The Local Geoid Model of Cameroon: CGM05

This paper deals with the geoid determination in Cameroon by a gravimetric solution. A number of data files were compiled for this work, containing about 62,000 points on land and ocean areas and also including data derived from satellite altimetry. A hybrid global geopotential model (EGM-GGM) supplied the longer wavelength components of this geoid model, CGM05. This global model is obtained by adjusting the GRACE model GGM02C to degree and order 360 using the harmonic coefficients of the model EGM96 beyond the maximal degree 200 of GGM02C. The medium wavelength components were computed using the best gridded residual gravity anomalies, by integration in Stokes’ formula. The digital terrain model GLOBE contributed to its short wavelength components. The residual terrain model (RTM) was applied to first determine a quasi-geoid model. This intermediate surface was converted to the geoid using a grid of simple Bouguer gravity anomalies. The validation of CGM05 is based on comparisons to global and regional geoids. A GPS/levelling geometric geoid computed in a small part of the target area shows that the absolute accuracy of this local geoid model is 14 cm. After a four-parameter fitting to the GPS/levelled reference surface, this absolute accuracy reduced to 11 cm

Accurate gravity anomaly interpolation: a case-study in cameroon, central africa

Many treatments in geodesy and geophysics require regularly gridded gravity anomalies. The gridding of gravity data needs interpolation. For the predicted data to be accurate, the smoothest type ofgravity anomaly should be used along with the most indicated prediction method. This paper presents the comparison of various prediction methods applied on different types of gravity anomalies andconsidering the relative geological complexity of the study area. Many algorithms are tested and the suitability of each type of anomaly and each prediction method discussed in a case-study in Cameroon (Central Africa), using a set of 43,000 gravity data points to determine the must accurate prediction technique

Evidence for upper mantle intrusion in the west African coastal sedimentary basins from gravity data: the case of the southern part of the Douala basin, Cameroon

The Bouguer anomaly map of the region between latitudes 30N and 3045’N and longitudes 9030’E and 10010’E and which forms the southern part of the Douala Basin, shows ring-like positive contour lines. The Bouguer gravity profiles obtained across the gravity anomaly contour lines in the region have been interpreted using 2D1/2 gravity modelling. The results reveal that in the southern part of the Douala Sedimentary Basin, two major structures exist: a half-dome of mantle material explained by isostatic compensation and a pillar of high density rocks probably representing an upper mantle intrusion down to a depth of about 14 km. The similarities of these results with those obtained in the coastal sedimentary basin of Mauritania-Senegal suggest a more extensive movement that would have affected the whole of West African coast.KEYWORDS: Bouguer anomaly, dome, graben

The crustal structure along the Mbere trough in South Adamawa (Cameroon) from spectral analysis and gravity modelling

The Mbere Cretaceous trough is located in the southern part of the Adamawa province. A gravity interpretation based on data obtained from three NW-SE profiles on the residual anomaly map has been carried out using a 2.5D modelling program. Spectral analysis has been used to estimate the depth of geological structures. The results obtained show that the metamorphic formations consist of gneiss and migmatite which are located in the north of the trough at a mean depth of 9 km. The gradient observed on the Bouguer anomaly map along the trough might not only be due to the northern fault but also due to the contact between the gneiss (and migmatite) and granite under the trough. The thickness of sediments along the trough varies between 1500 and 2250 m with a minimum at Yariban and a maximum at Djohong. The trough is bounded on its sides by faults related to the Central African Shear Zone. Basaltic rocks associated with the Cameroon Volcanic Line are present as intruded rocks lying at a minimum depth of 7200 m. Keywords: bouguer anomaly, residual anomaly, fault, trough Global Journal of Pure and Applied Sciences Vol. 12(1) 2006: 111-11

Moho Discontinuity Depth Estimates for the Cameroon Volcanic Line from Gravity Data

In this work, the Moho depth in a part of the Cameroon Volcanic Line (CVL) between the latitudes 3°30´ to 6°33´N and the longitudes 8°50’ to 11°27’E, has been estimated covering two distinct regions: the Mount Cameroon and the Bamenda by the use of polynomial separation of gravity data and spectral analysis along two profiles. The Moho is uplifted in the Mount Cameroon region, where the crust is thinned to about 24 km. In the Bamenda region, the crustal thickness is found to be normal at about 31 km. The high positive gravity anomalies of up to 100 m Gal observed in this area indicate the thinning of the crust in the Mount Cameroon region. Seismic and gravity data indicate a crustal thickness of 30-34 km along the continental parts of the Cameroon Volcanic Line, except in the Adamawa plateau, where the crust’s thickness ranges between 20-23 km. The crustal thickness of about 31 km in the Bamenda region is an evidence of perfect isostatic compensation, which suggests a deep seated source for the negative anomaly, resulting in a general asthenospheric uplift along the Cameroon Volcanic Line

Programme Fortran 77 d\'ajustement du modèle de géoïde EGM96 sur la surface de référence de points GPS/nivelés.

Les altitudes recherchées lors du nivellement sont mesurées par rapport à une surface de référence : le géoïde. Le nivellement géométrique est un processus long et onéreux, on préfère désormais procéder par nivellement GPS (Global Positioning System). La surface de référence est alors réalisée en ajustant le modèle de géoïde dont on dispose sur la référence obtenue d'un ensemble de points GPS/nivelés. Les modèles de géoïde locaux ne sont pas encore disponibles dans toutes les régions. Un modèle de géoïde global peut alors être utilisé. Cet article présente un programme exécutable sur un environnement Fortran, qui permet d'ajuster le modèle de géoïde global le plus utilisé EGM96 par moindres carrés dans une région, à partir d'une modélisation par régression linéaire. Il fournit en sortie tous les résultats dans un fichier texte. Le programme crée également une grille de géoïde ajusté dans laquelle l'utilisateur peut interpoler l'ondulation ajustée en un nouveau point où il vient de faire des mesures GPS. Les résultats d'un test à partir des données sur le Cameroun montrent les résidus escomptés, avec un écart-type 0,08 m. Ce programme présente ces avantages qu'il utilise la valeur exacte de l'ondulation EGM96 en chaque point du fichier d'entrée de l'utilisateur et a un temps d'exécution optimal quel que soit le nombre de points pris en compte.Heights obtained from levelling are determined above a reference surface: the geoid. Spirit levelling is a timeconsuming high cost process. An alternative leveling process by GPS (Global Positioning System) is now used instead. The reference surface is therefore realised by fitting the available geoid model to the surface obtained from a set GPS/levelled points. However, local geoid models are still not available in many parts of the world. A local geoid can be replaced by a global one to achieve this. The program presented in this paper is executable on a Fortran surrounding and it permits to fit the must widely used global geoid EGM96 in the least squares sense in a region, from a linear regression modelling. The results are presented in an output text file and an output grid where the user can interpolate the adjusted geoid undulation at a point where he made new GPS measurements. An experimental test obtained form data in Cameroon produced reasonable residuals, with a standard deviation of 0.08 m. Some advantages of using this program are that it computes the exact value of the EGM96 geoid undulation at all points of the user's input file and its execution is very fast, regardless the number of input data points. Keywords: Ajustement, nivellement, géoïde, moindres carrés, interpolation; Fitting, levelling, geoid, least squares, interpolation. Journal des Sciences Pour l\'Ingénieur. Vol. 9 2008: pp. 11-2

The structure of the Goulfey-Tourba sedimentary basin (Chad-Cameroon): a gravity study

Se reporta un estudio gravimétrico de la cuenca Goulfey-Tourba, sureste del Lago Chad, norte del Camerún, cuyo objetivo ha sido establecer las principales características estructurales de la corteza somera. Se interpreta, en terminos de la información geológica disponible, las anomalias regional y residual. Análisis espectral y modelado 2.5D de tres perfiles seleccionados del mapa de anomalía residual proporcionan profundidades al basamento de 4.0, 2.0, y 2.9 km. Estas profundidades constriñen los modelos gravimétricos a lo largo de los perfiles, e indican que el relleno sedimentario tiene espesores variables. De esta forma, se tienen tres sub-cuencas a lo largo de la depresión de Goulfey-Turba (Goulfey, Tom-Merifine, y Tourba). Estas subcuencas son de origen tectónico. Ellas están asociadas con la tectónica de extension que ha afectado la region que va de la depresion de Benue hasta la cuenca de Chad. La sub-cuenca de Goulfey es la más profunda. El relleno sedimentario está probablemente constituido por arenas arcillosas de depositos aluviales, areniscas o margas. Las profundidades al basamento fueron constreñidas por los resultados del análisis espectral. El basamento comprende granite y gneiss, con intrusiones basálticas. Las fallas inferidas son de tipo normal. doi: https://doi.org/10.22201/igeof.00167169p.2010.49.4.12