Recent technology development has enabled the use of electric potential difference (EPD) in the telluric electric frequency selection method (TEFSM) geophysical approach for groundwater exploration. As a new approach, there is a need to build knowledge and experience in the application of the TEFSM geophysical approach in groundwater exploration in different hydrogeological settings. Hard rock granite aquifers are one of those aquifers where the TEFSM geophysical approach to exploring groundwater is yet to be investigated. In this study, a geophysical survey to identify four drilling sites for community boreholes was first conducted using the TEFSM approach. Vertical electrical profiles (VEP) of EPD up to 120 m per station were analyzed and interpreted to identify the depths of potential aquifers prior to drilling. The EPD VEP was then corroborated with borehole lithology data collected from the drilling to provide hydrogeophysical meaning to the data. The results show that groundwater occurs in the weathered granite layers. However, the water strikes appear to occur at the contact plane between the overlying weathered granite and the underlying fresh amphibolite. This suggests that the groundwater is stored in the weathered granite while the contact plane at fresh amphibolite is a preferential flow path. The granite aquifer at the study site is characterized by the EPD ranging from 0.018 to 0.068 mV. However, not all geological materials in this EPD range had water, some were just a reflection of weathering. The TEFSM geophysical approach was able to delineate layers of weathered granite aquifers and impermeable amphibolite based on low and high EPD contrasts, respectively. The findings assist in improving the practical understanding of the application of TEFSM to delineate aquifers and site boreholes in granite aquifers.
Hinweise
The original online version of this article was revised due to correction in article title.
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Introduction
Due to the heterogeneous and isotropic nature of the earth’s subsurface, groundwater exploration for locating production borehole sites remains a daunting task. The task is much more challenging in hard-rock aquifers such as granite where groundwater occurrence and flow are controlled by the presence of weathered and/or fractured zones. Although hard-rock aquifers are not renowned for high yield (Courtois et al. 2010; MacDonald et al. 2012; Maurice et al. 2019), they are a major source of potable water in Africa where over half of the continent is occupied by over 40% of weathered crystalline basement aquifers (MacDonald et al. 2008). Groundwater quality is generally good in most of the hard rocks (Gurmessa et al. 2022); thus, they constitute a vital resource for meeting the current and future water needs of many African communities.
Globally, research to improve the identification and evaluation of groundwater potential continues to receive considerable attention with the goal of improving drilling success rates (Díaz-Alcaide and Martínez-Santos 2012; Achu et al. 2020, 2022). Electrical resistivity is one common method that has been successfully used to explore groundwater in hard-rock aquifers such as granite (Muchingami et al. 2012; Naiyeju et al. 2021; Nagaiah et al. 2022; Oyeyemi et al. 2023; Lubang et al. 2023). However, due to the high equipment costs, many groundwater practitioners and government departments in developing continents such as Africa cannot afford the equipment. It is due to this challenge that the telluric electric frequency selection method (TEFSM) equipment has been widely received in many of the developing continents as a low-cost and rapid approach (Gomo 2023). Due to urgent needs for exploration, the equipment is already in wide use in groundwater development projects but with limited understanding of its strengths and limitations.
Anzeige
The TEFSM is a recently developed technology based on the approach of natural electric field geophysical prospecting approach (Hunan Puqi Geologic Exploration Equipment Institute 2017; Yulong et al. 2023). Based on the principles of the natural electric field frequency selection approach, a variety of equipment have been developed over the years (Yang 1982; Lin et al. 1983; Han and Wu 1985; Hunan Puqi Geologic Exploration Equipment Institute 2017; Han and Han 2020). In principle, the approach uses the audio band frequency spanning from 0.1 Hz to 20 kHz, which is typically under the magnetotelluric (MT) frequency range spectrum. The MT is a high-resolution geoelectrical geophysical technique that measures changes in naturally occurring electromagnetic (EM) waves over the earth’s surface to investigate the subsurface electrical properties (Cagniard 1953; Vozoff 1990; Chave and Jones 2012). The technique can investigate up to tens of kilometers in depth (Cagniard 1953; Vozoff 1972; Wang et al. 2022; Inoue et al. 2022). The subsurface electrical property of interest is normally apparent resistivity, which is evaluated based on Maxwell’s equations. However, the TEFSM groundwater prospecting technology investigates the subsurface geology using the electrical potential difference (EPD) parameter. A theoretical background of the principles of the TEFSM approach from what has been already provided by the developers of the technology (Hunan Puqi Geologic Exploration Equipment Institute 2017) is presented under the methods and materials section.
A few of the published case studies on the application of the TEFSM geophysical approach in groundwater exploration have highlighted the great potential of the approach and encouraged further research (Melchinova and Pavlova 2022; Gomo 2023; Yulong et al. 2023). Yulong et al. (2023) showed that TEFSM was effective in delineating shallow aquifers of up to 250 m depth. Gomo (2023) successfully delineated aquifers in sandstone and bedding plane contacts in the typical Karoo sediments using the TEFSM approach. Due to the limited knowledge about the TEFSM approach, Gomo (2023) referred to the approach as the electric potential difference-audio magnetotelluric (EPD-AMT). It is however the same approach.
Despite the potential, the lack of more published case studies to demonstrate the capabilities and limitations of the TEFSM geophysical approach is a limiting factor to its meaningful and wider application. This is particularly true in Africa where the technology has the potential to offer a low-cost alternative groundwater exploration method. As part of the efforts to improve the practical understanding of the application of the TEFSM geophysical approach in groundwater exploration, research on different hydrogeological settings is of paramount importance. This paper is therefore designed to fill this research gap by testing the technology at local scales using hard rock granite aquifers in Eswatini (formerly Swaziland) as a case study. Granite aquifers are of interest given the role of water supply in many African countries and due to the challenges associated with the location of groundwater in typical fractured rock aquifers.
Study area description
The study area is in Eswatini (formerly Swaziland) which is in the Southern African Development Community (SADC) (Fig. 1). The Kingdom of Eswatini is one of the smallest countries in Africa, covering an area of 17,360 km2. The country is bounded on the east by Mozambique and by South Africa on the west, south, and north. This study uses boreholes drilled in three communities: Moyeni, Mawombe, and Mkhuzweni communities located in the Northern Hhohho administrative region.
Fig. 1
Location of the study area in Eswatini; the inset shows the location of Eswatini in the SADC region
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The climatic conditions of Eswatini are classified as subtropical with four climatic and topographical zones also known as the agroecological zones: Highveld, Middleveld, Lowveld, and the Lubombo plateau. The study area falls under the Middleveld zone. This zone is generally characterized by slightly humid conditions with annual rainfall ranging between 550 and 850 mm/annum. The rainy season often occurs between October and April and the dry season between May and September, with average temperatures ranging between 22 and 29 °C. This region is generally known to be warmer and drier than the Highveld region.
The Kingdom of Eswatini is generally known to be a mountainous country, with the highest elevated areas located in the Highveld (average elevation: 1 300 m). The low-lying areas are generally found in the Lowveld, with an average elevation of 200 m. The Middleveld where the study sites are located is generally characterized by high-relief areas with altitudes ranging between 500 and 1050 m above the sea level. The elevated areas are associated with undulating granitic terrains incised by rivers and streams.
General geology
The geological map of Eswatini is presented in Fig. 2. The Karoo Supergroup dominates the eastern part of the country, where the Ecca and Lower Stormberg groups underlie the Lowveld, and the Lebombo Rhyolites (Robins 1978). From geophysical investigation, Burley et al. (1970) inferred that the thickness of the Karoo is of the order of 11,500 m.
The western part of the country is typically dominated by the Archean-age granites and gneiss. The granites in Eswatini are typically grouped into three granitic units: the Lochiel granites (G3), the Mswati granites (G5), and other granites (GR). The Lochiel granites occur in the Northwest Highveld. These granites are intruded by dolerite dykes and sills and are in contact gneiss. The Mswati granites are the youngest granites in the country mainly characterized by deeper weathering than that of the other granites. The other granites mainly include the Hlatikhulu granites, Kwetta, and Mtombe plutons. These granites are also characterized by dolerite sill intrusions (CIDA 1992). The study area falls under the Northern Hhohho region which is characterized by medium- to coarse-grained granite.
Hydrogeology
Despite the role of groundwater in Eswatini communities, there is not much documented literature on groundwater. Groundwater in Eswatini occurs in both crystalline Precambrian rocks and the Karoo Supergroup (Robin 1978). The study area is however dominated by crystalline granite secondary aquifers. In such environments, groundwater occurs in the shallow permeable weathered zone and fractured/weathered granite at depths. In hard crystalline rocks, groundwater occurrs and flow are mainly controlled by secondary, porosity and permeability (Maurice et al. 2019; Lachassagne et al. 2021). Isolated dolerite intrusion also contributes to groundwater flow particularly along dykes which tend to form preferential flow paths. Fractured and/or weathered dolerites are a common feature in the typical hydrogeology of the Eswatini Highveld region (Swaziland Ministry of Natural Resources, Land Use and Energy and Canadian International Development Agency (1992). Field observations during the survey also show the presence of dolerite structures intruding into the granites (Fig. 3).
Fig. 3
Example of dolerite intruding the granite observed during the field survey
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Springs occur mostly in the Highveld and Middleveld. The springs occur in areas of surface elevation discontinuity, which tends to create groundwater discharge zones. Due to smaller catchments caused by topography discontinuities, the springs are mostly perennial. The spring water is for local-scale domestic use and sometimes to water household gardens. According to Manyatsi and Brown (2009), groundwater yield in the Middleveld region ranges between 0 and 20 L/s with a recharge of 5% of the annual precipitation. The groundwater quality in this region is regarded as good quality due to the high groundwater recharge rate.
Methods and materials
Basic principles of the telluric electric frequency section method (TEFSM)
This theoretical background gives only a summary of the principles of the approach from what has been already provided by the developers of the technology. Detailed information about the principles and the hardware components from data collection, amplification, filtering, transfer functions, and modeling is already available (Hunan Puqi Geologic Exploration Equipment Institute 2017). There is, therefore, no reason to rewrite what is already available. The likely limitation is that the report documenting the principles and development of the approach has been difficult to access, leading to many questions. Recently, more studies have been published about the theoretical application of the TEFSM approach from the Republic of China where the technology originated (Song et al. 2021; Lu et al. 2023; Yulong et al. 2023).
The development of the TEFSM is based on the approach of natural electric field geophysical prospecting (Hunan Puqi Geologic Exploration Equipment Institute 2017). The approach uses the audio frequency band spanning from 0.1 Hz to 20 kHz, which is typically under the MT frequency range spectrum. Detailed principles of the MT geophysical method can be found in wider literature (Cagniard 1953; Vozoff 1972, 1990; Rodriguez and Sampson 2010; Chave and Jones 2012).
The natural electric field groundwater surface geophysical prospecting approach is a passive-source electromagnetic approach proposed in China in the late 1970s (Hunan Puqi Geologic Exploration Equipment Institute 2017). The approach is also known as the “audio earth electric field method or frequency selection of earth electric method” (Hunan Puqi Geologic Exploration Equipment Institute 2017). The method only utilizes the natural electric field which is the electrical component of the electromagnetic field. However, the approach is still classified as MT because the source of the natural electric field is the natural EM waves of the audio frequency band.
The earth is surrounded by several natural sources of time-varying electromagnetic fields such as lightning activities and geomagnetic storms (Takeuti et al. 1976; Heidler and Hopf 1998; Nor et al. 2020; Mohammad et al. 2022). The time-varying EM fields whose plane wave distribution is approximated to be perpendicular to the ground are subject to the Maxwell’s equations. Secondary EM fields are induced within the earth by the time-varying fields in such a way that the generation of secondary fields is dependent on the electrical conductivity or resistivity of the earth. However, in the natural electric field approach, the generation of secondary fields is rather controlled by the shielding effectiveness of the earth. The shielding effectiveness is defined as the ability of subsurface geological materials to absorb the induced EM waves (Hunan Puqi Geologic Exploration Equipment Institute 2017). In the uniform earth, the impedance is related to the frequency (f) and resistivity (ρ) of the earth and the electromagnetic field to evaluate the earth’s apparent resistivity using Eq. 1 as follows:
where E is the electric field component, H is the magnetic field component, and f is the frequency of the EM waves.
In a smaller area, the magnetic field can be considered stable and therefore assumed to be constant, such that the qualitative relationship between the electric field component and resistivity is used to assess the electrical properties of the geological bodies.
The penetration depth (δ) of the propagating EM wave according to the attenuation properties of the plane is evaluated using the frequency and resistivity as shown in Eq. 2:
$$\delta \approx 503\sqrt{\frac{\rho }{f,}}$$
(2)
From Equation 2, when the resistivity of the underground materials is assumed to have a negligible effect and therefore constant, the smaller frequency results in a large penetration depth. Therefore, varying the operating frequency can be used to achieve the purpose of changing the penetration depth (Hunan Puqi Geologic Exploration Equipment Institute 2017).
According to the Hunan Puqi Geologic Exploration Equipment Institute (2017), research has shown that:
“Observation on the surface of the earth for the strength of electric field brings information at certain depths of the earth’s electric distribution.”
When the magnetic field is assumed to be constant over a small area, multiple-frequency electric field values are only observed on the earth’s surface.
The different frequencies of the electric field strength reflect the earth’s electrical structure at different depths.
If observations are only made for the electric field strength, it is not possible to calculate the resistivity of the earth’s structure; thus, only qualitative analysis of the earth’s subsurface structure can be made by observing the shielding effect on the electrical field signals.
Signals of the electric field of different frequencies and therefore different depths are measured as the electrical potential difference between two grounded copper electrodes spaced at a recommended separation of 10 m (Fig. 4). With this approach, it is possible to have a 1-m station spacing. The electric potential difference output readings reflect the shielding effectiveness of earth’s subsurface materials to electric field waves at certain depths and are used as the investigation parameter. The depth of investigation is determined from different selected frequencies of the electric field. It should therefore be clear that this approach does not evaluate the subsurface resistivity. There is therefore no need for inversion which is commonly done in the resistivity approach. The EPD output readings are therefore directly used as the investigation parameter.
Fig. 4
Field layout of the groundwater detector TEFSM equipment used for the survey
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Due to the composition of polar molecules, water has greater capacity to absorb the propagating induced EM waves and hence high shielding effectiveness (Hunan Puqi Geologic Exploration Equipment Institute 2017). Because the generation of induced EM waves by time-varying natural EM from lightning activities and geomagnetic storms is controlled by the shielding effectiveness, high shielding effectiveness results in low EPD difference being measured at the surface. When looking at the vertical profile, of EPD, aquifers are likely indicated by low EPD anomalies. The same can be said for weathered and/or fractured zones of the hard-rock aquifers due to their potential to store and transmit groundwater.
Field data collection
The investigation sites are in communities that are under traditional leadership and boundaries. The first step of the field survey involved a walkover of the sites where the water committee will show the land designated for the project and the traditional boundary for that community. Although we know that groundwater has no boundary, the issue of community boundary is very important because it relates to the ownership of the borehole and who must take responsibility for the management of the facility. This is particularly important in many rural areas where the water schemes are managed by the community. The surveys would then have to take place within the community boundary. The project was conducted in Moyeni (Fig. 5), Mawombe (Fig. 6), and Emkhuzweni (Fig. 7) communities.
Fig. 5
Google Earth map showing the investigated sites at the Mawombe community
Fig. 6
Google Earth map showing the investigated sites at the Moyeni community
Fig. 7
Google Earth map showing the investigated sites at the Emkhuzweni community
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The second step of the field survey involved a site walk to identify sites that can be subjected to detailed geophysical investigation. Field observations during the walk involve landscape observations for places with topographic discontinuities, outcrops, and vegetation change, among other general attributes. The investigated sites are distributed to try and cover the designated land within the community boundaries. The survey traverse length also varies from one site to another depending on the availability of the space. The number of sites investigated in each community also varies with the groundwater potential of the area. In areas where groundwater potential was judged to be good, one site is sometimes sufficient. Whereas in areas with poor groundwater potential, one has to investigate more sites relative to the size of the community. A total of 24 sites were investigated to select the 4 drilling sites. At each of the investigated sites, at least four measurements were repeated and an average was computed. Inconsistent or abnormal readings were discarded as part of quality control. The locations of the investigated sites are shown in Fig. 5 (Mawombe), Fig. 6 (Moyeni), and Fig. 7 (Emkhuzweni).
The geophysical survey consisted of an EPD vertical profile (VP) at identified sites collected using the ADMT-300S Groundwater Detector of the ADMT series products (Shanghai Aidu Energy Technology 2020). The Groundwater Detector is one of the TEFSM-based equipment available in the market. The geophysical equipment measures EPD at 5 m depth intervals up to 300 m. The main specifications of the 300 S Groundwater Detector used in the field are as follows:
Electric potential difference measurement range of 0–300 mV with a 0.5-μV resolution,
Measures EPD at 5 m depth intervals up to 300 m depth,
Input impedance is ≥ 10 MΩ,
Sixty frequency channels, and
Power frequency of > 80 dB.
Data processing and data analysis
After the survey, the EPD VP data were downloaded onto the computer, and the data file was converted into Microsoft Excel. Graphs of depth against EPD were plotted to ensure improved visualization of the depth profiles for further analysis. The study area is characterized by granite rocks, and these can only become aquifers after they are altered by processes such as weathering and/or fracturing which create storage and flow path for groundwater. In general, low EPD suggests softer geological formations which have good groundwater potential, while high EPD typically corresponds to less disintegrated or harder geological formations with limited to no groundwater potential. Prior to drilling, the main practical challenge is to define the EPD values that correspond to an aquifer.
Bearing in mind that this is probably the first instance this geophysical technique has been applied in typical hard rock granite aquifers, we had to come up with a way to constrain the EPD VEP data to identify potential drilling targets for this area. The only reported studies showed that electric potential difference ranges from 0.001to 0.05 mV (Abderahman 2019) and 0.005to 0.019 mV (Gomo 2023) characterized by sandstone aquifers. Our previous groundwater exploration in granitic aquifers of Eswatini with this equipment showed that the EPD of the weathered granite aquifer in this region ranges from 0.010to 0.085 mV. This EPD range is therefore used as the basis for the delineation of a potential aquifer and therefore drilling sites.
Results
This section is split into two parts. First, the EPD VEP data from all the 24 sites per community are presented followed by a discussion of drilling site selection through the data analysis. In the second part, the geophysical data are corroborated with the lithology data to evaluate the ability of the TEFSM to delineate the granite aquifers and general changes in lithology. This is a qualitative evaluation in which the geophysics data before drilling are compared with the drilling data/lithology to improve the understanding of the ability of the TEFSM approach to site boreholes at a local scale granite aquifer.
Selection of drilling sites
A total of 24 sites were investigated during the project. According to the project requirements, the drilling depth was not supposed to exceed 120 m; hence, only the EPD VEP data up to 120 m depth profile are presented in Figs. 8, 9, and 10. However, drilling would stop if aquifers of yield greater than 1 L/s were intersected before reaching 120 m depth.
Fig. 8
The EPD VEP data from the nine sites investigated at the Mawombe community
Fig. 9
The EPD VEP data from the nine sites investigated at the Moyeni community
Fig. 10
The EPD VEP data from the nine sites investigated at the Emkhuzweni community
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Mawombe community
The EPD VEP data from the nine sites investigated at the Mawombe community are presented in Fig. 8. The profiles from all the nine sites had some EPD data at various depths meeting the local criteria of 0.010–0.085 mV. This suggested the likelihood of weathered zones across all sites and therefore chances of intersecting the aquifers. Factors such as convenience for community hand pump location on open land and proximity to homestead were then considered for the final selection of the drilling sites. Sites 8 and 3 were selected for the drilling of Mawombe 1 and Mawombe 2 boreholes, respectively.
Moyeni community
The EPD VEP data from the six sites investigated at the Moyeni community are shown in Fig. 9. The EPD VEP data between the sites had a good correlation of at least 98%, suggesting that the subsurface geological conditions were likely to be similar. From 10 to 40 m depth, EPD VEP datasets from the six sites were able to meet the set criteria of 0.010–0.085 mV. Site 3 was selected based on the convenience of being located at the edge of the maize field rather than for scientific purposes. Being located on the edge of the field implied that there would be limited disturbance to disrupt the crop production. The selection was done by the community members themselves.
Emkhuzweni community
Nine sites were investigated at the Emkhuzweni community, and their EPD vertical profile data are displayed in Fig. 10. Out of all the 9 sites, only site 5 had the EPD readings meeting the local criteria of 0.010–0.085 mV, and therefore selected for drilling purposes. The rest of the investigated sites at the Emkhuzweni community had high EPD ranging from 0.128to 3.201 mV, which would suggest less chances of disintegration/weathering and hence low groundwater potential. Due to its closeness to the homesteads, site 5 location was also more suited for the hand pump type of water scheme.
Figure 11 shows the location of boreholes drilled at the study area in relation to surface topography. It can be seen that the study is characterized by areas of high relief. The elevated areas are typically comprised of mostly granite dwalas of different sizes. The low-lying valleys mostly form streams that are fed by runoff from the surrounding hills.
Fig. 11
Location of the boreholes drilled at the study area in relation to the surface topography (BH1 Mawombe borehole 1, BH2 Mawombe borehole 2, BH3 Moyeni borehole, and BH4 Emkhuzweni borehole)
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Corroborating geophysical data with drilling results
Mawombe community
Borehole 1
The EPD VEP and lithology profiles of borehole 1 at the Mawombe community are presented in Fig. 12. The first 6 m of the lithology intersected in borehole 1 comprises light-gray coarse-grained soils characterized by an EPD of ≥ 0.1 mV. Their light-gray color suggests that the soil has gone through high levels of leaching due to drainage. From 6 to 100 m, the lithology consists of alternating zones of amphibolite and granite. Groundwater was intersected (henceforth referred to as water strike) at 81 m and 95 m depth, and these correspond to a low EPD of 0.044 mV and 0.059 mV, respectively. The water strikes occur in weathered granite, but very close to the contact with the underlying amphibolite. These low-EPD zones generally correspond to the highly weathered granite across the study sites. The borehole had a final blow yield of 0.32 L/s measured at the end of its development.
Fig. 12
Electric potential difference VEP and lithology profiles of borehole 1 at the Mawombe community
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Borehole 2
The first 5 m of the subsurface lithology is characterized by loose soil followed by gravel sand (5–7 m) that is likely from highly weathered granite consisting of quartz (Fig. 13). The rest of the lithology comprises alternating zones of granite and amphibolite shown by the varying EPD and lithology. Three groundwater water strikes were intersected at 24 m, 30 m, and 45 m depths with an EPD of 0.03 mV, 0.025 mV, and 0.033 mV, respectively. The water strike depth correlates with zones of lower EPD contrast that occurs toward the end of weathered granite. It is important to note that the low EPD at water strikes at 30 m and 45 m depth is followed by an elevated EPD, likely suggesting that the amphibolite is less disintegrated. The final blow yield of 1.5 L/s was measured after the development of the borehole.
Fig. 13
Electric potential difference VEP and lithology profiles of borehole 2 at the Mawombe community
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Moyeni community
The lithology intersected during the drilling of the borehole at the Moyeni community is presented in Fig. 14. The first 8 m depth consists of yellow soils. Such soils typically occur under poor drainage conditions. Poor drainage could also suggest limited vertical groundwater recharge. From 8.0 to 24.0 m, the lithology comprises thick sand formation. The sand formation appears to be a by-product of granite weathering. Two groundwater strikes were encountered at 31 m and 54 m depth. The strikes appear to occur at the contact of weathered granite and fresh-hard amphibolite. The whitish color of the lithology suggests that the granite is rich in feldspar, while the black to dark gray reflects the hornblende-rich amphibolite. The water strikes are associated with an EPD of 0.018 and 0.068 mV, respectively. The borehole was drilled to 90 m depth below the ground surface and had a final blow yield of 1.1 L/s.
Fig. 14
Electric potential difference VEP and lithology profiles of the borehole drilled at the Moyeni community
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Emkhuzweni community
Groundwater strikes occurred at 35 m and 55 m below the ground surface. The first groundwater strike at 35 m occurred toward the end of the weathered dolerite, where it contacts the amphibolite. The water strike at 35 m suggests the existence of a preferential flow at the contact plane of dolerite and amphibolite. This shows the influence of dolerite intrusion on groundwater occurrence in typical hard-rock aquifers as previously demonstrated in the literature (Chandra et al. 2010; Senger et al. 2015). The second strike occurred toward the end of the weathered granite zone, and again where it contacts the amphibolite. It is likely that the weathered granite stores the groundwater while flow occurs at contact with the underlying amphibolite. The EPD of the lithology at the 35 m and 55 m water strike depths is 0.065 mV and 0.038 mV, respectively (Fig. 15). The borehole had a final blow yield of 1.7 L/s.
Fig. 15
Electric potential difference VEP and lithology profiles of the borehole at the Emkhuzweni community
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Discussion
The goal of the study is to investigate the application of the TEFSM groundwater detector surface geophysical approach for groundwater exploration in typical granite aquifers and to understand the factors that influence the occurrence of groundwater in such aquifers.
The groundwater appears to occur in confined conditions in the weathered granite aquifer underlying fresh amphibolite. This suggests that groundwater is likely stored in the weathered granite zones, while some of the flow occurs at the top of the underlying fresh-harder amphibolite. This occurs because groundwater in the weathered layer drains downward due to gravity and accumulates on top of the fresh amphibolite, which can trap the water. The contact plane between the weathered granite and the underlying amphibolite forms a preferential flow path for groundwater and is an important target for groundwater exploration in such environments.
The results suggest that the occurrence of alternating weathered granite and fresh amphibolite zones had an important contribution to groundwater occurrence. Fresh zones of amphibolite are typically hard and impermeable, thus on their own have limited potential for groundwater, if any. Weathered zones of granite have the potential to store and allow flow to make aquifers, but without a hard and impermeable underlying amphibolite layer, the groundwater would be difficult to access because it will continue to drain vertically downward due to gravity. It is, therefore, possible that the weathering of granite is associated with joint disintegration (Lachassagne et al. 2021) at the bedding plane with amphibolite. It is also important to note that in the four wells, groundwater did not occur in the upper weathered regolith. This is contrary to the existing model of groundwater occurrence in typical hard rock granite aquifers, which suggests groundwater occurrence is mainly associated with the weathered and/or fractured zone overlying the fresh hard rock (MacDonald et al. 2008, 2012). This study appears to show a layered weathered granite aquifer that is separated by competent amphibolite. The borehole blow yield ranges from 0.32 to 1.7 L/s. The variation shows that the granite aquifer is heterogeneous in nature.
The results show that the TEFSM groundwater detector geophysical approach was able to delineate both weathered granite and impermeable amphibolite zones. This is possible due to the contrasting properties of the weathered (soft) granite and (fresh) hard amphibolite resulting in anomalies of low and high EPD being measured, respectively. Weathered granite does have better shielding effectiveness to absorb the EM waves in comparison to hard and fresh amphibolite. The EPD of the weathered granite aquifer ranges from 0.018to 0.068 mV. At this point in time, it is not possible to compare these results with those of previous work because this is the first time the approach is being applied in a granite aquifer. It is likely that the EPD values will vary from one region to another given the variation in natural electric fields. The approach used in this study is therefore recommended for local-scale application.
The criteria of geophysical profiles with an EPD of 0.01–0.085 mV being targeted for analysis and identification of drilling target depths assisted with in-field decision-making on whether to continue or not with the survey. Sites that did not conform to the EPD criteria were regarded as having poor groundwater potential. This feature shows that the TEFSM approach is a rapid tool that is suited for many groundwater investigations where some in-field basic data analysis and decision-making are necessary.
It is also important to mention that not all the low EPD contrasts and contact zones of weathered and fresh granite yielded groundwater, some were completely dry. Even without water, weathered granite does have high shielding effectiveness to absorb EM waves, resulting in low EPD recordings. It does however appear that where dry weathered granite is encountered at shallow zones, there was deeper groundwater potential. The ability of the TEFSM groundwater detector geophysical approach to delineate anomalies closely related to the vertical changes of lithology is quite remarkable and a very important aspect that needs to be further explored in the quest to improve success rates in groundwater exploration projects. Although this study focuses on granitic aquifers, the findings could also be applicable to other hard-rock aquifers where groundwater occurs in weathered and/or fractured lithology.
Conclusions
The study seeks to understand the application of TEFSM surface geophysical approach to explore groundwater in granite hard-rock aquifers. The results show that the groundwater appears to occur at the contact zone between the overlying weathered granite and the underlying fresh amphibolite. The TEFSM groundwater detector geophysical approach was able to delineate weathered granite aquifers and impermeable amphibolite zones, and this makes it an important technology for groundwater exploration. The groundwater is stored in the weathered granite aquifer, but major flow happens at the contact plane with the underlying hard amphibolite. The granite aquifer appears mostly to be layered. The granite aquifer is characterized by an EPD ranging from 0.018 to 0.068 mV; this could be applicable to other hard-rock aquifers where groundwater occurs in weathered and/or fractured zones. However, not all geological materials with this EPD range had water, some were just a reflection of weathering. The TEFSM groundwater detector geophysical approach was able to delineate the vertical changes in lithology which are important for the identification of bedding plane contact zones which typically act as groundwater flow paths. Extensive field research to enhance experience and knowledge on the application of the TEFSM geophysical approach in groundwater exploration in other hydrogeological settings is highly recommended.
Acknowledgements
The data used in this study were collected during the WaterAid Eswatini project “WaterAid Eswatini and Vusumnotfo Hydrogeological Assessment” in which the authors participated. We would like to thank WaterAid Eswatini and Vusumnotfo Swazi Not-for-Profit Organisation.
Declarations
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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