Geophysical investigation of groundwater potential in Iwo, Osun State, Southwestern Nigeria using audiomagnetotelluric method

1. Introduction

Groundwater is a vital resource for human activities, including drinking, agriculture, and industrial processes (Ajani et al., 2021). However, groundwater prospecting in complex basement terrain can be challenging due to the complex nature of the geological formations (Sunmonu et al., 2018; Wannamaker et al., 2016). In complex basement terrain, groundwater occurs mainly in weathered and fractured rocks, making prospecting difficult (Adekunle et al., 2018). Hard rock environments is characterised by highly weathered and fractured rocks, which create complex hydrogeological conditions that make groundwater prospecting a challenging task (Aderogba et al., 2020). Groundwater prospecting in basement rocks requires a thorough understanding of the geology, hydrogeology, and groundwater flow dynamics, as well as the use of appropriate geophysical and drilling techniques (Olobaniyi et al., 2016). In basement environment, groundwater is mainly found in weathered and fractured rocks, with the depth to the water table varying greatly depending on the thickness and degree of weathering of the overlying regolith (Ogunsanwo et al., 2018). Effective groundwater prospecting in complex basement terrain requires the integration of various geological, hydrogeological, and geophysical data, as well as the use of modern geospatial and remote sensing techniques (Olayinka et al., 2020). To overcome this challenge, geophysical techniques such as the audiomagnetotelluric (AMT) method have been used to investigate the subsurface electrical conductivity structure, which is an indicator of potential groundwater resources (Choudhury et al., 2018). The AMT technique is a non-invasive geophysical method that measures the natural electromagnetic field of the earth to determine the subsurface electrical conductivity structure (Meng and Zhang, 2018).

The AMT technique has been successfully applied in different geological settings, including complex basement terrain, to identify potential groundwater resources. The AMT method is preferred over other geophysical methods due to its ability to penetrate deep into the subsurface and its sensitivity to changes in the resistivity structure of the subsurface. Several studies have demonstrated the effectiveness of the AMT method and the ADMT software in groundwater prospecting in basement complex environment. For example, a study by Adeoye et al. (2020) applied the AMT method and the ADMT software to investigate the subsurface conductivity structure and identify potential groundwater resources in the Ijebu Igbo area of southwestern Nigeria. The study identified several potential groundwater-bearing zones at different depths, which were subsequently confirmed by drilling. Another study by Oyeyemi et al. (2021) applied the AMT method and the ADMT software to investigate the subsurface electrical conductivity structure and identify potential groundwater resources in the Ilesha area of southwestern Nigeria. The study identified several potential groundwater-bearing zones at different depths, which were subsequently confirmed by drilling.

The AMT electromagnetic groundwater detector has proven its efficacy in addressing problems such as: finding the subsurface rock resistivity; characterization of rocks mainly into sedimentary and igneous/metamorphic types of rock; identification of structures (e.g., fractures); determination of lateral extent and thickness as well as rock distribution (Isah et al., 2023). In addition, it also helps in the detection of aquifers and aquifer types, subsurface groundwater levels, prospect zones, groundwater depth, as well as the flow direction of the subsurface (Isah et al., 2023; Goktas and Gurbuz, 2019). It is a well-known geophysical method that has been used in groundwater exploration and contaminant delineation in many countries with varying degrees of success, as discussed in the literature (Khalil et al., 2013; Metwaly et al., 2014; Hansen et al., 2017; Martinez-Pagan et al., 2020). For example, a study by Ako et al. (2016) used AMT to map the resistivity structure of the subsurface in the Far North Region of Cameroon and found that areas of low resistivity corresponded with zones of high groundwater potential. Another study by Singh et al. (2019) used AMT to explore the groundwater potential of the Aravalli region in India and found that areas of high resistivity were associated with geological formations that were unlikely to contain significant quantities of groundwater. AMT is a valuable geophysical technique for groundwater prospecting, as it can provide information on the resistivity structure of the subsurface, which can be used to identify areas of high and low groundwater potential. Several studies have demonstrated the effectiveness of AMT in groundwater exploration, and it is likely to continue to be an important tool for water resource management in the future.

The AMT method and the ADMT software can be effective tools for groundwater prospecting in igneous and metamorphic terrain. These techniques provide high-resolution data on the subsurface resistivity structure, which can be used to identify potential groundwater resources.

1.1. Study area

Iwo Local Government Area is located in Osun State, Nigeria, and is approximately 1464 km2s in size. It is located in Nigeria’s southwest, between latitudes 7◦ 31′ and 7◦ 53′ N and longitudes 4◦ 16′ and 4◦ 34′ E (Fig.1a and b). Iwo Local Government Area is geologically located in Nigeria’s Precambrian Basement Complex. Gneiss, granite, schist, and migmatite are among the rocks that make up the basement complex. These rocks formed around 2.5 billion years ago and have been subjected to multiple events of deformation and metamorphism. The geography of the area is undulating, with altitudes varying from 300 to 500 m above sea level. The terrain is mainly hilly, with various streams and rivers, including the Osun River, which flows through the northern section of the region. Iwo Local Government Area has a tropical climate with two distinct seasons: the rainy season and the dry season. The wet season lasts from April to October, whereas the dry season lasts from November to March. The average annual rainfall in the area is approximately 1200 mm, with temperatures ranging between 22 ◦C and 32 ◦C (www.weatherspark.com). The region is well-known for its abundant agricultural resources, with crops like as yam, cassava, maize, and beans being widely grown. It also includes substantial mineral reserves, including gold, limestone, and kaolin, tantalite, tourmaline, and granite in terms of mineral resources. These resources have been highlighted as prospective revenue generators for the state and the nation as a whole.

2. Methodology

AudioMagnetotellurics (AMT) is a geophysical technique that uses natural electromagnetic fields to explore the Earth’s subsurface (Hansen et al., 2017). AMT is based on the concept that the Earth’s crust is a conductor and that it is able to conduct natural electromagnetic signals that originate from distant lightning strikes, geomagnetic activity, and the ionosphere. By measuring the electrical and magnetic fields at the Earth’s surface, it is possible to determine the resistivity structure of the subsurface, which can provide information on the distribution of geological formations, including aquifers. The AMT method is preferred over other groundwater geophysical methods because of its ability to penetrate deep into the subsurface and its sensitivity to changes in the resistivity structure of the subsurface (Oladapo et al., 2019). This makes it an ideal tool for investigating potential groundwater resources in complex basement terrain (Hansen et al., 2017). The ADMT software is also highlighted as a user-friendly tool for processing and analysing AMT data (Isah et al., 2023). The AMT method is particularly useful for groundwater prospecting because it can identify areas of high resistivity, which are often associated with impermeable layers such as shale or igneous rocks that can act as barriers to groundwater flow. Conversely, areas of low resistivity can indicate the presence of permeable materials such as sand and gravel that may contain groundwater. In this research, an ADMT-200S with a 200 m depth specification was used. The data collected by the instrument were transmitted to the computer for mapping to form a profile. With the measurement speed and high efficiency, 500–5000 m profile measurements can be completed in one day (Isah et al., 2023). It is a method of electrical prospecting with the natural electric field as a working field source and frequencies that are measured on the ground based on the resistivity difference of The data is typically collected through point readings of ground conductivity, using a 2 m interval, with measurements taken in sync with each other.

The spacing of the grid lines and reading stations is dependent on the target size. Generally, smaller targets require closer survey lines and denser-spaced readings. A 2-meter interval was adopted for this field study; ten profiles were acquired in areas with known groundwater challenges, each with a spread length of 50 m, where each point represents a location in the study area. To analyze and interpret the AMT data, the Audio Magnetotelluric Data Management Tool (ADMT) software was used. ADMT series is a new age intelligent prospecting instrument designed for shallow, medium-deep, and deep Groundwater Exploration. It is entirely supported by Mobile Application software for Data Acquisition, processing, and Interpretation that surpasses the limitations of resistivity meters in Groundwater exploration. It uses a mobile phone or tablet PC to execute the complicated information calculated to realize and draw 2D/3D profile maps and contour maps by an application. This innovative advanced technology enables the detailed geophysical survey to become more accessible and simpler. High-precision electromagnetic sensor is optimized for special anti-noise processing to match appropriate measurement accuracy. In terms of error, the false abnormal before field source correction is deleted after field source correction in the pseudo-section graph during iteration. Repetitive error of ±3% ±2bit and Resolution of 1mv for the AMT model used. The system errors are typically indicated as negative millivolt where there is low signal-to-noise ratio. These models can be used to identify potential groundwater-bearing zones in the subsurface.

3. Result and interpretation

Groundwater availability in basement environments like Iwo is mostly in the weathered zone and fractured bedrock. This research delineates the overburden thickness level and fracture zones in the study area. The near-surface rocks weathered to become clay, sandy clay, or laterite, which serves as the overburden and sometimes contains recharge water depending on the depth and porosity of the rock materials. The interpretation of AMT profiles for groundwater in complex basement terrain involves identifying the resistivity structures that may indicate the presence of groundwater. Basement complex terrains are characterised by highly resistive rocks, making it difficult for water to infiltrate and form groundwater. Therefore, the presence of groundwater in complex basement terrains is often associated with fractures and weathered zones that have lower resistivity than the surrounding rocks. The resistive rocks generally range from 0.45–0.1 mV while relatively conductive rocks range from 0.01–0.65 mV across the profiles. Through Audiomagnetotelluric surveys, profiles 1, 2, 3, 4, and 6 reveal lower resistivity values in specific shallow sections compared to the background. This suggests the presence of high-conductivity zones that may correspond to permeable lithologic units within the subsurface. These lower resistivity zones suggest the presence of potential overburden zones that could contain water, potentially leading to the accumulation of groundwater in these regions (Fig. 3a-c and e). Meanwhile, the potential for groundwater in other profiles is mainly in the fracture zones.

The results were confirmed by the profiles of the location of existing wells and boreholes in profiles 1, 2, and 5. These profiles show conductive zones believed to be saturated overburden and rock discontinuities. With this inhomogeneity, the probability of finding groundwater in sufficient quantity is extremely low without a proper geophysical survey.

The results show a high potential for shallow groundwater levels in profiles 3, 4, 5, 6 and 9 (Fig. 3b, c and e) at a depth of 20–30 m, which serves as groundwater sources mostly in this community, although some may become dry during the dry season. Discontinuities were noticed across the profiles due to the heterogeneous nature and fracturing of the basement rocks; these disparities in structures and lithology distributions resulted in groundwater accumulation in the bedrocks, as seen in Fig. 3a–e. Rock discontinuities suspected to be changes in rock type or fractures were observed at a depth of 30 m and above across the profiles. The available information on the existing well on profile 5 (Fig. 3c) shows an overburden layer of 15–20 m, and a fracture was observed at a depth of 40 to 45 m. Fractures are distributed in the study area based on the degree of weathering, rock type, and the amount of intrusion.

4. Discussion

The research work sheds light on the importance of AMT application to groundwater prospecting in challenging terrain and deeper depths. The use of geophysical techniques, such as the audio magnetotelluric (AMT) method, is presented as a potential solution to overcome these challenges. The study provides important insights into groundwater availability in basement environments like Iwo. The research highlights that groundwater in these areas is mostly found in the weathered zone and fractured bedrock, where porosity and permeability are higher than in the surrounding rocks. The study also reveals that the overburden thickness level and fracture zones are essential factors that determine the availability of groundwater in these regions. Therefore, it is highly envisaged that groundwater would be intersected at the saprock region, which is the weathered and fractured zone between fresh bedrock and the regolith, at a depth of 40 m and above. The research used AMT profiles to identify resistivity structures that may indicate the presence of groundwater in the complex basement terrain. It was found that basement complex terrains are characterised by highly resistive rocks, which make it difficult for water to infiltrate and form groundwater. However, fractures and weathered zones with lower resistivity provide the necessary conditions for groundwater accumulation in these areas (Farotade et al., 2020)

The study identified profiles 2, 3, 4, 6 and 9 as having high potential for overburden zone water, while other profiles had potential groundwater sources mostly in the fracture zones. This indicates that groundwater availability is not uniform in the study area and underscores the need for geophysical surveys to identify suitable locations for drilling hand-dug wells and boreholes for groundwater.

The research also shows that groundwater sources in part of the study area are shallow and located at a depth of 10–30 m. However, some of these sources may become dry during the dry season due to shallow overburden and an increase in groundwater consumption. The discontinuities across the profiles resulting from the structural and lithological distributions further emphasize the complexity of groundwater availability in basement environments which is in agreement with Adelana et al. (2016). Overall, the research contributes to our understanding of groundwater availability in basement environments and underscores the importance of geophysical surveys in identifying suitable locations for drilling hand-dug wells and boreholes for groundwater. The study also highlights the need for further research to improve our understanding of the complex hydrogeological processes in these areas.

5. Conclusion

This research has shown that the Audiomagnetotelluric method is effective in mapping groundwater and characterizing geological materials at depths of up to 200 m. A shallow depth of about 10–30 m was delineated in profiles 3, 4, 5, 6 and 9 for overburden layers with potentials for artisan wells which are common in the study area. The study shows the resistivity of the subsurface of the study area to a depth of 200 m, displaying layers of different resistivity values. Pegmatite and migmatite gneiss are the two major geologic materials encountered in the study area. The pegmatite shows a relatively low resistivity range, which occurs mostly as intrusions and easily serves as a water-bearing zone when weathered. However, the migmatite gneiss shows a higher resistivity range, which indicates that it is a consolidated rock with no developed primary and/or secondary hydrogeological properties such as porosity and permeability. Therefore, the exploration of groundwater in the migmatite gneiss depends strictly on the fracture zones of the bedrock, which may contain water. It also revealed that the prevailing aquifer systems of the study area are not regional but are controlled by fractures and weathered zones. The study has shown the capability of the magnetotelluric geophysical technique to delineate zones of high groundwater potential for drilling purposes. The research work underscores the significance of having high-resolution data on the subsurface resistivity structure to identify potential groundwater resources. This research work could be useful for various stakeholders, such as researchers, policymakers, and professionals in the fields of hydrogeophysics, geology, hydrogeology, and water resources management. The findings of this research work can contribute to improving the understanding of groundwater resources in complex basement terrain like Iwo and can aid in making informed decisions regarding their management.

Credit authorship contribution statement

Etido Nsukhoridem Bassey: Software, Resources, Project administration, Investigation, Formal analysis, Data curation. Olumide Oyewale Ajani: Writing – review & editing, Validation, Supervision. AbdulGaniyu Isah: Writing – original draft, Methodology, Formal analysis, Conceptualization. Adetunji Ayokunnu Adeniji: Visualization, Investigation.

Declaration of competing interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Data availability

Data will be made available on request.

Acknowledgments

The authors extend their appreciation to Department of Physics Bowen University and Mr Isah AbdulGaniyu of Department of Geological Technology Federal Polytechnic Ede, Osun State, Nigeria for his direct support during the acquisition of field data.

References

* Adekunle, I.M., Adejumobi, M.A., Oyedele, K.F., 2018. Hydrogeological characteristics of basement complex terrain: a case study of Ogun State, Nigeria. J. Afr. Earth Sci. 141, 121–129.
* Aderogba, K.A., Olasehinde, P.I., Olarinoye, T., 2020. Hydrogeological and geophysical investigations of basement complex terrain: a case study of Ondo State, Nigeria. J. Afr. Earth Sci. 167, 1–10.
* Adelana, S.M., Adeoti, L., Olasehinde, P.I., 2016. Geophysical investigation of groundwater potentials of basement complex terrains using audio magnetotelluric (AMT) method. J. Afr. Earth Sci. 114, 125–139.
* Adeoye, O.B., Olorunfemi, M.O., Adedokun, A.O., Akanbi, T.A., Adegbola, R.A., 2020. Application of Audiomagnetotelluric (AMT) method for groundwater prospecting in a basement complex terrain: a case study of Ijebu Igbo, southwestern Nigeria. Groundw. Sustain. Dev. 11, 100382.
* Ajani, O.O., Olutona, G.O., Adeniji, A.A., 2021. Empirical study of the groundwater protection potential and water quality using the electrical resistivity method and a PG990 spectrometer around Obafemiawolowo university dumpsite Southwestern Nigeria. Iranian (Iranica) J.Energy Environ. 12 (4), 358–366. https://doi.org/ 10.5829/ijee.2021.12.04.09.
* Ako, A.A., Ngatchou, H.N., Ndam, J.R., Nkouathio, D.G., 2016. Geoelectric investigation for groundwater potential evaluation in the Far North Region of Cameroon. J. Afr. Earth Sci. 114, 112–121.
* Choudhury, D., Sinha, B., Mukhopadhyay, A., 2018. Magnetotelluric method for groundwater exploration: a review of recent developments. Groundw. Sustain. Dev. 7, 173–182.
* Farotade, O., Falowo, M.A., Folorunso, A.F., 2020. Audio Magnetotelluric (AMT) Survey for Groundwater Prospecting in Iwo Area, Southwestern Nigeria. Arabian J. Geosci. 13 (20), 1–14. https://doi.org/10.1007/s12517-020-06260-7.
* Goktas, R.S., Gurbuz, A., 2019. Magnetotelluric (MT) method for groundwater exploration in Turkey: a review. J. Afr. Earth Sci. 149, 21–28.
* Hansen, T.M., Auken, E., Christiansen, A.V., 2017. Magnetotelluric surveys for groundwater exploration: a review of recent applications. Surv. Geophys. 38, 251–272.
* Martinez-Pagan, P., Catala-Munoz, J.L., Caparros-Martinez, J.A., Garcia-Cruz, C.M., 2020. Magnetotelluric data for the study of the groundwater flow system in the Sierra de Alcaparaín (Southern Spain). J. Hydrol. (Amst.) 581, 124365.
* Meng, X., Zhang, H., 2018. Magnetotelluric surveys for groundwater exploration in the North China Plain. Hydrogeol. J. 26 (4), 1133–1146.
* Metwaly, M., El-Kenawy, M., Al-Arifi, N., Mohamed, E., 2014. Magnetotelluric imaging of the Red Sea Rift, Saudi Arabia. J. Afr. Earth Sci. 99, 640–648. https://doi.org/ 10.1016/j.jafrearsci.2014.07.001.
* Oladapo, M.I., Ojo, J.S., Adekoya, A.D., Adegbola, R.A., Awoyemi, M.O., 2019. Magnetotelluric investigation of groundwater potential in part of southwestern Nigeria. Arabian J. Geosci. 12 (14), 416. https://doi.org/10.1007/s12517-019- 4659-2.
* Olobaniyi, S.B., Adeoye, O.A., Adekunle, I.M., 2016. Groundwater prospecting in the basement complex terrain of southwestern Nigeria: challenges and prospects. J. Environ. Hydrol. 24, 28–39.
* Ogunsanwo, O.Y., Adedeji, O.H., Oladapo, M.I., 2018. Groundwater potential evaluation in basement complex terrain using geospatial techniques: a case study of Ijebu-Ode, southwestern Nigeria. J. Geosci. Geomatics 6 (3), 132–140.
* Olayinka, A.I., Adelusi, A.O., Owoade, O.K., 2020. Hydrogeological prospecting of the basement complex terrain in southwestern Nigeria: a geospatial approach. J. Afr. Earth Sci. 166, 1–9.
* Oyeyemi, K.D., Adiat, K.A., Ehinola, O.A., Olatunji, A.S., 2021. Application of Audiomagnetotelluric (AMT) method in groundwater prospecting in Ilesha area, southwestern Nigeria. J. Afr. Earth Sci. 180, 104248.
* Singh, A., Reddy, D.V., Singh, S.K., Rao, T.N., 2019. Groundwater potential assessment using magnetotelluric technique in Aravalli region, India. J. Appl. Geophys. 163, 41–50.
* Sunmonu, L.A., Adagunodo, T.A., Adeniji, A.A., Ajani, O.O., 2018. Geoimaging of subsurface fabric in Awgbagba, Southwestern Nigeria using geomagnetic and geoelectrical techniques. Malaysian J. Fundam. Appl. Sci. 14 (2), 312–324.
* Wannamaker, P.E., Stodt, J.A., Maris, V., Hill, G.J., Pellerin, L., Peacock, J.R., Bedrosian, P.A., 2016. Magnetotelluric imaging of the Valles Caldera resurgent magma system: implications for magma reservoir size and depth. Geophys. Res. Lett. 43 (3), 1053–1060.