Geoelectrical Characterization of Subsurface Structures in Dala Hill, Kano, Nigeria: A Comparative Study with Ground Magnetic Data

Exploration of the Earth's subsurface using geophysical methods has become an important tool for discovering its secrets. These minimally invasive methods serve as an effective investigative tool and provide important information about its structure and organization. Environmental and engineering studies, mining and resource exploration, geological mapping, archaeological research, and many other academic fields are among the problems that have been solved by geophysical techniques [1], [2], [3], [4]. The versatility of the methods lies in their ability to measure physical properties at the Earth's surface that are influenced by the internal distribution of these properties within the subsurface. Examples of these properties include density, electrical conductivity, gamma-ray emission, chargeability, magnetism, etc. Geophysical investigations of the earth's interior apply the principles of physics by taking measurements at the surface of the earth that are influenced by the internal distribution of those physical properties. By analyzing these measurements, the physical properties of the earth’s interior can be revealed [5]. Electrical tomography techniques are one of the geophysical methods commonly used in geophysical surveys to acquire large amounts of data quickly and also to provide a high-quality image of subsurface structures [6], [7].

Shehu et al. [8] conducted a ground magnetic study to delineate the subsurface structures of Dala Hill. Two classes of anomalies were discovered, with the shallow anomalies correlated with the anthropogenic activities of ancient settlers and the deep ones associated with internal geologic features. However, all geophysical methods are affected by equivalence and suppression, which is why Shehu et al. [8] recommended the execution of other geophysical studies. Among the methods recommended was ground penetrating radar (GPR) as well as other electrical methods. To provide more complementary information, electrical methods, namely, resistivity, IP, and self-potential, were exploited. The methods have the advantages of controlling the power source, estimating the depth of investigation and determining the lateral extent of influence. In addition, noise can be minimized in the course of data acquisition.

The study area, which is Dala Hill, lies between latitudes 12.008611°N and 12.009722°N and longitudes 8.505833°E and 8.507222°E. It shares the same geology as Kano State, lying on the rocks of the Basement Complex in northwestern Nigeria. The rocks of the Northern Nigerian Basement Complex consist of three groups: migmatites and high-grade gneisses, derived from the main Birri sedimentary rocks through high-grade metamorphism and granitization; migmatites and finally gneisses during the Pan-African orogeny; and the older granite series intruded during the Pan-African orogeny [9], [10].

Dala Hill rises to a height of about 40 meters above the surrounding area and can be climbed via stairs attached to the front of the hill. Although the hill is rough on the sides due to the weather, it is flat on top. The flat top of the hill is covered by coarse-grained lateritic sediments and a few rocky outcrops, as shown in Figure 1. Lateritic sediments form the uppermost part of the laterite cover. Because the particles are so small, they trap water, giving them a very good water storage capacity. After rainfall, water slowly penetrates the ground. According to the study of Minjibir [11], it covers a land mass of 289,892 square meters. Dala Hill is located in the Dala local government area of Kano State, which is one of the eight local government areas within the Kano metropolis (Figure 2).

Generally, the theory of the resistivity method is derived from Ohm’s law, which provides the relationship between the magnitudes of the electric field and current density [12], [13], [14].

where, σ is the conductivity of the medium, J is the current density, and E is the electric field intensity.

However,

where, $\rho$ is the resistivity.

By adopting Ohm’s law, it can be shown that the apparent resistivity is given by Eq. (3).

It can also be written as

where, K (K=$2 \pi r$) is the geometric factor that depends on electrode configuration, and R is the actual resistance of the material.

The IP method is based on the ability of certain geological materials, e.g., metallic minerals in a rock, to take on an electrical charge. After an excitation current pulse is introduced into the ground, the measured voltage shows a slow material-dependent decay instead of instantly returning to zero. Their addition to resistivity investigation improves the resolution of resistivity data analysis and can be used to distinguish geological layers that do not respond well to the resistivity method [15], [16], [17].

The IP method utilizes the capacitance of the subsurface to identify areas containing conductive minerals dispersed within their host rocks [5], [18], [19].

The chargeability M is the parameter most frequently measured, and it is calculated as the area A under the decay curve during a specific time period (t1–t2) divided by the steady state potential difference ∆Vc. This is shown in Figure 3 and can be expressed mathematically.

where, V(t) is the residual voltage at time t, and V_c is the steady voltage during the current flow interval.

The imaging was achieved by laying the multi-core cables along the profile with take-outs which were uniformly spaced at 10m spacing. The take-outs were connected to the grounded electrodes via metallic jumpers, as shown in Figure 4. The cables were connected to the Lund imaging device top panel, which was linked to Terrameter SAS 1000 by a cable. The set-up was powered by a 12V, 60W battery and was continuously charged by a 100W solar panel.

After switching on the terrameter, the “LUND Imaging System” was selected from the start-up menu. The preferred mode (SP, resistivity, or IP) was selected from the Record Manager and a record file was created for the first profile, with the electrode spacing set to 10m. To mitigate the effects of poor ground contact, the grounding electrodes were filled with saltwater. Additionally, the integrity of the cable connections was regularly evaluated to ensure optimal data acquisition. A current of 1000 mA was selected and the current mode was set to automatic. WENNER L was selected as protocol 1 in order to acquire data from deeper sections of the subsurface. The acquisition systems automatically check the electrode contacts and scan through a pre-defined measurement protocol to check the possibility of current flowing through all the electrodes. Measurements were automatically taken by the equipment and stored in the terrameter. The terrameter took about 20 minutes to complete, taking readings along each profile, with chargeability measurements being the most time-consuming.

The measured apparent resistivity/chargeability data files for all the imaging survey lines were downloaded from the ABEM ES464 Terrameter via cable into a computer in which ABEM file conversion (SAS4000 Utilities) and RES2DINV were installed. The SAS4000 Utilities software was used to convert the original data file (in. s4k format) to the appropriate (.DAT format) input file readable by the inversion software RES2DINV. A total of twelve files were converted (three resistivity filesand three IP files). RES2DINV V3.54.44 processed both the resistivity and IP data.

Interpretation of geophysical data entails translating information acquired from geophysical field measurements into geological terms. To obtain such geological results, available and reliable geological controls were necessary to be sourced for a reliable interpretation of geophysical data. According to the study of Tahir et al. [20], such controls were often obtained from borehole data near the study area, as shown in Table 1. In addition, Table 2 and Table 3 show the standard values of measured electrical parameters of rocks. This study also exploits the results of previous works within the area and has a good knowledge of its geology.

Figure 5 shows the resistivity and chargeability cross sections of Profile 1. The profile covers a lateral distance of 280m, from latitude 12.008641°N, longitude 8.506214°E to latitude 12.010795°N, longitude 8.506521°E. The top layer on this profile has a resistivity range of 2142Ωm to 6798Ωm, and the high resistivity could be associated with the intrusion (dyke) observed between 80m and 190m marked A1. This intrusion covered an area of size 40m along the profile and extended deep down beyond 48m. Towards the end of the profile, a very low resistivity layer with a range of 6.64Ωm to 67.8Ωm was identified and this could be interpreted as a clay deposit due to its low resistivity (Table 2). Along the profile, the chargeability observed can be divided into two main ranges: the first range, which consists of 0.506 to 2.43 msec, was observed between 20m and about 100m below 10m to 39.6m depth; the same range was also observed at 110m to 200m trending down to about 27m. The second chargeability range is 4.35 to 8.19 msec, which is observed at 20 to 80m, approximately from 0 to 10m depth. The high resistivity zone labeled A1 associated with the dyke intrusion correlates with the low chargeability zone labeled B1, which has a chargeability range of 0.506-4.35 msec. This profile shows a good possibility of the presence of magnetite by comparing the resistivity/chargeability values with standard values of minerals (Table 2 and Table 3).

Figure 6 shows the resistivity and chargeability cross sections of Profile 2. The profile covers a lateral distance of 360m with a spacing of 10m between the electrodes. It starts at latitude 12.008418°N, longitude 8.505930°E, and ends at latitude 12.010909°N, longitude 8.507331°E. This profile reveals three distinct layers. The top layer has a resistivity range of about 1698Ωm to 3874Ωm between 90m and 250m along the profile. The second and third layers can be seen at 70m and 260m along the profile, with resistivity ranges of 326Ωm to 744Ωm and 12Ωm to 62.7Ωm, respectively. The profile displays two major chargeability ranges. The range, which has a range of 0.911 to 3.58 msec at 40m to 75m and 120m to 300m, has a chargeability that is trending along the profile to a depth of 51.3m. The second range is 6.24 to 11.6 msec and it occurs below 5m depth at 80 to 130m and at 170m to 210m. The high resistivity zone, which occurs between 80 and 200m, is marked A3 along the profile and correlates with the low chargeability zone, marked B3. Thus, the observed values at this location fall within the range typically seen for magnetite (Table 2 and Table 3), suggesting its possible presence.

Figure 7 shows the resistivity and chargeability cross sections of Profile 3. The profile covers a lateral distance of 260m with a spacing of 10m between the electrodes. It starts at latitude 12.008153°N, longitude 8.507970°E, and ends at latitude 12.010082°N, longitude 8.506935°E. The profile reveals three layers. The first layer, which was identified between 70m and 240m along the profile and at a depth of about 12m to 20m, has a high resistivity range of 651Ωm to 1340Ωm. Thus, the high resistivity is associated with the lateritic nature of the hill. Below the top layer is the second layer, which has a resistivity range of 150Ωm to 319Ωm and is extended to a depth of 48m. Thus, it was interpreted as a weathered layer containing a sandy clay deposit. The third layer has a low resistivity of less than 100Ωm, as depicted on the profile around 40m. This could be interpreted as clay. A low chargeability zone having a range of -0.201 to 3.04 msec was observed at 190 to 220m along the profile at a depth of 10m; also, the same range was observed at 140m to 240m from a depth of about 30m to 39.6m. At 70m to 140m, a chargeability range of 6.27 to 19.2msec was observed. The high chargeability value observed at 290 to 315m labeled B1 correlates with the low resistivity zone of the third layer (ranging from 4.93 to 54.1Ωm) and this high chargeability indicates the possibility of pyrite in the region as the observed values closely agree with the standard values listed in Table 2 and Table 3.

The inverse sections of the resistivity profiles reveal three layers, with the highest resistivity being the top layer (mainly laterite). The second one contains a weathered basement, and the third one is mainly a highly weathered layer. The IP models reveal the areas of high/low chargeability. The iron ore was identified based on the chargeability model as having zones of low chargeability (0.1-5 msec) for magnetite and hematite as their chargeability values fall within this range (Table 3). Chargeability values of 3-5 msec could indicate an average grade of 10%-20% of iron, while 0.1-3 msec could indicate an average grade of 20%-40% of iron ore [22]. On the basis of this observation, the iron ore within the study area could be inferred to have an average grade of 20%-40%.

Considering Profile 1, which was taken from the northwestern part of the study area, an intrusion can be observed at the top layer and extends into the weathered layer, covering a total depth range of 2-48 m. This intrusion is characterized by a high resistivity of iron ore, and seems to contain possible iron ore as it shows a good correlation with low chargeability inversion. Additionally, the presence of this intrusion aligns with findings from the ground magnetic survey conducted by Shehu et al. [8], further supporting the potential for iron ore mineralization in this area.

However, as this survey identified potential iron ore deposits successfully, extensive exploration activities may face hindrance due to nearby residential areas. The utilization of the acquired data for knowledge gathering and resource confirmation is crucial, rather than immediate large-scale exploration, as underscored in this context. This study provides a valuable opportunity to enhance the comprehension of historical human activities in the area. The presence of iron ore-associated minerals, along with low chargeability anomalies, may indicate past utilization of the area by ancient societies for iron ore extraction. Therefore, the geophysical techniques utilized serve as effective and non-destructive investigative tools, especially in constrained environments. This study showcases the applicability of such techniques in confirming the existence of iron-ore-related minerals, potentially offering insights into the resource procurement methods of ancient civilizations.

Geoelectrical surveys conducted on Dala Hill have utilized resistivity and IP techniques to identify positive correlations between high resistivity and low chargeability zones across three profiles. These findings, in conjunction with the identification of shallow subsurface structures, indicate the possible existence of iron-ore-rich zones that may contain magnetite. Furthermore, a shallow intrusion displaying resistivity characteristics consistent with iron ore has been detected, aligning with previous magnetic studies and providing further evidence for the potential presence of iron ore mineralization.

However, the extensive exploration of these areas may be hindered by the presence of nearby residential areas. Despite this limitation, this study underscores the significance of utilizing the acquired geophysical data for confirming the availability of resources and gathering knowledge. Moreover, it has the potential to shed light on past human activities associated with iron ore extraction in the region. The non-destructive geophysical techniques employed in this study have proven to be effective in restricted environments and can serve as valuable tools for confirming the presence of iron-ore-related minerals. Additionally, they offer the possibility of uncovering insights into the resource acquisition practices of ancient civilizations.