Unveiling the subsurface geological structure of the centre region, cameroon, with aeromagnetic data analysis

Abstract

This study made use of aeromagnetic data analysis to identify potential geological structures in the magnetic field and map subsurface geological structure, which could serve as potential resources in the Centre Region of Cameroon. The satellite aeromagnetic dataset used was acquired from the Earth Magnetic Anomaly Grid 2 (EMAG2). The application of Reduction to Magnetic Equator (RTE) filter was to remove asymmetries in the data and position the peaks of magnetic anomalies directly over their sources. Low magnetic zones indicated by blue colours from the total magnetic fields possibly reveal the sedimentary intrusions into the area, moderate magnetic regions having green, yellow and red colourations suggest metamorphic rocks and the Northeastern portion of the maps indicated with pink colours correlate to the basement rocks in the study area. Derivative filters and Euler deconvolution technique were applied as enhancement techniques to accentuate the inferred linear subsurface structures interpreted as contacts, fractures, lineaments, as well as their depths using a structural index of N = 0. These structures served as predicted mineral entrapments or contact/fault lines whose NE-SW orientation conforms to the Pan African Orogeny with depths of around 2.0 km.

Introduction

Understanding the subsurface geological structure is fundamental to understanding the tectonic evolution, mineral potential, and geodynamic framework of a region. In the Centre Region of Cameroon, which forms part of the broader Central African Orogenic Belt (CAOB), subsurface geological mapping remains challenged by limited surface exposure and complex tectonic units. This area is geologically significant due to its position within the Neoproterozoic Pan-African mobile Belt, where crustal reworking, shear zone dynamics, and magmatic intrusions have played a pivotal role in shaping its present architecture^1–3. Despite previous geological and structural studies, much of the region’s deep crustal features remain unresolved, necessitating the application of geophysical methods capable of penetrating beneath surface cover and revealing buried structures. This study therefore fills this gap by meticulously analyzing high-resolution aeromagnetic data to:

• Unravel the subsurface geological structures and rock types through detailed anomaly analysis.

• Identify and characterize lineaments, acting as signs of hidden structures, using advanced processing techniques.

• Estimate the depths of these structures to gain valuable insights into the region’s subsurface architecture.

Aeromagnetic data analysis has emerged as a powerful and indispensable geophysical tool for delineating subsurface geological features in regions with limited outcrop.

Reference⁴ used aeromagnetic data to delineate geological structures and estimate the depths of the structural contacts of Southeastern Cameroon. The results revealed several faults, with principal lineaments in the ESE-WNW directions and E-W for the minor lineaments. The results also suggested an open synclinal transposed on vertical foliations: with the major fault at the DJADOM axis found to be quasi-parallel to the Northern limit of the Congo Craton (CC) and parallel to the Sanaga Fault (SF) and the Central Cameroon Shear Zone (CCSZ). These features showed a base strongly affected by tectonic events, which characterizes the transition between the zone from the CC and the belt from folds of the Pan-African. The high susceptibility contrasts, was indication of strong magnetization (inferred magnetite, dolerite and ochre schist quartzite) and weakly magnetized anomalies would be due to the migmatites of the base complex series. In addition, other studies using aeromagnetic data around the study area revealed structural features trending in the E-W, ENE-WSW and NE-SW directions with the E-W trend more strongly developed than the other identified trends. The depth of the basement rocks ranged between 150 and 3000 m as the average range⁵. Other authors, within the study area used a combination of Tilt-Angle and Euler Deconvolution to determine structural features from aeromagnetic data over Centre-East, Cameroon. Their results showed depths of around for deep and for shallow features at the northern boundary of the Congo Craton and southern boundary of the Pan-African Belt with structural directions in WSW-ENE. The lineaments indicated the circulation of minerals⁶. However, studies show that the identification of lineaments in a study area can serve as potential sources for mineral depositions since mineral exploration is very essential for economic growth^7,8).

Aeromagnetic technique also exploits spatial variations in the Earth’s magnetic field, which arise from differences in the magnetic susceptibility and remanent magnetization of underlying rocks^9–11). The method is particularly useful in crystalline and metamorphic terrains where magnetic mineral content is often sufficient to produce distinct anomalies. This technique can be used to identify complex geological structures, including faults and fracture zones¹², that are not very visible on the surface but play a key role in the distribution of mineralization¹³. By applying aeromagnetic techniques, geoscientists can detect structural elements such as faults, shear zones, lithological contacts and intrusions which are features that are critical for reconstructing tectonic histories and identifying mineralized zones^8,14.

In this study, a suite of advanced processing and interpretation techniques was applied, including reduction-to-the-Equator (RTE), vertical and horizontal derivatives, analytic signal, tilt derivative, and Euler deconvolution. RTE transforms magnetic data to a form where anomalies are positioned directly over their sources, enhancing interpretability¹⁵. Vertical and horizontal derivatives emphasize shallow and edge-like features¹⁶, while the analytic signal highlights the total gradient of magnetic intensity, offering insights into both shallow and deeper sources irrespective of magnetization direction¹⁷. Tilt derivative enhances subtle linear features¹⁸, and Euler deconvolution provides depth estimates and structural indices for anomaly sources^19,20).

This study not only contributes to the structural mapping of the Centre Region but also provides valuable geophysical constraints for tectonic models, support further geophysical and mineral exploration initiatives as well as geothermal assessment in Cameroon. Through this approach, we aim to refine the geological understanding of the area and contribute to ongoing efforts in subsurface resource evaluation and regional geodynamic studies. Thus, the results will enable the identification of linear magnetic anomalies, fault zones, lithological contacts, and deep-seated basement features.

Furthermore, this study equally demonstrates a systematic approach to identifying deep-seated fault systems and lithological boundaries. The novelty of this approach lies in the integrated use of satellite aeromagnetic data and structural analysis techniques to interpret tectonic patterns and identify potential resource-rich areas, providing a cost-efficient and flexible method for regional geophysical investigations.

Geologic and tectonic settings

The Centre region lies between latitude 4° 45ʹ N and longitude 12° E (Fig. 1a). The Centre Region of Cameroon is characterized by diverse geological formations, including the Precambrian metamorphic rocks, including gneisses, migmatites, and schists²¹. The region is part of the Central African Rift System, which has significant implications for tectonic activity and mineral deposition. These rocks form part of the Yaoundé lithological domain, a key player in the Central African Fold Belt (CAFB)^2,22–25. Again, These rocks were derived from ancient granitized and metamorphosed sediments affected by the Pan-African orogeny (600 and 500 Ma)²⁶. This domain comprises thrust slices of metasedimentary rocks, estimated to be around 626 million years old²⁵. Additionally, the region incorporates the Yaoundé Belt (dating back 700–1000 million years)²⁴ and the Ntem Complex (Fig. 1b), boasting an even more ancient lineage of 3.1–2.5 billion years which provides a relevant geologic and tectonic setting of the Centre Region^1,2,24,27. The southern part of the study area forms part of the Nyong series of the Congo Craton (CC) (Fig. 1b), which consist of high-grade gneissic rocks, including biotite-hornblende gneiss, charnockite, garnet-amphibole-pyroxenite Banded Iron Formation (BIF)²². The study area also has numerous shear zones, including the Central Cameroon Shear Zone (CCSZ) and the Sanaga Fault (SF) (Fig. 1c)^26,27. While localized studies of the Yaoundé Belt have shed light on specific parts^5,28, a comprehensive investigation using aeromagnetic data across the entire region has remained elusive. The Centre Region of Cameroon therefore holds significant potential for mineral exploration, particularly in the search for metallic ores, petroleum, and groundwater. Therefore, an in-depth understanding of the geological variations beneath the surface could provide valuable insights for resource management and exploration. Additionally, the region’s geological structure may have important implications for its seismic activity and geo-hazard assessments, further underscoring the need for detailed geophysical surveys.

Fig. 1.

(a) Location map of study area created using QGIS version 3.34 “Prizren”²⁹. (b) Geological sketch map of Southern Cameroon showing its main lithological units (extracted from³⁰). (c) Geological map of study area showing localities (modified from³¹, licensed under CC BY 4.0 https://creativecommons.org/licenses/by/4.0/).

Methods

Aeromagnetic data acquisition and processing

The global EMAG2 grid, meticulously compiled from a multitude of sources including satellite, ship, and airborne measurements, served as the foundation for this study³². Geomagnetic reference fields obtained from IGRF-15 were removed to ensure accuracy³³. Data processing was carried out using Geosoft Oasis Montaj 8.4, involving a series of crucial steps:

Regional-residual separation

Typically, geophysical survey; magnetic data is the total of all the magnetic fields generated by all the subsurface sources^34,35. So, the final map has two major disturbances, ranked differently in terms of magnitude and often superimposed on top of each other. The major traits often manifest as long-lasting patterns, also called regional trends that stretch over miles. Subsidiary in size but paramount in significance are the smaller (local disturbances) that are superimposed on the regional field and are often concealed by them. These disturbances are indicative of the presence of reservoir-type structures or mineral ore deposits and they are referred to as residual anomalies.

• Regional Trend Removal and Residual Maps: Large-scale regional trends were eliminated and the residual magnetic field of the study area is produced by subtracting the regional field from the total magnetic field (TMI) using the Polynomial fitting method³⁶.

• Data Gridding: Minimum curvature gridding method was done which interpolates TMI anomaly values of the database to a square grid for accurate spatial representation³⁷.

• Projection: The data was projected into the Universal Transverse Mercator (UTM) zone 32 N for consistent analysis and integration with other geospatial data.

Edge detection and lineament analysis

These methods were aimed at estimating the boundaries of magnetic source bodies as well as to establish a structural map of the study area. To unravel the hidden secrets within the aeromagnetic data, a combination of tilt angle derivative, horizontal gradient magnitude, first vertical derivative and analytic signal techniques were employed:

Edge enhancement

Techniques like TDR and HGM were employed to sharpen anomaly edges and facilitate interpretation^18,38.

Tilt angle derivative (TDR)

This map is used to study the area’s underlying geology formations and characteristics (geologic structures/features)³⁹. As described by¹⁸, the calculated tilt angle of total field gradient, enhances visualization of structural features. This technique also involves finding places/discontinuities where the contour value is zero. The equation governing this technique is given as

1

.

where M is the magnetic field,, and are the derivatives of the magnetic field in the x, y and z directions respectively, FVD is the first vertical derivative. One way to calculate the tilt derivative (TDR) is to use the fast Fourier transform (FFT) technique to convert the values of the residual magnetic field into the frequency domain. After that, a TDR filter equation is multiplied by the converted data (1). The TDR values are acquired by transforming the product back to the space domain³⁹.

Horizontal gradient magnitude (HGM)

Similar to TDR, HGM focuses on sharp anomalies and potential contacts by computing the magnitude of the horizontal gradient of the total magnetic field^40,41. This technique also involves finding places/discontinuities where the local maxima in the HGM are aligned and the magnetic source body edges can be interpreted at these locations.

If M is the magnetic field, then the horizontal gradient magnitude (HGM) is given by

2

.

Analytic signal (AS)

This technique, introduced by⁴² highlights the contrast of magnetic susceptibility of the underlying rocks⁴³ and refines the resolution of anomalies by combining the vertical and horizontal gradients, facilitating the detection of noticeable edges and structural discontinuities⁴¹. In 2D situations, the amplitude typically does not rely on the direction of magnetization⁴⁴, and in 3D bodies, it is almost always independent³⁹.

In fact, the Analytic Signal anomaly over a 2D magnetic contact located at say at a depth, h, is written as

3

.

Where the term

4

.

is a constant which represents the amplitude factor, h is the depth to the top of the contact, M is the magnetization strength, D is the dip of the contact, I is the inclination of the magnetization vector, and A is the direction of the magnetization vector.

The equation used to compute the 3D analytical signal amplitude (ASA) is given by the equation³⁹.

5

.

First vertical derivative (FVD)

This technique helps to sharpen and enhance shallow magnetic anomalies³⁹. Equation (6) represents the first vertical derivative filter expressed as

6

.

A fast Fourier transformation (FFT) method is used to convert the residual field values from space domain into frequency domain in order to calculate the FVD values. The Fourier converted data are then multiplied by FVD filter, ^45,46, where denotes the frequency domain wave number and n is the filter’s order (being one for FVD)⁴⁶. Returning to the space domain, the FVD values are extracted from the final result.

3D Euler Deconvolution

This powerful method, developed by^19,20, estimates the depths and orientations of structures by analyzing the magnetic field anomalies in the frequency domain. By assigning structural indices (N = 0 for contacts, N = 1 for faults, N = 2 for dykes), it deciphers the hidden language of the anomalies, revealing valuable insights into the subsurface architecture⁴⁷.

The 3D form of Euler’s equation is given by

7

.

Where is the magnetic field’s regional value, ) is the location of the magnetic source that produces the total magnetic field M measured at and N is referred to as the structural index which helps to characterize the source.

Centre for exploration targeting (CET) analysis

The CET analysis has tools, which are most often applied on the FVD and TDR data. For the purpose of detecting different magnetic structures, these tools also include structural complexity, lineation vectorization, texture analysis, and lineation detection^48,49. In order to locate anomalies relative to their source bodies, texture analysis estimates the local variance in the reduced to the equator data at each point in the region⁵⁰. In order to improve the visibility of linear features, line detection makes use of phase-based methods that provide contrast-variant magnetic feature identification. This allows for the detection of any zones of discontinuity that resemble laterally continuous lines, including lineaments along ridges and edges. A linear feature’s discontinuities, or edges, may be located using phase congruency. Skeletonization is another morphological process that reduces objects to a single cell size by iteratively eroding their border cells from binary (black and white) set of analysed data⁴⁸.

Results and discussion

Magnetic anomalies and source characterization

Total magnetic intensity (TMI) map

The total magnetic intensity (TMI) map unveiled a captivating canvas of magnetic anomalies; both positive and negative (Fig. 2). These anomalies vary in intensity, shape, and size, hinting at variations in rock susceptibility and underlying lithology of the study area. This variation in the magnetic intensity values in the area ranges from a minimum value of − 166.260 nT to a maximum value of 57.862 nT. Areas having high magnetic intensity values are dominant in the N, NE and Centre of the study area. Meanwhile, those with low values are dominant in the S and SE of the study area. The southern part of the study area is characterized by low magnetic intensity values (Fig. 2). Whereas, the northern part is characterized by high magnetic intensity. In between the two sections are areas characterized by medium magnetic intensity values. These high magnetic intensity values, which dominate the northern part, are probably caused by near surface igneous rocks of high magnetic susceptibility values.

Fig. 2.

Total magnetic ıntensity (TMI) map.

Total magnetic intensity reduced to the equator (TMI-RTE) map

In order to improve the interpretation of the aeromagnetic data, the TMI was reduced to the equator using the MAGMAP module of the Geosoft Oasis Montaj version 6.4.2, obtained from the IGRF on the date of 07/08/2009 obtaining the values of the geomagnetic (I) inclination and declination (D) which were and respectively. This places the anomalies directly over their causative source bodies thereby making the interpretation easier (Fig. 3).

Fig. 3.

TMI-RTE map of study area.

Residual anomaly map

However, the residual map obtained (Fig. 4) which gives the undistorted picture of the anomalies in the study area was used for further analysis and interpretation. Figure 4 shows that the residual magnetic field values range from − 45.131 nT to 74.560 nT. The magnetic strength of the crustal rocks in the region is likely represented by these changes⁵¹. In the NE and SE sections of the area, the positive values shown by red and pink are predominant, whereas in the far SW, they are less so. In the far north, west, and SE, as well as in the eastern section immediately above Minta, we have low values shown by green, blue, and yellow are most predominant. It is possible that the presence of low magnetic zones suggests the presence of metamorphic rocks like schists found in studies carried out by²¹. Regions with high magnetic (values) could suggest the occurrence of mafic, felsic intrusions, metasedimentary units that intruded the country rocks in the region such as dyke-like bodies (intrusions)²¹. The SW and extreme SE, western and central area have low susceptibility values. The sedimentary region of the study area is probably mapped out by the portion with the predominant low magnetic intensity, whereas the basement rock is mapped out by the part with the strongest magnetic intensity found in the southern, NE and SE regions.

Fig. 4.

Residual magnetic map.

First vertical derivative map

This map (Fig. 5) shows values ranging between − 0.0037 nT/m to 0.0036 nT/m with positive anomalies showing trends from NE, SE directions.

Fig. 5.

First vertical derivative (FVD) map.

Analytic signal (AS) map

Figure 6 shows the analytical signal map, obtained from the residual magnetic anomaly map, reveals structural variation ranging 0.0004 nT/m to 0.0057 nT/m. Areas with high amplitude might indicate geologic structures that have a high magnetic susceptibility contrast. These structures could be mineralized zones that contain metallic minerals like iron ore in the study area. The southern part of the region is characterized by high amplitudes in the W-E and SE while in the eastern region they are predominant in the NE direction. Inferred as non-metallic mineral zones, low-amplitude regions such as the western and far northeastern parts of the study area, as well as locations like Obala and the area that goes down to Otélé, indicate geologic formations with little magnetic susceptibility contrast. The research area’s geology and FVD are mapped out by these amplitude areas (Fig. 5).

Fig. 6.

Analytic signal (AS) map.

Tilt derivative (TDR) map

The tilt-derivative map obtained from the residual map ranges from − 1.4225 radians to 1.3265 radians as seen in (Fig. 7). The red and pink colours on the map indicate that the rocks (geologic formations) have a high magnetic susceptibility, while the green and blue colours show the low magnetic susceptibility in rocks⁵¹. The northern, NE, eastern, and SE regions of the study area show the highest concentrations of magnetic intensity anomalies. Low magnetic values in the SW, where strong anomalies are distributed, may indicate the presence edge bodies and their alternative shapes. These anomalies align with the ones seen in FVD and AS.

Fig. 7.

Tilt derivative (TDR) map.

Horizontal gradient magnitude (HGM) map

The Horizontal Gradient Magnitude (HGM) map ranges from 0.0001 nT/m to 0.0040 nT/m as seen in (Fig. 8). This map shows local maxima or peaks of contacts/faults. High anomalies are predominant in the NE, eastern, southern and SE part of the study area, while low anomalies are predominant in the SW of the area.

Fig. 8.

Horizontal gradient magnitude (HGM) map.

Generally, TDR, HGM, AS, and FVD show the variation in the subsurface geologic structures of the study area. The observed anomaly patterns suggest the presence of contrasting magnetic units, potentially reflecting variations in rock types such as mafic and felsic intrusions, metasedimentary units, and basement rocks^3,26.

From the results, we see that the NE, SW and southern portions of the study area have predominantly high magnetic anomalies meanwhile the western and central portions have low magnetic anomalies. These different magnetic susceptibility contrasts show the variations in the subsurface geologic structures in the study area implying the different rock types that make up the area. Also, results from the rose diagram showed that the area is characterized by major and minor linear features which serve as fault zones, contacts that can provide for possible potential mineral exploration. The results of this study fairly agree with where the gravity dataset of the Yaoundé-Yoko area and environs was analysed to reveal the distinct features of Basement rocks and the Sanaga fault³¹. The results also agree with^12,26, who also reported a structural direction of NE-SW likely caused by the Pan-African orogeny.

Structural analysis

The structural investigation yielded two key sets of findings; which are the directions and the depths from Euler.

3D Euler deconvolution results

As mentioned earlier, this method predominantly identified faults/contacts (N = 0), revealing a dominance of NE-SW, W-E, N-S, WNW-ESE, and SW-NE trends (Fig. 9). The estimated depths of these faults reached a staggering 3.2 km, suggesting a complex and dynamic geological past characterized by significant tectonic activity. These trends and depths align with previous studies in the region, such as^28,52, who also reported major NW-SE and W-E trending faults. Additionally, the estimated depths are comparable to findings in similar geological settings elsewhere in Africa [53,54].

Fig. 9.

Euler deconvolution for contacts.

While 3D Euler Deconvolution is a powerful tool, it is important to consider its limitations. The accuracy of depth estimates can be influenced by factors like noise levels, data quality, and assumptions about the structural index (N)¹⁹. Reference⁵⁵ suggest employing multiple depth estimation methods and integrating them with geological constraints for better reliability.

In this method, a single negative depth value was obtained, characterized by low magnetic anomaly gradients. These low anomaly gradient results are generally non-physical, as negative depths imply a source located above the observation level (e.g., above the flight altitude). In this case, the negative value was most likely an artifact resulting from noise or minor misfits in the deconvolution process, rather than a geologically significant feature^19,20.

Lineament analysis results

Lineament analysis

Extracting trends and orientations directly from the magnetic anomalies, lineament analysis complements 3D Euler Deconvolution and provides additional details about the structural fabric of the region (Fig. 10).

Fig. 10.

Structural map obtained from Euler solutions (N = 0).

Confirming the findings of 3D Euler Deconvolution, lineament analysis identified major trends in the NNE-SSW, WNW-ESE directions, with additional minor trends in NNW-SSE, WSW-ENE, NE-SW, and NW-SE directions (Fig. 11). This concurrence strengthens our confidence in the structural interpretation and paints a comprehensive picture of the region’s tectonic fabric.

Fig. 11.

Rose diagram for the lineaments in the study area from the Euler solutions.

The identified trends likely reflect major shear zones, fracture zones, and regional foliation directions. Integrating these trends with existing knowledge of regional tectonics² can provide valuable insights into the geodynamic processes that shaped the region’s geological history.

Conclusion

The interpreted TMI and the residual anomaly maps clearly delineated areas of low, moderate and high magnetic values to be sedimentary rocks, metamorphic rocks and basement complex respectively. Accompanying rock units in the sedimentary regions are predicted to be clay stones, silt stones, limestone, etc. found around Yaoundé, Obala, Mfou, Otélé and environs while metamorphic rocks are sandstones, schist, gneiss, mica, amphibolites, quartz, etc. found in the neighbourhood of Ntui, Mbandjock, Akonolinga, Mengang and environs. The basement complex terrain is overlain by igneous rocks (intrusions) in Nkoateng, Nanga-Eboko, Minta, Abong-Mbang, Ayos and neighbourhoods. Structural evaluation of linear features of the area revealed the orientation of these lineaments to trend NE-SW, that is likely caused by the Pan-African orogeny, a tectonic event of the Neoproterozoic era (650 to 500 million years ago). These lineaments can be potential sources for mineral exploration; faults or fractures for groundwater exploration; geological boundaries as contacts zones. Therefore, future geophysical research should focus on investigating the physical extents or magnitude of delineated lineaments seismic and/or electrical geophysical surveys to verify if these linear features pose immediate threats to lives and infrastructures.

Overall, this study uses the application of Reduction to the Magnetic Equator (RTE), derivative filters and Euler deconvolution with a structural index of N = 0 to enable the precise mapping of NE–SW trending structures, which is consistent with Pan-African tectonic imprints, and the identification of lithological units at depths of approximately 2.0 km. This study provides new insights into the geological architecture of the region and suggest zones of potential mineralization. The study also demonstrates that satellite aeromagnetic data, when combined with modern interpretational frameworks, can serve as a powerful tool in guiding mineral exploration and geological risk assessments in underexplored terrains.

Author contributions

Desmond Vihywuseh conceived the research idea and Djam Ann-Marie wrote the introduction. Awa Rostand did the literature review and Tabod Charles proposed the methodology. Ndikum Eric extracted the data for this study and Desmond Vihywuseh did the analysis and interpretation of results. Desmond Vihywuseh wrote the conclusion and compiled a draft copy of the manuscript. Ndikum Eric did the first review of the draft manuscript, and Tabod Charles did the second review. Desmond Vihywuseh did the final review and submitted the manuscript for consideration.

Data availability

The dataset used and/or analysed during this study is available on the website. http://geomag.org/models/EsMAG2/acknoledgments.html.

Declarations

Competing interests

The authors declare no competing interests.