This study is based on the analysis and interpretation of aeromagnetic data using version 8.4 of the Geosoft Oasis Montaj Software, to map the subsurface or deep geological structures that affected the geological formations of the Ngaoundere area. The use of the standard aeromagnetic methods made it possible to draw up the maps of the residual magnetic field reduced to the equator (RTE), the horizontal gradient (HG), the analytical signal (AS) and that of the Euler solutions (ED) to find the main magnetic facies corresponding to these structures. The geological formations of the studied area thus appear to be intensely fractured by a NE-SW (N45°E) and ENE-WSW (N70°E) main orientation fault system, the depth of which has been estimated by combining the three-analytical methods HG, AS and ED. Advanced magmatic map analysis revealed dikes associated with vertical faults in the studied area. The development of an interpretative geological map taking into account the basic geology, the deep faults, the identified dikes and the mineralization index made it possible to extract a correlation between geological structures and mineralization of the studied area. The 2.5D modelling of two magnetic profiles plotted on the reduced residual map at the equator was performed to approximate the geometry and depth of the dikes sector, which are potential sources of mineralization here.
Since 1970, the Adamawa plateau in general and the studied area has been the subject of several geological studies by several authors, such as the studies conducted by Eno Belinga [
Aeromagnetic data have very often been used in the mining sector to detect areas of magnetic anomalies that may correspond to mineral concentration. They are currently widely used for geological land reconnaissance, especially in large areas where land or plant cover is important. The magnetic susceptibility technique from airborne data is therefore used to specify geological interpretations at regional scales. In the case of this study, the main objective is to produce a structural mapping from the aeromagnetic data of the studied area to highlight a correlation between the geological structures (faults, geological contacts) and the possible mineralization of the studied zone.
This study will therefore be unique in that research studies in the study area have so far consisted either of studying the general geology or measuring the average thickness of the earth’s crust. They therefore did not provide information on the potential sources of near surface anomalies that are generally important for mineral research. All the results that will come from this study will provide a basic document on the very solid study area to base further investigations in mineral research.
The studied area covers an area of approximately 4647 Km2. It is limited by longitudes 12˚30' - 13˚30'East and latitudes 6˚45' - 7˚15'North. It’s a plateau area of altitudes between 875 m and 1325 m.
The Adamawa plateau is largely covered by large basaltic and basalto-andesitic volcanic effusions of essentially Tertiary age, which extend into the middle part of the southern moat [
This plateau has an altitude of 1100 m above sea level, with large basaltic flows accompanied by trachytes and trachyphonolites [
A thematic map of the mineral resources of Cameroon published in 2001 by the Ministry of Mines, Water and Energy (MINMEE) shows several indices of mineralization (Gold, Aluminum, Germanium, Tin, Thermal springs) in the studied area (
The data set used in the present study was obtained an aeromagnetic survey covering some parts of the Cameroon territory [
Indeed, unlike the gravitational attraction field, which is vertical and always directed downwards, the magnetization field vector and the inductive field vector are generally inclined, thus causing an asymmetry in the shape of the anomalies. To correct this error, a reduction to the pole or the equator is often applied to the basic data. In the case of this study, given that Cameroon is moving from
low-latitude areas (−15˚ and 15˚), it was necessary to calculate the reduction at the equator in order to bring anomalies back into line with the structures which generates them, since any analysis of anomalies is somewhat influenced by the inclination of the magnetic field and that of the magnetization direction of the magnetic fields sources. This reduction filter is calculated by choosing a point located at the center of the magnetic anomaly map (13.028˚East and 7.04˚North) with inclination and declination values from the model of the normal or theoretical field of the magnetic anomaly map IGRF at January 1, 1970. From the TMI anomaly map (
A set of methods were used in the processing of the present aeromagnetic data. These included the gradient method, the analytic signal, the Euler deconvolution method, and modelling methods.
The horizontal gradient method is in many ways the simplest approach to estimate contact locations of the bodies at depths. The biggest advantage of the horizontal gradient method is its low sensitivity to the noise in the data because it only requires calculations of the two first-order horizontal derivatives of the field [
HGM ( x , y ) = ( ∂ M ∂ x ) 2 + ( ∂ M ∂ y ) 2 (1)
This function gives a peak anomaly above magnetic contacts under the following assumptions [
5) the sources are thick. Violations of the first four assumptions can lead to shifts of the peaks away from the contacts. Violations of the fifth assumption can lead to secondary peaks parallel to the contacts.
Absolute analytic signal magnitude (ASM) according to [
‖ ASM ( x , y ) ‖ = ( ∂ f ( x , y ) ∂ x ) 2 + ( ∂ f ( x , y ) ∂ y ) 2 + ( ∂ f ( x , y ) ∂ z ) 2 (2)
The advantage of this method of magnetic data enhancement is that its amplitude function is always positive and does not need any assumption of the direction of body magnetization [
Depth estimation by Euler deconvolution technique was used for delineating geologic contacts. This technique provides automatic estimates of source location and depth. Therefore, Euler deconvolution is both a boundary finder and depth estimation method. Euler deconvolution is commonly employed in magnetic interpretation because it requires only a little prior knowledge about the magnetic source geometry, and more importantly, it requires no information about the magnetization vector [
( x − x o ) ∂ M ∂ x + ( y − y o ) ∂ M ∂ y + ( z − z o ) ∂ M ∂ z = N ( B − M ) (3)
where B is the regional value of the total magnetic field and (x0, y0, z0) is the position of the magnetic source, which produces the total magnetic field M measured at (x, y, z). N is so called structural index. For each position of the moving window, on over-estimated system of linear equations is solved for the position and depth of the sources [
The 2.5D modeling shows a vertical section of the geological formations of the subsoil and the magnetic property (magnetic susceptibility) of each of the formations encountered. The objective of this modeling is to obtain a model calculated from an observed model of a magnetic field. All the programs use a 2.5D Talwani algorithm to calculate the magnetic field anomaly produced by the causative bodies [
The equation associated with this modeling is:
Δ B y ( r 0 ) = − 2 J j y ∑ ( tan − 1 u i + 1 + Y w i R i + 1 − tan − 1 u i Y w i R i ) (4)
where the profile is in the x plane, z is down and y is orthogonal to both. J is the magnetization of the body. R i = ( u i 2 + w i 2 + Y 2 ) ; r i + 1 = ( u i + 1 2 + w i 2 ) ; w i = − sin ∅ i x i + cos ∅ i x i + 1 and u i = cos ∅ i x i + 1 + sin ∅ i z i
The TMI-RTE map (
Indeed, the extension consists in artificially moving the observation plane and calculating the field that one would observe in these new points from the data collected in the field. Extension can be up or down. But in this study, we will focus only on the upward extension. The latter makes it possible to pass the anomaly of the altitude Z = 0 to an altitude Z > 0. This operator acts as a low-pass electronic filter by attenuating the short wavelengths; highlighting the anomalies of deeper and deeper structures, depending on the altitude of the extension [
The upward extension, which is a powerful low-pass filter, is used here to determine the regional; however, this method also has a handicap that lies in the difficulty in determining the appropriate extension altitude in order to have the best approach to the regional. In the case of this study, we adopted the Zeng [
The map of regional anomalies thus obtained is subtracted from the TMI-RTE map, which made it possible to obtain the residual anomaly map which is a map freed from the influence of the very deep structures (
Reduced residual map at equator shows smoothing and stretching of anomalies on the horizontal plane. It also shows the magnitudes of anomalies that vary between −100 and 80 nT. The superimposition (
In the South-East of the studied area, the identified ENE-WSW positive anomalies are perfectly superposed on the large fault (thick black line in
At the center of the studied area, the NE-SW direction of the anomaly coincides with the tectonic line (thick green line in
Based on previous correlations, the negative anomalies observed at the Marlok Zone Center (represented by the blue colour in
The horizontal gradient is an excellent way to locate geological contacts in the subsoil, including faults by determining their alignments, dips, and degree of importance [
Ø The most rectilinear contacts and strong intensities are grouped in the center of the zone between Minim and Tékel for several kilometers and characterize the signature of the faults in the covers.
Ø Contacts of circular to sub-circular shapes with high intensities are much more grouped north-west of Minim and also discontinuously in other areas of the map. These contacts characterize the limits of the intrusive bodies (dikes or veins of magmatic rock forming a depression).
To highlight the geological contacts associated with faults or fractures on the map of the Horizontal gradient magnitude (HGM), the map of the local maxima of the magnitude of the horizontal gradient of the residual anomaly (
by the irregular distribution of maxima associated with faults in the basement. In the center of this map, the configuration of ENE-WSW direction maxima suggests the presence of significant structural deformation in the base.
In order to better understand the relatively deep geological structures, the method of Analytical signal (AS) has been applied to the map of the residual anomaly. The map obtained (
Horizontal gradient magnitude (HGM), the map of the local maxima of the analytical signal (
contacts are highlighted on this map namely the contacts: NE-SW; ENE-WSW and E-W.
HGM and AS association is very important in this study for the final interpretation of the geological contacts (
Ø When the maxima of the horizontal gradient are isolated on the map of the two superimposed methods, they represent the real contacts.
Ø When the maxima of the analytical signal and the horizontal gradient are almost parallel and not coincidental, then those of the analytical signal represent the true contacts and those of the horizontal gradient indicate the direction of dip of these contacts.
Ø When the maxima of the two methods are merged, then they materialize the vertical contacts.
The rosette of fracture directions (
The inversion by the Euler method was carried out using the Euler 3D calculation program incorporated in Geosoft oasis montaj. This is a very effective method that can locate the magnetic contacts in the horizontal plane and their different depths. This is the reason why it has been applied to the residual map. The application of the Euler deconvolution requires the knowledge of three parameters which are: the structural index N, the dimensions of the filter window W
and the tolerance Z. Since the objective was to highlight the solutions related to geological contacts, in this study, several tests were carried out by varying various N, W and Z parameters. All these made it possible to notice that the best results or groupings of the Euler solutions are obtained for a window size equal to 3, a selection criterion of the solutions which is the tolerance of 12% and a structural index of 0.
The superimposition of this map (
By superimposing on the map of residual anomalies reduced to the equator the mining index maps, and deep faults in the studied area (
Ø Some clues (Au, Sn, Ge) are found at the edges of rivers and these are related to geological structures.
Ø Gold (Au) clues found in the area appear to be superimposed on strong NE-SW magnetic direction signals recorded north of Marlok.
Ø To the west of Tékel, the Aluminum (Al) index, as well as the portion of the bauxite plateau of Minim-Martap (represented by the blue circle in
Faults | Dips | Approximate depths (m) | Faults | Dips | Approximate depths (m) |
---|---|---|---|---|---|
F1 | Vertical | 1200 - 1500 | F12 | Vertical | 900 - 1000 |
F2 | Vertical | 1100 - 1200 | F13 | SE | 1000 - 1300 |
F3 | Vertical | 1000 - 1300 | F14 | Vertical | 1000 - 1500 |
F4 | N-S | 1000 - 1500 | F15 | S | 650 - 850 |
F5 | Vertical | 800 - 1000 | F16 | Vertical | 1200 - 1700 |
F6 | Vertical | 1500 - 1700 | F17 | Vertical | 950 - 1100 |
F7 | NW | 1200 - 1500 | F18 | SE | 900 - 1000 |
F8 | SE | 600 - 700 | F19 | SE | 600 - 680 |
F9 | Vertical | 900 - 1000 | F20 | N-S | 700 - 1000 |
F10 | NW | 1000 - 1500 | F21 | N-S | 800 - 900 |
F11 | N-S | Unknown |
Ø The other elements namely tin (Sn) and Germanium (Ge) appear to be related to recent sedimentary formations outcropping in the north-western part of the studied area (North-West of Minim).
In summary, the reduction of magnetic data to the equator yielded a map that reflects the degree of concentration of magnetic minerals at the studied area scale’s. This map has been superimposed on that of the geology of the studied area. Its examination makes it possible to realize that the highlighted lineaments represent, for the most part, sub-meridian magnetic structures, offset by transverse faults with dextral plays.
The very elongated shape of most anomalies represents the magnetic signature of also elongated sources that may correspond to veins or other lenses. However, the very high amplitude of the magmatic signals of these anomalies makes it possible to exclude the hypothesis of the quartz, which cannot present such a signal. The elongated shape of these anomalies as well as their high amplitude is therefore suitable for bodies meeting these criteria, namely: the dikes. The dikes are actually veins of magmatic rocks that have infiltrated the various fractures of the country. It is therefore an intrusive phenomenon in an open crack. This phenomenon is very visible on the residual map of the studied area (
Taking into account the faults obtained, the dikes observed on the residual map as well as the mineralization index, and integrating them with the map of
The 2.5D modelling was performed on two magnetic profiles (P1 and P2) of NNW-SSE directions. These profiles plotted on the residual map reduced to the
Equator (
Previous interpretations show that the mineralization indices are associated with the geological contacts or structures of the sector and in particular with the dikes (Paragraph 4.2.4) which are folded fractures by newcomers of magmatic fluids within a host. The objective of this modeling was therefore to highlight these dikes in a vertical section, thus giving their geometry, their approximate depth, as well as the other associated geologies.
The association of the horizontal gradient method with that of the analytical signal and the analysis made on other magmatic maps shows that the dikes of the studied area are associated with vertical faults (
For the P2 model, in addition to sediment, we have granite and dikes. From NNW to SSE, two (02) dikes intrusions also occurred associated with vertical contacts at depths between 450 and 1400 meters below the topographic level.
The apparent magmatic susceptibility of the dikes, obtained from modeling (S = 0.1278), is similar to that of diorite (S = 0.13) according to the classification of rocks and minerals according to their magmatic susceptibilities of Hunt et al. [
The treatment and interpretation of the aeromagnetic data made it possible to
know the structure of the subsoil of the studied area from the analysis of the magnetic surface signals. Indeed, the combination of analytical methods (HG, AS, ED) has made it possible to highlight the deep faults and their approximate depths. The major directions of these faults namely ENE-WSW (N70˚E) and NE-SW (N45˚E) coincide with those found by [
The authors declare that there is no conflict of interest regarding the publication of this paper.
Arsène, M., Hervé, G.D., Theophile, N.M., Mouhamed, N.N. and Igor, O.A.O.U. (2019) 2.5D Modelling of Aeromagnetic Data and their Mining Implications over the Ngaoundere Area (Adamawa Province, Cameroon). International Journal of Geosciences, 10, 173-192. https://doi.org/10.4236/ijg.2019.102011