Introduction

The Ewekoro Formation houses one of Nigeria’s major limestone deposits, which serves as raw material for cement manufacturing plants in the southwestern part of Nigeria. Ewekoro limestone is sometimes overlaid by hard shale and underlined by calcareous sandstone. Limestone is pale white to buff-colored, medium to coarse-grained. Occasionally, thin bands of siliceous limestone are also observed. Limestone is an example of a sedimentary rock mainly of calcite (CaCO3) mineral, formed as a result of either biological or chemical processes (Serra 2006). Nigeria has vast limestone resources in the country (Felix and Yomi 2013).

Mineral deposits have been successfully identified and delineated by geophysical methods and have extremely reduced ambiguities in mineral exploitation (Haile and Atsbaha 2014). The electrical resistivity method of geophysical prospecting is an economical non-invasive method, that can provide 2D or 3D models of the subsurface (Lara and Giovanni 2019; Hussain et al. 2020; Gobashy et al. 2021; Ameloko and Ayolabi 2020; Ameloko et al. 2020; Obihan et al. 2021). This provides a key advantage over the intrusive sampling method, providing information only at isolated locations (Ayolabi et al. 2009).

Mapping of the shallow-weathered zone and cavities employing geophysical techniques has gained wide interest in the past few decades (Sum et al. 1996; Mohamed and Fouzan 2013). The occurrence of natural cavities and voids in the limestone can cause serious problems for miners, environmental management, and civil engineering. One of the major factors affecting limestone is porosity, which is the volume of pores to the volume of bulk rock (limestone) and determines the amount of moisture contained in it (Rita and Adedeji 2021). Rocks contain inter-grain spaces sometimes occupied with fluid or solution cavities (Oluwaseun et al. 2012; Golian et al. 2020), thereby decreasing the electric resistance of that rock. Hence, electrical resistivity tomography can delineate limestone as well as overburden thickness provided sufficient contrast exists between the overburden (Ammar and Kamal 2018) and the limestone.

Electrical resistivity imaging is one of the most frequently utilized geophysical methods in mapping weathered zones and cavities. It offers a cost-effective and quick exploration of the near subsurface with acceptable results. Therefore, the objective of the study is to map the subsurface geology of the concession area from their electrical conductivity distribution, as a means of defining the main lithologic units, the geometry of the limestone bands, and possible cavitation zones.

Geomorphology of Study Area

The study area is located in Yewa Local Government Area, Ogun State in Nigeria. It has geographical coordinates of latitude 773,212.93 mN and 772,566.58 mN and longitude 503928.1 mE and 504663.04 mE (Fig. 1). The site is generally flat with minimum and maximum elevations of 38.12 m and 57.59 m, respectively, above Lagos Datum (mean sea level). It has an almost flat topography with localized low-lying areas. These low-lying areas are generally waterlogged during rainy season because of a poor drainage system and flat ground. The highest elevation is towards the southeastern part, about 57.59 m above sea level (Fig. 1), generally sloping towards NW-, and the drainage joins the river Iniya which is flowing towards NW to join the major river Yewa. A seasonal stream running NW-SW was observed to drain the area.

Fig. 1
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Topography of the study area

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Ibese is located at the southern edge of the Ewekoro depression in Dahomey basin and is overlooked by the escarpment capped by the Ilaro Formation (Fig. 2), which forms a conspicuous feature (Ozebo et al. 2008). Limestone deposit of economic interest is the thick limestone body near the base of the Ewekoro Formation observed at Shagamu, Ewekoro town, and the North of Ibese, which occurs at varying depths. Ewekoro Formation consists of algae limestone of varying thickness, overlain by finely laminated black and gray shales interbedded with marl. The grayish shales at the top of the sequence grade into thick reddish clay and sand. These have been tentatively referred to as Akinbo and Oshosun Formations, respectively (Ozebo et al. 2008).

Fig. 2
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Stratigraphic of Dahomey basin (geological survey of Nigeria 1963)

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This limestone deposit is free from any major structural disturbances. The general strike of the limestone is NW to SE and dipping towards the South; hence, the thickness of the overlain shale increases towards the south. Since it is a shallow marine deposit, many karst topographic features and cavities have been observed in the limestone zone.

Previous field studies and available drill logs confirmed that the sediments in Ibese consist essentially of Oshsun Formation—lateritic clay and conglomerate, sandstone, and the Akinbo Formation is dark gray/black shales with discontinuous limestone and marl bands of the Ewekoro Formation.

Methodology

The electrical resistivity technique was selected as a suitable approach in delineating the geology due to the expected contrast in the electrical resistivity representation of the subsurface geology. The 2D ERT data used for the investigation of the Ewekoro Formation was acquired via the Super Sting R8 Earth Resistivity/IP meter (automated multi-electrode system). The instrument used 84 electrodes and was deployed alongside an electrode selector and other accessories for the study. Eight profiles, at 2-m inter-electrode spacing, were occupied in the entire study area using the dipole–dipole and pole-dipole configuration (Fig. 3). The choice of these electrode spreads lies in their high sensitivity to horizontal changes in resistivity and high signal strength, respectively (Loke and Barker 1996).

Fig. 3
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Dipole–dipole electrode array with apparent resistivity and pole-dipole electrode array with apparent resistivity (after Morrison and Gasperikova 2012)

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The 2D data obtained from the survey were processed and inverted using the Earth Imager inversion algorithm (AGI 2003), to produce the resistivity models. The apparent resistivity was determined via a forward modeling subroutine, with the inversion scheme performed by the software by means of the least-square optimization routine (Loke 2003). The computer program determines the resistivity of a series of rectangular cells so that the measured values agreed with the calculated apparent resistivity values.

The processed data were correlated with borehole logs and interpreted, thereby resulting in the identification of the various lithologies and the limestone deposits in the study location. The variation in resistivity and thickness values of the overburden rock, and the limestone deposits were interpolated using the Surfer software to produce the isoresistivity, overburden, and limestone deposit isopach maps.

Results and discussion

The results of the 2D ERT survey along with the profiles are presented in Fig. 4. A critical observation of these models indicates that they are essentially similar in terms of their signatures except for the higher depth of investigation achieved with the pole-dipole array and a better lateral resolution with a dipole–dipole array. A maximum depth of investigation of 34.8 m was achieved by the dipole–dipole array, while a depth of 57 m was probed by the pole-dipole array. The observed electrical resistivity ranged from 1.0 to 13,000 Ωm, and it is the characteristic of varying conductivity and mineral composition in the subsurface. Available drill hole logs were used to constrain the interpretation of the 2D ERT sections, and strong positive correlation was established. (Table 1). Generally, four geoelectric stratums were delineated in all the 2D ERT sections except for traverse four which was characterized by three bands (Table 1). The low resistivity (1.0–50 Ωm) around the middle of some of the profiles may be attributed to the possible dissolution of limestone, thus creating a cavity or void spaces within the rock. Physical observation shows that regions of cavitation are impregnated with groundwater and may result in flooding during mining.

Fig. 4
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2D ERT along traverse one and three. (a) dipole-dipole array along traverse one. (b) pole-dipole array along traverse one. (c) dipole-dipole array along traverse three

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Table 1 Summary of borehole the detail versus geo-electric characterization
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The orientation of the 2D models in Fig. 4a, b (traverse one) is towards the southeast-northwest direction, and four geoelectric layers/strata were delineated. The first layer is mostly clay having resistivity values around 1.0 to 100 Ωm occurring between 0 and 3 m in depth. The underlying stratum, interpreted as shale occurs at 3–16 m at the subsurface from about 64 m to the end of the traverse, and has resistivity ranging from 1.0 to 50 Ωm. The third layer occurs at depth of 4.0–28 m from the origin of the profile to about 112 m along the surface. It has resistivity values ranging from 200 to 13,000 Ωm and is interpreted as limestones. The fourth geoelectric layer is present at a depth of 26–57 m beneath the surface, with a resistivity of 100–400 Ωm. When correlated with borehole logs, it was deduced to be composed of marl.

The limestone is characterized by relatively high resistivity values at variable depths suggestive of a hard limestone bed, except for an important feature having low resistivity values between 64 and 85 m along the profiles indicating limestone as well. This feature is interpreted as a cavity, and it extends from the shallow-weathered zones into the hard limestone and could have resulted from the weathering effects of surface and groundwater inside the limestone (Mohamed and Fouzan 2013).

Traverse three (Fig. 4c) has its orientation along the northwest-southeast direction. The first geoelectric layer here has resistivity values ranging from 1.0 to 1200 Ωm and occurs at a depth of 0–3 m. It is predominantly sand/topsoil. When correlated alongside the borehole log, the second geoelectric layer is found to have a depth of 1.0–7 m, with resistivity ranging from 1.0 to 80 Ωm. It is interpreted as shale/marl with some isolated high resistivity (1000–13,000 Ωm) due to the possible presence of limestone/intercalation of limestone. The third geoelectric component extends from a depth of 8.7–26.5 m, with a resistivity range from 200 to 13,000 Ωm. It is composed predominantly of limestone when tied to the lithology log. The fourth geoelectric stratum extends from 26 to 34.8 m, with resistivity ranging from 100 to 400 Ωm and suggestive of the presence of marl. Also conspicuous on the 2D-inverted section is the presence of low resistivity cavity structure at a lateral distance of 82–88 m.

The thicknesses of the overburden rock and limestone was determined at each data location using 2D sections and a lithology log. The overburden isopach map (Fig. 5) consists of the thickness of all the materials above the limestone. The map shows the overburden is thickest in the eastern part of the map with a value of 14. 5 m and thinnest in the northernmost region. The limestone isopach map (Fig. 6) shows that the limestone is thickest around the north-western and south-eastern parts of the map. The general thickness of limestone is between 5.9 and 31.45 m with 21 m on average, and it is thinnest at the southwestern and the central northern region where a noticeable structural depression trending north-eastern—south-western part of the survey area. The limestone resistivity values were also interpolated to produce the isoresistivity map presented in Fig. 7. The resistivity of limestone generally ranges from low to high, i.e., 80–13,000 Ωm. The limestone resistivity map shows that the limestone bed is predominantly characterized by a very high resistivity value (2000–13,000 Ωm) with few exceptions. The low resistivity is indicative of shallow groundwater level, the presence of clayey material, increased porosity due to karst structures, and the presence of fluid-filled cavities/karst structures. Figures 8a and 8b, demonstrate a lateral continuity of the limestone deposit and inverted 2D sections superimposed on lithologic section of the boreholes in the study region, correlate well and further suggest the presence of limestone of commercial relevance.

Fig. 5
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Overburden isopach map

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Fig. 6
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Isopach map of the limestone deposit

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Fig. 7
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Limestone average apparent resistivity map

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Fig. 8
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a Cross-sectional view of boreholes along the East–West direction. b Cross sectional view of boreholes along the north-south direction

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Conclusion

2D electrical resistivity imaging has been integrated with borehole information in Ibese mines to determine the vertical and the lateral extent of limestone deposits. The study area is predominantly characterized by four geologic units from the interpreted 2D ERT sections. The results of overburden and limestone thicknesses, suggest an enormous limestone resource of economic interest mineable in an environmentally friendly manner using surface mining technique rather than the blasting method. However, the occurrence of low resistivity (1–50 Ωm) in some parts of the limestone beds may constitute geohazards during mining, as these may be cavities or fractured zones as a result of a possible dissolution of the limestone. The 2D geoelectric models show that the regions of cavitation are highly impregnated with groundwater and may result in flooding during mining. Further assessment/evaluation of these cavities is recommended, and precautionary measures should be put in place to ensure safe operation within the mining site.