Journal of Geosciences and Geomatics

Joint inversion of body wave receiver functions and dispersion data was used to model the shear wave velocity distribution of the crust and upper mantle below VLC (44.16° N, 10.39° E), a broadband seismographic station in Italy. Receiver functions are primarily sensitive to the shear wave velocity contrasts and vertical travel times, whereas the surface wave dispersion measurements are sensitive to absolute vertical shear wave velocity averages and changes as function of depth. Each data set has inherent lapses but by jointly inverting both we are able to draw on the capabilities of one to compensate the imperfections of the other and this provides better S-wave velocity constraints than we would obtain by inverting either data set individually. The receiver functions were computed from the teleseismic earthquakes recorded by VLC station between 2005 and 2012, while the dispersion curves at regional scale were determined by the Frequency Time Analysis and have been used to obtain tomography maps, using the two-dimensional tomography algorithm developed by Ditmar and Yanovskaya in 1987. The inversion results include a crust with a sharp gradient near the surface (shear velocity changing from 2.15 to 3.4 kms^-1in 5 km) underlain by a 13-km-thick layer with a shear velocity of and 3.4 kms^-1 another 15-km- thick layer with a shear velocity of 3.72 kms^-1, and an upper mantle with an average shear velocity of 4.4 kms^-1. The crust–mantle transition has a significant gradient, with velocity values varying from 3.72 to 4.4 kms^-1 at about 32 km depth. This result is also in agreement with shear wave velocity cross-section of the area obtained from ITA-LSO data sets.

Journal of Geosciences and Geomatics. 2015, 3(4), 109-115. DOI: 10.12691/jgg-3-4-3
Pub. Date: September 01, 2015

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Mineral Alteration and Chlorite Geothermometry in Platinum Group Element (PGE)- Bearing Meta-ultramafic Rocks from South East Cameroon

by Ako T. A., A. Vishiti, K. I. Ateh, A. C. Kedia and C. E. Suh

Original Research

The meta-ultramafic rocks that are part of the Paleoproterozoic unit termed the Nyong Series in SE Cameroon were investigated in this study. The lithologic assemblage mapped is exposed on a cliff face and consists of distinguishable horizons that include least, moderately to intensely altered pyroxenite to amphibolite units. The rocks are partially to completely serpenitinized and foliated. The main mineral phases identified under the microscope include pyroxenes (clinopyroxene and orthopyroxene), olivine, hornblende, plagioclase, garnet and sulphides. The rocks depict variable alteration of the pyroxenes and other primary minerals such as olivine to actinolite, chlorite, serpentine, talc, epidote and tremolite. Electron microprobe analysis on chlorite show that the principal chlorite type ranges from talc-chlorite to penninite. Using the chlorite geothermometer it is observed that the hydrothermal alteration temperatures vary between 160-180°C. This has been overprinted by surface temperatures (20 - 40°C) during the process of weathering.

Journal of Geosciences and Geomatics. 2015, 3(4), 96-108. DOI: 10.12691/jgg-3-4-2
Pub. Date: August 21, 2015

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Determination of the best-fit Tropospheric Delay Model on the Nigerian Permanent GNSS Network

by Dodo Joseph Danasabe, Ojigi Lazarus Mustapha and Tsebeje Samuel Yabayanze

Original Research

The Federal Government of Nigeria through the Office of the Surveyor General of the Federation (OSGoF) set up surveying infrastructure throughout the country known as the NIGerian Reference GNSS NETwork (NIGNET). The NIGNET is a network of Global Navigation Satellite System (GNSS) Continuously Operating Reference Stations (CORS) set up at different locations in Nigeria for surveying and mapping. They are satellite tracking stations operating 24 hours a day providing positional solutions. As signals from the satellite pass through the different layers of the atmosphere (ionosphere and troposphere), they are refracted thus, causing delay on the arrival of the signal, which in-turns affect positioning in the horizontal and height component. The most dominant spatially correlated bias is the tropospheric effect on the GNSS satellite signals. Several global tropospheric delay models are in use by different countries to mitigate the biases cause by the troposphere. This study therefore aim to determine the best-fit tropospheric delay model for the NIGerian GNSS Reference NET work (NIGNET) using data collected from the NIGNET stations across Nigeria. Three different global tropospheric models, namely; the Saastamoinen model, Hopfield model and Niell models were used, and results compared. Four processing strategies were adopted. The first strategy was without the application of any of the models, while in the second, third and fourth strategies, the GNSS data were processed with the application of each of the models. The results indicate that, the Niell model has the lowest mean zenith tropospheric delay (ZTD) of 2.330m with root mean square error (rmse) of 0.45m, while the Hopfield and Saastamoinen models have ZTD of 2.386m and 2.398m with rmse value of 0.60m and 0.71m respectively. On the overall, the Niell model has better performance in the network. This suggests that, the application of Niell model in the processing of all GNSS data will give a more reliable result in the position domain as well as the height component. The results are very useful to surveyors and geodesist engaged in surveying and mapping, and spatial positioning of infrastructures. It will enhance the effectiveness and reliability of the tropospheric delay resolution process for regional Global Positioning System (GPS) network users.

Journal of Geosciences and Geomatics. 2015, 3(4), 88-95. DOI: 10.12691/jgg-3-4-1
Pub. Date: August 16, 2015

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