logs. For Hole 1095B, the remanent anomalies for the two passes of the GHMT tool were calculated independently and are similar (Fig. F5).
The remanent anomaly (Bfr) is the field in the borehole caused by remanent magnetization of the sediment, projected along B0 (Fig. F3). Bfr is determined by subtracting the other individual fields that contribute to the total field measurement B0:
All the components of B0 are a function of depth (z). The time-dependant magnetic field (Bt), generated in the ionosphere, also contributes.
Br, the field generated in the Earth's core, forms the largest component of B0 by about two orders of magnitude. It is estimated from the International Geomagnetic Reference Field (IGRF) (Table T1).
Ba is the regional magnetic field (seafloor and other crustal anomalies). The field caused by the metal (magnetic) drill pipe (Bpipe) is also included within Ba. It decays with distance (r) away from the pipe (Bpipe = a/r3). The pipe effect is determined by iterative forward modeling until a satisfactory fit to the data is obtained. The pipe effect for Holes 1096C and 1095B is shown in Figures F4 and F5.
Bt, the time dependant field, is generally neglected. Two passes of the GHMT were run so that the repeatability could be checked. Generally the differences were slight and could be neglected. The "spikes" in the Hole 1095B magnetic field log can be considered to be part of Bt, and they have been removed from the log where the tool voltage falls below 1.5 V (Fig. F5).
Bfi, the induced field anomaly, is calculated using Equations 3, 4, and 5, with values for B and I taken from the IGRF values for the site (Table T1).
There is some uncertainty in the values of the component fields. For example, Br changes with time and depth. A major uncertainty is in the part of Ba due to local crustal anomalies, which is the main reason the GHMT average field value differs from the IGRF value (Table T1). Hence, after Br, Ba, and Bfi have been subtracted from the measured total field B0 to leave Bfr, the result can be offset from zero by a few hundred nT. However, we know that for a mixed polarity sequence the remanent field anomaly (Bfr) will average out to near zero, and therefore we can correct for the offset. For Hole 1095B, a linear fit was subtracted from the Bfr curve to give a corrected Bfr curve. For Hole 1096C, a constant value was subtracted (395 nT) (a linear fit was not appropriate for this hole because the logged interval contains essentially only one normal and one reversed polarity zone). The corrected Bfr curves are shown in Figures F4 and F5, along with the uncorrected field anomaly Bf (= Bfi + Bfr) and the original B0 and logs. For Hole 1095B, the remanent anomalies for the two passes of the GHMT tool were calculated independently and are similar (Fig. F5).
