AIRBORNE GEOPHYSICAL DATA MANAGEMENT AND INTERPRETATION

Transcription

1 Geoscience for Society 125 th Anniversary Volume Edited by Keijo Nenonen and Pekka A. Nurmi Geological Survey of Finland, Special Paper 49, , 2011 AIRBORNE GEOPHYSICAL DATA MANAGEMENT AND INTERPRETATION by Meri-Liisa Airo*, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi Airo, M.-L., Hautaniemi, H., Korhonen, J. V., Kurimo, M. & Leväniemi, H Airborne geophysical data management and interpretation. Geological Survey of Finland, Special Paper 49, , 6 figures. Nationwide airborne geophysical surveys undertaken by the Geological Survey of Finland (GTK) were completed in 2007 and altogether covered about 1.90 million line kilometres. Magnetic, radiometric and multi-frequency EM survey data were systematically acquired at 200 m line spacing and 30 m nominal terrain clearance, along north-south or east-west trending flight lines. Consistent countrywide data grids have been generated at a 50 m grid cell size and incorporate GIS-compatible source data for integration with other geoscience data in mapping, exploration and for environmental purposes. Airborne geophysical data archives include both survey line data and grid products. Throughout the 35 years of surveying, direct first-pass interpretation of these data has been carried out on a map sheet basis. Rock samples from the entire country have been gathered for physical property determinations to establish a petrophysical database. An interpretation package has been introduced for accessing detailed geological and geophysical information, and map compilations of larger areas have formed the basis for understanding regional geology. Interpretation is in progress to deliver countrywide geophysical imagery for ArcGIS applications. These could be used to further customize interpretation and ground surveys for local investigation, as well as predictive targeting using neural networks. GTK s expertise in aeromagnetic mapping and map compilation of diverse datasets is acknowledged in an international project that is compiling the first magnetic anomaly map of the world (2007) at the scale 1: The project is a joint effort between numerous scientific organizations including the IAGA, CGMW and world geological surveys. The project at GTK is planned to be extended until 2011 to allow the compilation and release of a new edition of the map and databases. Keywords (GeoRef Thesaurus, AGI): geophysical surveys, airborne methods, data management, interpretation, petrophysics, magnetic anomalies, aeromagnetic maps, Finland * Geological Survey of Finland, P.O. Box 96, FI Espoo, Finland * meri-liisa.airo@gtk.fi 349

2 Meri-Liisa Airo, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi INTRODUCTION Finland has been covered by airborne geophysical measurements in two successive mapping programmes. The first, so-called high-altitude mapping programme started in the early 1950s and lasted for about 20 years. It was followed by more high-resolution surveys of the so-called low-altitude mapping programme. The Geological Survey of Finland (GTK) conducted these programmes in a systematic way with the goal to create constant countrywide airborne geophysical databases to be used as a reference in geological mapping and exploration. The first two chapters in this report describe the data collection and the techniques adopted in the integrated interpretation. A characteristic of GTK surveys is the 3-in-1 approach: simultaneous measurement of magnetic, electromagnetic (EM) and radiometric data. In particular, GTK has focused on developing the frequency-domain EM method, which is well suited to mapping electrically conductive sulphide deposits close to the surface. Typical applications of 3-in-1 surveys nowadays include bedrock mapping, environmental monitoring and raw-material investigations. The combination of 3-in-1 data and the national petrophysical database offers a unique resource for geophysical and geological modelling and GIS-compatible interpretation. The high quality and resolution of GTK s aerogeophysical datasets allow the application of statistical and neural network approaches to encourage data mining processes and mapping of the mineral potential. One of the original aims of countrywide airborne geophysical surveys was to create a general picture of geophysical anomalies in Finland. These data also enable the integration of Finnish data into crossnational aeromagnetic datasets. The third chapter of this report describes how local aeromagnetic datasets have been integrated into multi-national and global magnetic anomaly maps. This report provides a brief description of GTK s airborne geophysical databases, their maintenance and interpretation. Articles providing more detailed information and further references can be found in Airo (2005). LOW-ALTITUDE AIRBORNE GEOPHYSICAL DATA COLLECTION The National Airborne Geophysical Mapping Programme During , the Geological Survey of Finland (GTK) conducted a systematic programme of high-resolution, low-altitude airborne geophysical mapping of the entire country (Hautaniemi et al. 2005). The methodology was based on the experi- ence and know-how acquired during the first national airborne geophysical programme in The low-altitude mapping programme and resulting countrywide maps have been summarized by Moore (2008). Methodology The geophysical system included three geophysical methods: magnetics, electromagnetics (EM) and gamma-spectrometry. The key points in magnetic surveys were 1) the use of two sensor systems (horizontal gradiometry) as a wing-tip installation to improve data interpolation between flight lines and 2) systematic levelling of all surveys to the reference year 1965 by a survey base station and two geomagnetic observatories in Finland, and then subtracting of IGRF-65. The frequency domain EM system was designed and built entirely in-house at GTK, and the 3 khz frequency was selected to meet the demands of exploration for base-metal bearing sulphide deposits in Precambrian shield areas. The radiometric data face challenges with the thick overburden spread out during glacial periods. Data collection The whole country was covered by a strict systematic survey during a 36-year period. The flight line spacing was fixed to 200 metres and the mean terrain clearance to 30 metres. The survey areas were selected according to the Finnish map sheet system. Throughout the survey period, the geophysical parameters were basically kept constant. 350

3 Airborne geophysical data management and interpretation The selection of the annual survey area was based on GTK needs and the fact that the survey season is very limited in northern Finland (Figure 1). The flight season was started in the south after the snow had melted; in midsummer, flights were then carried out in the north, slowly returning to the south in the autumn. The annual survey flight hours were maximized with two full flights daily and a 6- to 7-day working week (Figure 2). The safety of low altitude flying was carefully considered, and often two pilots were accompanied by a navigator. The aircraft was leased from an aviation company with long-term agreements to ensure the safety and professional skills of the pilots. Figure 1. Annual survey areas of the National Mapping Project. 351

4 Meri-Liisa Airo, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi Figure 2. The annual survey coverage of the National Mapping Project. The quality of measured data was carefully monitored during the survey, and after the flight the data were checked as soon as possible. At first, the daily flight tapes were sent by air freight to the GTK office; later, the data were checked in the field immediately after each flight. The annual calibrations were carried out using constant calibration areas, above a deep part of the Baltic Sea for EM and a permanent calibration line near Helsinki for radiometrics. The instruments were upgraded and modernized continuously. For the radiometrics, the crystal size was increased from 27 litres to 41 litres and the number of spectrometer channels from 36 to 256. The magnetometer accuracy improved when proton sensors were replaced with caesium sensors and the resolution was increased from 2/s to 10/sec. The EM development steps included the adding of a second frequency in 1996 and the implementation of a four-frequency system in 2006 (Leväniemi et al. 2009). In 1993, navigation accuracy was im- proved by GPS. The processing software was under continuous development, the most important steps being during the 1990s with data visualization and field laptops. The raw data from the aircraft and base station, with positioning and other auxiliary information, were collected daily and archived. The geophysical data were methodologically corrected with calibration and positioning data, as described in Hautaniemi et al. (2005). All filtering was avoided in order to keep the data as original as possible. EM levelling is always problematic, and the methodology has been developed together with increased technical capabilities. The final results include digital, corrected data in two formats: corrected data along true flight lines in ASCII XYZ format and with original sampling intervals, and interpolated grids with a 50 metre cell size. Data maintenance and archiving The quality of the early data is variable and computer and software technology during the 1970s and 1980s had limitations. The calibrations were inadequate in comparison to current standards. Three different aircraft and two coil configurations in EM were used in data collection, causing a challenge in the combining of the data. The amount of data is huge, comprising over 300 separate flight areas, about flight lines and ca.1.90 million line kilometres of magnetic, radiometric (Tot, K, U, Th), EM (1 4 frequencies) and apparent resistivity data. The first-phase radiometric correction included re-levelling, the correction of a few malfunctions, false anomalies and coordinate problems. Radiometric components are often presented as ratio maps and their efficient use requires the further fine tuning of re-levelling, as well as radon verification and removal for the entire data (U, Tot). 352

5 Geological Survey of Finland, Special Paper 49 Airborne geophysical data management and interpretation The first phase of correcting the 3 khz EM data was carried out in The zero levels were relevelled for the first time throughout the entire country (real and imaginary components, and apparent resistivity, Figure 3). The approximated coefficients for the oldest non-calibrated data will then be determined and a second levelling carried out on the whole dataset. The high noise levels are also a problem for some areas. Coordinate errors and malfunctioning of equipment also need to be corrected for in many areas. The entire magnetic data set will also be corrected in the near future, mainly by re-levelling the data and recalculating the secular corrections. Figure 3. The calculated ratio of real and imaginary components of the airborne electromagnetic field (3 khz). 353

6 Meri-Liisa Airo, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi INTEGRATED INTERPRETATION ASSISTED BY PHYSICAL ROCK PROPERTIES Interpretations of airborne geophysical survey data are carried out on an integrated basis, correlating geological, petrophysical or geochemical data, and incorporating terrain data and satellite imagery. Interpretations aim at finding geological reasons for geophysical anomalies and at explaining the survey results and the calculated models based on them. Depending on the objective of the interpretation, airborne data are used in data and grid analysis for surface mapping and characterization, or 2D and 3D modelling to add a depth dimension and to understand the subsurface geometry. Mapping and monitoring GTK s airborne geophysical dataset contains extensive and versatile geophysical and geological information to be used in mapping and characterizing the survey area and in identifying features that may be critical for resource exploration. In some areas, airborne surveys have been conducted repeatedly and comparison of the datasets from different years may be valuable in environmental monitoring. Various tools, filtering and enhancement methods are applied in grid texture analysis, lineament and edge detection, and the classification of geological provinces possessing different geophysical properties. A variety of processed airborne geophysical images are used to characterize the lithology and locate structures of interest, to describe the configuration of rocks or soils and structures in the ground and to extend the known geological features to areas where the outcrop information is limited. When looking for local features, the analysis of profile data from flight lines may provide access to more detailed analysis. For example, subtle local magnetic intensity variations related to fracture and joint settings have been highlighted by directional trend analysis and discontinuity structure detection of magnetometer data along survey lines (Airo & Wennerström 2010). The local radiometric response attributed to hydrothermal alteration associated with mineralization may also require detailed analysis of flight line data (Airo 2007). To help in the utilization of the wide range of airborne geophysical datasets, an interpretation package has been developed for lateral mapping of surface geophysics. The package provides a general picture of the variation in the lithology and soil in the study area and formulates a first-pass interpretation that helps in the selection of targets for more detailed investigations. The interpretation package introduces processed easy-readable geophysical images: the example in Figure 4 is from southern Finland. Classification of radiometric and AEM datasets is represented as overlays on aeromagnetic images: either total magnetic intensity (TMI) or derivative data. The radiometric classification shows the highest K, Th and U radiation counts as cut-off values. Some of the radiometric anomalies reflect the bedrock, but soil and infrastructure strongly disturb the geological response. The AEM colour categories in Figure 4 are based on analysis of in-phase (Real) and quadrature (Imaginary) components and are related to the variation in the electrical conductivity of the ground. The negative in-phase response in frequency-domain AEM data is related to high magnetic permeability due to an abundant magnetite concentration. The map compilation of the AEM Re/Im ratio for the whole of Finland in Figure 3 demonstrates the distribution of high electrical conductivity associated with schist belts and high magnetic susceptibility mainly associated with igneous rock rich in magnetite. In particular, granitoid areas and ultramafic rock sequences are emphasized in Finnish Lapland. In southern Finland, the gabbro intrusion of Hyvinkää and the highly magnetic granitoids in the Häme region are distinguished by their negative EM real component. 354

7 Geological Survey of Finland, Special Paper 49 Airborne geophysical data management and interpretation Figure 4. The interpretation package. Geophysical easy-reading interpretation images: an aeromagnetic grey-scale image as the background, with high-radiation regions (top panel) and AEM categories (bottom panel) as overlays. Modelling Supplementary petrophysical rock parameters improve the modelling when creating numerical estimates of the depth and the dimensions of the anomaly sources. Creating conceptual models of the subsurface involves the quantification of geophysical properties and matching of properties to rock types, mineralization types or ore grades. Investigation of the magnetic signature, i.e. the magnetic anomaly texture and intensity variation, may be of key importance in resource exploration, because rock magnetic properties record the magnetic and the geological history of rock. The remanent and induced magnetizations play a role in producing the magnetic signature and describe the magnetic mineralogy of rock. Remanent magnetization correlates with grain size and induced magnetization depends on the ferrimagnetic mineral content. Figure 5 compares remanent and induced magnetizations determined in the laboratory for samples representing different rock types. A predominance of induced magnetization causes a soft, consistent magnetic anomaly signature, often related to magnetite. Prevailing high remanence causes sharp, discontinuous magnetic anomalies and may indicate monoclinic pyrrhotite or fine-grained magnetite. Hydrothermal alteration or other geological processes related to mineralization affect the magnetic mineralogy and thereby the magnetic properties (Airo & Mertanen 2008). The interpretation and identification of alteration zones also incorporates radiometric and EM signatures. For data mining and predictive targeting based on statistical or neural network methods, GTK s airborne geophysical datasets provide an endless resource. Interpretation is in progress to classify geophysical clusters with similar spectral signatures in different geological provinces in Finland and to create prospectivity models for different mineralized areas. Classification and prospectivity modelling have been applied to GTK s survey data collected in international airborne geophysical projects. 355

8 Meri-Liisa Airo, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi Figure 5. Physical rock property data (total magnetization) from the petrophysical database. The background image is TMI. Comparison of magnetization components: warm colours indicate a predominance of remanent magnetization (top panel) or induced magnetization (bottom panel). A MULTINATIONAL AND GLOBAL VIEW OF MAGNETIC ANOMALIES For almost two decades, Finnish aeromagnetic surveys were based on analog data processing and drafting methods. The numerical work was begun by digitizing hand-drawn anomaly maps and by measuring long, digital tie lines across the country in This led to two branches of aeromagnetics at GTK: new digital national airborne mapping at a lower altitude and digital compilation and interpretation of magnetic anomalies from regional to international scales (Hautaniemi et al. 2005, Korhonen 2005). The Finnish high altitude aeromagnetic map set at the scale of 1: was digitized to a 1 km x 1 km grid. The aim was to prepare an overview of the magnetic anomalies of the country, and to facilitate using Finnish aeromagnetic data jointly with neighbouring and global datasets. In ten years the first Finnish aeromagnetic summary was ready (Korhonen 1980). A printed anomaly map was presented to IGC26 for geological studies and a numerical grid to the IAGA for the European and global compilation of anomaly grids. During the following decades, GTK developed co-operation with neighbouring countries, finally compiling and providing access to a joint magnetic (1 km x 1 km) and gravity (2.5 km x 2.5 km) anomaly set on the Fennoscandian Shield and its close margins (Korhonen et al. 2002). A geological result of sub-continental scale was that the shield borders could not be outlined from either of the sets. Hence, it was concluded that the present exposure of the crystalline rock units is not solely due to the properties of the Precambrian units of the basement, but for an essential part due to later balancing of the lithosphere. The Finnish anomaly grid was the first national grid to be reported for the IAGA World Magnetic Anomaly Map in At the end of this work in 2003, GTK was given the responsibility to co-ordinate the final stage of the map compilation, which was jointly carried out with several scientific teams and data owner organizations. The map was published and grid and data sets released at the 2007 IUGG General Assembly in Perugia, 30 years after the first IAGA resolutions to create the map 356

9 Airborne geophysical data management and interpretation (Korhonen et al. 2007). The second edition of the global magnetic anomaly map is in preparation by complementing the datasets and reduction methods. A basis for the third edition is being established by preparing for the launch of the SWARM satellite constellation by ESA. The global magnetic anomaly map clearly outlines major geological units on the sub-continental scale. Figure 5 presents a European and North Atlantic window to the map. The youngest, Cenozoic part of the Earth s crust of the Atlantic Ocean floor causes a well known striped signature of magnetic anomalies, mainly due to the varying direction of total magnetization (left). The oldest, Precambrian lithosphere causes a more irregular and stronger pattern of magnetic regional anomalies above the East European basement area, mainly due to variation in the intensity of total magnetization between major geological units (upper right). The crust in Central and Southern Europe is younger, magnetically thinner and less magnetized (from the centre to the lower edge of the figure). This anomaly picture presents anomalies at an elevation of 5 km above the geoid, and the widest parts of the anomalies (> km) have been removed by the anomaly definition of the map. To represent these missing wider features, a global magnetization model of the Earth s lithosphere would be required. This task is one of the future challenges of the scientific geological and geomagnetic communities. To explain the geological sources of magnetic anomalies in Finland, petrophysical measurements of bulk density, magnetic susceptibility, the intensity of remanent magnetization, and to minor extent its direction, have been carried out for decades. A general conclusion is that a considerable proportion of the smooth anomalies are due to sources in the deeper part of the crust, above the Curie isotherm of magnetite, and some are influenced by the direction of remanent magnetization, different to that of the present main field. A view has been presented that geological interpretation of the sources of the anomalies, and the establishment of sets of numerical models to work out the results of future studies should be among the key tasks in applying aeromagnetics in Finland, as well as internationally. Figure 6. Atlantic-European window to the World Digital Magnetic Anomaly Map 2007 (WDMAM 2007, Korhonen et al. 2007, available at: projects.gtk.fi/wdmam/). The upright dimension of the window (N-S) corresponds to 3200 km of arc length. 357

10 Meri-Liisa Airo, Heikki Hautaniemi, Juha Ville Korhonen, Maija Kurimo and Hanna Leväniemi REFERENCES Airo, M.-L. (ed.) Aerogeophysics in Finland : Methods, System Characteristics and Applications. Geological Survey of Finland, Special Paper p. Airo, M.-L Application of Aerogeophysical Data for Gold Exploration: Implications for the Central Lapland Greenstone Belt. In: Ojala, V. J. (ed.) Gold in the Central Lapland Greenstone Belt, Finland. Geological Survey of Finland, Special Paper 44, Airo, M.-L. & Mertanen, S Magnetic signatures related to orogenic gold mineralization, Central Lapland Greenstone Belt, Finland. Journal of Applied Geophysics 64, Airo, M.-L. & Wennerström, M Application of aeromagnetic data in targeting detailed fracture zones. Journal of Applied Geophysics 71, Hautaniemi, H., Kurimo, M., Multala, J., Leväniemi, H. & Vironmäki, J The three in one aerogeophysical concept of GTK in In: Airo, M.-L. (ed.) Aerogeophysics in Finland : Methods, System Characteristics and Applications, Geological Survey of Finland, Special Paper 39, Korhonen, J Suomen aeromagneettinen kartta / Flygmagnetiska kartan over Finland / The Aeromagnetic Map of Finland 1: Espoo: Geological Survey of Finland. Korhonen, J. V Airborne magnetic method: Special features and review of applications. In: Airo, M.-L. (ed.) Aerogeophysics in Finland : Methods, System Characteristics and Applications. Geological Survey of Finland, Special Paper 39, Korhonen, J. V., Aaro, S., All, T., Nevanlinna, H., Skilbrei, J. R., Säävuori, H., Vaher, R., Zhdanova, L. & Koistinen, T Magnetic Anomaly Map of the Fennoscandian Shield 1: Geological Surveys of Finland, Norway and Sweden and Ministry of Natural Resources of Russian Federation. Korhonen, J. V., Fairhead, J. D., Hamoudi, M., Hemant, K., Lesur, V., Mandea, M., Maus, S., Purucker, M., Ravat, D., Sazonova, T. & Thébault, E Magnetic Anomaly Map of the World, Equatorial scale 1: Map published by the Commission for the Geological Map of the World, supported by UNESCO, 1st Edition, GTK, Helsinki, Leväniemi, H, Beamish, D., Hautaniemi, H., Kurimo, M., Suppala, I., Vironmäki, J., Cuss, D., Lahti, M. & Tartaras, E The JAC airborne EM system AEM-05. Journal of Applied Geophysics 67, Moore, G Finland s national airborne geophysical mapping programme and the 3-in-1 approach. First Break, 26, Special Topic on Airborne Geophysics, November