An electrical resistivity and electromagnetic emission survey was carried out involving the use of vertical electrical soundings (VES) and natural pulse electromagnetic field of the earth (NPEMFE). The use of this new methodology managed to detect the fracture flow system rupture zones in the underground, also answered the questions about the deferent subsurface water bodies. The present study focuses on Marsaba-Feshcha sub-basin in the northeast of the Dead Sea. Due to the scarcity of boreholes in the study area, several geophysical methods were implanted. The combination of these two methods (VES and NPEMFE) with the field observations and East-West transversal faults with the coordination (624437/242888) was determined, cutting through the anticlines with their mainly impervious cores with fracture length of >400 m. These transversal faults saddle inside Nabi Musa syncline (Boqea syncline), leading to a hydraulic connection between the Lower and the Upper Aquifer. Due to the identified transversal fault, the water of the Upper and Lower Aquifer mixed and emerged as springs at Ein Feshcha group.
Groundwater resources in the Middle East surrounding areas were deteriorated noticeably in the last 50 years. Salt concentration in the aquifer systems along Jordan valley which is covered by Lake Lisan (ancestor of the Dead Sea) indicates a general trend of increasing salinity which reach its maximum in the lower part of Jordan Rift Valley and threaten the groundwater resources and consequently the stability of the whole system in the area [
This catchment area is considered as part of the eastern drainage basin of the Jerusalem hills and the outlet of this basin is Ein Feshcha spring group which is located on the upper North-Western shore of the Dead Sea at an elevation of approximately 413 m below sea level, that is, a head of 860 m over a lateral distance of about 20 - 25 [
Geologically the West Bank is located on the Northern edge of the Nubian-Arabian Shield. The Shield belongs
Location map (a) Location of the background of the Near East Countries; (b) Wells and spring location in Marsaba chatchment; (c) Wells and springs downstream
to the Precambrian age and consists of complex crystalline plutonic and metamorphic rocks. The Shield metamorphic rocks are mainly of sedimentary origin (metasediments) and usually referred to as the basement comple. These Sedimentary rocks overlaying the Shield are known as the shelf deposits. Two environments of sedimentation can be recognized within the shelf rocks: the stable and the unstable environments. The stable environment is characterized by continental deposits inter-fingering with neritic and littoral deposits. The unstable environment is characterized, in general, by carbonate sediments deposited in marine environment. Locally, the unstable shelf is divided into basins of Euxinic conditions and swells of continental conditions [
The thickness of Ajlun Group (Judea) is about 800 - 850 m [
This study deals with the aquifer beneath the Jerusalem Desert (Judean Desert), a thick carbonate aquifer comprised of an upper phreatic unit overlying a confined unit, separated by a thin aquitard. The spring complex of Ein Feshcha stretches about 4 km along the North Western Dead Sea shoreline. At this location the distance from the fault escarpments to the Dead Sea ranges from 100 to 500 m. The territory comprises strata of down faulted Cretaceous, Lisan Formation as well as recent conglomerates, gravel, sand, silt and clay. Recent regression of the Dead Sea in past decades resulted in the formation of a mudflat enveloping the sea. It predominately
Generalized geological columnr section indicating the aquiferial chractaristics of the various formations [17]
Geological formations exposed in Ein Feshcha Study area [19]
consists of grey, brown, dark-green and black clays composed of detritic illite, smectite and kaolonite in similar amounts and some minor palygorskite [
The stratigraphic succession exposed in the study area varies from Lower Cenomanian age (Cretaceous) to young Holocene formations, attaining a thickness of approximately 1200 m. The important sequence with respect to hydrogeology is the Ajlun (Judea) Group Aquifer of Cenomanian-Turonian age with a total thickness of about 800 - 850 m in the Jerusalem Mountains (Judea Mountains) and decreases towards the south; east and west to a minimum of 600 m near Ein Gedi. It is built of karstic limestones and dolomites, separated by layers of marl and cherts.
The presented stratigraphic column includes formations of the Ajlun (Judea), Belqa (Mt. Scopus) and Lisan (Dead Sea) Groups of Lower Cretaceous to Holocene ages (
The predominant elements governing the geological structure and flow regime are the anticlinal and synclinal structures crossing the entire study area (
Structural map showing the main folds and faults in the study area
Geophysical resistivity techniques are based on the response of the earth to the flow of electrical current. In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of the resistivity is a measure of the amount of water saturation and connectivity of pore space. Increasing water content and increasing salinity of the underground water will decrease the measured resistivity. So, increasing porosity of rock and increasing number of fractures will tend to decrease measured resistivity if the voids are water filled. The geophysical methods (VES and NOEMFE) were applied in this study, to bring information about fresh, brain or saline water bodies in the underground, but also information about the geological structure until 100 m depth. While the special electromagnetic measurements were applied to bring direct information about fractures and rupture zones in the underground. By this VES method (Schlumberger sounding), the variation of the resistivity with depth is measured, depending on the electric properties of the geologic sequences in the subsurface. The electric properties are influenced by the lithology, the saturation degree and the salinity of the fluids involved, salt water can be easily distinguished from almost any lithological combination, having a resistivity below 1 Ohm/m. In the year 1997 in unpublished report Goldman proposed guidelines for comparing water quality by resistivities in this specific area:
. In the shallow subsurface, the presence of water controls much of the conductivity variation. Measurement of the resistivity is a measure of the amount of water saturation and connectivity of pore space. Increasing water content and increasing salinity of the underground water will decrease the measured resistivity. So, increasing porosity of rock and increasing number of fractures will tend to decrease measured resistivity if the voids are water filled. The geophysical methods (VES and NOEMFE) were applied in this study, to bring information about fresh, brain or saline water bodies in the underground, but also information about the geological structure until 100 m depth. While the special electromagnetic measurements were applied to bring direct information about fractures and rupture zones in the underground. By this VES method (Schlumberger sounding), the variation of the resistivity with depth is measured, depending on the electric properties of the geologic sequences in the subsurface. The electric properties are influenced by the lithology, the saturation degree and the salinity of the fluids involved, salt water can be easily distinguished from almost any lithological combination, having a resistivity below 1 Ohm/m. In the year 1997 in unpublished report Goldman proposed guidelines for comparing water quality by resistivities in this specific area
Resistivity [Ohm/m] | Label | TDS [mg/l] |
---|---|---|
>3 | Fresh | 1000 - 2000 mg/l TDS, |
3 to 2 | Brackish | 2000 - 10000 mg/l TDS, |
<2 | Saline | >10000 mg/l TDS, |
<1 | Brines | Dead Sea saltwater intrusion. |
The NPEMFE method (CERESKOP) is depending on the measurement of natural pulsed low frequency electromagnetic radiation (EMR) signals. Generally in geosciences the detection and analysis of these signals can assist to conceive and understand deformation processes. This method detects peak values in the geogenic electromagnetic field clearly exceeding the background noise and registers the orientation of the corresponding electromagnetic waves which produced by nanofractures, and by piezoelectric, turboelectric or pyroelectric effects [
The present study was carried out due to the scarcity of boreholes, which could provide information about the main groundwater flow paths, the fresh-saline water interface, the freshwater bodies, the saline water bodies and the depth of these bodies. At Ein Feshcha study area three profiles were modeled and studied (
A total of 19 sounding points were carried out along the three profiles. By using the Zhody software the final resistivity values for different depths were obtained in order to look for anomalous regions. These 19 sounding points were plotted on Google satellite map (
The vertical section along profile-I (
Location of the three profiles and the main topography of the area
Satellite map with the localization of the VES-soundings on three profiles
Vertical section along profile I
large development of resistivity values ranges between 3 - 15 Ohm/m can be seen, suggesting the existence of fresh water over the whole investigated depth interval of 100 m. This domain corresponds to the area around Ein Feshcha main spring (1 + 2). This fresh water is being mixed with trapped saline waters leading to dilution of these saline pockets. Mixing tacks place at the vicinity of the spring at least at two points, this can be seen from the chemistry of the borehole log Feshcha-4. The chemical analysis and the lithology of borehole Feshcha-4 (
Lithological section presenting the lithological setting of Borehole Feshcha-4 and 10
This block acts as a barrier in the advancing of saline water intrusion from the Dead Sea to the west not only but also it prevent the trapped water to retreat to the east, evidenced by low resistivity values, <1 Ohm/m. At sounding point 4, at distance of 300 to 350 m on profile-I, local low resistivity values suggest possible trapped saline/ brackish water, behind the assumed clay-limestone block.
Profile-II extends also from the west (624263/242910) to the east (624114/243290), from the road toward the Dead Sea (
Profile-III is located along the road, in the North-South direction (
Vertical section along profile II
Vertical section along profile III
water until a depth of 100 m. To the south of the main spring, on sounding points 14, 15 and 16, higher resistivity values were measured, suggesting the existence of sedimentary rocks overlaid with late Pleistocene-Holo- cene sediments (tills and clays) block, forming a barrier in the fresh groundwater flow along the North-South oriented main fault system. In the northern part of this section, at sounding point 19, the measured resistivity values suggest fresh water at greater depth, below 80 m and rising up in the southern direction toward the main spring. Between the sounding points 18 and 19, at distance of 25 to 325 m on profile-III. The soil was very dry may be due to the existence of tills filling the main fault system, the 34 resistivity measurements were not possible, therefore with the help of the available data and Zhody software the expected missing part were simulated. The results at this section suggests the existence of tills and limestone blocks, forming a barrier in the fresh groundwater flow along the North-South oriented main fault system, were exposed sedimentary rocks at the field site can be seen. At sounding point 18 low resistivity values were identified ranging between 1 - 3 Ohm/m indicating existence of localized brackish water zone at the depth of 30 up to 60 m. Very close to the main spring between sounding points 12 and 13 a mass of brackish water was identified with resistivity value ranging between 1 - 3 Ohm/m indicating existence of localized brackish water zone at the depth of 30 up to 100 m finding its way out in the main spring. Since these two localized zones are surrounded with fresh water then it can only be explained by a salt dissolution from the surrounding rocks (salt layer) and/or conduction of saline waters through associated fractures.
Along Ein Feshcha natural reserve borders in a North-South direction along the main fault system a profile was constructed in order to localize the fractures and rupture zones in the underground. This profile was explored using the Electromagnetic Radiation (EMR) reflecting the orientations of active crustal stresses in the uppermost lithosphere. The beginning of the explored profile was at the coordinate (625000/242800) and the end of it was at the coordinate (624000/242800). Along this profile several scans East-West direction was carried out using the high-sensitive geophysical electromagnetic instrument (CERESKOP) depending on the NPEMFE-method. Through the NPEMFE-method a lot of fractures were identified, mainly along the main fault system of the Dead Sea Rift, but also two transversal faults perpendicular with the main fault, one of them crossing the main fault at the main spring (1 + 2). The first transversally fault can be seen in the field, extending in an East-West direction heading to the Dead Sea cliffs (
The results obtained by the applied methods (VES and NPEMFE) and the field evidence gave more information about, the interfaces between fresh, brackish and saline waters. The lithological setting in the study area and localization of the faults and fractures exist in the study area. The combination of these two methods (VES and NPEMFE) with field observations gave more specific information which lead to more understanding of the mixing mechanisms. The VES measurements detect low resistivity (<1 Ohm/m) representing brines and the interface between them (brackish water) as well as the overlying fresher water bodies. In addition, high resistivity values >3 Ohm/m representing freshwater are also detected, underlying the brines. The presence of fresh water until a depth of 100 m in the area around the main spring (1 + 2) was shown also by the obtained distribution of the measured resistivities on profile-I. The only explanation for the fresh water source in Ein Feshcha is from Judea Mountains finding its way through the West-East transversal fault number 10. The occurrence of localized low resistivity water bodies (1 - 3 Ohm/m) in sounding points 2 and 12 on profile-I can be interpreted as a result of the existence of fracture number 1 and 2. These fractures may conduct waters with lower resistivities leading to mixing of the two water types. The source of the brackish water could be related to ascending brines across the fractures/salt dissolution. In the southern direction, immediately south of the main spring on profile-III, the resistivity values indicate the presence of a limestone block, displaced along a transversal fault, forming probably a barrier in the flow direction of the fresh water, obliging it to flow out and forming the main spring. Such a block was also signalized on profile-I, forming barrier in the advancing of the saline water from the Dead Sea toward the west. The occurrence of low resistivity water bodies until the investigated depth of 100 m, on profile-II, can be interpreted as trapped residual brines or salts, related to earlier Dead Sea base levels. These brines are mostly found in the East, closed to the Dead Sea base level and it has a horizontal development.
(a) East-West transversal fault crossing the main spring; (b) East-West transversal fault at Ras Feshcha
Fractures localized by combination of the stress variations using NPEMEF and VES method
The outcome of the geophysical analyses method presented the existence of late Pleistocene sediments (tills and clays) block, forming a barrier in the fresh groundwater flow along the North-South oriented main fault system. The localized fractures and faults found working as barriers and/or conductors as well as the lithology barrier explain some of the water mixing mechanisms. The combination between all of what have been analyzed and noticed in the field, a schematic cross section can lead to fresh groundwater inflow from the aquifer to the spring area, the ascending brines along the fault forming a soft salt body in the shallower combined aquifer with an interface causing the overflow of the fresh water, a possible diaper shown here reasonably in larger depth as well as the discharge of the mixed brackish water in the Dead Sea sediment sequence. It can be concluded that the existence of brackish water above and below the saline domain and the variation of water quality even over short distances implies multiple hydrological aquifer systems and a fracture flow system controlling the Ein Feshcha springs. Understanding this concept will help to evaluate the impact of the groundwater extraction on the heads and flow patterns in the aquifer and to locate extraction wells.
We gratefully acknowledge the financial and logistic support provided by SMART project at KIT University which is funded by the German Federal Ministry of Education and Research (BMBF). Special regards to the staff of natural reserve Ein Feshcha for their support and understanding. Special thanks also go to the reviewers for their efforts for improving and finalizing the outcome manuscript.