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Refinement of Bouguer anomalies derived from the EGM2008 model,impact on gravimetric signatures in mountainous region:Case of Cameroon Volcanic Line,Central Africa

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Earth and Planetary Physics 2020年第6期4卷 639-650页

作者： Paul Gautier Kamto Cyrille Mezoue Adiang Severin Nguiya Joseph Kamguia loudi yap Geodesy Research Laboratory National Institute of Cartography(NIC)YaoundeCameroon Department of Physics Faculty of ScienceUniversity of Yaounde ICameroon Laboratory E3M Faculty of Industrial EngineeringUniversity of DoualaCameroon Department of Physics Faculty of ScienceUniversity of DoualaCameroon

Global geopotential models have not included the very high frequencies of the Earth’s external gravity *** is called omission *** omission error becomes more important in mountainous areas(areas with highly variable topography).The work reported here consists in reducing the omission error in measurements of Bouguer gravity anomalies,by refining the global geopotential model EGM2008 using the spectral enhancement *** method consists in computing the residual terrain effects and then coupling them to the gravimetric signal of the global geopotential *** compute the residual terrain effects,we used the Residual Terrain Model(RTM)*** refine it required a reference surface(ETOPO1)developed up to degree 2190(the maximum degree of the EGM2008 model)and a detailed elevation model(AW3D30).Computation was performed with the TC program of the GRAVSOFT *** topography of the study area was assumed to have a constant density of 2670 kg/*** the inner and outer zones,the respective integration radii of 10 km and 200 km have been *** obtained very important RTM values ranging from−53.59 to 34.79 *** values were added to the gravity anomalies grid of the EGM2008 model to improve accuracy at high *** a part of the Cameroon Volcanic Line and its surroundings(mountainous area),we made a comparison between the residual Bouguer anomalies before and after *** report differences ranging from−37.40 to 26.40 *** conclude that the impact of omission error on gravimetric signatures is observed especially in areas with high variable topography,such as on the Cameroon Volcanic Line and around the localities of Takamanda,Essu,Dumbo,and *** finding illustrates the great influence that topography has on accurate measurement of these gravity anomalies,and thus why topography must be taken into *** can conclude that in preparing a global geopotential model,a high resolution DTM must be used to decrease

关键词： residual Terrain Model EGM2008 Omission error refined Bouguer anomalies mountainous area

Vertical accuracy evaluation of freely available latest high-resolution(30 m)global digital elevation models over Cameroon(Central Africa)with GPS/leveling ground control points.

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International Journal of Digital Earth 2019年第5期12卷 500-524页

作者： loudi yap Ludovic Houetchak Kande Robert Nouayou Joseph Kamguia Nasser Abdou Ngouh Marie Brigitte Makuate Research Laboratory in Geodesy National Institute of CartographyYaound3Cameroon Department of Physics Faculty of SciencesUniversity of Yaoundé1YaoundéCameroon

Digital Elevation Models(DEMs)contain topographic relief data that are vital for many geoscience *** study relies on the vertical accuracy of publicly available latest high-resolution(30 m)global DEMs over *** models are(1)the ALOS World 3D-30 m(AW3D30),(2)the Shuttle Radar Topography Mission 1 Arc-Second CBand Global DEM(SRTM 1)and(3)the Advanced Spaceborne Thermal Emission and Reflection Global DEM Version 2(ASTER GDEM 2).After matching their coordinate systems and datums,the horizontal positional accuracy evaluation was carried out and it shows that geolocation errors significantly influence the vertical accuracy of global *** this,the three models are compared among them,in order to access random and systematic effects in the elevation data each of them ***,heights from 555 GPS/leveling points distributed all over Cameroon are compared to each DEM,for their vertical accuracy *** and robust statistical measures,normality test,outlier detection and removal were used to describe the vertical quality of the *** test of the normality rejected the hypothesis of normal distribution for all tested global *** vertical accuracies obtained for the three models after georeferencing and gross error removal in terms of Root Mean Square(RMS)and Normalized Median Absolute Deviation(NMAD)are:AW3D30(13.06 m and 7.75 m),SRTM 1(13.25 m and 7.41 m)and ASTER GDEM 2(18.87 m and 13.30 m).Other accuracy measures(MED,68.3% quantile,95% quantile)supply some evidence of the good quality of AW3D30 over ***,the effect of land cover and slope on DEM vertical accuracy was also *** models have proved to be worse in the areas dominated by forests and shrubs *** 1 and AW3D30 are more resilient to the effects of the scattering objects respectively in forests and cultivated *** dependency of DEMs accuracy on the terrain roughness is *** all slope intervals,AW3D30 is performing

关键词： DEM AW3D30 SRTM 1 ASTER GDEM 2 Cameroon GPS/leveling orthometric heights vertical accuracy

Reference Ellipsoid Parameters of Cameroon from GPS Data

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International Journal of Geosciences 2016年第8期7卷 1049-1056页

作者： Ludovic Houetchak Kande Joseph Kamguia loudi yap Louis Foyang National Institute of Cartography Yaounde Cameroon

The standard spheroidal reference surface also referred as the reference ellipsoid is a part of mapping basic infrastructures of a country. In Cameroon, the ellipsoid Clarke 1880 with unknown parameters used for a long time as reference system in basic mapping, has lead to the use of World Geodetic System 1984 (WGS 84) whose parameters are well known. Meanwhile, the latter is not accurate locally due to the fact that the best ellipsoid is the one that fits very well with the local geoid. In order to look for the parameters of the local ellipsoid that fits best to the local geoid (Cameroon Geoid Model 2011 (CGM11)), the Global Positionning System (GPS) data made of 525 geodetic ground control points of the new geodetic network of Cameroon set up in 2011 were used. These GPS measures provide for each point the values of the ellipsoid height and the elevation which are used to determine the parameters of the local ellipsoid model for Cameroon through the least square form of the Molodensky analytical method. The results are given as the difference in ellipsoidal height relative to the parameters of the WGS84 ellipsoid assuming the two ellipsoids in their parallel. These results show that the obtained ellipsoid fits better to the local geoid with 1.072 m as the standard deviation value, is improved considerably in comparison to the previous studies conducted in Cameroon whose standard deviation was fairly equal to 1.679 m.

关键词： Ellipsoid Parameters Geoid GPS Molodensky Analytical Method Cameroon

Evidence for the Upwelling of Mafic Bodies into the Upper Crust beneath the N40-50°E Branch of the Pan-African Central Cameroon Shear Zone from Gravity Studies

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Open Journal of Geology 2018年第3期8卷 278-297页

作者： loudi yap Robert Nouayou Depesquidoux Tchaptchet Tchato Joseph Kamguia Apollinaire Bouba Department of Physics Faculty of Sciences University of Yaoundé 1 Yaoundé Cameroon Research Laboratory in Geodesy National Institute of Cartography Yaoundé Cameroon Department of Earth Sciences Faculty of Sciences University of Yaoundé 1 Yaoundé Cameroon Department of Physics Advanced Teacher’s Training College University of Maroua Maroua Cameroon

The existence of mafic bodies at mid crustal level beneath the Pan-African Central Cameroon Shear Zone is still a matter of debate. To provide additional constrains on this issue, the crust of the west region of Cameroon has been investigated using gravity data. Analyses of these data show N40-50°E oriented iso-anomal contours in the Bafoussam area, interpreted as the N40-50°E branch of the Central Cameroon Shear Zone. In addition, spectral analysis and 2.5D gravity modeling reveal intrusions of mafic bodies at depth between 3.2 and 14.2 km under N40-50°E aligned volcanic centers, namely Mt Bambouto and Mt Mbapit. The above observations suggest a structurally controlled emplacement of the mafic bodies. In the light of the recent geophysical data, the interaction between the NE-ward channel flow operating at the bottom of the lithosphere or the asthenosphere upwelling and the Cameroon Shear Zone could better explain the magma upwelling in the upper crust. This result is the novelty of the present work.

关键词： West Region Mafic Bodies Central Cameroon Shear Zone Gravity 2.5D Modelling

Subsurface Structural Mapping Using Combined Terrestrial and Grace Gravity Data of the Adamawa Plateau (North-Cameroon)

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International Journal of Geosciences 2017年第7期8卷 869-887页

作者： Bouba Apollinaire Kamguia Joseph Tabod Charles Tabod yap loudi Nouayou Robert Kande Houetchak Ludovic Oyoa Valentin Department of Physics Higher Teachers’ Training College University of Maroua Maroua Cameroon Department of Physics Faculty of Science University of Yaounde I Yaounde Cameroon National Institute of Cartography Yaounde Cameroon

In order to analyze and determine the geological structures of the Adamawa plateau, the terrestrial gravity data were combined to data computed from GGM02C gravity model. The dense gravity net obtained were further introduced into qualitative and quantitative interpretations. The resulting Bouguer anomaly map obtained from combined data shows NE-SW direction which nearly coincides with the main direction of the fractures affecting the basement in the region and indicates strong gradients marking the presence of discontinuities between heavy and negative gravity anomaly. In order to conduct the quantitative interpretation of the combined gravity data, three profiles were drawn on the residual Bouguer anomaly map and therefore were interpreted using spectral analysis method and 3D density inversion. The knowledge of the depth and density of the geological structures show an uplift of dense rocks under the granite-gneiss substratum. This dense material found in the ENE-WSW direction of the Adamawa Plateau is interpreted as basaltic intrusion probably resulting from tectonic processes. According to this study, the depths of 3.83 km and 9.62 km are the new values of depths obtained for futures investigations in the Adamawa plateau.

关键词： GGM02C Adamawa Plateau Bouguer Anomaly 3D Density Model Spectral Analysis