Downhole Geophysics for Iron-Ore Exploration in the Kiruna Area

Downhole geophysics is used to support the exploration drilling carried out by LKAB in the Kiruna area, both for the actively mined Kiirunavaara ore, and for the Per Geijer Deep iron mineralization. The former is almost totally consisting of magnetite, whereas the latter is a mix of magnetite and hematite, also having associated phosphorus and REE’s. These deposits are dipping 50–60 degrees, which in combination with access limitations make seismic imaging challenging. Various downhole geophysical techniques have therefore been used for exploring the surroundings of the drill holes, including magnetics, gravity, time-domain electromagnetics (TEM), pole-dipole resistivity, magnetometric resistivity, and walkaway vertical seismic profiling. The merits and drawbacks of these methods are discussed, looking also at how to build subsurface models via forward modelling or inversion. Wireline logs of physical parameters are used to support the interpretations. Downhole magnetometry is the most common method, and it can give quick guidance to the drilling. Downhole TEM has also been used in numerous holes and can be effective to localize off-hole mineralization. The remaining methods are due to logistics not used on a routine basis, although all show distinct responses from the exploration targets.