A new high-efficiency and low-background system for the measurement of natural gamma radioactivity in marine sediment and rock cores retrieved from beneath the seabed was designed, built, and installed on the JOIDES Resolution research vessel. The system includes eight large NaI(Tl) detectors that measure adjacent intervals of the core simultaneously, maximizing counting times and minimizing statistical error for the limited measurement times available during drilling expeditions. Effect to background ratio is maximized with passive lead shielding, including both ordinary and low-activity lead. Large-area plastic scintillator active shielding filters background associated with the high-energy part of cosmic radiation. The new system has at least an order of magnitude higher statistical reliability and significantly enhances data quality compared to other offshore natural gamma radiation (NGR) systems designed to measure geological core samples. Reliable correlations and interpretations of cored intervals are possible at rates of a few counts per second.

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.

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