Application of Electrical Prospecting Methods for Monitoring the Condition of Earth Dam in the Almaty Region of Kazakhstan

Abstract

This article deals with the issue of diagnostics of the physical condition of earthen dams, taking into account seasonal changes in the water level of hydraulic structures using electrical exploration methods. The simplicity of the method, the accuracy of measurements of geophysical parameters, and the availability of software packages for the processing, interpretation, and visualization were the basis for the choice of method. The method of electrical resistivity and self-potential was chosen. The methodology, technique, technology of field surveys, processing, and geological interpretation of the study results are given. A comparative analysis of the obtained geophysical parameters of seasonal measurements is given. The research results are given in the form of sections of the resistivity model and self-potential isolines.

1. Introduction

A malfunctioning of hydraulic structures often leads to accidents, posing risks to human life and causing significant economic and environmental damage. It has been established that nearly 80% of dam failures are associated with poor technical conditions [1,2,3,4,5,6]. Therefore, developing methods for monitoring the stability and predicting the physical condition of hydraulic structures is highly relevant. The application of standard geological engineering methods (pitting and drilling) for this purpose does not fully address these issues due to the high cost of work and, most importantly, the ambiguity of the geological model obtained. Recently, increasing attention has been paid to creating new technologies for the operational non-destructive monitoring of earth dam conditions based on geophysical methods.

The condition of dam foundations is strongly influenced not only by the geology and climate but also by virtually any human activity (anthropogenic influence). Assessing geological conditions using geophysical methods and forecasting their possible development is an important component of the efficiency of structure operation. In other words, the state and dynamics of changes in the physical and geometric characteristics of the dam rock mass must be known during the monitoring. The operational monitoring of the condition of earth dams is most effective, based on geophysical monitoring. However, the methodology for its application requires certain development and improvement in the theoretical and experimental base, as well as an interpretation of monitoring geophysical research. The development and improvement in the methodology of geophysical monitoring significantly reduce the costs of monitoring hydraulic structure conditions and promptly solve the problems to ensure the effective operation of engineering structures.

Analysis of the application of geophysical methods for monitoring the condition of dams has shown that the most frequently used methods are the electrical exploration methods of resistivity and self-potential (SP), seismic surveys in various modifications, and ground-penetrating radar (GPR) surveys [7].

Table 1. Physical prerequisites for the application of geophysical methods for the determination of weakened zones.

Effects and Processes	Manifestation in Physical Characteristics	Study Methods
Weakened water-saturated zones, cracks	Elastic waves and density speed reduction, porosity increase	Seismic exploration
Wet areas in the dam body	Local increase in electrical conductivity and polarizability	Electrical Resistivity Exploration
Filtration processes in the dam body	Decrease or increase in filtration field	Self-potential exploration
Wet areas in the dam body	Increasing of dielectric permittivity	Gound-penetrating radar sounding

presents the physical prerequisites for the application of geophysical methods in the examination of earthen dams.

The research discussed in this publication aims to identify zones of increased water saturation in the body of earth dams using electrical survey methods. To this purpose, a complex of self-potential and dipole-sounding geophysical methods is used. These methods use the following measurable parameters: apparent resistivity ρ_a and self-potential ΔU_SP. They react quite well to the soil moisture in the dam and the fluid filtration in the structure. The main principle of forming the complex is to increase the unambiguity of solving the inverse geophysical problem and the physical and geological interpretation of obtained measurements.

The physical basis for conducting electrical exploration methods is changes in the petrophysical properties of soils related to their water saturation [8] (

Table 2. Electrical resistivity of some sedimentary soils.

Rock Name	Minimum, Ohm*m	Typical, Ohm*m	Maximum, Ohm*m
Clay	5	10	15
Loam	15	30	50
Sandy loam	30	50	80
Saturated sands	50	80	200
Slightly wet sands	100	150	500
Dry sands	200	500	10,000
Coarse sand	30	50	500
Permafrost soils of various ice content	500	-	80,000

).

The choice of the complex of electrical exploration methods for assessing the condition of dams is based on the experience of similar thematic studies previously conducted. In particular, the authors of [9] noted that the electrical exploration allows us to determine the structure of the section based on the differences in soil resistivity, depending on the soil moisture and groundwater mineralization. The researchers highly appreciate the role of electrical exploration in analyzing the physical properties of earth dams: “the apparent resistivity ρ_a depends on the temperature of soils, their degree of water saturation and mineralization” [10].

The self-potential method is based on the study of natural constant electric fields. Constant fields are defined as those with a period of up to 1 Hz. The term “self“ means that the potential is not generated by an externally controlled source. Constant fields arise during the geological section’s redox, filtration, and diffusion–adsorption processes. Recording of these fields is the purpose of the SP method, and geological interpretation of parameters of sources of these fields is the goal of interpreting the SP data. One of the features of the SP method is its sensitivity directly to the groundwater flow. Fields of filtration origin arise during charge separation when charge carriers are displaced by water flow during filtration through a porous medium. For a filtration field to occur, the following are necessary: (1) contact of substances in solid and liquid phases, (2) fluid flow (pressure gradient) in the medium, and (3) porous structure of the solid phase. The groundwater flow, seeping through the porous media, creates an electrical field on the surface of the earth [9,10]. Also, as shown in [10], similar anomalous potential effects occur due to the formation of diffusion–adsorption processes associated with the differences in soil porosity and pore fluid mineralization. According to [11], “natural galvanic elements” are formed in them in the form of unevenly oxidizing conductive bodies. This manifests as the local negative anomalies in the SP field, which may also be similar to the filtration ones in configuration. The main advantage of the SP method is the ability to determine not only the preferred water flow path but also its intensity [12]. The effectiveness of the method has been confirmed by the results of studies by various researchers [13,14,15,16,17].

Electrical Resistivity Tomography (ERT) is a popular, recent geophysical method used to solve various engineering–geological, hydrogeological, ecological, archeological, and other problems. Essentially, ERT is not a separate direction of electrical exploration but represents a combination of electrical sounding and profiling. The ERT measurements can be called continuous-sounding with some reservations. However, unlike the long-standing vertical electrical soundings, fairly dense observation systems with a linear step are used in the ERT. The essence of the measurement technique lies in the repeated measurements of signal in the receiving lines at the different positions of the power line. In this way, a kind of “illumination” of the geological section from the different positions of the source and the projection of signal changed by the geological objects onto the receiving lines is implemented. Due to the use of this principle and modern inversion algorithms, ERT allows the study of complex two-dimensional and three-dimensional environments, significantly expanding the range of tasks solvable by electrical exploration. Its applicability continues to expand due to the simplicity of its observation techniques, its low cost, and the environmental friendliness of its fieldwork, as well as continuous improvement in its techniques and methods of interpretation. It is used in mineral exploration [18]; the survey of landslide-prone slopes [19,20,21,22], buried wastes, and roads [22]; the identification of aquifers in arid areas [23]; the assessment of soil salinization [24]; the diagnosis of dam conditions [25,26,27]; etc. The method operates with large volumes of data, ranging from the first thousands of measurements for the two-dimensional inversion to the tens and hundreds of thousands for the three-dimensional inversion. This implies using high-performance, multi-electrode or multi-channel-switching equipment and exploration arrays. The depth of study is determined by the geoelectrical section and the greatest spreads. The resolution of ERT is determined by the distance between electrodes in the array and, like for other electrical exploration methods, decreases with depth. The ERT is successfully applied in the routine studies of dams, and the results of research by foreign scientists unequivocally prove the effectiveness of the method in analyzing the physical characteristics of soils composing the hydraulic structures [28,29,30,31,32,33].

The application of the complex of geophysical methods allows for obtaining data for the comprehensive analysis of the behavior of individual geophysical parameters depending on the specific physico-mechanical properties and processes occurring in the dam over time [10]. The complexes of geophysical methods are used at the various stages of the construction and operation of hydraulic structures [34]. The combined use of the SP and ERT methods enables the reconstruction of seepage paths in the earth dam [35,36].

The electrical exploration study was conducted over two sessions: October 2022 and April 2023. The complex of electrical exploration methods consisted of the SP method, with the gradient potential device, and the ERT method, with the dipole–dipole modification.

2. Object of Study

The experimental and methodological studies were carried out on the earth dam of the K-28 reservoir, located in the Almaty region in the south of the Republic of Kazakhstan (Figure 1).

The dam is located in the northern part of the Zailiyskiy Alatau mountain range. The low-lying soils are characterized by alluvial deposits (loams, sands). Unfortunately, several administrative reforms over the last 30 years have resulted in a lack of information on the construction project, including sections and plans.

The dam was built in 1971. The total length of the dam is 340 m, with the maximum height being 20 m. The dam body consists of bulk loamy soils. The upper slope is reinforced with rock fill. Over 50 years of operation, the soils of the dam body have given a slight settlement, and at the moment, they can be characterized as soils with an undamaged structure. The dam has a shaft spillway made of reinforced concrete and a pumping station with a lift height of 18 m.

The electrical exploration work was preceded by topographic surveying. The profiles and points were tied using the Garmin GPS navigator with coordinates X and Y recorded in the UTM WGS84 system. Ten profiles were marked with a distance of 5 m between them, and points were placed at intervals of 5 m (Figure 2).

3. Methodology

Self-potential method: The field investigations were conducted using the Mary-24 m, which has increased noise immunity and enhanced measurement accuracy [37]. Examples of the use of this device in solving various problems are described in [38,39]. Non-polarizing electrodes were used for grounding of receiving electric survey lines during geophysical works. Non-polarizing electrodes are characterized by a small value and constancy of the electrode potential. This is achieved by the fact that the contact of the copper rod with the ground is carried out through an ionic medium, forming an electrochemical equilibrium system of the first kind with the electrode metal. Measurements of SP were performed using the gradient potential setup. The distance between profiles was 5 m, the distance between electrodes MN was 10 m, and the survey step was 5 m. Figure 3 shows the measurement scheme used for the SP method.

The total number of observation points was 380. The control measurements with the same instruments were performed by another operator to check the correct operation of measuring equipment and the correctness of the measured values. The proportion of control observations was 7%. The Surfer 23.1.162. software package (Golden Software) was used for the data processing and map drawing. The contour maps of gradient potential changes were constructed for the period from autumn 2022 to spring 2023 based on the results of electrical exploration work.

Electrical Resistivity Tomography: The main task in analyzing data is to represent the objects being mapped. This task is achieved through the inversion transformations applied to the digital resistivity models. In essence, the inversion represents a modern implementation of solving the inverse task in geophysics. The inverse problem is solved in the inversion algorithms through the iterative fitting of the physical model based on the observed distribution of potentials at the receiver dipoles, considering the locations of current sources. Initially, the lower half-space is divided into numerous elemental cells approximating the physical parameters of the lower half-space. The two-dimensional resistivity inversions were used in the data processing. The two-dimensional inversion algorithm converts the observed electrical field into a corresponding two-dimensional resistivity distribution in the cross-section. Since this task is ill-posed, the regularization of the solution is applied using the models with smooth changes in resistivity. This approach allows for the formal interpretation without regard to a priori data. Due to the integral nature of the resistivity method, the obtained solutions typically simplify and smooth the real details of the geological structure cross-section, often overestimating the layer thicknesses. Additionally, false anomalies may appear on the cross-section due to the nearby objects and inversion instability. The results of surface geophysical surveys determine depths by estimation, due to the equivalent relationships between the resistivity and rock thicknesses. All the aforementioned complexities of geophysical interpretation are common to most geophysical methods.

Measurements of signal amplitude were carried out using the standard dipole–dipole method on 3 profiles, #10, #25, and #40, with a survey step of 5 m (Figure 4). A constant electric current was applied to the feeding electrodes AB with a distance between them of 10 m using the generator Astra-100 [40]. The distance between the feeding electrode B and the first receiving electrode M was 5 m. The ∆U was recorded by the MERI-24 meter alternately at 6 receiving dipoles at 5 m each. The MERI-24 was moved by 5 m at each measurement. After data were received on 6 dipoles, the generator was switched off and moved 10 m forward along the profile, and the algorithm of measurements was repeated. This achieved the required depth of sounding at all points.

The stainless steel electrodes, 50 cm long, were used as the grounds on lines AB and MN. The current magnitude in feeding line AB ranged from 50 mA to 100 mA. The total number of observation points was 114, with the control observations accounting for 5.5%.

The pseudo sections were used to display the field measurements, representing the two-dimensional distributions of apparent resistivity in the form of contour maps. ZondRes2D 7.0. software was used to calculate the electrical resistivity [41]. This is a certified system that allows the automatic qualitative and quantitative interpretation of field measurements. As a result, the numerical values of electrical resistivity are obtained for drawing geoelectric sections.

ZondRes2D implements a direct modeling procedure that supports all common array types used in resistivity imaging. The array configuration and the number of measurement points are user-defined. The program can be used to determine the optimal imaging scheme at the research-planning stage. For this purpose, the software allows the resolution of a particular array to be analyzed. The sensitivity study allows for making an optimal choice of the type and parameters of survey array for a specific geological problem.

For solving the forward and inverse problem, the mathematical apparatus of the finite element method is used, which gives better results than finite differences methods. When modeling the field of a pointwise source, the medium is partitioned by a set of triangular cells with different resistivities. The distribution of those resistivities is evaluated iteratively by solving the minimization problem. The cost function of that problem is defined as the discrepancy between the field measurements and the solution of the direct problem for each iteration step with an additional regularization term.

As a result of resistivity calculation, numerical resistivity values were obtained to construct geoelectric sections. Examples of calculations of resistivity using the ZondRes2D program for solving various engineering and geological problems are given in [42,43].

4. Results and Discussions

Self-potential method: During the engineering–geological investigations, attention was drawn to the natural local electric fields on the earth’s surface, caused by the contacts of heterogeneous sedimentary rocks, aquifers, groundwater filtration, and diffusion of solutions with varying mineralization.

The characteristic feature of the earthen dam’s self-potential is that its parameters are mainly determined by water filtration in its permeable soils, and no flow occurs if the ΔU_SP equals zero. For uniform infiltration throughout the dam, the SP increases toward the drainage cut, while the frontal and flank leakage inhomogeneities in the downstream wedge of the dam are characterized by local SP anomalies. Figure 5 shows the results of the processing of SP values received in autumn 2022 and spring 2023 in the form of the map of potential gradient isolines.

The field of the measured potential gradient values varies from 2 to 62 mV/m. Visually, a trend in ΔU_SP values from high values (from 22 to 62 mV/m) on profiles #0 and #5 (profiles close to the upper cut of the dam) to low ΔU_SP values on profiles #35–45 at the base (lower part) of the dam can be observed.

Some regularities in the anomalous field of ΔU_SP for both seasonal sessions are observed. For example, there is a zone of increased-gradient-potential ΔU_SP values on profiles #40 and #45 at the points 75–80 on both resulting maps. The increased ΔU_SP values are also recorded at points 170–180 of profile #25 and points 10–15 of profiles #10–20.

Assuming that the material composition of the dam body remains unchanged during the measurement period, the revealed dynamics of anomalous effects in the ΔU_SP parameter are interpreted as having a filtration origin when the water level in the reservoir changes.

According to the results of the EP method, in a first approximation, the state of the embankment can be as follows. The differentiation of soils in its body exists due to differences in composition and structure, water saturation, and technological factors. The clear channels and filtration paths that would pass through the entire dam were not identified and traced, as the corresponding geophysical anomalies were not recorded on the parallel profiles. Some deviations of the EP were identified in the loamy core, possibly related to the heterogeneities in its composition.

Electrotomography method: As a result of processing the field data, sections of the distribution of specific electrical resistance were constructed while considering the topography. Figure 6, Figure 7 and Figure 8 show sections obtained in the spring of 2023 along profiles #10, 25, and 40.

The analysis of resistivity distribution along the profile shows homogeneity of the soil composition below the profile level at a depth of 5–10 m, having low values of resistivity (less than 10–25 Ohm*m). A noticeable increase in resistivity up to 50–80 Ohm*m was found at the base of the embankment at a depth of 12–15 m.

Profile #25 is located in the middle part of the downstream slope of the dam. Two levels of resistivity are clearly distinguished in the resistivity section: The upper part (745–755 m) is characterized by low resistivity values, in the order of 10–35 Ohm*m, which is composed of bulk clay soils. The lower part (735–745 m) is an area of medium-to-high resistivity values (70 to 160 Ohm*m) and is characterized by the basement rocks of the dam, represented by boulder–pebble deposits.

Profile #40 is located in the downstream part of the dam, 12–13 m below the top of the embankment. The resistivity section mainly characterizes the dam foundation rocks, reflected in the resistivity values, which range from 80 to 170 Ohm*m. Only the upper part of the section, at depths of 1–2 m, has low values (40–50 Ohm*m). In the center of the section, at points 90–120, the core is clearly distinguished, where the resistivity values are maximum (120–180 Ohm*m).

The resistivity data for profiles #10, #25, and #40 obtained in autumn 2022 and spring 2023 were analyzed (Figure 9 and Figure 10).

The following conclusions were made based on the results of the analysis:

Autumn 2022:

-

Along profile #10, located closer to the upper reservoir, there were weakly expressed zones of specific electrical resistivity, indicating that the water saturation of soils is distributed in a fragmented manner.

-

The differentiation of resistivity by depth was noticeable in profile #25.

-

On all three profiles, the influence of the operational spillway was noticeable in the form of low resistivity values at pickets 75–85.

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Less expressive changes in electrical resistivity, correlated over all profiles, were obtained in the central and eastern part of the dam in the interval of pickets 130–150 (zone 1).

-

Lower resistivity values were observed on profiles #10 and #25 in the range of pickets 160 and 180 (zone 2).

Spring 2023:

The results of processing and interpretation of the Spring 2023 ERT data largely confirm the data obtained in Autumn 2022.

-

On profile #10: there are no sharp deviations in resistance values;

-

On profile #25: fragmentation in the near-surface zone appeared;

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On profile #40: the resistivity distribution is almost unchanged;

-

In addition to the previously detected zones 1 and 2, zone 3 of low resistivity was additionally detected on profiles #10 and #20 in the range of pickets 20–50.

The consecutive ERT survey works made it possible to trace the state of detected anomalous zones of resistivity, their development, and variability depending on the season.

The used complex of geophysical methods allows for a comprehensive analysis of the behavior of individual electrical parameters depending on the various physico-mechanical properties and processes occurring in the dam over time. The main conclusions from the results of the survey are as follows:

-

The earth dam body, under the influence of natural and anthropogenic factors, is constantly undergoing changes that are causing fluctuations in the physical fields, including those associated with filtration processes, and the directionality of these processes will be established during subsequent regime observations.

-

The anomalies detected by the EP and ERT data show their activation in the instrumental parts of the dam.

-

The reliability of the conclusions on the geoelectric characteristics of the dam is confirmed by their comparability with the results of foreign scientists [35,36,44].

Thus, the relative stability of the electrical parameters of dam soils was established based on the results of instrumental measurements over two years. The comparative analysis of data from the geophysical measurements using ERT and SP methods showed their good comparability. Still, deviations were observed in certain sections, necessitating further study of their nature.

In general, the electrical survey data provide a general characterization of the seepage conditions in the K-28 dam. The zones presumably connected with filtration processes are revealed, which require specification of their nature. In this connection, it is expedient to use the electrical survey in modification of ERT for primary assessment of inhomogeneity of earth dam bodies as the most express method for specification of the range of tasks to be solved when assessing their condition.

For an unambiguous assessment of the degree of manifestation of filtration flows accompanied by suffusion processes that reduce the stability of dams, information on the presence and spatial location of zones with anomalously low values of their elastic-deformation parameters, which can only be obtained from seismic data, is required. This information will be obtained in the course of further investigations of the dam.

5. Conclusions

Experimental data on structural and material peculiarities of the internal structure of the dam body of the K-28 reservoir were obtained. The method of dipole electric sounding showed high efficiency in identifying areas with increased water permeability. The informativeness of resistivity parameters in identifying zones of increased water saturation in the dam body and their location in the plan was assessed.

The area electrical survey conducted using the method of a natural electric field showed high efficiency (in the variant of mode observations) in assessing soil heterogeneity.

Based on the work results, an electrical survey should be used for the initial assessment of the dam condition, which is the most common method. For a more detailed study of the structural structure of the site, longitudinal and transverse-wave seismic surveys should be used.

The applied complex of electrical survey methods determines, with sufficient accuracy, the condition of earth dams and can be used by organizations operating hydraulic structures to monitor their technical condition to ensure their safe operation.