the subsurface conductivity structure. The results show that the conductive zones revealed by inversion of VLF-EM data are in a very good agreement with the conductive zones of the electrical tomography. The combined use of multi-dimensional DC resistivity and VLF-EM methods have potential for improved structural characterization of the subsurface.
The application of integrated geophysical methods revealed that the southern and southeastern parts of the study area are the most promising potential zones of, relatively, better quality groundwater occurrence. The results shed valuable light on the sources of groundwater associated with two aquifers. The Oolitic limestone (Pleistocene-Holocene) aquifer is restricted to the northern parts of the study area. This aquifer is recharged, mainly, from the infiltrated direct precipitation over the porous and permeable Oolitic limestone. The fractured chalky limestone and argillaceous limestone of the Middle Miocene Marmarica Formation are considered the main source of groundwater in the basin, especially southeastwards of the coastal plain as seen at multiple VES points including VES 1, VES 2, VES 3, VES 4 and ERI images at Site 2 and Site 4. The thickness and resistivity of this aquifer increases towards the south and southeast. This aquifer is primarily composed of chalky and argillaceous limestone interbedded with layers of marl. Indeed, VLF-EM profiles revealed evidence for a highly fractured system over the study region which presumably forms an internal interconnected network of caves and conduits through which the groundwater moves. Fracture zones (high electrical conductivity) acts as conduits or pipes along which groundwater flows. These structures play a vital role in improving the hydrogeological setting.
Additionally, the results also demonstrate the capability of integrated 1D-
VES, 2D-ERI and VLF-EM techniques to understand and delineate groundwater occurrence in arid heterogeneous/karst environments. The integration of three geophysical methods successfully identified potential zones of ground water, the saline water zones, and the major geological structures in El-Salloum basin.
In view of the results of the present study, the following is recommended:
1- To deal with anisotropic aquifers (such as fractured limestone aquifers), it is necessary to apply detailed and integrated exploration techniques.
2- Due to the relatively limited spatial extension and thickness of the fractured limestone aquifers, drilling decisions should be made according to detailed exploration to avoid erroneous location of the production wells.
3- Calculation of the depth of investigation (DOI) gives better and reliable geoelectrical images of the subsurface and avoids wrong decisions based on erroneous data.
4- Application of induced polarization (IP) method could help to better discriminate between saline water and clay layers in future studies.
5- The best locations for drilling water wells are in the southern and southeastern parts, e.g. VES 3 and Sites 15 and 16 of 2D- ERI.
6- Desalination plants can be deployed in the middle and northern parts of El- Salloum Basin to supply the local communities with the needed drinking water.