Improved application of remote referencing data in aeromagnetic processing: insights and applications from global geomagnetic modelling

Measuring the geomagnetic field is an important tool in prospection for economic resources. Aeromagnetic or marine magnetic surveys are comparatively cheap and easy to conduct, but interpreting the data collected is not straight forward. The field is made up of contributions from the dynamo in the Earth's core, currents flowing in regions external to Earth (the ionosphere and magnetosphere), and magnetised material in the Earth's crust and mantle. It is this last that is of potential economic interest, but in order to use magnetic measurements, the other sources must be corrected for. It is particularly important to correct for the rapidly varying external field. Most commonly, this is achieved by a process called 'Remote referencing'. A fixed magnetometer is installed near or within the area to be surveyed, and the readings from that fixed instrument are subtracted from the survey measurements. For this to work, it is assumed that the external field must be the same (or close) at the base station and the survey location. This assumption is not bad, and has served the industry well for many years, but it less good in some geographical areas of great current economic interest (near the equator, and also at high latitudes above about 60 degrees latitude). However, a further complication is provided by electromagnetic induction: the external fields penetrate electrically conducting regions of the Earth's crust, inducing electric currents that in turn generate more magnetic fields. Because electrical conductivity can vary on short length scales, the assumption that the field in the two locations is the same breaks down. Incorrect remote referencing risks changes in the external or induced field being misinterpreted as static magnetic anomalies, so providing a false picture of the potential economic resources. In this project, we will apply the increased understanding of all components of the geomagnetic field that has arisen from recent dedicated satellite missions. Both the Danish Oersted spacecraft (launched in 1999) and the German CHAMP spacecraft (launched in 2000) are still collecting magnetic data from low Earth orbit. These data have revolutionised our detailed understanding of the various components of the geomagnetic field; the aim of this proposal is to bring this new understanding to bear in improving methods of processing of exploration magnetic survey data, in particular to improve the process of remote referencing with both predictions from field models and simultaneous measurements from ground and spacecraft platforms. Data from the upcoming ESA multi-satellite mission Swarm will be of particular use; this mission is due to be launched in 2010, and so will overlap will with the period of the studentship.