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Magnetic Airborne Survey – Geophysical Flight Geosci. Instrum. Method. Data Syst., 5, 181–192, 2016 www.geosci-instrum-method-data-syst.net/5/181/2016/ doi:10.5194/gi-5-181-2016 © Author(s) 2016. CC Attribution 3.0 License. Magnetic airborne survey – geophysical flight Erick de Barros Camara1 and Suze Nei Pereira Guimarães2 1Brucelandair, Ontario, Canada 2Prospectors Aerolevantamentos e Sistemas Ltda, Rio de Janeiro, Brazil Correspondence to: Erick de Barros Camara ([email protected]) Received: 16 December 2015 – Published in Geosci. Instrum. Method. Data Syst. Discuss.: 2 February 2016 Revised: 9 May 2016 – Accepted: 19 May 2016 – Published: 6 June 2016 Abstract. This paper provides a technical review process in utilized both magnetic and radiometric methods. The fixed- the area of airborne acquisition of geophysical data, with em- wing aircraft PBY-5 (Catalina) was used in the survey. It was phasis for magnetometry. In summary, it addresses the cali- equipped with a Fluxgate magnetometer in the tail of the air- bration processes of geophysical equipment as well as the craft, which measured the total magnetic field (Hildebrand, aircraft to minimize possible errors in measurements. The 2004). The system was entirely analogue type, constructed corrections used in data processing and filtering are demon- using electromechanical units and an infinite series of valves. strated with the same results as well as the evolution of these All the data processing was done manually because, at that techniques in Brazil and worldwide. time, analogue data were recorded, tabulated, corrected, in- terpolated, and plotted on a cartographic base. The data were then presented in the form of a profile overlay on contour maps. All tracing was also manually carried out. In Fig. 1 an 1 Introduction example of aircraft used in the geophysical data acquisition is shown. Geophysics is the branch of science that involves the study of The acquisition methods more commonly used in airborne the Earth’s physical measurements. There are many types of surveys are the magnetometric and gamma spectrometric geophysical measurements that can be made. Airborne geo- methods. Both methods require a data acquisition at low al- physics deals with one of them. It uses data acquired in air- titudes to allow the survey to show the study area in great borne surveys in assessment of mineral exploration potential detail. The acquired data are processed to obtain images or over large areas. Measurements are usually made at an early maps of a region, where the key areas are those with anoma- stage of the exploration process, which can be of consider- lous magnetic fields (magnetometric case) and radioelement able help also in classification of soil types in the area. levels (gamma spectrometric case). The features depicted can The first geophysical airborne research method was the then be used to determine intrusions, faults, and lineaments magnetic method. Discovered by Faraday, Sect. XIX, the associated with subsurface geology. They can also provide method was initially employed by the USSR (currently Rus- indications of depth anomalies and possible mineralization sia) in 1936 (Hood and Ward, 1969) and adapted later with areas. Therefore, these methods have significant economic better modifications by the United States in 1940 (Hood value, particularly for mineral exploration. and Ward, 1969). Both countries had vested interests in Aerogeophysics technology development has undergone military and technology, particularly for submarine applica- several cycles over the past 5 decades. The most important tions. After some improvements, another early airborne sur- advancement has been the use of digital technology. How- vey was made in the US in 1944 using the Beech Stagger- ever, another massive technological step was made via the wing NC18575 (Morrison, 2004). use of navigational systems like satellite positioning and The first geophysical airborne survey in Brazil was carried GPS (Global Positioning System). This technology became out 60 years ago (1953) in the city of Sao Joao Del Rey, state available when the United States government opened their of Minas Gerais (Hildebrand, 2004). It was conducted by the satellite signal GPS to commercial users in the late 1980s Prospec Company, which later became Geomag. The survey Published by Copernicus Publications on behalf of the European Geosciences Union. 182 E. de Barros Camara and S. N. P. Guimarães: Magnetic airborne survey – geophysical flight Figure 1. Example of the C208B model geophysical acquisition aircraft (source: E. Camara, author private collection, Septem- ber 2014). (Hildebrand, 2004). Consequently, the development of auto- matic aeromagnetic compensators, color plotters, and Win- dows software, such as Geosoft from Oasis Montaj, soon fol- lowed. Figure 2. Natural localization model for satellites in the GNSS 2 Geophysical method magnetometry system (source: United States Government public domain, of- ficial US Government information about the Global Position- The magnetometry method measures small intensity vari- ing System (GPS) and related topics, 2014, http://www.gps.gov/ ations in the Earth’s magnetic field (Reitz and Milford, multimedia/images/, last access: October 2014). 1966). Thus, it measures rocks that exhibit variable mag- netism, which are distributed in the Earth’s crust above the Curie surface (Sordi, 2007). These variations are present in 3 Air localization system or navigation different types of ferromagnetic rocks, including magnetite (mineral magnetic more abundance in Earth) and basalt. In the early stages, air navigation for airborne surveys was These magnetic materials present in crust terrestrial exhibit performed using an aerial topographic map or aerial pho- magnetic variations in terrestrial magnetic fields (anoma- tographs and a video camera, which aided in future planning lous magnetic), magnetically active regions, and high terrains and management analyses. (Werner, 1953). Now, new and improved equipment is available for geo- Because of these multiple magnetic influences, airborne physics applications. Since the 1950s, large companies have data must be validated, and both external and internal influ- had access to microwave signal emitters. Multiple emitters ences must be removed from the data sets. Data removal is were installed on aircrafts, eventually becoming the Inertial conducted using diurnal variation calculus (diurnal monitor- Navigation System (INS) for large aircrafts. Combined with ing) and the internal terrestrial magnetic field (based on the a gyroscope, INSs calculate aircraft position. International Geomagnetic Reference Field (IGRF) mathe- However, the INS has been largely replaced by the GNSS matical model) (Ernesto et al., 1979). satellite. GNSS satellites are small, highly precise, relatively The IGRF model is approved by the International Associa- cheap, widely available, and use little energy, giving them a tion for Geomagnetism and Aeronomy (IAGA). It is a group distinct advantage over other systems (Bullock and Barritt, of coefficients developed using spherical harmonics (Gaus- 1989; Featherstone, 1995; Hakli, 2004; Haugh, 1993). sian coefficients and Legendre polynomials), and is semi- normalized to the 10th degree. Every 5 years, this model GNSS undergoes a recalculation process until a definitive model is developed for the next 5 years. This definitive model is The GNSS is currently composed of 31 satellites, which op- called the Definitive Geomagnetic Reference Field (DGRF). erate in orbit. After 2016, some satellites will provide net- The eleventh degree of these equations about the geomag- work measurements. In 2000, the US government disabled netic field model can be related to the spatial dimension of the selective availability (SA) filter, which controls the GPS, the Earth’s surface magnetic anomalies (Backus et al., 1996). resulting in an improved system precision. See illustation Other books and papers dedicated to just these topics can shown in Fig. 2. be used for review and reference (Airo, 1999; Barton, 1988; Antennas are arranged to capture two frequencies, but one Boyd, 1970; Elo, 1994; Hjelt, 1973; Parkinson, 1983; Pura- is reserved for military use. However, by receiving both sig- nen and Puranen, 1977; Reford and Sumner, 1964). nals, the measurement does not suffer degradation caused by the ionosphere. After 2020, new satellites will send two civil Geosci. Instrum. Method. Data Syst., 5, 181–192, 2016 www.geosci-instrum-method-data-syst.net/5/181/2016/ E. de Barros Camara and S. N. P. Guimarães: Magnetic airborne survey – geophysical flight 183 Figure 5. Example of the Earth’s magnetic field components, in- cluding the total magnetic field vector, which is measured by the equipment (source: S. Guimarães, author private collection, March 2006). Figure 3. Example of a localization system with selective avail- ability (a) and nonselective availability (b) (source: United States tips, so that mechanical or human factors do not affect the Government public domain, official US Government information measurements. Pilots must be wary of the performance loss about the Global Positioning System (GPS) and related topics, caused by the addition of sensors to the wing tips as they 2014, http://www.gps.gov/systems/gps/modernization/sa/data/, last affect aerodynamics. access: October 2014). Systems with off-board equipment typically carry the magnetic sensor, often called the bird, below the plane. This requires precise flying and a high level of compensation to attain reliable data. An example of the system offboard is shown
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