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Geophysical Investigation for Marl Exploration Using Vertical
Electrical Sounding in Akpokponke Ibii Afikpo Southeast Nigeria
1
Nwugha V.N.,
6
Ezebunanwa A.C.,
2
Onwuegbulam C.O.,
4
Mbagwu E.C.,
3
Chinaka A.I.,
5
Emeghara K. C
1
Physics,
3
Mathematics and
5
Human Kinetics and Sports Science Departments, Alvan Ikoku Federal
University of Education Owerri, Imo State, Nigeria
2
Department of Geosciences, Federal University of Technology Owerri, Imo State, Nigeria
4
Department of Geosciences, University of Calgary, Alberta, Canada
6
Department of Management, University of Hertsfordshire, Hatfield, united Kingdom
DOI: https://doi.org/10.51244/IJRSI.2025.120800192
Received: 07 Aug 2025; Accepted: 25 Aug 2025; Published: 19 September 2025
ABSTRACT
This study employs Vertical Electrical Sounding (VES) to investigate subsurface lithology with emphasis on
identifying marl deposits in the study area. Resistivity profiles from five locations reveal diverse geological
formations, including shallow weathered zones and deeper high-resistivity layers consistent with dense marl
and sandstone. Results indicate that marl-rich formations occur at extractable depths. Overlying clay and shale
horizons, identified as low-resistivity layers, may act as protective aquitards. These findings demonstrate the
effectiveness of VES in delineating marl deposits and assessing their spatial distribution. The integration of
recent advances in geophysical surveying and resistivity inversion enhances the accuracy of locating
economically viable marl deposits, supporting sustainable extraction and land-use planning in sedimentary
environments
Keywords: Vertical Electrical Sounding, Marl Exploration, Resistivity, Subsurface Lithology, Sedimentary
Deposits, Geophysical Survey, Resource Assessment, Hydrogeology
INTRODUCTION
The exploration and characterization of sedimentary deposits such as marl are critical for construction,
agriculture, and industry, particularly in regions with abundant sedimentary basins like Southeast Nigeria.
Marl, a calcareous sedimentary rock composed of clay and carbonate minerals, is widely applied in cement
production, soil stabilization, and as a building material (Akinluyi et al., 2020). Beyond its industrial value,
marl plays an important role in agriculture by improving soil fertility and reducing acidity, making it a dual-
purpose resource with economic and environmental benefits. Its local availability therefore presents a cost-
effective alternative to imported raw materials for cement plants in Nigeria (Enesi et al,2023).
Within Nigeria, the Benue Trough is especially notable for its marl occurrences interbedded with shale,
sandstone, and claystone (Ofoegbu, 1984; Nwankwo et al., 2019). Afikpo, located in the southeastern portion
of the Benue Trough, contains a complex stratigraphy of marl, sandstone, and limestone. Despite the economic
importance of these resources, the subsurface distribution of marl in Afikpo remains poorly mapped due to the
heterogeneity of the formations and the limitations of conventional exploration techniques (Eze et al., 2018;
Eze et al., 2022). The lack of detailed mapping limits the ability to evaluate the lateral continuity and thickness
of marl beds, which are essential parameters for determining their long-term economic viability in quarrying
and cement production. Traditional geological methods are often inadequate for delineating such
heterogeneous deposits, creating the need for improved geophysical approaches.
Vertical Electrical Sounding (VES), based on the measurement of apparent resistivity variations with depth,
has proven effective in distinguishing lithologies due to their contrasting electrical properties (Ajakaiye et al.,
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1994). The method has been successfully employed across Nigeria to delineate aquifers, locate mineral
deposits, and map sedimentary layers (Oladipo et al., 2019). Recent advancements, including multi-electrode
arrays, automated data acquisition, and robust inversion algorithms, have significantly improved the resolution
and reliability of resistivity-based subsurface models (Olatunji et al., 2021).
Resistivity contrasts provide a useful framework for differentiating Afikpo’s lithologies. Marl typically
exhibits moderate resistivity values, generally ranging from 100 to 1000 Ωm, depending on moisture content,
clay mineralogy, and cementation (Ojo et al., 2021). Limestone, by contrast, often exceeds 1000 Ωm when dry
and well-cemented, though fractures and karst features may lower its resistivity (Akinluyi et al., 2021).
Sandstone displays a broader range, from as low as 10 Ωm in saturated, clay-rich conditions to over 2000 Ωm
in dry, cemented settings (Olatunji et al., 2022). These differences in resistivity reflect variations in porosity,
cementation, and fluid content, which have important implications for both resource exploitation and
groundwater flow.
To overcome the limitations of traditional exploration methods, this study applies VES to investigate the marl
deposits of Afikpo. Specifically, it seeks to:
1. Delineate the depth, thickness and lateral extent of marl deposits.
2. Differentiate marl from adjacent sandstone and limestone formations using resistivity contrasts.
3. Assess the suitability of marl deposits for cement production and agricultural use by integrating
geophysical results with geological and borehole data (Olatunji, et al., 2020).
The findings will provide a scientific basis for the sustainable quarrying and utilization of marl in the region,
supporting both local economic development and infrastructure growth.
Complementing the scientific and technical efforts, the human kinetics expert plays a vital role in supporting
the geophysical exploration team by optimizing physical performance, ensuring safety, and promoting well-
being during field operations. This role enhances overall productivity and minimizes health risks, addressing
the human element of exploration activities. Recent studies emphasize the importance of incorporating human
performance optimization in fieldwork to improve efficiency and safety outcomes (Smith & Johnson, 2022).
Geology Of The Area
Akponkponsi Ibii is situated within the Ozara Shale/Amasiri Sandstone member of the Ezeaku Formation (Fig.
1). The geological setting of this area is characterized by a complex stratigraphy comprising various
sedimentary lithologies indicative of shallow marine depositional environments during the Late Cretaceous
period.
The Ezeaku Formation mainly consists of thick, calcareous and non-calcareous shales, interbedded with sandy
and shelly limestones, as well as calcareous sandstones. Near Akponkponsi Ibii, the formation includes the
Ozara Shale/Amasiri Sandstone member, notable for its massive sandstones exhibiting planar-tabular cross-
bedding—an indicator of high-energy depositional processes such as river channels or deltaic systems
(Adegoke et al., 2022). The shales and limestones often display lateral facies changes, reflecting a dynamic
environment influenced by fluctuating energy levels and sediment supply.
The limestone horizons within the formation are fossiliferous, containing marine fossils such as pelecypods,
gastropods, echinoids, brachiopods, and ammonites, primarily of Turonian age. These fossils suggest that the
area was once a shallow marine habitat capable of supporting diverse marine life (Ojo et al., 2021). The
presence of shelly limestones and calcareous sandstones further supports this interpretation.
Structurally, the formations in the area exhibit evidence of folding and faulting, indicating tectonic activity that
has affected the stratigraphy and surface expression of the rocks (Eze et al., 2020). Lateral facies changes and
the presence of cross-bedded sandstones highlight a depositional environment that ranged from quiet, deep
marine settings to high-energy shoreline or deltaic environments.
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Economically, the limestone horizons within the Ezeaku Formation are significant for cement manufacturing,
while the sandstone units may serve as valuable construction materials. Archaeologically, the area contains
rock shelters and artifacts, demonstrating its cultural importance and illustrating how geological features have
influenced human activity (Nwachukwu & Okezie, 2022).
Overall, the geology of Akponkponsi Ibii reveals a rich and varied depositional history that provides valuable
insights into paleoenvironmental conditions and the resource potential of the region.
Figure 1: The Geological Map of the study Area
METHODOLOGY
The Research Area and Methodology
The study was conducted in Akponkponsi Ibii, within the Afikpo area of Southeast Nigeria. This environment
is characterized by various sedimentary formations, including marl, limestone, and sandstone. Due to the
geological complexity of the area, geophysical methods were employed to effectively map the subsurface
lithologies. Vertical Electrical Sounding (VES) was used as the primary technique to investigate the subsurface
resistivity distribution and delineate marl deposits.
Field Procedure
The fieldwork utilized the Schlumberger array, which comprises four electrodes arranged in a specific
configuration. A controlled current and voltage were injected through the current and potential electrodes,
respectively, and the resulting resistance of the rocks at various depths was measured. Multiple readings were
taken with a maximum electrode spacing (AB/2) of 350 meters to improve data reliability. A total of five VES
points were surveyed to ensure comprehensive coverage of the area.
Data Processing and Interpretation
The apparent resistivity data obtained from the field were processed through inversion and modelling using
IPI2Win software, which generated true resistivity models of the subsurface layers and their corresponding
depths. These models facilitated the identification of distinct lithological layers based on their resistivity
characteristics.
Lithological Identification
Resistivity ranges associated with marl, limestone, and sandstone were established from existing literature and
local geological data. The interpreted resistivity profiles enabled the mapping of marl deposits and other
lithologies across the study area.
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MATERIALS AND METHODS
Theory of Resistivity (DC) Method
Resistivity surveys are founded on the principle that different subsurface materials exhibit varying electrical
resistivity levels, which measure a material's ability to resist the flow of electric current (Telford et al., 1990).
The resistivity (ρ) of a material depends on factors such as lithology, porosity, moisture content, and mineral
composition (Dahlin & Zhou, 2004).
In the direct current (DC) resistivity method, an electric current is introduced into the ground via a pair of
electrodes, and the resulting potential difference is measured with another pair. The fundamental relationship
relates the measured potential difference (ΔV) and the injected current (I) to the apparent resistivity (ρa)
through the geometric factor (G):
ℓa=2πGΔV/I
where:
ℓa is the apparent resistivity in ohm-meters (Ω⋅m),
(G) depends on the electrode array configuration (Loke, 2013),
ΔV is the measured voltage difference,
(I) is the current injected into the ground.
The apparent resistivity reflects the average resistivity of the subsurface layers within the investigation zone.
Varying the electrode spacing allows exploration at different depths, aiding in the interpretation of layered
structures and lithologies (Dahlin & Zhou, 2004).
Electrode Arrays
Multiple electrode configurations exist; among these, the Schlumberger array is widely used due to its
efficiency and depth penetration capabilities (Telford et al., 1990). In this array, the current electrodes (C1 and
C2) are moved outward symmetrically, while the potential electrodes (P1 and P2) remain fixed near the center,
enabling investigation of deeper layers as the electrode spacing increases.
Data Acquisition
Fieldwork Overview
A total of five VES points were surveyed within the study area to characterize the subsurface resistivity
distribution. The site locations were strategically chosen to sample different geological features, with electrode
spacing (AB/2) tailored to target specific depths (Figure 2):
Figure 2: Location of The Study Area
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The maximum electrode spacing was selected based on preliminary surveys and regional geological
information to ensure comprehensive depth coverage.
Surface Mapping
Surface outcrops were mapped using GPS and measuring tapes, covering an area of approximately 69,284.96
m² (Figure 3). These surface features provided important geological context for interpreting the resistivity data.
Figure3: Showing an outcrop within the study Area
Equipment and Procedure
Data collection was performed using an ABEM Terrameter SAS 300B, which offers high precision and
automation features (Loke, 2013). During measurements, electrodes were systematically moved following the
Schlumberger array protocol—current electrodes (C1 and C2) moved outward symmetrically while potential
electrodes (P1 and P2) remained fixed—allowing exploration at increasing depths (Telford et al., 1990).
Instrumentation
The ABEM SAS 300B Terrameter is equipped with features such as a digital liquid crystal display and
microprocessors for automatic signal averaging, which reduce noise and improve data reliability (Loke, 2013).
Additional modules, such as the SAS 2000 Booster and SAS Log, can be attached to enhance signal strength
and facilitate data recording, respectively.
Figure 4: Showing Data acquisition by the prospecting Team
Data Processing
Data Handling and Analysis
Field data were first corrected for instrument and environmental factors. The corrected data were then
processed using i2iwin software (Loke et al., 2015), which enables inversion modeling of resistivity data to
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generate detailed 2D and 3D subsurface resistivity images. These images facilitated the interpretation of
geological structures and lithological distributions. The Surfer 12 software was employed to visualize the
spatial distribution and resistivity models of the subsurface within the study area.
PRESENTATION OF RESULTS
The plotted sounding curves show apparent resistivity versus half-current electrode spacing on double
logarithmic graphs, aiding in the identification of layered subsurface features (Telford et al., 1990), Figure 5.
Figure 5a: The Modelled Curve of VES 1
Figure 5b: The Modelled Curve of VES 2
Figure 5c: The Modelled Curve of VES 3
Figure 5d: The Modelled Curve of VES 4
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Figure 5f: The Modelled Curve of VES 5
Coordinates and elevation data of the outcrops were contoured to produce spatial distribution maps, enhancing
geological interpretation. The outcrops are massive in clusters. The thicknesses are estimated to be from 4m
(13ft) to greater than 20m (66ft) above ground surface, Figure 6a.
Comparative Analysis and Interpretation of VES Data
The five VES profiles offer key insights into the subsurface lithology, resistivity variations, and depth
distribution across different locations in the study area. The data indicate geological heterogeneity, reflecting
variations in soil and rock types, which are crucial for geological, hydrogeological, and engineering
evaluations (Table 1; Figures 7a, 7b and 7c).
Table 1: The Resistivity, thickness, Depth and Probable lithology of the modelled layers.
VES No
Layers
Resistivity (Ωm)
Thickness (m)
Probable Lithology
1
1
119
0.271
Topsoil + Marl
2
2125
0.693
Sand+ limestone
3
16.3
3.02
Marl
4
1129
5.86
Limestone
5
46089
30.4
Dry Sandstone or Basement
6
4065
19.8
Sandstone/ Fractured Basement
7
54894
Dry Sandstone or Basement
2
1
99.1
0.461
Topsoil + Marl
2
1182
1.63
Limestone
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3
49.5
5.54
Marl
4
684
12.2
Limestone
5
103
45.2
Marl
6
8548
82.2
Sandstone
7
21.5
Marl/ Shale
3
1
150000
0.198
Topsoil + Sandstone
2
502
0.598
Limestone
3
13006
2.03
Dry Sandstone or Basement
4
2155
1.37
Sand+ limestone
5
933
1.8
Limestone
6
12.1
7.82
Marl
7
401
Limestone
4
1
93.5
0.333
Topsoil + Marl
2
2040
0.707
Sand+ limestone
3
136
3.19
Marl
4
58781
13.2
Dry Sandstone or Basement
5
85516
Dry Sandstone or Basement
5
1
278
1.16
Topsoil + Marl
2
32.5
1.16
Marl
3
3263
2.09
Sand+ limestone
4
14964
5.59
Dry Sandstone or Basement
5
290000
Dry Sandstone or Basement
Three profiles were drawn traversing VES 4 and VES 5 in the NE-SW, VES 4 and VES 2 AND VES 4 and
VES 1 as shown in Figures 7a, 7b and 7c. A preliminary quantifications of rock types using thicknesses of the
respective rock units, as illustrated in Figure 8.
Figure 7a: Profile of Geo-Electric section across VES 4 and VES 5 in the NE-SW direction
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Figure 7b: Profile of Geo-Electric section across VES 4 and VES 2 in the NW-SE direction
Figure 7c: Profile of Geo-Electric section across VES 4 and VES 1 in the NE-SW direction
DISCUSSION
The analysis of Vertical Electrical Sounding (VES) profiles across five locations in the study area reveals
significant heterogeneity in subsurface lithology, resistivity distribution, and depth to various formations. Such
variability is typical of sedimentary basins and has important implications for geotechnical engineering,
hydrogeology, and resource management.
Lithological Variability and Geological Implications
Resistivity values in the profiles ranged from as low as 32.5 Ω·m in clay/shale horizons to as high as 290,000
Ω·m in dense sandstone or marl formations. These variations align with previous studies that highlight the
usefulness of resistivity contrasts in delineating lithological boundaries (Smith et al., 2023; Zhang et al., 2024).
Low-resistivity zones typically indicate clay-rich or weathered materials that are permeable and prone to water
retention (Kumar & Patel, 2024). In contrast, high-resistivity values correspond to dense, unweathered rocks
such as sandstone and marl, which are mechanically competent and less prone to deformation.
Depth profiles further reveal that the weathered overburden is relatively shallow, generally between 0.33 and
1.16 m. This agrees with findings by Lee et al. (2024), who observed similar shallow weathering profiles in
sedimentary environments. More resistive layers at depths greater than 17 m represent consolidated formations
that form stable geological horizons (Zhang et al., 2024).
Hydrogeological Significance
The resistivity data also point to potential aquifer zones within the high-resistivity sandstone and marl layers.
These units are commonly associated with high porosity and permeability, making them favorable for
groundwater storage and flow (Ojo & Adeyemi, 2024). Conversely, low-resistivity clay and shale layers at
shallow depths may function as aquitards, restricting recharge and influencing groundwater distribution
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(Zhang et al., 2024). Understanding the spatial extent of these lithologies is therefore critical for sustainable
groundwater development, particularly in regions where groundwater is the main water supply (Kumar &
Patel, 2024).
Engineering and Construction Considerations
From a geotechnical perspective, the dense, high-resistivity sandstone and marl formations represent
competent and stable ground suitable for construction, as also observed by Lee et al. (2024). However, the
presence of shallow weathered zones and clay/shale horizons introduces challenges such as differential
settlement or slope instability. These issues necessitate careful site-specific assessment and, in some cases,
ground improvement measures. The integration of VES data with geotechnical testing has been shown to
enhance the reliability of construction site evaluations (Zhang et al., 2024).
Methodological Advances and Future Directions
Recent developments in geophysical techniques, including multi-electrode resistivity arrays and machine
learning-assisted inversion, have significantly improved the resolution of subsurface models (Zhang et al.,
2024; Kumar & Patel, 2024). Incorporating these methods into future studies will allow for more accurate
characterization of subsurface heterogeneity. Furthermore, combining VES results with borehole logs, remote
sensing, and geotechnical investigations provides a holistic framework for subsurface analysis, as
recommended in recent studies (Ojo & Adeyemi, 2024; Zhang et al., 2024).
CONCLUSION
The VES data across the five locations demonstrate significant lithological variability that has direct
implications for geotechnical, hydrogeological, and engineering applications within the study area. The
presence of shallow weathered zones overlain by deeper, high-resistivity formations suggests suitable zones
for foundation support and groundwater extraction, provided that detailed site investigations are carried out.
The evolving geophysical techniques and integrative approaches documented in recent studies underscore the
importance of comprehensive subsurface evaluation for sustainable development.
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