Abstract The aeromagnetic data of Idogo, Southwestern Nigeria, have been used to study the lithology and to determine the magnetic source parameters within Idogo and its environs. Idogo lies between latitudes 6°30′N and 7°00′N and between longitudes 2°30′E and 3°00′E. The magnetic anomaly map, the regional geology, the analytic signal and the local wavenumber were used to identify the nature and depth of the magnetic sources in the region. Data enhancement was carried out to delineate the residual features relative to the strong regional gradients and intense anomalies due to the basin features. The estimated basement depth using the horizontal gradient method revealed depths ranging between 0.55 km and 2.49 km, while the analytic signal amplitude and local wavenumber methods estimated depth to the magnetic sources to range from 0.57 km to 4.22 km and 0.96 km to 2.43 km, respectively. Depth computations suggested the presence of both shallow and deep sources. The total magnetic intensity values ranged from 3.1 nT to 108.3 nT. The area shows magnetic closures of various sizes in different parts of the area trending West, with prominence at the centre and distributed East–West.
[1] This article presents the first results regarding the investigation of the response of the equatorial ionospheric F region in the African sector during geomagnetic storm periods between April 2000 and November 2007 using GPS-derived vertical total electron content observed at Libreville, Gabon (0.35°N, 9.67°E, dip latitude −8.05°S). We performed a superposed epoch analysis of the storms by defining the start time of the epoch as the storm onset time. During geomagnetic storms, the altered electric fields contribute significantly to the occurrence of negative and positive ionospheric storm effects. Our results showed that the positive storm effects are more prevalent than the negative storm effects and generally last longer irrespective of storm onset times. Also, the positive storm effects are most pronounced in the daytime than in the premidnight and postmidnight periods.
The greatest source of error in position estimate and precise time transfer using Global Positioning System (GPS) satellites is range delays, which degrade GPS signals as they propagate through the ionosphere. In order to improve the accuracy of GPS position fixing, it is important to have a good understanding of ionospheric time delay. In this paper, we have studied the diurnal and seasonal variations of ionospheric time delay of radio signals from GPS satellites and their dependence on solar flux (10.7 cm) index during low solar activity period (March 2008 to December 2009) over an equatorial station, Lagos, Nigeria (6.5°N, 3.4°E; magnetic latitude 3.03°S). It is found that diurnal variation of ionospheric time delay show maximum values around 14h00-16h00 LT during all the months considered. The study reveals that during low solar activity, the time delay values are high in equinox months, least during summer and moderate in winter. The correlation between average daytime peak ionospheric time delay and the solar F10.7 flux shows low positive correlation, with Correlation Coefficient of R = 0.31 for 2008 and R = 0.15 for 2009. Keywords: Ionospheric time delay, Total electron content (TEC), GPS, navigation
This work examines the performance of four ionospheric models for estimating Total Electron Content (TEC) over Nigeria.The observed Vertical TEC values retrieved from five GPS stations located between geographic latitudes 4.80° and 12.47°N were analyzed and compared with corresponding values obtained from the International Reference Ionosphere (IRI-01corrected option), the IRI-Plasmasphere 2017, NeQuick-2 and Nigerian Total Electron Content (NIGTEC) models.NIGTEC model used in this work is a neural network based model developed at the Nigerian Centre for Atmospheric Research as a regional model, while the IRI-01cor, IRI-Plas 2017 and NeQuick are well-known global ionospheric models.We evaluated TEC from the four models at hourly levels for all the days of the year 2012 (sunspot number Rz = 84.4).TEC directly derived from the five GPS stations under consideration was also evaluated for all the days of the year.The paper also considered the performance of the models under geomagnetically quiet condition (Ap ≤ 5).The NIGTEC model has shown better agreement with the observed VTEC when compared with the IRI-01cor, NEQUICK and IRI-Plas 2017 models especially during the pre-Sunrise period.With mean values of RMSE as 3.378 TECU and 3.8403 TECU for all days and geomagnetically quiet condition respectively, NIGTEC returned the lowest RMSE values at all conditions.The modelled TEC obtained from all the four models, at both geomagnetically disturbed and quiet times, follow the diurnal pattern of the observed GPS-TEC with variations in magnitudes.The NIGTEC model performed better than IRI-01cor, NEQUICK and IRI-Plas 2017 even when only geomagnetically quiet condition was considered.The NeQuick and NIGTEC model correctly captured the annual distribution pattern of the observed GPS-TEC across the stations.This distribution was over-smoothed by the IRI and IRI-Plas models.
This research analyzed the Transient (Quiet and disturbed conditions) variation of Total Electron Content (TEC) within the Equatorial Ionization Anomaly (EIA) region and outside the anomaly region in the area of ground Global Positioning System (GPS) stations at Federal University of Technology Akure (FUTA) in Nigeria (a station in the EIA region) and Matera in Italy (a station in the middle latitude region) for period of 2008 to 2010. The work shows variation of TEC in function of the daily timing, geographical positioning of the studied area and seasons. The study correlates the TEC to the development of the investigated regions and to the variation of ultraviolet solar radiation and neutral winds. The data were collected using special equipment (Novatel GSV 4004B GPS SCINDA system installed at FUTA Nigeria by the US Air Force Research Laboratory (AFRL) USA (PI-keith Groves)) and the International GNSS Service (IGS) GPS system at Matera Italy. The diurnal variation shows TEC maximum on afternoon, for quiet days and almost similar variations, with small changes for the disturbed days. The difference observed between quiet and disturbed days are correlated with different space weather conditions. A predawn minimum (low in EIA region) and pre-midnight enhancement (higher at EIA region) were observed in each area investigated. Similarly the monthly and seasonal variations were analyzed. The study shows that TEC value is usually higher at the EIA region than the middle latitude region. But when storms occurs TEC Value at the middle latitude is higher. It observed from the study that TEC in the EIA region is mainly controlled by the EIA development, while the TEC variation in the middle latitude region is controlled by extreme ultraviolet solar radiation and neutral winds.
Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Tomography (ERT), with Schlumberger and Wenner electrode array configurations respectively were employed to investigate the groundwater potential of Laderin Housing Estate located at Oke-mosan, Abeokuta, Ogun state, southwestern Nigeria. The area is underlain by the basement complex of the southwestern Nigeria. The research aimed at determining the aquifer/groundwater zone and characterizes the lithology of the study area. The geophysical survey involving nine VES and four profile of 2-D ERT lines with lengths varying from 100 m to 150 m were carried out. The field data from both the ERT and VES were processed and interpreted using RES2DINV and WINRESIST software respectively. Geometrical effects from the pseudo-section were removed and an image of true depth and true formation resistivity were produced. Three to four geo-electric layers were revealed in all, which are; topsoil, weathered layer, fractured and fresh basement rock. The result of 2 D inversion provide lithologic unit, weathering profile and geological structure favourable for groundwater potential. The results show that the basement rock was delineated with resistivity values that range between 701.3 Ωm and 9459.3 Ωm. The bedrock topography has a variable thickness of overburden between 3 m and above 16.4 m, which is fairly shallow. The geophysical survey show that VES 2, VES 3, VES 4, VES 5, and VES 6 are possible location for groundwater extraction. The difficult terrain, where thick overburden are located are also promising target for groundwater development.
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