During Leg 182, a large amount of data were collected to compare the paleomagnetic records of cores obtained using the experimental nonmagnetic assembly and shoe with cores obtained using standard APC assemblies. The nonmagnetic shoe, including the flapper valve, was made of 15-15-LC steel. The magnetic fields of the nonmagnetic shoe and assembly were first compared with a standard assembly.
During coring, the effect of the whole nonmagnetic assembly, consisting of the shoe and two nonmagnetic barrels, was first compared with standard assemblies. Then comparisons were made between the effect of the nonmagnetic shoe and the whole nonmagnetic assembly. Finally, numerous comparisons were made between the shoe and standard assemblies. When possible, the comparisons were made in two ways: (1) the paleomagnetic records in alternate cores in a single hole were compared and (2) cores from equivalent depths in the A and B holes were compared. In the single-hole studies, the effects in cores immediately above and below the core taken with the nonmagnetic tool provide controls. When studying different holes at the same site, the effects in material at identical depths were compared.
The principal means of comparison used in this study is the departure of the observed horizontal component of magnetization declination from the fiducial line on the core liner. This 0° declination was the expression of the radial moment that originally drew attention to the phenomenon, but it is an imperfect measure of the effect because the core could be shot so that the fiducial line is near geomagnetic north. We therefore need an independent determination of the orientation of the field with respect to the core liner to compare how well the geomagnetic field is recorded in the cores obtained with standard and nonmagnetic assemblies, or shoes.
In principle, the tensor tool should give the necessary information, but there is a history of difficulties with this observation and uncertainties remain in the interpretation of the tensor tool data. A second possible method of orientation is provided by the magnetization of discrete samples taken from near the center of the core. These have been found to be much less susceptible to the radial moment than the half cores and, therefore, provide a means of orienting the core for comparisons with the tensor tool. Eventually, both methods will be used, but the discrete sample measurements must largely await shore-based work because many of these samples are so weakly magnetized that they cannot be measured with the shipboard magnetometer. They should, however, be measurable in the University of Hawaii magnetometer, which has similar direct-current superconducting quantum interference devices, but has a smaller pick-up coil volume, providing higher sensitivity for discrete samples.
The nature of the comparison and, consequently, the assessment of the effect and the possibility of improving the quality of the record are clearly not straightforward. Given this situation, final interpretations must await shore-based work. However, for a preliminary discussion the departure from the fiducial line of the observed magnetization is useful. In addition, we know the geomagnetic axial dipole (GAD) field inclination at the sites and we have some discrete sample measurements.