Delineation of structural lineaments of Southeast Nigeria using high resolution aeromagnetic data

1 Introduction

Many geoscience applications require knowledge of the lateral boundaries of subsurface features in rift environments like the Benue Trough, Chad Rift, Baikak Rift, East African Rift, Rio Grande Rift, Rhine Graben, the Dead Sea transform fault, etc., [1], because the morphologies of the structures [2], rather than their depths, are reflective of the tectonic condition and the history of such locations [3–7]. Potential field data as well as other geophysical data can be used to determine the subsurface geologic formations [8–12]. The magnetic data contains anomalies with a wide dynamic amplitude range that is dependent on the source’s geometry, depth, and magnetization [6]. The edge enhancing filters aim to highlight structures of the magnetic field, making it easier to recognize geologic structures in magnetic anomaly data [13–17].

To highlight the source edges, there are numerous edge detectors based on magnetic data derivatives [18]. Some edge detection methods such as total gradient [19], gradient amplitude [20], and enhanced total gradient [21] are frequently applied to extract source boundaries, even though their results are predominated by shallow structure signals [22–24]. Several methods have been created to balance the diverse signals, including the tilt derivative [25], the gradient amplitude of the tilt derivative [26], theta [27], and normalized horizontal derivative [28]. Nevertheless, supplementary boundaries in the edge maps are often created by these filters [15]. The tilt derivative of the gradient amplitude [29], the normalized total gradient [30], the enhanced theta method [31], the improved tilt derivative [32], and the total directional theta technique have all been proposed as solutions to this problem [33].

Recently, a enhanced version of the horizontal gradient amplitude filter (EHGA) has been introduced to delineate the source horizontal boundaries [34]. This filter brings clearer results compared to the traditional filters such as the total gradient, tilt derivative, gradient amplitude of the tilt derivative theta, normalized horizontal derivative, and tilt derivative of the gradient amplitude [34]. In the rift environments like the Nigerian Benue Trough, edge-enhancing filters have proven to be quite effective in retrieving the lateral limits of subsurface structures [2,35].

The Southern Benue Trough, which is a part of the failed rift located in Southeast Nigeria [36], is a vital location that has witnessed post-depositional Santonian tectonism. The degree of expression and development of geological structures is controlled by tectono-magmatic activity [37]. Fractures, fissures, and dike swarms created by tectonic processes are common in the Benue Trough’s Cretaceous deposits. With abundant mineral resources and the recent discovery of commercial hydrocarbon in the Anambra basin, the structural framework and tectonic setting of the Benue Trough have attracted more interest from geoscientists. High-resolution airborne magnetic data are now widely used in mineral exploration projects [1,38,39]. Dentith et al. [40] found that the magnetic method is remarkably useful as a mapping tool whenever magnetism is comparably strong and when there are clear differences among the various rocks. The objective of this study involves the appraisal of the efficacy of advanced filtering techniques like enhanced horizontal gradient amplitude (EHGA) involving a 3D model and tilt depth method (TDM). The EHGA is a modern high-resolution detection filter for the delineation of source edges from magnetic and gravity data [34]. The efficacy of its application is evaluated through both real and synthetic magnetic data of the study site. Furthermore, the obtained results are expected to help map structural lineaments of the research area which will offer a better understanding of the advanced processing filters, and improve the knowledge of the geological structures and sediment thickness of Southeast Nigeria.

2 Geologic setting of the study area

The research site is located (Figure 1) in the Southeastern region of Nigeria, between latitude 5°00′N to 6°00′N, and longitude 7°30′E to 8°30′E. The region encompasses two geological provinces, including the Precambrian Oban Massif (OM) and a portion of the Cretaceous Southern Benue Trough (SBT). The OM is located at the eastern end of the study area and is surrounded to the north by the Ikom-Mamfe Rift, to the south by the Calabar Flank, to the west by the Afikpo rift, and the east by the Cameroon Volcanic Line. The SBT encompasses sub-basins like the Afikpo and Ikom-Mamfe rifts as well as the Calabar Flank.

Figure 1

Map of Nigeria showing the location of the study area (after Abraham et al., [61]).

The regional geologic framework of the study area is related to the West African Rift System, which includes multiple fault-bounded groupings of basins (such as the Benue Trough) and subordinate basins. The Benue Trough (Figure 2) is a rift system that has been classified into the Northern Benue Trough, Central Benue Trough, and Southern Benue Trough [41]. The primary rift basins in the Southern Benue Trough include the Abakaliki, Anambra, and Afikpo rift basins as well as the Calabar Flank. Past researchers have paid close attention to the geologic history and stratigraphic succession of these rift basins. According to previous researches [42,43], the rift basins occurred during the late Jurassic–early Cretaceous tectonic periods that marked the split of the South American and African plates and subsequent heavy sediment deposition, and magmatic intrusion [44].

Figure 2

Abridged map of Nigeria showing the Benue Trough and location of the study area.

Hoque and Nwajide [45] reported that the Albian depositional environment was marine, witnessed the transgressive deposition of arkosic sands, laminated shales, and limestones, followed by igneous intrusive and volcanics, all known as the Asu River Group (ARG). During the Turonian period, the region witnessed another incursion, which resulted in the deposition of the Eze-Aku Group of sediments. This group of sediments is dominated by marine shales intercalated with sandstone and siltstone strata. The Eze-Aku Group is overlaid by the Coniancian Awgu Group. The Awgu Group, which consists of limestone, shales, and fine sands, is a deposit of the renewed regression that occurred as a result of an intensive tectonic event during the Santonian period [46]. The Santonian tectonic episode was marked by widespread faulting, fracturing, bending, and magmatism, which shifted sediments into the nearby Anambra and Afikpo rift basins.

The OM, which consists of Neo-Proterozoic crystalline basement complex rocks, is located in the eastern section of the research region (Figure 3). Porphyritic granites, migmatite gneiss, and banded gneiss dominate the basement rocks, with thin dykes of mafic intrusive compositions [47]. The western section of the research region is dominated by younger Campanian-Paleogene deposits. These sediments are from the basins of Afikpo and Anambra. These basins were formed as a result of the considerable uplift that typified the Santonian period, which was caused by a broad compressional event [1]. The following deposits, in order of age, are the Nsukka Formation, Ajali Formation, Mamu Formation, Enugu Shale, and Nkporo Group [48] (Figure 3).

Figure 3

Geological map of the study area.

The structural and stratigraphic origins of the Afikpo basin (Figure 3) have been thoroughly described in the past [49,50]. After rifting halted, gravity dominated the depositional and deformational processes in the mid-Eocene-Pliocene, resulting in the creation of the hydrocarbon-rich Niger Delta basin sediments. This basin’s sediments are composed of layers up to 20 km thick of upward coarsening regressive association of Tertiary clastics that are significantly diachronous (Figure 4) [51]. The Niger Delta Basin sediments include the Benin Formation, Agbada Formation, Akata Formation, Ogwashi-Asaba Formation, Ameki Group, and Imo Group.

Figure 4

Stratigraphic setting of the Cretaceous-Paleogene succession in Southeast Nigeria.

3 Methods

The EHGA is given by ref. [34]

where the horizontal gradient amplitude HG is given by [20]

and p is a constant greater or equal to 2 [34]. In this study, p = 3 was used to create a simulated magnetic model to examine the sharpness of the EHGA detector. The model includes three prismatic bodies (Figure 5), whose parameters are presented in Table 1. The magnetic anomaly related to the model is shown in Figure 5b. Figure 5c shows the EHGA of data. We can see that the peaks of the EHGA are placed instantly over the source boundaries. It was observed that the EHGA method can produce sharp and clear edges for bodies A, B, and C.

Figure 5

(a) Three-dimensional view of the model, (b) its magnetic anomaly, and (c) the EHGA of the model. The dashed lines show the true edges of the prismatic sources.

A commonly applied improvement method for the potential field (PF) data F is the tilt-angle (T) [25], which normalizes the amplitude of the vertical-derivative of the field utilizing its horizontal derivatives.

When the mathematical formulations for the vertical and horizontal gradients of the magnetic field over a vertical contact were entered into equation (3), this equation is simplified to ref. [52]:

where Δ z and Δ x are the vertical and horizontal distances from the prevailing estimation point to the center of the top of the boundary. The TDM suggested estimating the distance within the ±45° values of T, which was twice the depth Δ z [52].

4 Results

Figure 6 shows aeromagnetic data of Southeast Nigeria. Here, a depth approximation approach utilizing tilt depth solutions was used to quantitatively describe the sedimentary pile of the investigated region (Figure 7) [53]. This approach, like source parameter imaging, is an improved procedure for determining the depths of shallow and deep magnetic entities [37]. This approach is appropriate for delineating single and multiple magnetic source geometries such as an isolated pole, dipole, dyke (prism), lines of poles, vertical contact, and susceptibility disparity [54]. Figure 7 shows the depths calculated from total magnetic intensity data. The gridded tilt depth solution displays a variety of colors that represent varying depths to magnetic bodies found in distinct locations in the subsurface of the study area.

Figure 6

Aeromagnetic data of Southeast Nigeria.

Figure 7

Tilt depth solutions.

Depths to shallow (pink-yellow) and deep (lemon green-blue) magnetic sources vary from ∼500 to ∼2,500 m and ∼3,000 to ∼5,000 m, respectively, as shown in Figure 7. The highest depth obtained using tilt depth solutions is ∼5,000 m, which is within the depth values recorded in the area by previous investigations [8,37,55–57]. As a consequence (Figure 7), the eastern flank, which is distinguished by thin sedimentation and strong magnetization [55], corresponds to the Precambrian basement. Isolated thin sedimentary bodies observed in the SBT area were caused by post-depositional intrusions associated with the Santonian Abakaliki Anticlinorium [55,58] and persisting minor tectonic events associated with the Quaternary-Recent Cameroon Volcanic Line [55,59].

The EHGA filter was also applied to aeromagnetic data to extract the lineaments. The EHGA (Figure 8) map reveals a substantial occurrence of shorter local subsurface structural lineaments with limited lateral extents that ranged from ∼0.29 to ∼1.48 km that are primarily constrained to the same geological location. However, a lesser number of regional linear structures (∼2.8 to ∼24.5 km) that span several geological provinces are found largely on the western flank of the study area. These structural entities, found in the underlying basement complex rocks and the overlaying sedimentary layer, have major NE–SW, NNE–SSW, and N–S structural orientations, as well as minor E–W and NW–SE trend directions (Figures 9 and 10).

Figure 8

EHGA map of the study area.

Figure 9

Lineament map of the study area.

Figure 10

Rose diagram showing dominant and minor trend directions of subsurface lineaments.

The NNE–SSW, and NE–SW bodies are the continental extensions of the pre-Cretaceous Chain and Charcot fracture zones oriented parallel to the basin axis [60].

The construction of the N–S and NW–SE linear bodies, on the other hand, might be attributed to Santonian crustal tectonism, which distorted the overlying sedimentary rocks and resulted in the generation of linear features that were oriented in the same direction as the regional basement rocks [61]. These lineament bodies appear as fractures and faults, and are densely and tightly dispersed in the studied region of Odukpani, Akamkpa, Ugep, Essabang, Ediba, Apiapum, and Afikpo (Figures 9 and 10). A majority of these linear features or faults occur at deeper subsurface intervals. The presence of many structural lineaments within these areas suggests the presence of active basins that form linear structures that might trap minerals related hydrothermal fluids in a rift environment like the Benue Trough.

5 Discussion

The EHGA proposed by Pham et al. [34] was used to simulate a magnetic model to evaluate the sharpness of this filter. The magnetic anomaly related to the model of prismatic bodies with characteristic parameters given in Table 1 is shown in Figure 5. The projected magnetic anomaly related to the model is shown in Figure 5b. The enhanced EHGA which places peaks over source boundaries generated sharp and clear edges for bodies A, B, and C (Figure 5a). This filter has been observed to have outperformed several edge detection filters [5,34].

The TDM (Salem et al. [52]) considered as an improved depth determination technique revealed a maximum depth of ∼5,000 m. This value correlates relatively well with results obtained by previous workers that engaged methods like source parameter imaging, standard Euler deconvolution, spectral depth analysis, and 2D forward modeling [37,55–57].

Potential field investigations involving magnetic datasets and enhanced source edge detectors have been carried out by numerous researchers [5,18–28]. Mapping of geologic structures using enhanced edge detector filters in rift environments and Precambrian basement is for the exploration of polymetallic-magmatic hydrothermal deposits [2,8]. Hydrothermal alterations related to tectonic events are usually associated with base metal, massive sulfide, shear-hosted gold, and some other deposits [62,67]. The structural map shows an extensive existence of shorter local subsurface geologic structures that ranged from ∼0.29 to ∼1.48 km (Figure 9). Furthermore, a few regional linear structures (∼2.8 to ∼24.5 km) that cut across some geologic regions are observed more at the western flank of the study area. The rose petals in the NE–SW, NNE–SSW and N–S, and E–W and NW–SE trend directions represent the major and minor strike directions respectively (Figure 10). Generally, the investigated area is dominated by hydrothermal alterations [1,46], Santonian and Quaternary-Recent igneous intrusions [2], brine fields, chain, and Charcot fault zones [63], and related geologic lineaments from thermo-tectonic perturbations [59]. Some of the main geologic structures are believed to have perhaps extended to the upper mantle [8,64]. On the whole, previous studies have shown that structural control and hydrothermal alterations are largely connected with mineralization [62,65,66].

6 Conclusion

We have applied the enhanced techniques such as the enhanced horizontal gradient amplitude EHGA and TDM to high-resolution airborne magnetic data to extract lineaments of Southeast Nigeria. The EHGA detector generated sharp and clear edges for magnetic sources. The TDM which is an advanced depth determination technique revealed thin and thick sedimentations that vary from ∼500 to ∼2,500 m and ∼3,000 to ∼5,000 m, respectively. Geologic structures of the investigated area were extracted from tilt depth and EHGA maps. The lineaments map shows a widespread occurrence of short-ranged (∼0.29 to ∼1.48 km) linear structures. Furthermore, they existed more regional linear structures (∼2.8 to ∼24.5 km) at the western flank of the study area. Most of these structures are trending in the NE–SW, NNE–SSW, N–S, E–W, and NW–SE directions. Thermo-tectonic events connected Santonian Abakaliki Anticlinorium, Tertiary-Recent Ikom-Mamfe Rift, and structural deformations of the basement rocks associated with the OM. These structures serve as a pathway and a host for hydrothermal fluid and base metals.