There is generally good repeatability in both absolute value and shape of the susceptibility and the magnetic field logs, indicating that the logs are of good quality (Fig. F2). However, there are some differences in the details between the two passes, such as the presence of spikes (discussed below) and different depth positioning of some features, which we ascribe mainly to the high ship heave during logging.
The magnetic field logs for Holes 1095B and 1103A contain brief jumps in the field values (spikes). The magnetic field logs from Hole 1096C contain far fewer spikes. There are likely two origins for the spikes because some spikes repeat at the same depth in successive logging runs (implying an origin in the formation), but others do not (implying a transient problem with the tool). It is known that spikes can occur when the tool is struck sharply (this was found during Leg 188 to be caused by a loose fitting inside the tool), and this could certainly happen downhole if the tool hit the borehole wall. Most of the spikes are accompanied by a drop in the tool voltage at the same depth. We have used this observation to clean spikes from the Hole 1095B magnetic field log used in the polarity analyses. Values at depths where the voltage dropped below 1.5 V were removed. The topography of the borehole wall might be such that impacts, and hence spikes, could repeat in the same location. Another possible source for the spikes in the formation itself are large dropstones composed of basalt or another rock type rich in magnetite. If they were close to the borehole wall, they could cause a repeatable spike in the magnetic field log. During logging at Hole 1095B, the metal caliper arm from the density logging tool broke off leaving metal debris in the hole during the GHMT run which would cause the same effect.