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Core Orientation and Susceptibility Logging |
Department of Geological Sciences, University of Plymouth, Plymouth PL4 8AA, UK
The magnetic field associated with a core barrel is almost entirely restricted to a small volume near to its tip and is insignificant within the barrel, except immediately adjacent to the wall. At the tip of the barrel the field is lowest along its axis and directed almost entirely parallel to it. As the volume occupied by the field is small, the processes operating to produce any drilling imposed remanent magnetization can only be effective while this volume of borecore is passing through the field in this restricted location. This means that the borecore is only subjected to this field as it passes through about 2 cm length and consequently can only be affected by it for a few minutes during normal drilling operations. Consequently, viscous remanences are likely to be minimal and the processes by which the drilling imposed remanence are acquired (thermal, chemical or vibrational) appear to be associated with the actual cutting process. It should thus be possible to minimize the magnitude of these remanences by inserting a short (a few centimetres long), non-magnetic barrel immediately behind the drill bit, on to which the later barrels then screw. This would effectively result in the field being displaced away from the volume of rock near the crown where the processes producing the remanence occur, enabling more successful orientation and palaeomagnetic analyses to be undertaken of the natural remanence.
