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
SUMMARY Three‐dimensional (3‐D) electromagnetic (EM) inversion is increasingly important for the correct interpretation of EM data sets in complex environments. To this end, several approximate solutions have been developed that allow the construction of relatively fast inversion schemes. We have developed a localized quasi‐linear (LQL) approximation that is source‐independent and, therefore, appropriate for multisource array‐type surveys, typical in many geophysical applications, such as airborne EM, cross‐well tomography, and well logging. This method is based on the assumption that the anomalous electric field within an inhomogeneous domain is linearly proportional to the background electric field through an electrical reflectivity tensor λ^. This tensor is determined by solving a source‐independent minimization problem based on the integral equation for the scattering currents. We have also developed a new, fast 3‐D EM inversion method, based on this new approximation, and applied it to synthetic and real helicopter‐borne EM data. The results demonstrate the stability and efficiency of the method and show that the LQL approximation can be a practical solution to the problem of 3‐D inversion of multitransmitter frequency‐domain EM data.
SUMMARYNew highresolution airborne geophysical surveys of the UK, undertaken with the system developed under the Joint Airbornegeoscience Capability programme, established between the Geological Survey of Finland and the British Geological Survey, will provide large 4frequency airborne electromagnetic data sets. These data sets will be used to characterise the conductivity distribution of the subsurface for environmental and exploration purposes. To invert these large data sets in a fast and robust manner we have developed "LC1DINV", a laterally constrained onedimensional inversion algorithm. This algorithm inverts simultaneously for all observation points along a profile and regularises the inverse problem by requiring that differences between model parameters at adjacent points be small. We use the conjugate gradient method for minimising the data misfit subject to the lateral constraints and a priori model terms. We have inverted 4frequency data obtained over Suurpelto, a test area in southern Finland, characterised by conductive clays overlying a highly resistive granitic shield. The results show that LC1DINV can successfully locate the depth extent and variations of the clays. Comparison of these results with those obtained with two other types of inversion shows that LC1DINV produces welldefined layer boundaries and laterally smooth crosssections.
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