Drill core orientation using palaeomagnetism

Abstract

Palaeomagnetic orientation of drill core from the Sydney-Bowen Basin is feasible. A consistent magnetisation is observed in coal measures in the basin enabling the remanence of an unoriented sample to be utilised to orient the sample. Palaeomagnetic orientation of drill core for fracture analysis has been used successfully in the Sydney-Bowen Basin. The direction of magnetic remanence in the sediments of the southern Sydney Basin is consistent across a wide area, this direction being north and up. Similarly, the remanence direction observed in Bowen Basin coal measures is consistent from the south (Moura mine) to the north (Goonyella mine) of the basin, and this direction is also north and up. The consistency of the remanence direction is lithology dependent. Fine sandstone and siltstone/mudstone units gave the most reliable results. Coarse sandstones gave inconsistent remanence directions and should not routinely be used for drill core orientation using palaeomagnetism. A longcore magnetometer constructed by the CSIRO Rock Magnetism Group enables measurement of the remanence of HQ drill core without the need to subsample. The measurement of the remanence of HQ drill core using a longcore magnetometer shows similar results to the remanence measured using laboratory magnetometers indicating that drilling does not alter the NRM. Thus, the noninvasive orientation of drill core by measurement of remanence using a longcore magnetometer is feasible for samples from the Permian coal measures of the Sydney-Bowen Basin. As well, storage of the drill core has no or limited effect on the remanence of the drill core. A small component is sometimes present but is easily removed by AF demagnetisation. Weathering has a severe effect on the remanence of samples, inhibiting orientation by palaeomagnetism. Weathering can produce hematite/goethite which retains a strong chemical remanent magnetisation and thus dominates the NRM of the sample.