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Collaborating Authors
ABSTRACT This study is part of a larger survey flown as part of a project of the United Nation Development Program (UNDP) in Burkina Faso. A combined aeromagnetic and time-domain electromagnetic survey (GEOTEM) was flown by CGG-GEOTERREX. After delivery of the raw data and the preliminary maps, ground follow up and geophysical modelling were carried out. It soon became clear that it was necessary to develop new interpretation techniques. Using the geophysical data acquired in Goren, new data processing and interpretation techniques have also been used to increase the benefits of the airborne survey.
Nigeria's Nationwide High-resolution Airborne Geophysical Surveys
Reford, Stephen W. (Grant & Watson Limited) | Misener, D. James (Grant & Watson Limited) | Paterson, Paterson (Grant & Watson Limited) | Ugalde, Hernan A. (McMaster University) | Gana, Jacob S. (Nigerian Geological Survey Agency) | Oladele, Olaniyan (Nigerian Geological Survey Agency)
Summary Nigeria is nearing completion of an ambitious program for nationwide airborne geophysical coverage and interpretation, providing significant positive contributions to both the minerals and oil exploration sectors. It forms part of a broader strategy to stimulate investment through the Sustainable Management for Mineral Resources Project (SMMRP). Introduction Airborne geophysics, particularly aeromagnetic and gamma-ray spectrometer (radiometric) surveys, form a critical component of geological mapping and mineral resource inventory programs in many African countries (Reford et al, 2009). In the 60โs and 70โs, regional aeromagnetic surveys were fairly widespread over much of the continent, in both sedimentary and hard rock terrains (Barritt, 1993). In the 80โs and 90โs, higher resolution surveys, incorporating radiometrics, were carried out in certain countries, particularly in southern Africa. In the last decade, a number of national initiatives (e.g. Madagascar, Mozambique, Namibia, Morocco, Mauritania, Uganda, Ghana) have seen the high-resolution geophysical coverage greatly improve. The surveys form part of larger initiatives to improve the geological knowledge of a country or region, with the ultimate objectives to increase mineral investment and develop a sustainable mining industry. These geoscience programs are typically accompanied by reforms in the mining law to promote such investment. They contribute to tectonic reconstruction, groundwater and environmental applications, and petroleum exploration, all of which ultimately assist societal development. International funding agencies such as the World Bank, European Community and African Development Bank have seen the value in such programs, and ensure that airborne geophysics receive a large share of project budgets. In jurisdictions throughout the world, it has been demonstrated that high-quality geophysical coverage leads directly to increased and more focused exploration. A trend in the last few years has been the inclusion of an airborne electromagnetic follow-up component to the airborne geophysical program. Nigeria has gained near nationwide airborne geophysical coverage, through high resolution horizontal gradiometer magnetic and radiometric surveys, flown at 500 m line spacing and 80 m mean terrain clearance and totaling almost 2 million line-km. The data acquisition required as many as seven aircraft at once. This, coupled with the multi-year and multi-season campaigns, required innovative approaches for survey planning, instrument calibration, data compilation and grid merging. Processing and Modelling The resolution of the magnetic data, incorporating the measured horizontal gradients, affords a range of processes that highlight the high-frequency responses. These are useful for accurately locating contacts, tracing horizons and delineating structure. Vertical derivatives, horizontal gradients, the analytic signal amplitude and tilt derivative have all played a role in the interpretation, and contributed to semi-automated techniques for tracing contacts and anomaly peaks. The magnetic inclination for the country varies from 7ยฐN to 13ยฐS, which is problematic for computation of a clean grid of the pole-reduced magnetic field. Consequently, a nationwide grid of the reduced-toequator magnetic field was prepared, incorporating the variations in the magnetic inclination and declination. Figure 2 shows the first iteration of the depth-to-magnetic sources from northeast Nigeria, prepared using Source Parameter Imagingโข (Thurston and Smith, 1997). For the most part, this image represents the depth-to-magnetic basement.
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Africa > Niger > Lake Chad Basin (0.99)
- Africa > Nigeria > Lake Chad Basin (0.99)
- Africa > Chad > Lake Chad Basin (0.99)
Distances between profiles can vary from 50 m to Electromagnetics, magnetics and radiometrics are used.
Exploration and development of resources, mineral or hydrocarbon, in new areas is often hampered by a lack of geophysical data such as seismic. Rugged terrain and remote, inaccessible locations make the collection of this type of data time consuming and expensive. In addition, geologic factors can absorb or redirect seismic energy resulting in very poor seismic imaging. Examples of this are steep dips of the edges of salt bodies, rugose top of salt, salt structures with embedded sediments, multiples, and velocity insensitivity which affect near salt, intra-salt and sub-salt imaging. In recent years gravity gradient technology has been incorporated into the interpretation and processing workflow.
- Geophysics > Seismic Surveying > Seismic Processing (0.62)
- Geophysics > Gravity Surveying > Gravity Acquisition (0.54)
- Geophysics > Seismic Surveying > Seismic Modeling (0.40)
In the summer of 2001, GEDCO and Sander Geophysics (SGL) carried out an airborne gravity survey using Sander's AIRGrav system in the overthrust belt in the Turner Valley area near Calgary, Alberta. The final results image all of the structures known from older ground surveys and reveal more complex details. This comparison demonstrates the exploration value of high quality airborne gravity surveys flown in severe foothills environments. Geological Setting The Turner Valley region is a well-established area for oil and gas production and was the site of the first Alberta oil boom in the 1920's. New discoveries are still being drilled in the structure and in sub-thrust plays where accurate depth mapping from seismic data is a challenge.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.48)
- Geology > Structural Geology > Tectonics > Compressional Tectonics > Fold and Thrust Belt (0.35)