A cascade method of instrumental drift correction to gravimetric data which ensures flexibility of field procedure is presented. The method strikes a compromise between the requirements of a least squares adjustment and the inadequacy of a reference drift curve method of drift removal from data obtained in complicated field procedure. The basic principle is that when all observations in a day are referred to the same time origin during a day's work, repeat observations at the same station after drift correction should be equal. The underlying assumption is that the drift of the instrument is a linear function of time over a short time interval. The method requires that measurements should be done in loops and that observations terminating the outermost loop must either be made at the same station or at different stations whose absolute gravity values are known. Linear drift corrections are then carried out from one loop to another in a cascade manner. A test of the model was made using the data of the Primary Gravity Network of Nigeria. The results of the drift determinations with the cascade method compare favourably with those obtained with the least squares method. The major advantages of the cascade model are summarized. It is further deduced that the cascade model can be used to determine drift corrections in similar surveys where instruments other than the gravimeters are used
Journal of Environmental Hydrology Volume 16 Paper 29 September 2008 1 Resistivity and induced polarization imaging was conducted at an operational municipal solid waste disposal site in Kaduna metropolis (north nentral Nigeria) with the aim of investigating groundwater contamination as a result of leachate accumulation.Ten survey profiles were laid around the perimeter of, as well as outside the waste disposal site to map the contaminated zones and to investigate the subsurface features responsible for the migration of the leachate. The survey was performed with multi channel (42) ABEM Field equipment. Interpreted resistivity/induced polarization models from survey profiles at the margins of the dump showed a contamination plume extending below the groundwater table while water quality analysis from existing hand dug well near the dump showed elevations in pollutant concentrations exceeding the permissible limits. There is a strong correlation between the geophysical measurements and the physiochemical analysis. Migration of the contaminants is believed to be through interconnected pore spaces which are evident from the characteristics of the conductive nature of the geo-electric layers. ELECTRICAL RESISTIVITY AND INDUCED POLARIZATION INVESTIGATION AT AN OPEN SOLID WASTE DUMPSITE. CASE STUDY: KADUNA, NIGERIA
Low field magnetic susceptibility measurements have been carried out within the Zaria granite batholith. The result has shown that magnetic susceptibility within the batholith varies between 29 10–6 SI to 3506 10–6 SI, with an average value of 684 10 –6 SI. The large variation in the measured values of the susceptibility is as a result of large variation of magnetic mineral content within an outcrop and the diversity of rock types within individual suite. Thin section observation of representative samples shows the major mineral to be feldspar, quartz and biotite, while magnetite, ilmenite and hematite occur as trace minerals. The frequency distribution shows a bimodal distribution, which is typical of granites due to a low-k peak for paramagnetic dominated specimens and high-k peak for magnetite/hematite dominated specimens. The occurrence of hematite and ilmenite may be due to the alteration of magnetite.
Gravimetric method of geophysical prospecting requires a methodical combination of both field techniques and data processing. For example drift correction in data processing cannot be successfully carried out without an a priori well-structured observational sequence during field observation that takes cognisance of both the instrumental and diurnal drift effects. The rigour of gravimetric data collection, and the non-availability of comprehensive data reduction software that takes care of local peculiarities, have always constituted hindrance to the application of the gravity method of geophysical studies. However, in recent time, the importance of the gravity method in mineral prospecting and geodynamic studies, has spurred researchers into the use of it. This programme is designed bearing in mind the need to include all the requirements for an all-embracing, up-to-date and exhaustive data reduction scheme. Therefore, the programme begins with systematic conversion of the gravimeter scale readings into an appropriate gravity unit, followed by a comprehensive drift correction routine, which employs the cascade model. The programme has a built-in mechanism, which automatically ties the survey to the international reference standard and computes the theoretical absolute gravity value at every station projected to the reference ellipsoid at the given latitude. In addition, the programme also calculates the observed absolute gravity value at every station, from which the various gravity anomalies are determined. A portion of the programme makes use of altimeter data, if available, to compute the elevation of the gravity station about the mean sea level, and return the result to an output data file. The programme is structured in modular form in order to enhance its flexibility, and with appropriate comment facilities in order to aid its comprehensibility. Keywords: Gravimetric, data, reduction, computer, programme, sequence, modular, cascade, model Nigerian Journal of Physics Vol. 19 (1) 2007: pp. 121-128
A 0.5 0 x 0.5 0 representation of the geomagnetic elements and their rates of change have been predicted for the Nigeria sector of the Niger Delta basin using the spherical harmonic analysis.The geomagnetic elements computed are the total field (F), horizontal field (H), north component (X), east component (Y), vertical component (Z), declination (D), inclination (I) and their rates of change,
Gravimetric studies in Nigeria date back some four decades, but it was only in the last twenty years that the scope of activities was broadened. During that period, most gravimetric studies were initiated by the universities. However, due to the dwindling resources available to the Nigerian universities in recent times, appropriate government research departments became involved in gravimetric studies in the country. This led to the formation of the Gravity Network Project Committee, which comprised Federal Government institutional bodies. The immediate priority of the Committee is to consolidate the Primary Gravity Network of Nigeria which was established under the first phase of the Gravity Network Project. The long-term objective of the Gravity Network Project Committee is to integrate the various gravimetric data in Nigeria to date into the consolidated network and then to produce regional gravity anomaly maps for the various parts of Nigeria. The prospects and problems of the project are discussed. Many recommendations are made, including the establishment of absolute gravity bases in a number of locations within Nigeria so as to strengthen the existing primary gravity network. It is pointed out that, for the nation to realise the full benefit of gravimetric studies, the establishment of a national geodetic and geophysical institute, and the setting up of a gravimetric data bank are expedient.
A two-dimensional electrical resistivity survey was carried out at the periphery of the impounding reservoir of the Ahmadu Bello University farm dam in order to investigate the subsurface seepage conditions and identify possible weak zone that could serve as seepage paths within the subsurface in the vicinity of the dam. Eight profiles were acquired, using Abem Lund imaging system, along each of the northern and southern abutment (flanks) of the reservoir. The RES2DINV interpretation software was used to analyze the data and the resulting tomograms, generally, it shows a resistivity variation from 23 to 926 â¦m. Anomalous low resistivity zones within the bedrock are interpreted as zones of weakness representing preferential flow pathways for water from the impounding reservoir.
Key words: Two-dimensional, electrical resistivity, subsurface seepage condition, artificial lake, Nigeria.
A Direct Current electrical resistivity survey was carried out in Unguwan Dosa open dumpsite in Kaduna metropolis, North Western Nigeria. The dumpsite is the typical non-controlled waste facility that lack bottom liner. 8 vertical electrical soundings (VES) employing the Schlumberger electrode array were conducted with maximum electrode spacing of 100 m. Interpreted resistivities were obtained by iterative computer modeling of the apparent resistivity data. The VES data were plotted as pseudo and resistivity cross-sections in order to look at the spatial distribution of the contaminant plumes. The interpreted VES data measured inside the dumpsite showed contamination plumes as low zones with resistivity values ranging between 1 and 12.9 ohm-m extending from the surface down to the aquifer of shallow groundwater of less than 5 m. Calculated hydraulic conductivity ranges between m/s and for shallow subsurface layers of interpreted VES points located inside and outside the dumpsite. This moderately conductivity value of the subsurface materials is believed to facilitate movement of the leachate plume through the soils and migration of the contaminants outside the dump and into the shallow aquifer in the study area. Elevation in concentrations of the measured parameters of the physio-chemical analysis of water samples from existing hand dug wells indicate contamination of the groundwater as a result of solid waste leachate accumulation, consequently, complimenting the geophysical data. (Journal of American Science 2010; 6(8):540-548). (ISSN: 1545-1003).
The durability of roads is dependent on the proper screening of the variations in subsurface geological characteristics and conditions through geo-engineering investigations and good construction practices. In this study, electrical resistivity tomography (ERT) technique was used to investigate the subsurface defects and potential failures along the substrate of Etioro-Akoko highway, Ondo State, southwestern Nigeria. Results of the inverse model resistivity sections generated for the two investigated traverses showed four distinct subsurface layers. The shallow clayey topsoil, weathered layer, and partially weathered/fractured bedrock have resistivity values ranging from 4–150 ohm-m, 10–325 ohm-m, and 205–800 ohm-m, with thickness values of 0–2 m, 0.5–12.5 m, and less than few meters to > 24 m, respectively. The fresh bedrock is characterised by resistivity generally in excess of 1000 ohm-m. The bedrock mirrored gently to rapidly oscillating bedrock troughs and relatively inclined deep penetrating multiple fractures: F1–F’1, F2–F’2 and F3–F’3, with floater in-between the first two fractures. These delineated subsurface characteristic features were envisaged as potential threats to the pavement of the highway. Pavement failures in the area could be attributed to the incompetent clayey sub-base/substrate materials and the imposed stresses on the low load-bearing fractured bedrock and deep weathered troughs by heavy traffics. Anticipatory construction designs that included the use of competent sub-base materials and bridges for the failed segments and fractured zones along the highway, respectively, were recommended.
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