Vertical electrical sounding for characterizing subsurface tectonic features and their impacts on groundwater occurrences: a case study of basaltic Jbab uplifting in southern Syria

The tectonic and faulted zones, characterized by a pronounced change in rock physical properties can be directly delineated and detected by the application of geophysical techniques.

The geoelectrical direct current (DC) technologies are the most suitable tools for characterizing the faulted and tectonic zones. Several DC configurations, such as the combined resistivity profiling have been applied for locating the subsurface faulted and tectonic zones (Mares 1984). Information on timing and Quaternary fault geometry was successfully provided by conducting a large number of high-resolution seismic reflection prospecting surveys (e.g., Williams et al. 1995; Palmer et al. 1997; Van Ardsal et al. 1998). The ground-penetrating radar (GPR) technique, which bridges the gap between the trenching and high-resolution seismic surveys, has been practiced in the San Francisco Bay area for very shallow investigations (Cai et al. 1996).

A high-resolution image down to 4 to 6 m is provided by applying the GPR technique, however, its real disadvantage is the high number of GPR diffractions and reflections caused by the presence of tectonic features and complex sedimentary, which permits to locate the fault ambiguously (Demanet et al. 2001). The interpretations of the GPR data quantitatively and qualitatively offer important information on the deformation features near the fault and its location, particularly, when this fault is localized by other appropriate geophysical techniques.

Different geophysical technologies have been applied at the border of California and Nevada (electromagnetic, magnetic and seismic reflection) to determine the Parhump Valley fault zone extension (Shields et al. 1998)). Different geophysical techniques (seismic reflection, GPR, electromagnetic and electrical profiling) have been also carried out along the Bree fault scrap (Western border of the Roer Graben), to locate and image an active faulted zone at a depth varying between some decimeters to some tens of meters (Demanet et al. 2001). Combined geophysical techniques have been applied in an investigation site including a fractured faulted zone in Managua, Nicaragua (Parrales et al. 2003).

Chwatel et al. (2005) have provided some examples of mapping active faults from the Central Vienna basin, by using the geophysical techniques of high resolution.

Electric resistivity tomography carried out by Fazzito et al. (2009) was used to characterize the Quaternary faults in the Andean Precordillera of Western Argentina. Massoud et al. (2009) have already applied in El Fayoum near Lake Qaroun in Egypt the directional azimuthal resistivity sounding and the joint inversion technique of the VES-TEM data to trace the shallow subsurface structure there.

Asfahani (2007a, 2007b, 2010, 2011a, 2011b) have carried out different geoelectrical DC approaches in the Khanasser Valley region in northern Syria, particularly the vertical electrical sounding (VES) technology to evaluate the water resources. Those approaches have allowed the author to delineate the subsurface structures and outline the distribution of the fresh, brackish and saline water accumulations by interpreting the data VES with the Pichgin–Habibullaev technique (1985). Asfahani & Radwan (2007) have also enhanced the capability of the Pichgin–Habibullaev method to make it suitable for application in relief regions, even in a pronounced topography. Young and shallow subsurface structures were accordingly localized, to be used thereafter as a basis for explaining the tectonic origin of the Khanasser Valley region. Several geoelectrical techniques have been also carried out by Asfahani et al. (2010) in the Al-Lujj study area, Northwestern Syria to explain the origin of the subsurface tectonic of the Kastoon Dam in the Ghab basinal depression. A geoelectrical combined sounding-profiling array (GCSPA) has been invented and applied by Asfahani (2018) for delineating and describing the subsurface tectonic conditions of a phosphatic deposit in Al-Sharquieh deposits mine in Syria.

Asfahani & Al-Fares (2023) developed and calibrated a new technique of geoelectrical VES technique for tracing and characterizing the active tectonic features, the Northern Dead Sea Fault System, Syria. Such acquired calibrated geoelectrical data are regarded as a reconnaissance tool before the trenching.

The available VES soundings are interpreted in this paper by using three interpretative techniques for characterizing the subsurface tectonic of a basaltic environment.

The first is the traditional inversion technique with the use of the Resist software of Velpen (2004) to characterize the optimum models of resistivity and thickness for the (VES) points measured in the study region. The second is the enhanced Pichgin–Habibullaev technique (1985) for delineating the different subsurface tectonic features in the Jbab area, southern Syria. The 1D results obtained by the first technique are superimposed on the subsurface tectonic model obtained by the second technique, in such a manner, a subsurface lithological and tectonic model is established along the studied profile.

The third is the application of fractal concentration–number (C–N) modeling to identify the apparent resistivity populations existing in the area of study.

The knowledge of the apparent resistivity populations is a helpful tool to determine the lithological boundaries under a studied profile. The originality of this research paper is the conjoint utilization of those mentioned three techniques for describing and identifying the basaltic environment, characterized by sharp changes laterally and vertically. Such a characterization is not an easy task and therefore requires us to employ different suitable interpretative approaches to get the best results as possible as we can.

The use of the three above techniques as an integrated approach is aimed at identifying and describing the subsurface basaltic environment as accurately as possible.

The aim of this paper is therefore to realize the measurements of VES in the Jbab area, southern Syria, and to interpret those VES data points by several interpretative techniques. The Pichgin and Habibullaev approach (1985) is used to characterize different subsurface tectonic features, particularly Jbab uplift in the study area. The fractal approach of the C–N model is applied herein to determine the apparent resistivity populations, dominating in the study area.

This aquifer forms the contact boundary between the lower Quaternary and the Pliocene with its contents of fractured and impermeable rocks. The average basalt thickness ranges up to 100 m, with an intermediate average aquifer thickness of up to 70 m. The groundwater moves within this aquifer according to the northeast and southeast currents; the first is characterized by greater water abundance.

It is within the overlap of worn-out basalt levels and consolidated basalt. It is characterized by having different recharges related to the degree of fracturing, decay of basalt, and the clay levels that form impermeable zones. The thickness of this aquifer ranges between 50 and 155 m.

It is located between the chalky limestone and fractured broken flint zones, associated with the faulted zones. This aquifer gets its recharge from the vertical infiltration of water from the upper aquifers. This aquifer exists at great depths of about more than 400 m.

It is located at depths of more than 650 m, and appears within the fractured dolomitic limestone levels. The information about this aquifer is generally limited.

Geoelectrical VES measurements were realized in the Jbab region by using the Schlumberger array. The acquired VES data were interpreted qualitatively and quantitatively in order to characterize the best geoelectrical models for all the measured VES points. In addition to the quantitative inversion interpretation of the VES data, two main interpretative techniques are involved in this study; the Pichgin–Habibullaev technique and the C–N fractal modeling technique.

Schlumberger array is applied to establish the VES data at 18 VES points in the Jbab region.

The measured apparent resistivity values obtained at each VES sounding station were thereafter plotted on a bilogarithmic graph against the half-electrode current separation (AB/2) to construct the VES curves. Any noisy data in the established VES curves must be firstly removed by smoothening procedure (Chakravarthi et al. 2007; George et al. 2015). Any discontinuities, observed in the smoothened curves were subsequently attributed to the vertical variation in apparent resistivity with depth. The traditional curve matching technique is used to quantitatively interpret every smoothened VES curve (Zohdy et al. 1974) to obtain the preliminary layer thicknesses and resistivities model, which can be used thereafter as an initial input model in the computer-aided interpretation program ‘WINRESIST’ (Velpen 2004). Least-squares inversion procedure of the initial input model of VES data is applied within this software to produce the final 1D resistivity model curves.

The green continuous line represents the theoretical model obtained by the software, while the ‘ + ’ indicates the apparent resistivity measured data points. The inverted 1D geoelectrical results allow the final resistivity-depth model to be obtained as shown on the right of Figure 6. On the whole, the root mean square (RMS%) error of the fitting is between 1.9 and 5.8%.

The quantitative geoelectrical results of the 18 VES points in the Jbab area are indicated in Table 1.

The Pichgin–Habibuleav method is the most sophisticated one for describing the subsurface structural features (Asfahani 2011b). It is applied to interpret the VES measured along a given profile (Pichgin & Habibullaev 1985).

The summary and the basis of this technique are explained as follows:

When VES sounding is carried out at a given point, the injected electric current boundary conditions at a contact between two outcropping formations of resistivities ρ1 and ρ2 is explained through the main principle of the Pichgin–Habibullaev approach as follows:

• 1. If the center point of the VES sounding is located over a vertical contact between two formations of resistivities ρ1 and ρ2, and if the electrode configuration is perpendicular to this contact the measured apparent resistivity ρk is written by the following expression:

  

  (2)
• 2. If the configuration is parallel to such a contact, the measured apparent resistivity ρ'k is written by the following expression:

  

  (3)

The measured apparent resistivity in the above cases is independent of the distances between the potential electrodes (M and N) or between the current electrodes (A and B).

The locations of the VES points measured along a studied profile, and the distances between those VES are plotted on the abscissa, and the corresponding apparent resistivities (ρk or ρ'k) are plotted for each given AB/2 on the ordinate as indicated in Figure 7.

A PC computer software developed earlier by Asfahani & Radwan (2007) is applied in this study to compute and determine the locations of the PNH. The following assumptions of Pichgin–Habibullaev method are used to get the geological interpretations of PNH:

• 1. The presence of an inhomogeneous lithologic contact is indicated by PNH distributed along oblique lines located at shallow depths.

• 2. The presence of a fractured zone is indicated by PNH, arranged along oblique lines dipping at an angle exceeding 30° at depth.

• 3. A homogeneous lithology is indicated by PNH scattered randomly near the surface.

• 4. The presence of some geological structures, such as anticlines, synclines or horizontally layered strata is indicated by PNH, arranged in regular form.

The above four assumptions have been previously calibrated and verified through some field applications in Syria with several structural architecture and lithologies (Asfahani & Mohamad 2002; Asfahani 2007a, 2007b, 2024; Asfahani & Radwan 2007; Asfahani et al. 2010; Asfahani & Al-Fares 2022).

The present work is mainly concentrated on applying this technique with its different assumptions in a tectonic case study taken from the Jbab area, southern Syria, to characterize the tectonic subsurface of this region. The findings are discussed in light of the geology of the study Jbab area.

This (C–N) semi-quantitative approach has been recently proposed by Asfahani (2021) to interpret (VES) measurements in the Khanasser Valley area, northern Syria. This technique was also applied to characterize the basalt distributions of Deir El-Adas, southern Syria, where encouraging results were obtained (Asfahani & Al-Fares 2022).

The suppression and equivalence problem common in electrical resistivity data interpretation is overcome by comparing the 1D results interpretation shown in Table 1 by geological and hydrological sections available in the study Jbab area, where suitable, comparable and acceptable results are obtained.

The apparent resistivity distribution lines for AB/2 spacing of 1.5 m up to 1,000 m indicate lithological conditions of the Neogene and Quaternary basalts, where the low and high resistivity reflect the degree of the basalt weathering.

The apparent resistivity values oscillate between low resistivity at the VES sounding points V5 and V8 (except for the values taken at AB/2 of 700 and 1,000 m) and V3 (except for the values taken for AB/2 of 1,000 m) and high resistivity. The transition zones from low to high resistivity are governed by the tectonic conditions and the faults dominating the study Jbab area. These transition limits are suitable locations for occurrences of groundwater.

The two hundred apparent resistivity data values measured along the LP for the AB/2 spacings varying from 1.5 m up to 1,000 m are treated by Pichgin and Habibullaev technique (1985) to identify the general features of the subsurface tectonic setting related to the study area.

The different established geoelectrical profiles discussed above provide clarity not only about the geology and the subsurface tectonics of each profile but also aid in the construction of the subsurface lithological and tectonic section under a given profile.

The stratigraphic sequence of the interpreted geoelectrical layers in the study Jbab region indicates a succession of layers of high resistivity with layers of low resistivity, reflecting the effects of basalt alteration and weathering with the associated clay products.

The conductive weathered Neogene basalt aquifer (layer 3 or 4 marked in bold as shown in Table 1) in the Jbab area sandwiched between two basaltic resistive layers is located at a depth (D) varying from 12.40 to 291.8 m with an average of 66.22 m. The thickness of this aquifer-weathered basalt ranged between 32 and 277 m, with an average of 140 m, and its resistivity ranged between 12.5 and 47 Ω·m, with an average of 30 Ω·m. This aquifer basaltic horizon is considered the optimum one from a groundwater point of view, in terms of thickness on the one hand, and its lithological nature on the other.

The locations of C1, C2, C3, and C4 allow consequently to determine several apparent resistivity ranges as follows:

• 1. Less than 10 Ω·m.

• 2. Between 10 and 45 Ω·m.

• 3. Between 45 and 74 Ω·m.

• 4. Between 74 and 126 Ω·m.

• 5. Bigger than 126 Ω·m.

One of the advantages of the fractal (C–N) modeling technique as practiced in this paper is to trace and follow easily the apparent resistivity contrast between the different litho-resistivity ranges. These contrasts of apparent resistivity ranges and the boundaries of different lithologies cannot be obtained and seen by the traditional interpolation Kriging technique as shown in Figure 9.

Eighteen VES were measured by applying a Schlumberger array to characterize the main subsurface tectonic features and their impacts on groundwater occurrences of the uplifted Jbab area in southern Syria.

The main salient findings of the present geoelectrical research are as follows:

• 1. The new integrated interpretative approach (IIA) composed of different geoelectrical techniques, as used in this paper (Pichgin and Habibullaev and the fractal modeling C–N) proves its efficacy, when it is practiced conjointly with the ID conventional inversion VES interpretation technique, to identify and describe the basaltic structures, characterized by rugged resistivity variations. The combination of the 1D geoelectrical inverted results with those of Pichgin–Habibullaev technique allows the establishment of a 2D subsurface lithological and tectonic model of the study profile as done by analyzing the different longitudinal and transverse profiles.

• 2. The fractal modeling of (C–N) technique allows distinguishing between different populations of apparent resistivity by finding the locations of the break points and their line segments with different apparent resistivity populations. Each range population represents a specific lithology, where the constructed 2D profile shows clearly the boundaries of different lithologies (Figure 14). The advantage of this C–N is that it does not need prior constraints. The technique itself is also sensitive to lithological variations, where the lithological boundaries are well determined under a studied profile. This (C–N) method overcomes the limitations of the interpolation Kriging variogram method, which is used for analyzing the autocorrelation function of the apparent resistivity target process as indicated and presented in Figure 9.

• 3. According to the above-described approach of IIA, the tectonic and geological structure of the study Jbab region to depths of up to 500 m has been well determined and documented along the different longitudinal and transverse. Profiles.

• 4. A set of VES points arranged according to their importance and priority are identified for drilling wells for groundwater extraction. The discharges in the study Jbab area and its neighborhood are expected to be about 15 to 20 m³/h.

• 5. It is believed the boundary between the two Neogene basaltic systems and the Paleogene sedimentary system is penetrated, especially at the VES point (V4) at a depth of 498 m. It must be noted that this boundary is expected to appear at depths exceeding 400 m in the study area, and this requires lithological verification and characterization by drilling a well at VES point (V2).

• 6. The integrated geoelectrical techniques applied in this paper can be applied to characterizing and studying similar basaltic environments worldwide.

Thanks to Professor I. Othman, General Director of the Syrian Atomic Energy Commission, for his approval to publish this research work. The three competent reviewers are cordially thanked for their skill and professional critiques and remarks that improved considerably the final version of this paper. My thanks to the editor of Water Practice and Technology Journal for handling this paper at its different stages.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.