Ground-penetrating radar imaging of subsurface karst features controlling water leakage across Wadi Namar dam, south Riyadh, Saudi Arabia

Saad S. Alarifi; Elkhedr Ibrahim; Khaled Al-Kahtany

doi:10.1515/geo-2025-0781

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Ground-penetrating radar imaging of subsurface karst features controlling water leakage across Wadi Namar dam, south Riyadh, Saudi Arabia

Saad S. Alarifi , Elkhedr Ibrahim und Khaled Al-Kahtany

Veröffentlicht/Copyright: 16. April 2025

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Abstract

This article presents a study conducted at the Wadi Namar dam in southern Riyadh, Saudi Arabia, using ground-penetrating radar (GPR) to investigate underlying karst features influencing water leakage. Through the acquisition of 38 GPR profiles downstream of the dam, the investigation reveals diverse structural irregularities primarily within the Jubaila carbonate rocks beneath the dam. The processed GPR data delineated vertical fractures dissecting the carbonate bedrock, as well as parallel sheet-like lateral fracture zones. These features enhance groundwater permeability, facilitating both vertical and lateral flow, thus impacting water leakage from the dam. Additionally, the presence of dissolution and karstic cavity structures is indicated by high-resolution GPR reflections, potentially acting as underground water reservoirs. The findings underscore the significance of karst features in governing water seepage at the dam site, emphasizing the importance of geological and geophysical investigations for dam safety and management.

Keywords: GPR; karst features; water seepage; Namar dam; south Riyadh

1 Introduction

Saudi Arabia, situated in an arid region with limited rainfall and surface freshwater resources, has implemented numerous dams across its streams to collect and manage rainwater, thereby mitigating flood risks. However, recent instances of groundwater seepage have highlighted vulnerabilities in these structures. Among them is the Namar concrete dam, which was built across the northeast-oriented Wadi Namar, a western tributary of the 80-km-long Wadi Hanifa, located to the south of Riyadh, the capital city of KSA (Figure 1). The Namar dam has a surface water lake that covers a total area of 200,000 m², measuring 2 km in length, 170 m in width, and 20 m in depth. Resting on the carbonate bedrock of the Upper Jurassic karst Jubaila Formation (Figure 1), the dam and its upstream lake directly interface with this karst formation, known for its fractures and dissolution cavities [1,2]. This karst formation, at various locations in the Kingdom, displays fractures and dissolution cavities [3,4]. In karst terrains, rainfall and subsurface groundwater flow, coupled with extensive fracturing, lead to the development of karst features like solution-widened fractures and cavities. These features play a critical role in controlling vertical infiltration and shallow water flow, occasionally causing partial collapses and groundwater seepage. Therefore, the safety of dams and the potential impact of geological karst features necessitate thorough geological and geophysical investigations [5,6]. Water leakage, a common issue at dam sites, typically occurs through the bedrock, especially in dams constructed on karstic bedrock [7–11]. Milanović [12] discussed the leakage problem beneath many dams constructed on karst formations, some of which suffer from heavy leakage in the dam lake. Among the variety of geological and hydrological features, karst features are the most vulnerable, allowing groundwater to flow across the fracture and void system [13,14]. The development of flow conduits within karst formations leads to high permeability, allowing rapid transfer of groundwater beneath the dam body [7]. The Namar dam site, where groundwater emerges downstream of the dam (Figure 2), is particularly susceptible to leakage through karst conduits, potentially leading to subsurface collapse (Figure 3). This problem has not yet been definitively resolved and requires more in-depth studies to be understood. Hence, this study aims to address this issue and provide a comprehensive understanding of subsurface geological features and their impact on potential water leakage at the dam site. For this purpose, detailed ground-penetrating radar (GPR) imaging is used. Previous studies have demonstrated the efficacy of the GPR method in karst environments and groundwater flow investigations. Mellor et al. [15] applied the GPR method to detect the effect of solution cavities in North Carolina. Alhumimidi et al. [16] verified the fracture pattern of the Al-Khuff Formation outcrops in the Al-Qassim area, Saudi Arabia, using GPR and ERT geophysical surveys. The results revealed the presence of laterally extensive and vertically oriented fractures, identified as vertical to sub-vertical, across the measured sections. Almadani et al. [17] investigated near-surface karstification and fractured subsurface features beneath the site of the El-Elb dam northwest of Riyadh, using GPR and 2D resistivity techniques. Romano et al. [18] Romano et al. [19] used integrated geophysical techniques to verify the existence of underground caves and voids and provided useful information on their possible extension and depth; in addition, they detected likely pathways of water infiltration. The karst features in the limestone formations within and around the Riyadh region have been studied by many authors (e.g., [2,20–25]).

Figure 1

(a) Location of the studied site. (b) Simplified geologic map of south Riyadh showing the location of Wadi Namar dam. (c) A satellite image showing Wadi Namar dam and its upstream lake.

Figure 2

Groundwater appears on the surface on the downstream side of the Namar dam.

Figure 3

A satellite image showing the Wadi Namar dam (left) and a photograph showing partial collapse on southern side of the dam (center) with a close view showing the displacement of the collapse (right).

2 Geological setting

The Phanerozoic Arabian platform rests unconformably on the Precambrian basement rocks of the Arabian Shield and gently dips towards the northeast. The Upper Jurassic rocks of the platform are mainly shallow marine carbonates comprising, from base to top, the Hanifa, Jubaila, Arab, Hith, and Sulaiy Formations [26,27]. The Namar dam was established across Wadi Namar, which has carved its course through the Upper Jurassic (Kimmeridgian) Jubaila Formation. At the dam site, the lithology of the Jubaila Formation abruptly transitions from limestone in the lower part to dolomite in the upper part. Field investigation revealed that the Jubaila Formation is dissected by N-S, E-W, NE-SW, and NW-SE trending nearly vertical extensional fractures. The banks of Wadi Namar are steep, reflecting the structural control over its course. Fractures in carbonate rocks are effective elements that facilitate dissolution processes and fluid circulation during the development of karst features [28–31]. The soluble limestone can undergo dissolution due to the reaction of water (H₂O) and carbon dioxide (CO₂), leading to the formation of carbonic acid (H₂CO₃), which transforms solid carbonate (CO₃) into dissolved bicarbonate (HCO₃⁻). Al-Sayari and Zötl [32] and Jado and Johnson [33] proposed that karstified terrain in Saudi Arabia was developed during the intense pluvial intervals of the Pliocene or Pleistocene. The Jubaila Formation displays karst dissolution features. Occasionally, some solution cavities collapse, leading to undesirable environmental hazards.

3 Methodology and data acquisition

GPR is a non-destructive geophysical method that has been developed over the past 30 years for high-resolution subsurface investigation and characterization of soil and shallow subsurface layers [34]. The GPR method has been widely used to yield a more reliable image of subsurface structures in karst areas, where it has proven useful for mapping the bedrock, the geometry of fractures, and other dissolution features [9,35–40]. The GPR method detects contrasts in electrical properties in the subsurface, where electromagnetic (EM) waves are reflected and refracted upon encountering rock fractures; hence, the direction and amplitudes of EM waves can be used to delineate and characterize rock fractures and their fillings [41–43]. The penetration depth of GPR waves depends mainly on the frequency of the GPR antenna used, the electrical conductivity of the subsurface layers, and the moisture content of the investigated soil layers. The presence of high-conductivity materials, such as clayey or wet soil, attenuates GPR waves and hence reduces the penetration depth. Moreover, high-frequency waves produce higher-resolution images at shallow depths, whereas low-frequency waves produce lower-resolution images at greater depths. Thus, the proper selection of the antenna depends on the nature of the target being studied and the desired investigation depth.

In this study, a total of 38 GPR profiles were conducted on the downstream side of the dam (Figure 4) using an SIR-3000 GPR system with a 400 MHz antenna. The spacing between the collected GPR profiles is nearly 10 m. The data were acquired using the cart-pull system survey, with a wheel system that enables distance measuring mode for the GPR system. The GPR data were collected at a sampling rate of 50 scans per meter. The data were collected with about 2 cm trace spacing with 512 samples per trace. The data were filtered using frequency and spatial filters to attenuate noise and eliminate EM interference. In this study, a shielded antenna was utilized for GPR data acquisition, with both the transmitter and receiver enclosed within the same unit and placed at the air–ground interface. This arrangement is intended to enhance resolution and minimize the effect of external noise on the EM signals related to the subsurface targets.

Figure 4

A satellite image showing the Wadi Namar dam and its upstream lake (top) and the location of the GPR survey lines (bottom).

4 GPR data processing

The general objective of GPR signal processing is to produce a readily interpreted image. GPR data often contain noise or interference, which makes data interpretation difficult. Therefore, the raw data are processed using different steps to clean up the noise, making it easier to visualize and interpret. Data processing was conducted using REFLEXW 7.0 software [44], and a series of frequency filters were applied to the GPR sections. The typical sequence of processing steps applied to the collected field data is as follows:

Apply subtract-mean (de-wow): This process involves temporal filtering to remove very low-frequency components (de-wowing) from the data. The filter calculates a running mean value for each trace and subtracts it from the central value. This filter acts on each trace independently, with a running mean value calculated for every data point in each trace. As a filtering parameter, de-wowing is applied using a low-cut filter with a cut-off frequency set below the bandwidth of the recorded data.
Move start time (time cut), this filter acts on each trace independently to limit its duration to a pre-definable maximum time. Only the time range is reduced to a preset interval. For our data, we applied a maximum time of 9–10 ns to obtain better resolution for the surface layer.
Adjusting the gain filter: Through this process, energy decay gain is applied, where a gain curve in the y (time) direction is used for each profile, based on the mean amplitude decay curve. First, a mean decay curve is calculated from all existing traces. After applying a median filter to this curve, every data point of each trace is divided by the corresponding values of the decay curve.
Background removal: In this stage, antenna ringing and horizontal banding across the image are eliminated by applying the background removal filter to the average of all accumulated scans. This processing step enables effective radar image enhancement. In addition to removing banding and ringing effects, background removal can eliminate other horizontal features, such as flat geological layers and planar reflections. Special care was taken when applying this filter to ensure optimal results.
Band pass filter: In the frequency domain, this filter has an independent effect on every trace. A cosine taper was selected from among the three available bandpass filtering types. Four frequency values are specified to define the filter band for the cosine taper. The low-cut frequency is determined by the first point, and the beginning of the lower plateau by the second. A cosine window defines the filter between the low-cut frequency and the start of the plateau. The end of the plateau is determined by the third point, while the high-cut frequency is determined by the fourth. A cosine window defines the filter between these points. The frequency spectrum is reduced to zero above the high-cut frequency and below the low-cut frequency. Either a low-pass filter or a high-pass filter can be achieved approximately by selecting the appropriate bandpass positions. Unwanted reverberations from the filter operator are suppressed using the cosine taper. The spectral amplitudes present at the high- and low-cut frequencies should be taken into account while adjusting the cosine range. The cosine range can be reduced if the spectral amplitudes are very small. When the frequency content of the noise differs from that of the signal, it can be suppressed using the bandpass filter.
Subtracting average: The filter subtracts the average of a predetermined number of traces at each time step. Horizontally coherent energy is suppressed by the filter through a process known as sliding background removal. Its effect is to enhance signals that vary laterally, such as diffractions.
Finally, the average x–y filter was applied: The average in this filter is calculated over a number of samples (y) as well as a number of traces (x). A smooth average of arrival times from the GPR signals is achieved by centering the filter area on the current data point. The start time of each trace is determined by its beginning time, and the end time by its maximum duration. For the average x–y filter, we set the number of traces to be averaged to eight and the number of samples to four.

During GPR data analysis, the GPR propagation velocity within limestone rocks is found to range between 0.099 and 0.159 m/ns [45] for karstified limestone. Therefore, to interpret the present GPR data, a velocity value of 0.13 m/ns is used, which is reasonable for delineating the subsurface structure. Previous studies have confirmed that the velocity value for limestone ranges from 0.09 to 0.15 m/ns [45–47].

5 Results and discussion

A variety of geophysical techniques are employed to understand subsurface hydrogeology [48–52]. The GPR method is applicable to shallow hydrogeological investigations; therefore, this method was used in the present study to investigate the geological parameters that could affect groundwater flow beneath the dam site. GPR sections (Figure 5) indicate GPR reflection signals related to vertical fractures that affect the Jubaila carbonate bedrock. The higher intensity of GPR reflections along these fractures may indicate wet fractures contaminated with fine clays (Arifin et al. [53]). GPR reflections caused by fracture planes are enhanced when the fractures are fluid-filled [42,54,55] due to the strong contrast in electrical properties between the fracture filling and the surrounding rock matrix [56,57]. The clearest examples are visible along GPR sections 2, 3, and 6 (Figure 5); however, there are other examples throughout the study area, such as GPR sections 11, 16, and 21 (Figure 6), which reveal wide vertical anomaly zones interpreted as vertical fracture zones extending to different depths, exceeding 5 m. These vertical fracture zones are characterized by high-amplitude reflections and a vertical pattern (Figure 6), implying zones of fractured carbonate rocks with high moisture content. This observation suggests that fractures and cracks may be interconnected, making the subsurface carbonate rock matrix more permeable and facilitating the vertical infiltration of groundwater in the study area. Similar fracture zones are observed in nearby surface exposures (Figure 7).

$Figure 5 Representative examples of GPR sections across lines 2, 3, and 6, acquired with the use of the 400 MHz shielded antenna (black arrows indicate fracture structures).$

Figure 5

Representative examples of GPR sections across lines 2, 3, and 6, acquired with the use of the 400 MHz shielded antenna (black arrows indicate fracture structures).

$Figure 6 GPR sections along lines 11, 16, and 21 indicating fractured wet zones.$

Figure 6

GPR sections along lines 11, 16, and 21 indicating fractured wet zones.

$Figure ‎7 Field photograph showing the vertical (denoted by black arrows) and sheet-like fractures (denoted by white arrows) that dissected the Jubaila carbonate rocks around the Namar dam.$

Figure ‎7

Field photograph showing the vertical (denoted by black arrows) and sheet-like fractures (denoted by white arrows) that dissected the Jubaila carbonate rocks around the Namar dam.

The GPR sections (Figure 8) indicate the presence of parallel sheet-like fracture sets that form horizontal lineaments. In Figure 8, horizontal reflectors at depths of 0.5 and 2.5 m are interpreted as sheet-like fractures similar to those observed in nearby surface exposures (Figure 9). The presence of sheet-like fractures as horizontal lineaments enables the lateral migration of water from the saturated lineaments. These sheet-like fractures could act as potential groundwater reservoirs that may extend laterally over considerable distances; therefore, groundwater can flow horizontally in many directions, depending on the geometry of these sheet fractures. Understanding this fracturing system is critical for an accurate assessment of groundwater flow in the study area. Furthermore, the identification of such laterally continuous karstic fractures may act as groundwater flow conduits and could impact the problem of groundwater rising at the dam site. It is evident from GPR sections 1, 2, 4, and 11 (Figure 8) that sheet-like fractures have affected the Jubaila carbonate formation, as confirmed by surface geological exposures (Figures 7 and 9). It is expected that the interpreted vertical and sheet-like fracture networks are the main groundwater conduits within the Jubaila carbonate rocks at the dam site. Additionally, when examining the general outline of the studied GPR sections (Figure 8), high-amplitude horizontal reflections appear disconnected in some places at different distances and depths, possibly indicating the dissection of the sheet fractures by vertical fractures or the presence of weathered zones in the Jubaila carbonate rock.

$Figure 8 GPR sections along lines 1, 2, 4, and 11; black arrows indicate sheet-like fractures.$

Figure 8

GPR sections along lines 1, 2, 4, and 11; black arrows indicate sheet-like fractures.

$Figure ‎9 Field photograph showing the sheet-like fractures that dissected the Jubaila carbonate formation.$

Figure ‎9

Field photograph showing the sheet-like fractures that dissected the Jubaila carbonate formation.

In some areas, GPR sections show high-resolution and clear GPR signals indicating the existence of dissolution karstic structures, which extend vertically and horizontally (Figure 10). The interpreted dissolution cavities on GPR sections are marked by disruptions in the strong GPR reflectors, possibly indicating the dissection of the interpreted karst cavities by vertical fracturing, similar to those observed in nearby surface exposures (Figure 11). In certain GPR sections (Figure 10), GPR reflections reveal the lateral and vertical distribution of karst dissolution features, which are connected to vertical and horizontal sheet fracturing zones. These GPR sections (Figure 10) display repeated hyperbolic reflections, suggesting possible karstic cavities with vertical fractures serving as conduits to facilitate groundwater ascension. Figure 10 shows examples of GPR signals with a symmetric hyperbolic shape and relatively strong amplitude, indicating carbonate dissolution cavities. For example, GPR profiles 23 and 26 (Figure 10) show a relatively strong hyperbolic GPR reflector around distances of 20 m and 100 m, at depths extending from 1 to 3 m.

Figure 10

GPR sections along lines 1, 23, and 26 indicating reflections from karst cavity structures.

Figure 11

Field photograph showing the dominant cavity karst features in the Jubaila carbonate rocks (white arrow denotes cavity karst features).

6 Conclusion

The findings obtained through the analysis of the GPR sections underscore the effectiveness of this method in delineating subsurface geological structures that could contribute to water seepage issues in dams. The examination of GPR sections downstream of the Namar dam revealed a substrate of fractured and karstified carbonate rocks characterized by cavities, voids, and discontinuous fractures. These observations suggest that the carbonate bedrock underlying the dam lake floor is susceptible to dissolution by chemically aggressive water, leading to increased secondary porosity along fractures and joints, potentially forming conduits for groundwater flow.

Various abnormal features, including cavities, voids, and fractures, were identified along the GPR sections within the Jubaila limestone bedrock. These karst features appear to be significant factors contributing to leakage at the Namar dam, with water seepage from the dam lake posing a notable challenge at the site. Furthermore, the successful application of GPR imaging highlights its efficacy in mapping subsurface structures and delineating potential pathways for groundwater flow, making it an invaluable tool in hydrogeological investigations.

Further research and monitoring efforts are essential to enhance our understanding of subsurface hydrogeological dynamics in karstic terrains. The ongoing utilization of advanced geophysical techniques, in conjunction with geological studies, will play a pivotal role in devising comprehensive strategies for managing water resources and safeguarding critical infrastructure.

Acknowledgments

This research was supported by Researchers Supporting Project number (RSP2025R496), King Saud University, Riyadh, Saudi Arabia.

Funding information: This research was supported by Researchers Supporting Project number (RSP2025R496), King Saud University, Riyadh, Saudi Arabia.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and consented to its submission to the journal. SSA and EH wrote the draft of the manuscript. SSA, EH, and KA reviewed, edited the manuscript and approved the final version of the manuscript. SSA paid APC and collected the field data.
Conflict of interest: Authors state no conflict of interest.

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Received: 2024-06-28

Revised: 2025-01-25

Accepted: 2025-02-26

Published Online: 2025-04-16

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

https://doi.org/10.1515/geo-2025-0781

Schlagwörter für diesen Artikel

GPR; karst features; water seepage; Namar dam; south Riyadh

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