This paper describes the airborne electromagnetic (AEM) system operated by the Joint Airborne geoscience Capability (JAC), a partnership between the Finnish and British Geological Surveys. The system is a component of a 3-in-1, fixed-wing facility acquiring magnetic gradiometer and full spectrum radiometric data alongside the wing-tip, frequency-domain AEM measurements. The AEM system has recently (2005) been upgraded from 2 to 4 frequencies and now provides a bandwidth from 900 Hz to 25 kHz. The fixed-wing configuration of 4 dual vertical coplanar coils, offers a high signal/noise by virtue of the wingspan separation of the sensors. This unique configuration allows 3-in-1 surveys to be successfully performed at a variety of survey elevations when regulatory conditions are imposed. Its deployment on a twin-engine aircraft also permits low altitude surveying in countries, such as the UK, where this is a requirement.The development of the new AEM-05 system has been incremental and its history can be traced back over five decades. The AEM data acquired in the Finnish National Mapping project, and across northern Europe, have been used extensively in mineral exploration. More recent projects have investigated the application of the data to environmental, hydrogeological and land quality issues. These studies have been enhanced by reducing the flight line separation from 200 m (the national highresolution scale) to 50 m.Our surveys also increasingly involve the application of AEM across populated areas often with extensive infrastructure. Additional secondary instrumentation has been introduced to provide an increased understanding of the data and the AEM responses observed. The secondary systems include an accurate, high sampling rate laser altimeter, a downward-looking digital camera to record the flight path, a 50/60 Hz power line monitor and a GPS gyroscope. The paper is intended as an overview and provides descriptions of the new AEM system, the secondary systems now employed and some of the software used to provide accurate and levelled AEM data. Recent applications of the system are reviewed and the challenging nature of the new subsurface information being revealed is demonstrated.3
SUMMARYThis paper describes an airborne geophysical survey of Northern Ireland that is being conducted over a two year period. Measurements from a fixed-wing aircraft operating at 56 m include magnetic (gradiometer), radiometric and frequency-domain electromagnetic. The survey will complete over 80,000 line-km of coverage in the summer of 2006. The Phase 1 data, described here, comprise ~47,000 line-km obtained across the western and central areas of the province. The nature of acquiring geophysical data, at high resolution (200 m line spacing) in populated areas is distinct from that of other exploration contexts. The survey is being coordinated and conducted alongside a high public profile. The initial Phase 1 Tellus survey results have exceeded expectations. They have excited the interest of the planning, mineral and environmental communities.
This study considers a specific issue, often termed the canopy effect that relates to our ability to provide accurate conductivity models from airborne electromagnetic (AEM) data. The central issue is one of the correct determination of sensor height(s) above the ground surface (terrain clearance) to the appropriate accuracy. The present study uses the radar and laser systems installed on a fixed-wing AEM system to further investigate the effect. The canopy effect can arise due to a variety of elevated features below, and in the vicinity of, the flight line. The most obvious features are welldefined forest and copse zones together with domestic, commercial and agricultural buildings. Such features may cause the terrain clearance to be underestimated and this has the potential to introduce resistive artefacts and incorrect interface depths into conductivity models. Correct determination of terrain clearance is also important for the accurate processing of the other geophysical data sets acquired by airborne surveys. Radar and laser altimetry offer two very different physical measurements of height above ground. Airborne radars detect the range to the nearest reflecting object.They do this over a cone of influence that may have a radius (at the ground surface) of ~55 m (assuming a survey height of 60 m). Reflections from objects that are off-line are thus a distinct possibility. In direct contrast, a laser ranging device with low beam divergence provides a highly focussed measurement. Laser accuracies (typically < 2cm) are far greater than those of radars (typically ~0.5 m). In addition, the rapid sampling of laser ranging devices (e.g. up to 2 kHz) allows both real-time and post-3 processing algorithms to be applied to estimate the maximum range recorded across appropriate time/spatial windows. In our case a 2 kHz (maximum) dual-pulse laser
This paper studies gold prospectivity on the Palaeoproterozoic Häme Belt located in southwestern Finland. The Häme Belt comprises calc-alcaline and tholeitic volcanic rocks, migmatites, granitoids and mafic to ultramafic intrusions. Mineral exploration in the region has resulted in the discovery of several gold occurrences during the last decades, however, as of today prospectivity modelling for gold has not been conducted. This study integrates till geochemical and geophysical data to examine and extract data characteristics critical for gold occurrences. Modelling is guided by a selforganizing map (SOM) analysis to define essential data associations and to aid in model input data selection and generation. The final fuzzy logic prospectivity model map shows high predictability values for most of the known Au or Cu-Au occurrences but also highlights new targets for exploration.
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