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In order to produce a more detailed structural and geometrical information, and determine sediments thickness along the Kribi-Campo sub-basin, statistical spectral analysis and horizontal gradient analysis of residual anomalies coupled with the Euler deconvolution approach were applied on the gravity data in the area. The results obtained from the 2D spectral analysis on anomaly grids gave a depth to the basement rocks of the basin from 0.60 km to 3.93 km. This represents the thickness of the sedimentary formations overlying the basement. The interpretation of the spectral analysis results indicated that the potential hydrocarbon field areas are situated between Kribi and Lolabe and at Campo given that those areas have the highest sedimentary thicknesses values. From the analysis of the horizontal gradient, deep faults mainly striking SW-NE have been traced and a structural map of the area has been produced. By applying the Euler deconvolution method to the gravity data, information about the depth and trend of the main subsurface structures have been obtained.

The Kribi-Campo basin is located at the northern edge of the South Atlantic rift and is one of a series of divergent passive margin basins along the west coast of Africa. Covering a total area of about 6.2 × 10^{3} km^{2}, it was formed during the Mesozoic to Tertiary, as a result of the rifting of Africa and South America during the break-up of the Gondwana supercontinent [

The geological formations of Kribi in South Cameroon belong to four major lithological and structural units [

In general, the region has a complex and uneven tectonic structure. This tectonic seems to have given rise to a vertical movement of the basement with subsidence to the North and uplift to the South [

Two dataset have been combined to carry out this work: the existing ORSTOM data and the newly collected ones. In

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:

B ( ω ) = ∑ 0 N − 1 b ( x ) exp ( − i ω x ) ⋅ Δ x (1)

where b ( x ) represents the discrete N data array of gravity data obtained by sampling a continuous profile at evenly spaced intervals Δ x . 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:

B ( k ) z = 0 = 2 π Δ ρ G ⋅ exp ( − 2 π k t ) ⋅ F ( k ) z = 0 (2)

where B ( k ) z = 0 is the Fourier transform of the Bouguer anomaly profile b ( k ) z = 0 ; Δ ρ 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:

h = Δ L o g E 4 π Δ k (3)

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

When the square of the Fourier amplitude spectrum is plotted against 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 to basement of the anomaly sources.

The Euler Method is a technique generally used to locate the apparent depth to the gravity or magnetic anomaly source. Considering a degree of homogeneity, the gravity or magnetic field is related to its gradient component in order to trace the surface of the ground contact. The degree of homogeneity is expressed by the structural index which defines the measure of the fall-off rate of the field with distance from the source. The Euler homogeneity equation is given as:

( x − x 0 ) ∂ T ∂ x + ( y − y 0 ) ∂ T ∂ y + ( z − z 0 ) ∂ T ∂ z = N ( B − T ) (4)

where (x_{0}, y_{0}, z_{0}) is the position of the magnetic or gravity source whose total field (T) is detected at (x, y, z,). B is the regional gravity or magnetic field. N is the measure of the fall-off rate of the gravity field and may be interpreted as the structural index (SI). This value needs to be chosen according to a prior knowledge of the source geometry.

The Euler depth appear wherever there are lithological discontinuities in the geological formations. They represent the structural and/or stratigraphic changes of various geological formations [

According to [

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

H G M ( x , y ) = ( ∂ G ∂ x ) 2 + ( ∂ G ∂ y ) 2 (5)

where G is the Bouguer gravity field.

The horizontal gradient method is used to locate the boundaries of density contrast from gravity data. These results mark the top edges of gravity or density boundaries. Thus, the maximum value of the horizontal gradient anomalies is placed on top of the sources edges. However, offsets occur when edges are not vertical or when several anomalies are close together. 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 (x- and y-directions) of the gravity field [

The works of [

The gravity anomaly maps generally superpose the effects of deep, shallow, local and extended gravity contrasts. The effects of a local or shallow structure are often hidden in the signatures of regional structures. We carried out regional-residual separation using the polynomial fitting method with the aim of isolating the anomalies caused by deep and extended sources (long-wavelength anomalies) from those caused by local and shallow density contrasts (short wavelength anomalies). The residual field is obtained by estimating the regional gravity field and removing it from the observed field which is the Bouguer anomaly (

the wavelength of the residual anomaly decreases when the degree of the polynomial increases thereby revealing geological structures which appear closest to the surface [

In this work, we have used a polynomial of degree ‘1’, for spectral analysis, Horizontal gradient and Euler deconvolution, so as to have a better chance of locating the major contacts.

In

In

The first order residual map ^{3} [^{3}.

We applied a 2D spectral analysis on grids centered on positive anomalies in the basin situated on the western area of the map (A1, A2, A3, A4, A5, A6, and A7), which enabled us to determine the depths. The power spectrum has been

obtained from the energy values which derive from the anomaly values. The given values are presented in

We used the Oasis montaj 8.0 software to calculate the amplitude of the horizontal gradient of the residual data of the study area (

ANOMALY VALUES/ mGal | POWER SPECTRUM | LOG (POWER SPECTRUM) | WAVENUMBER/ km^{-1} |
---|---|---|---|

−50.00 | 87.44 | 1.95 | 0.02 |

−12.60 | 2.06 | 0.33 | 0.07 |

−4.19 | 0.40 | −0.40 | 0.09 |

−7.58 | 0.11 | −0.95 | 0.11 |

−9.22 | 0.04 | −1.40 | 0.15 |

ANOMALY ID | DEPTH TO BASEMENT/km |
---|---|

A1 | 2.62 |

A2 | 2.84 |

A3 | 3.48 |

A4 | 0.97 |

A5 | 3.93 |

A6 | 0.60 |

A7 | 1.19 |

to Nyabessan and from Lolabe to the south east of Campo. The east of Bipindi is also characterized by high gradient variation. This could mark the huge change in density between the Bipindi intrusive block and metamorphic formations surrounding it. Due to the broad nature of the high gradients, we suggest that the boundaries of density contacts in the Kribi-Campo basin are probably not

necessarily vertical and relatively deep or produced by several boundaries.

For the Euler method, the following parameters have been used to compute the Euler solutions: structural Index N = 0.5, maximum % of tolerance of 5 and a window size of 5 km × 5 km.

The combination of the above described results, namely the spectral analysis, horizontal gradient and Euler solutions coupled with the results published by [

FAULT ID | DIRECTION | DEPTH RANGE/km |
---|---|---|

f1 | SW-NE | 2 - 5 |

f2 | SSW-NNE | 2 - 4 |

f3 | SW-NE | 7 - 13 |

f4 | NS | 2 - 5 |

f5 | SW-NE | 2 - 6 |

f6 | NNW-SSE | 6 - 12 |

f7 | SW-NE | 2 - 8 |

basin (_{1}, C_{2} and C_{3}) corresponding to horizontal limits of intrusive bodies.

The results presented in the above sections are in accordance with the fact that the Kribi-Campo basin formations are relatively shallow compared to the Douala and Garoua basins. The general disposition of anomalies on the first order residual anomaly map (

obtained by spectral analysis, it can be seen that as we move from the North to the South of the study area, the sedimentary layer varies following a sinusoidal trend. This variation of the sediment thickness could be explained by the presence of high tectonic activities in the area [_{1}, C_{3} and C_{3} representing rocks intrusions amongst which one had been characterized and modelled in the works of [

also suggests that given the high sedimentary thickness, the area situated between Kribi and Lolabe and the Campo locality are of high potential in mining and/or hydrocarbon resources. This suggestion is supported by the fact that the presence of oil and gas in a basin might be due to two factors: in-situ generation and migration of fluids into the basin [

The aim of this study was to provide new insights on the structural setting and the geometrical characteristics of the Kribi-Campo basin. We used the polynomial fitting method to carry out the separation of the residual and regional components of the gravity field. We observed that the positive residual anomalies in the area are the effect of both high density rocks intrusions and sedimentary infill. The spectral analysis enabled to estimate the depth to basement on various parts of the Kribi-Campo basin which gives the sedimentary thickness. This thickness varies from 0.60 km to 3.93 km with the highest values obtained in some specific localities of the study area namely Campo and the area between Kribi and Lolabe. From the residual anomaly map and spectral analysis it can be deduced that the sedimentary infill presents a discontinued nord-south variation and also decreases from the west towards the east as we move from the coast into the continent. We applied the horizontal gradient analysis to the residual component. The residual structural setting of the zone from the Euler method is characterized by major faults and contacts mainly oriented SW-NE with the shallowest in the west (from 2 to 7 km deep) and the deepest in the east (right down to 20 km deep) of the region. The use of spectral analysis and euler solutions is very advantageous in the geometrical and structural caracterization of gravity anomalies in the sense that they help not just to determine depths to basement of causative tructures but also to evaluate their dip and their evolution in the longitudinal and transversal directions. The structural map of the basin provides the most relevant structural information in the area. This map can help in identifying the direction of fluid flow in the subsurface. The interpretation of the sedimentary thickness values can serve to identify areas with the highest mineral and hydrocarbon production potentials which correspond to areas with the highest sedimentary thickness.

The authors are thankful to the University of Yaoundé 1 and the Ministry of Higher Education for funding the data acquisition campaign. All reviewers of this manuscript are equally acknowledged.

Malquaire, K.P.R., Clotilde, O.A.M.L., Nfor, N., Théophile, N.M. and Eliezer, M.D. (2017) Determination of Structural and Geometrical Parameters of the Kribi-Campo Sedimentary Sub-Basin Using Gravity Data. International Journal of Geosciences, 8, 1210-1224. https://doi.org/10.4236/ijg.2017.89069