The Bouguer anomaly is the gravimetric response caused by heterogeneities in subsoil density [19] . In simple terms, it results from the difference between the gravity measurement that has undergone the Bouguer correction (Faye correction, plateau correction and relief correction) and the gravity value on the reference ellipsoid, at the vertical of the measurement station. Its expression is given by:

$\text{Δ}{g}_B=\left[g_{mes}+\text{Δ}{g}_F-\text{Δ}{g}_B+\text{Δ}{g}_T\right]-g_{th}$, (1)

where: $\text{Δ}{g}_B$ is the Bouguer anomaly; g_mes, the value of g measured at station S on the topographic surface; $\text{Δ}{g}_F$: the Faye correction; $\text{Δ}{g}_B$: the simple plateau or Bouguer correction; $\text{Δ}{g}_T$: the topographic correction; and $g_{th}$: the value of $g$ calculated on the ellipsoid (E) plumb with S.

Derivation filters amplify the effect of shallow sources by attenuating the effect of deep ones, focusing on anomalies and making it possible to better define the geometric limits of bodies.

The horizontal gradient is used to identify contacts that may be faults, fractures, or lithological boundaries in magnetic or gravity data. These contacts are very intense [20] [21] . This method has the advantage of not being affected by noise emitted by the data during the aeromagnetic or gravity survey, as it only takes into account the first derivatives of the magnetic field in the horizontal plane [22] . The horizontal gradient of a magnetic or gravimetric field M, also known as the HGM, is calculated by studying the horizontal derivatives in the x and y directions. We therefore have:

$HGM\left(x,y\right)=\sqrt{{\left(\frac{\partial M}{\partial x}\right)}^2+{\left(\frac{\partial M}{\partial y}\right)}^2}$ [22] (2)

If the following assertions are met, this function provides maxima over geological contacts. The regional magnetic field must be vertical or horizontal depending on the position on the earth’s surface.

When the first four assertions are violated, the local maxima are displayed on the map above the contacts, while violation of the fifth creates a second maximum parallel to the contact. In order to resolve these difficulties, it is necessary to convert the data using the pole reduction operator. The analytical signal, which does not depend on the directions of the magnetisation and the magnetic field, will make it possible to determine the definitive geological contacts.

Short-wavelength anomalies are amplified by the vertical derivative, making it possible to locate the impact of surface structures in the subsurface by attenuating the impact of long-wavelength anomalies. Mapping the vertical gradient is essential for structural analysis of the study [23] [24] provided the algorithm to determine this gradient for a magnetic field M.

$M\left(u,v\right)=M\left(u,v\right){\left({\left(u^2+v^2\right)}^{\frac{1}{2}}/n\right)}^n$.(3)

According to [25] - [27] , the analytical signal method is used to determine the depths and coordinates of magnetic sources. The special feature of the analytical signal is its ability to be independent of the direction of magnetisation of the source, given that the magnetic field is a vector whose direction and intensity vary according to the time and place of observation. According to [28] , the amplitude of the analytical signal is directly linked to the value of the magnetisation. According to the research carried out by [29] , the amplitude of the analytical signal is calculated by studying the squares of the derivatives of the total magnetic field as a function of the x, y and z directions. Its definition is given by:

$\left|A\left(x,y\right)\right|=\sqrt{{\left(\frac{\partial M}{\partial x}\right)}^2+{\left(\frac{\partial M}{\partial y}\right)}^2+{\left(\frac{\partial M}{\partial z}\right)}^2}$,(4)

where M is the total magnetic field.

The Tilt-angle transformation calculates the inverse of the tangent of the ratio of the modulus of the horizontal gradient with the vertical gradient of the gravity field [30] [31] [32] . The equation for this transformation is written:

$\theta =ta{n}^{-1}\frac{\frac{\partial M}{\partial z}}{\sqrt{{\left(\frac{\partial M}{\partial x}\right)}^2+{\left(\frac{\partial M}{\partial y}\right)}^2}}$, (4)

where M represents the gravity field. The values of the tilt angle are between $\frac{-\pi }{2}$ and $\frac{\pi }{2}$, this method delimits the variations in the amplitude of the angle within a certain range. The advantage of the transformation is that by calculating an angle, all the shapes will be represented in a similar way, whether the anomalies are small or large in amplitude [33] . The magnitude of the tilt angle is positive for gravity sources, almost zero at the edges of these sources, and negative outside of these magnetic sources [34] . This operator is applied to the Bouguer map.

Upward extension and horizontal gradient are two methods used in multi-scale analysis. The use of a multi-scale analysis is an effective means of locating in the subsurface the linear contacts corresponding to faults and the circular contacts representing the limits of intrusive bodies [35] , on the one hand, and evaluating their trajectory, inclination and even their importance [36] [37] , on the other. Indeed, the line indicating the local maxima of the values of this gradient highlights the boundary between two blocks with different densities or susceptibilities.

A curve representing the magnetic or gravimetric anomaly above a vertical contact shows a minimum on the side of rocks with low densities or susceptibilities and a maximum on the side of rocks with high susceptibilities. The point of inflection of the curve is at the vertical contact, which represents the maximum of the horizontal gradient [20] [21] .

This is a filtering technique which allows the origins of anomalies to be identified. [38] [39] represents the total magnetic field intensity at the observation point P: $f\left[\left(x-x_0\right),\left(y-y_0\right),\left(z-z_0\right)\right]$ [20] [40] developed the homogeneity equation, which takes the following form:

$\frac{\left(x-x_0\right)\partial M}{\partial x}+\frac{\left(y-y_0\right)\partial M}{\partial y}+\frac{\left(z-z_0\right)\partial M}{\partial z}=N\left(B-M\right)$,(5)

where (x, y, z) are the coordinates of the observation point, $\left(x_0,y_0,z_0\right)$ are the coordinates of the magnetic source. The total magnetic field at the observation point is M, the regional value of the total magnetic field is B and the structural index N is used to define the type of source and the rate of change of the field as a function of distance. Euler deconvolution is based on solving the previous equation, which has four unknowns: x, y, z and B. Solving a system of equation with four unknowns requires four measurement points, and then the least squares method is used [23] .

$N=-n$ is the structural index, where n represents the degree of homogeneity. It varies according to the shape of the source. Therefore, a homogeneous source with a structural index of $N=3$ is a linear source (such as a line of dipoles or poles, a homogeneous cylinder, a rod, etc.). In the case of $N=2$, the source is an intrusive body such as a dyke, thin layer, etc. There is a contact when $N=1$, while a block is present at the top of a pyramid when $N=0$.

2^3/4-D modelling is based on three important parameters: the susceptibility or density contrast of the subsoil rocks, the depth of burial and the shape of the geological structures that are the source of the anomalies observed. The constraints on these parameters are derived from the geology of the study area and the analysis of magnetic or gravimetric data. The principle of this direct modelling method is to calculate the theoretical anomaly using a model of a simple structure such as a cylinder, sphere or prism, and compare it with the observed anomaly. The best model is the one that corresponds to the structure whose calculated anomaly is as close as possible by adjustment to the observed anomaly. However, the model selected should take geological clues into account as far as possible [41] - [43] .

Researchers at the National Geospatial-Intelligence Agency (NGA) have developed the Earth gravity model known as EGM 2008 [44] . This model is a combination of terrestrial gravity data from Champ/Grace satellite data.

The various gravity anomalies identified are separated by zones of strong gradients, indicating the presence of density discontinuities that generally represent tectonic faults.

The map below ( Figure 2 ) shows the Bouguer anomalies obtained with the EGM 2008 data. It shows several anomaly shapes. This map is characterised by a heavy anomaly (with a value greater than the average anomaly, i.e. −32 mGal) covering the south-western and central part (from Oulidik to Doumé) with a break-up to the north of Diang. This anomaly has an E-W to ESE-WNW direction in the West-Centre part and a N-S direction towards Diang. It has a long wavelength and appears to be bowl-shaped in the south-western part, while in the northern part, it has an ellipsoidal shape. This zone of heavy anomalies obviously indicates the presence of high-density materials such as granulites in the subsurface. This anomaly is thought to represent the gravity signature of a fault in the study area, which appears to be parallel to those observed on a regional scale and is thought to be the result of faults that acted as shear zones at the end of the Pan-African, and then during the Cretaceous.

The succession of positive anomalies and negative anomalies suggests that there have been accidents and/or clear contacts between lithological units that appear to be independent of one another.

The vertical derivative enhances the features of surface structures by amplifying short wavelength anomalies and attenuating the effect of long wavelength (low frequency) anomalies. This filter shows a lateral separation of anomalies and an amplification of the gravimetric effect of surface density contrasts to the detriment of depth contrasts.

The map below ( Figure 3 ) shows anomalies along the Oulidik-Doumé axis that separate from east to centre and are characterised by a positive vertical gradient greater than 1.36 mGal/km. They trend ESE-WNW. In the same area, there is also a positive gradient anomaly that crosses the Ankom-Ngamendouma axis in an ENE-WSW direction. This anomalous zone is thought to be due to a dense mass upwelling that was observed on the Bouguer anomaly map. Similarly, to the north of Diang, there is an anomaly with a gradient greater than 1.36 mGal/km trending ESE-WNW, a direction recognised by the structures of the Adamaoua plateau and followed by the tectonic lines observed in the geological map of the study area. The area between Viali and Badissé to the north is also covered by a similar anomaly.

The southern part consists of an anomaly with a very low gradient (less than −1.63 mGal/km) in an E-W direction, characterising the presence of weakly dense structures, which are in agreement with the results obtained in the Bouguer anomaly map. The Mbangue zone also has an anomaly with a very weak gradient (less than −1.63 mGal/km) in a NE-SW direction. This could be linked to the passage of the Sanaga River through the area. The depression observed in the Viali area in the Bouguer anomaly map is well individualised in the present map. It follows the ENE-WSW direction.

This map also highlights two major corridors of sub-vertical and sub-horizontal positive gradient anomalies that pass almost through the centre of the study area. These anomalies could represent the transition between the two main units, the Congo Craton and the Pan-African Central African Chain.

The analytical signal is a transformation that highlights shallow anomalies. It highlights the major tectonic directions and makes it possible to limit contacts between geological formations by their density contrast and to expose intrusive bodies [38] .

The map of the analytical signal above ( Figure 4 ) highlights three major sectors with strong gradients, notably the South-West sector (Oulidick, Ankom, Abong-Mbang, Doumé), and the Nlang and Viali sectors.

The South-West sector has an E-W to ENE-WSW direction, characteristic of regional anomalies. The Vilali-Nlang magnetic zones are separated by anomalies with intermediate gradients, and trend ENE-WSW. This gives the impression that they formed a single block that was separated by a tectonic event of Pan-African age.

The map in Figure 5 shows the dip angle anomaly, which highlights the intersecting lineaments and dykes. The orientations of the structures mentioned in the previous maps are clearly visible, which only confirms the results given above.

The Oulidik-Doumé fault still stands out, as do the regional E-W and N-S trending anomalies identified on the vertical gradient map, proving that these anomalies represent regional geological faults probably associated with subduction of the Pan-African in the Congolese Craton.

The map below ( Figure 6 ) shows local maxima of the horizontal gradient extended upwards at several altitudes, in particular 10 km, 20 km and 30 km. According to Blakely & Simpson (1986), this superposition of maxima highlights the geological contacts associated with the faults or fractures suspected in the previous maps. This method also highlights the direction of dips. Vertical dips will show a superimposition of maxima, while oblique dips will show a shift of maxima in the direction of dip. This reveals a structural complexity marked by the alignment of maxima, indicating the existence of several linear structures that have affected the basement. These include vertically dipping faults:

- to the south of the Ankom study area, along the E-W axis;

- between Viali and Doumé, running N-S;

- between Bika and Nlang, running NE-SW;

- and finally between Nlang and Mbangue, running NW-SE.

In addition to these vertically dipping faults, there are others that dip obliquely. These can be seen:

- to the south between Abong-Mbang and Ankom, trending ENE-WSW;

- Between Bika and Oudilik, there is an NW-SE accident;

- to the south of Boulembe, an ESE-WNW accident;

- and between Boulembe and Badissé, where there is an ESE-WNW accident;

- and from Yoko Betougou to Ebaka, with an ESE-WNW directional accident.

The lineament map ( Figure 7(a) ) is obtained by plotting the lineaments resulting from the superposition of the horizontal gradient maxima in Figure 6 . These lineaments follow several structural directions, notably ESE-WNW, ENE-WSW,

NW-SE, NE-SW, N-S and E-W. These directions are in good agreement, firstly with those observed in the analysis of the magnetic data, and secondly with the deformations observed on a regional scale, such as fractures, dykes and faults.

The map below ( Figure 7(b) ) shows the directional rosette of the various gravity lineaments. It shows that the lineaments preferentially follow NW-SE, ESE-WNW, NE-SW, ENE-WSW directions and, to a lesser extent E-W and N-S directions.

This gives an idea of the depths of tectonic faults on the one hand, and on the other, an idea of the behaviour and lateral and vertical variations in the susceptibilities and densities of subsoil rocks.

Euler solutions were calculated using the following parameters: structural index 0, tolerance 10 and window size 15. The structural index 0 used here highlights the magnetic contacts observed in the study zone.

Analysis of the Euler solution map ( Figure 8 ) highlights several tectonic faults.

It allows us to differentiate between intrusive bodies and deep faults. The shape of the groupings of Euler solutions confirms the tectonic faults mentioned in the previous maps on the one hand, and reveals new faults on the other.

To the south of the study area, particularly in the vicinity of Abong-Mbang, we note the presence of deep faults located at depths greater than 14 km and oriented ENE-WSW, ESE-WNW and NE-SW. The Oudilik-Ngamendouma zones are characterised by a swarm of shallow solutions that can be assimilated to intrusions.

The N-S-trending tectonic fault between Doumé and Diang mentioned in previous maps is still clearly defined here. It is thought to be located at depths greater than 7 km.

There is an NW-SE-oriented lineament between Bertoua and Mbangue, which would indicate the presence of a 7 km deep fault, which extends to the south-east of the study area in the same direction. It appears to correlate well with the tectonic fault that divides the study area diagonally in the analysis of the previous magnetic data.

Between the villages of Nlang and Viali, there is an ENE-WSW fault 10 km deep. This fault was not mentioned in the aeromagnetic study. The Badissé area is crossed by an NW-SE tectonic fault that can be likened to a fracture.

Models can be used to observe lateral and vertical variations in rock density in the

subsurface. They can also be used to locate major discontinuities in the subsurface. The three gravity models plotted here each show highly deformed contours, bearing witness to intense folding, shearing and fracturing, proof that the area bears the imprint of the collision of two major geological units, namely the Pan-African formations and those of the Congo Craton. The figure opposite ( Figure 9 ) is the Bouguer anomaly map showing the profiles.

Analysis of gravity profile P1

Gravity profile P1 ( Figure 10(a) ) runs NW-SE from the locality of Bika to the south of the study area, crossing the locality of Ngamendouma at the points (283316 m; 512299 m) and (341004 m; 440446 m). It is approximately 92 km long and centred on a zone of high anomalies. Gravimetric topography along this profile reveals four layers, including ancient and delayed tectonic granites, embrittled gneisses, two-mica gneisses and micaschists. The bedrock here is granitic. The 2^3/4 D model of this gravity profile shows that it crosses a disturbed gravity relief with heavy to light gravity anomalies.

This profile has very irregular shapes which would correspond to intense folding or shearing with crushing of the structure’s characteristic of a collision.

Analysis of gravity profile P2

Gravity profile P2 ( Figure 10(b) ) runs from Bika to Viali in an ENE-WSW direction, ending at coordinates (286651 m; 522869 m) and (344052 m; 532547 m). It is approximately 58 km long and centred on a zone of high anomalies. Gravimetric topography along this profile reveals four layers made up of syntectonic granites, biotite embrechites, two-mica gneisses and biotite gneisses. The bedrock here is granitic. The 2^3/4-D model of this gravity profile shows that it crosses a disturbed gravity relief with heavy to light gravity anomalies.

This profile has very irregular shapes resembling deformed sinusoids, which would characterize the intensity of the folding that affected the study area.

Analysis of gravity profile P3

Gravity profile P3 ( Figure 10(c) ) runs from Boulembe to Viali in a NW-SE direction, ending at coordinates (369182 m; 520120 m) and (350770m; 550307m), is approximately 35 km long and is centred on a zone of high anomalies. Gravimetric topography along this profile reveals five layers, including ancient syntectonic granites, late syntectonic granites, late alkaline-tending granites, sericite-gneisses and embrechites. The bedrock here is granitic. The 2^3/4 D model of this gravity profile shows that it crosses a disturbed gravity relief with moderate to slight gravity anomalies.

This profile shows a tabulation of the layers. The area near Viali is the one most affected by the tectonic movements that affected the basement, and is characterised by a contact between geological formations of different densities. It is thought to be the site of a major discontinuity assimilated to fracturing.