Coring-induced magnetization of recovered sediment

Abstract

Shipboard measurements of archived advanced piston corer (APC) cores during Leg 157 revealed a preponderance of radially inward-directed magnetization. The nature of the radial magnetization was investigated with tests on a wash core. The coring-induced magnetization has higher coercivity than a simple isothermal remanent magnetization (IRM). Well-lithified samples do not have this coring-induced, high-coercivity contamination. The coring-induced moment, therefore, appears to be related to the coring or recovery process in relatively poorly lithified sediments. The magnetic fields of bottom-hole assemblies (BHAs), APC barrels, and APC cutting shoes were measured as possible source fields for the drill moments. Fields of tens of mT were found in the core barrels, with fields only 1 order of magnitude smaller at the cutting edge of the shoes. The fields associated with the BHAs were weaker and fell off rapidly on the scale length of the drill bit. The coring-induced magnetization appears to be caused by the mechanical disturbance of sediments during coring, pull-out, or passage up the drill string in the presence of the field of the APC barrel.