When measuring sections of core in the MST or the cryogenic magnetometer, paleomagnetists must find a balance between having measurement intervals as closely spaced as they would like and the potential that making large numbers of such measurements slows the flow of cores in the core laboratory, thereby preventing shipboard sedimentologists and other scientists from performing their duties. When APC coring is conducted in shallow water, cores can arrive on deck 15–30 min apart. There is finite space for storing the cores on the catwalk and in the core laboratory and for writing site reports. Thus, strategies must be devised that provide the necessary data within the available time. This usually means that a reduction in the number of demagnetization steps or an increase in the distance between measurements must be made. On high-recovery legs (>3 km of core), it is likely that all measurements to be conducted on a core section will have to be completed in <30 min and sometimes in <15 min. The measurement strategy (i.e., the number of demagnetization steps and measurement spacing) needs to be discussed with the Co-Chief Scientists and the shipboard party to determine a strategy that permits cores to be described and measured optimally.
MST susceptibility measurements are generally made at 2- to 5-cm intervals along whole-core sections. The spacing between susceptibility measurements and the other MST instruments can be selected for the most efficient use of time while collecting the highest quality and quantity of data possible. Trial runs can be conducted to determine the time it will take to finish measuring a section for variable MST measurement settings. Additional susceptibility measurements can be made with the point susceptibility probe on the AMST track should higher resolution data be needed for some intervals.
For the SRM, measurements are also generally made at 2- to 5-cm intervals. Often the paleomagnetists will ensure that similar intervals are measured for the SRM as were for the MST susceptibility. The NRM (prior to demagnetization) should be measured on all archive-half sections that are sufficiently long and contain relatively undeformed core material. Given the length of the response functions of the sensors, it is unlikely that much useful information will be gained from sections that are shorter than ~20 cm or from sections that contain only scattered pieces of core (e.g., pebbles, cobbles, or small chunks of sediment separated by large voids). Similarly, completely deformed core sections (e.g., cores that are dominated by "suck-in") will unlikely contribute useful information. Collecting marginal quality or meaningless data results in wasted effort on the part of the paleomagnetist, particularly because additional time will be required to explain or cull these data for site reports. Additionally, future users may inadvertently incorporate these bogus data in their studies.
For the SRM, the most frequently asked question concerns the rules for demagnetization of the archive-half sections. In September 1992 the Information Handling Panel (IHP) recommended that shipboard paleomagnetists be allowed to partially demagnetize the archive half of any core to as high a level as is necessary to isolate the characteristic remanence. The in-line AF demagnetizer that is part of the SRM can demagnetize cores up to 80 mT, so this is ultimately the upper limit. The IHP also recommended that the maximum demagnetization step size be limited to 15 mT to prevent the destruction of core magnetization without a record of intermediate data. In practice, many high-recovery legs have time for only the NRM step and one demagnetization step. Because the drilling overprint is sometimes only partially removed at 15 mT, the demagnetization step chosen is often 20 or 25 mT. Although this violates the IHP recommendation for step size, it may be the only means for the shipboard paleomagnetist to obtain an unbiased and timely paleomagnetic record that can be used for magnetostratigraphy, devising sampling strategies, or other purposes. If time is available, every effort should be made to obtain data at an intermediate demagnetization step, again with the maximum step size being 15 mT.
Experiments with discrete samples can be used to devise demagnetization strategies for the split cores. Shipboard paleomagnetists are encouraged to take discrete shipboard samples that can then be demagnetized either thermally or with AFs up to 80 mT with the in-line AF unit or up to 200 mT with the D-Tech unit. These results can then be compared to the results of demagnetization of archive halves to determine whether or not secondary magnetizations were adequately removed from the archive halves. Drilling overprints may, however, be more resistant to demagnetization for the split cores because core material near the periphery is often more strongly magnetized by than that near the center of the core and discrete samples are typically taken from the interior of the core (Fig. F26)
Occasionally, requests are made to conduct demagnetization experiments on whole-round core sections. Because the entire cored interval is affected by whatever demagnetization is conducted, great care should be taken in selecting demagnetization steps. Such measurements are not part of standard shipboard procedures and so requests for demagnetizing whole-round sections need to be approved by the Sample Allocation Committee, as is the case for other nonstandard measurement or sampling strategies.