Three-component magnetic logging in the Outokumpu deep drillhole

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One of the aims of the Outokumpu deep drilling project in Finland is to obtain an understanding of the deep structure of the Cu-Co-Zn ore deposits. The ore is hosted by the Outokumpu formation, consisting of black shales, serpentinite and skarn rocks, which was transsected by the drillhole between 1300 and 1500 m depth. We carried out threecomponent magnetic logging and we will derive the rock magnetisation from the measured magnetic field. The integrated interpretation of our results with data from laboratory investigations carried out in a parallel project, will enable us to test various hypotheses on the tectonic evolution of the Outokumpu assemblage. We aim at an estimation of the magnetisation vector, and in particular its declination, which is generally considered difficult. We use the Göttingen borehole magnetometer (GBM), which is equipped with fiber-optic gyros allowing an independent orientation of the tool. The favourable conditions at the Outokumpu hole, an open section between 40m and 2500m, continuous coring, cool temperatures and a highly magnetised section embedded between weakly magnetised layers, will allow an unprecedented accuracy. The detailed evaluation of the results based on data redundancy and laboratory measurements, and the identification of the most critical error sources will have important implications for the application of borehole magnetometers in general.