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A new gravity survey was carried out in the northern part of the onshore Kribi- Campo sub-basin in Cameroon. The data were incorporated to the existing ones and then analyzed and modeled in order to elucidate the subsurface structure of the area. The area is characterized in its north-western part by considerably high positive anomalies indicative of the presence of a dense intrusive body. We find, 1) from the analysis of the gravity residual anomaly map, the high positive anomalies observed are the signature of a shallow dense structure; 2) from the multi-scale analysis of the maxima of the horizontal gradient, the structure is confined between depths of 0.5 km and 5 km; 3) from the quantitative interpretation of residual anomalies by spectral analysis, the depth to the upper surface of the intrusive body is not uniform, the average depth of the bottom is h
_{1} = 3.6 km and the depths to particular sections of the roof of the intrusion are h
_{2} = 1.6 km and h
_{3} = 0.5 km; 4) and the 3D modeling gives results that are suggestive of the presence of contacts between rocks of different densities at different depths and a dense intrusive igneous body in the upper crust of the Kribi zone. From the 3D model the dense intrusive igneous block is surrounded by sedimentary formations to the south-west and metamorphic formations to the north-east. Both formations have a density of about 2.74 g/cm
^{3}. The near surface portions of this igneous block lie at a depth range of 0.5 km to 1.5 km while its lower surface has a depth range of 3.6 km to 5.2 km. The shape of the edges and the bottom of the intrusive body are suggestive of the fact that it forms part of a broader structure underlying the Kribi-Campo sub-basin with a great influence on the sedimentary cover.

According to [^{2} offshore and 45 km^{2} in a triangular onshore area [

which covers a greater part of the study area. The other part of the area is covered by sediments; Metagranodiorite, and Metamorphic rocks of the Yaounde Group, which are part of the Ntem Unit. Until now, oil and gas production and exploration are focused on the offshore portion of the sub-basin. But recent studies are suggesting the possibility of the presence of oil, gas and minerals in the onshore portion [

The Kribi-Campo sub-basin, is a sedimentary basin which lies both onshore and offshore on the cost of Cameroon between latitudes 2˚20'N - 3˚20'N and longitudes 9˚15'E - 10˚00'E covering a total surface area of about 6195 km^{2} (

The northern area of the basin is occupied by the Oubanguide Belt which consists of Precambrian rocks that were remobilized by the Panafrican episode (600 to 500 Ma). These rocks are mainly schists and gneisses that have been intruded by granites and diorites [

The southern domain of the basin is occupied by the northern edge of the Congo craton, represented by the Ntem complex which consists predominantly of Precambrian rocks of granulite facies formed during the Archean and rejuvenated during the Eburnean orogeny. The Ntem Complex also carries imprints of past magmatic activities, which are characterized by several occurrences of dense rocks such as gabbros [

The coastal area of the basin is composed of Cretaceous sediments, mostly sandstones and small amounts of limestone and shales [

Many studies, [

craton and the Oubanguide belt have reported that the northern margin of the Congo Craton resulted from a convergent collision with the Panafrican Mobile Belt (PMB), which has been thrusted southwards onto the Craton. These same studies revealed that the area mainly underwent brittle deformations related to multistage compressional and extensional tectonics that give rise to major faults. These are mainly characterised by the Kribi-Campo Fault (KCF) system which is herein defined as a continuation of the Sanaga Fault [

The gravity data used in this work are the combination of new and existing data. The existing data were collected during gravity surveys of central Africa by ORSTOM and referenced in [

Putting these new and existing data points together, gives a total of 409 gravity data points.

Free-air and Bouguer reductions based on a mean density of 2.67 g/cm^{3} were applied on the data and the simple Bouguer anomalies derived. The resulting Bouguer anomalies as shown in

the context of the present study. The Kriging method was applied on the gravity data for interpolation using kriging gridding algorithm implemented in Oasis Montaj 8.0 software. The maximum value of the Bouguer anomaly in the grid is −68.7 mGal while the minimum value is −2.0 mGal.

In order to delineate the structures of the subsurface in our study area, two main approaches have been used namely:

- the maxima of the horizontal gradient upward continued to locate the anomalous body in the subsurface,

- the spectral analysis to determine the average depth of this body source.

Results of these methods, coupled with previous findings in the area [

According to [

The horizontal gradient is an operation that measures the rate of change of a potential field in the x and y directions [

where G is the Bouguer gravity field.

The works of [

This method is carried out through 2D Fast Fourier Transform which transforms gravity data from the space domain to the wavenumber domain to estimate the depths of the structures responsible for the measured anomaly. It has been used extensively by many authors, namely [

The finite discrete Fourier transform is given by the equation:

where b(x) represents the discrete N data array of gravity data obtained by sampling a continuous profile at evenly spaced intervals Δx. i is the complex operator, ω = 2πk is the spatial frequency and k = λ^{−}^{1} is the wavenumber in the x direction.

The expression of the Bouguer Slab Effect is then given by the equation:

where B(k)_{z}_{=0} is the Fourier transform of the Bouguer anomaly profile b(x)_{z=0}; Δρ is the density contrast between two layers; F(k) is the Fourier transform of f(x), the derivation of the interface from the mean depth z; G is the gravitational constant. The mean depth can then be calculated using the following equation:

where E is the power spectrum of B(k).

The square of the Fourier amplitude spectrum is plotted versus the radial frequency. The slope of the relationship between the wave number of the gravity field and the logarithmic power spectrum provide information about the depths of the source bodies.

The Bouguer anomaly map presented in

The first domain which covers the western part of the map is characterized by high values of gravity anomalies. The form of these anomalies suggests that it marks the limit of a large structure to the left of the study area. In the field, this area corresponds to low altitudes and its position near the Atlantic Ocean shows that this anomaly has it source from highly dense rocks. This domain, with anomaly values ranging between −40.1 mGal and −2.0 mGal presents four prominent peaks, one at the Lolabe locality, another one in Kribi and the last two to the east of Kribi. All these peaks can be interpreted as high density or basic intrusive bodies within the main formation. The two peaks observable to the east of Kribi may be considered as a unique, very high-value anomaly, which according to [

discontinuity.

The second domain, located at the northeastern side of the map displays two apparent ring shapes, characterized by very low amplitude anomalies trending NW-SE from Bipindi to the eastern part of Akom II, these anomaly values range from −68.7 to −54.7 mGal. There are interpreted as due to the presence of intrusive low density bodies in the subsurface.

The third domain which is situated in the middle of the map, from Bipindi to Nyabessan, consists of average anomaly values ranging from −50.9 mGal to −40.9 mGal and is separated from the two other domains by high gradients marking discontinuities between two structures in the subsurface. The correlation with surface geology shows that this area is the signature of charnockites and green rock belts of the Ntem Unit.

The Bouguer anomalies are the combination of deep and shallow sources, a separation of these anomalies into regional (deep sources) and residual (shallow sources) components was carried out in order to clearly identify the anomaly sources. This separation was performed using the polynomial fitting method. The procedure computes the mathematical surface, which gives the best fit to the gravity field within specific limits [

peaks observed around the Kribi area are seen to have merged to form a single oval peak indicating the prominence of the dense intrusive body at depth. At depth the characteristics of the structure surrounding this intrusive body extend southeastwards from Kribi to Akom II and become more pronounced again between Nyabessan and Ma’an. Since the effects of the mantle and lower crust are not of interest in this work, a third order residual anomaly is used for modelling in order to have a better chance of locating the depth to bottom of the dense intrusive body.

In order to determine the nature and shape of the Kribi intrusive body, the residual field obtained here will be studied using three methods namely: the multi-scale analysis of gradients method, which is usually employed for the analysis of the multi-scale residual anomalies; the spectral analysis method and the 3D modelling.

After computing the horizontal gradient of the third order residual anomaly, the resulting map is upward continued at 0 km, 1 km, 2 km, 3 km, 4 km and 10 km. The local maxima are then calculated and superimposed. The choice of the high of the upward continuation is determined by the types and depth of the structures that we intend to highlight. Given that, the higher we upward continue, the deeper the structures are highlighted. In our study, we intend to model a shallow formation. The maxima of the gradient of the residual anomaly upward continued could be observed on the map until the depth of 10 km. after 10 km, no effect of the anomaly featured on the map. That is why we choosed to stop at 10 km. The Maxima of the horizontal gradient of the third order residual anomaly map upward continued to 0 km, 1 km, 2 km, 3 km, 4 km and 10 km as presented in

The vertical limits of such an intrusive body can be predicted by the method developed by [

A profile (P) was chosen and drawn on the third order residual gravity map. The data used to carry out the spectral analysis were from this profile shown by the black line crossing the main positive anomaly (

This profile was drawn with a NE-SW orientation and traversing through the suspected area of the intrusion.

3D modelling was carried out using GRAV3D software on the residual field with the aim of delineating and caracterizing the dense intrusive body responsible for the observed gravity anomalies in the study area. The GRAV3D library consists of three major programs and one utility. The facilities include: GM-DATA-

VIEWER: this utility was used for viewing the observed gravity data, error distributions, and for comparing observed to predicted data directly or as difference maps; MESHTOOLS3d: this utility was used for displaying resulting 3D models as volume renderings.

The modelling process of the intrusive body consist of constructing the body block by block in a predefined mesh. Every block is a combination of cubeoids of the same volume. Every block, with a constant density is defined along a vertical axis and along a horizontal axis. The coordinates of the blocks have been given taking as origin the point O (1095, 300, 0) bring latitude, longitude and altitude respectively all in kilometers. The body was constructed taking into account all the results provided by the multi-scale analysis of the maxima of gradients and spectral analysis.

After constructing the body, we used the GRAV3D program to calculate the gravity signature of the body, this signature is presented in the form of a map. This map is then compared with the map obtained from the observed gravity data. The best model is the one for which the two maps can approximately be superposed.

The intrusive body was depicted to be located between the depths of 0.5 km and 2 km from the surface on its Eastern side and between 0.5 km and 5 km on its Western side. The result provided by the spectral analysis, giving the depth to

the top of the Kribi intrusive body was also considered.

The various views of the 3D model of the Kribi intrusive body are presented in Figures 10-15. These models consist of a major block having a density of 2.74

g/cm^{3}, a depth from the ground surface varying between 0.5 km and 1.5 km. The latitudinal and longitudinal extensions of the body are about 12 km and 30 km respectively.

The forward modelling of gravity data can have many models developed from an anomaly. In order to obtain a model that best reflects the subsurface structure, one has to consider certain parameters that would limit this uncertainty. As part of the constraints, the multi-scale analysis of the maxima of the horizontal

gradient of the third order residual anomaly has been used to locate and determine the depth range of the anomalous body. The spectral analysis has also been used to determine the average depths to the tops and bottoms of the anomalous body. It is observed that the depth varies between 0.5 km and 5 km from the surface of the earth. These constraints give an assurance of the validity of the model. The parameters of the model (depth to the top of 0.5 km and 1.5 for some sections of the body, and depth to the bottom of 4.8 km and 5.2 km for a slide section of the center of the body) are in accordance with the results obtained by [^{3}. A superposition of the residual anomaly map with the geological map and previous studies [

Given all these, the intrusive igneous body obtained by gravity modelling may be composed of gneiss, and granodiorite because their mean density is close to that of the modelled body. Assuming the value of the density of the surrounding metamorphic rocks to the North-East of 2.67 g/cm^{3} and the value of the density

Rock name | gneiss | Alkaline syenite | Nepheline syenite | granodiorite | dolerite | tonalite | peridotites |
---|---|---|---|---|---|---|---|

Density range/ g/cm^{3} | 2.60 - 2.90 | 2.60 - 2.95 | 2.53 - 2.70 | 2.67 - 2.79 | 2.70 - 3.50 | 2.62 - 2.96 | 2.78 - 3.37 |

Mean density value/ g/cm^{3} | 2.75 | 2.78 | 2.61 | 2.73 | 3.10 | 2.79 | 3.08 |

of the surrounding sedimentary formations to the south-west mainly limestones and sandstones with mean densities of 2.55 g/cm^{3} and 2.35 g/cm^{3} respectively, the 3D model possibly consist of an intrusive igneous body (gneiss, granodiorite) with a density estimated at about 2.74 g/cm^{3} surrounded by other metamorphic formations to the north-east and sedimentary formations to the south-west. The density contrast between this body and the sorrounding formations varies from 0.07 g/cm^{3} to 0.39 g/cm^{3}.

According to [

The analysis of the third order residual anomaly map and the superposition of horizontal gradient maxima from the residual anomaly and its upward continuation at several heights shows quasi-circular disposition of many maxima indicating the presence of a dense intrusive body in the Kribi area. The multi-scale analysis of the maxima of the horizontal gradient of the third order residual anomaly led to the location of this body at a depth to bottom ranging 0.5 km and 5.0 km. The power spectrum method used gave the depth to the top from the surface at 0.5 km and 1.6 km and to a mean depth to the bottom of 3.6 km. The 3D model obtained using the GRAV3D software and taking into account the results provided by the two previous methods, shows a block with part of its top located at 0.5 km and other sections located at about 1.5 km. Its bottom lies at a depth ranging from 4.8 km to 5.2 km. The identified body of density 2.74 g/cm^{3} which is surrounded by other lower density metamorphic formations to the north-east and sedimentary formations to the southwest suggests that it is an igneous intrusion. The observation of the shape of this body coupled with the results obtained by [

We greatly appreciate constructive and insightful comments of reviewers whose remarks and critique have led to a significant improvement of the work. We equally want to thank the team of Geophysicists from the Laboratory of Physics of Earth’s Environment of the University of Yaoundé 1 who made the data acquisition campaign possible, Prof. Njandjock Nouck Phillipe, Dr. Evariste Ngatchou and Mr. Abate Marcel are gratefully acknowledged for leading this campaign.

Malquaire, K.P.R., Louise, O.A.M., Nfor, N. and Eliezer, M.-D. (2017) 3D Modelling from New and Existing Gravity Data of an Intrusive Body in the Northern Part of Kribi-Campo Sub- Basin in Cameroon. International Journal of Geosciences, 8, 984-1003. https://doi.org/10.4236/ijg.2017.88056