Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Location: West Antarctic Peninsula Rise (Antarctic Ocean)
Latitude: 67° 34.009' S
Longitude: 76° 57.82608' W
Logging date: March, 1998
Bottom felt: 3163.7 mbrf
Total penetration: 3771.4 mbsf
Total core recovered: 409.9 m (84.2 %)
GHMT Logging Runs
Two passes were recorded. Processing was performed on pass 2, open-hole section (361.7-499.5 mbsf). Wireline heave compensator was used to counter ship heave due to the mild sea conditions.
No caliper measurements is available for the interval logged with the GHMT;. considering the great hole size variations observed in the upper part of the hole, the raw susceptibility is corrected for hole diameter assuming an average diameter of 16 inches.
The GHMT data from pass 1 have been interactively depth shifted with reference to NGT from GHMT/NGT pass 2 and to the sea floor (- 3164.5 m). This value corresponds to the water depth observed on the GHMT/NGT pass 2 and differs 0.8 m from the drillers' "bottom felt" depth. The program used is an interactive, graphical depth-match program that allows to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma ray curve (SGR or HSGR) from the NGT/HNGS tool run on each logging string is used to correlate the logging runs most often. In general, the reference curve is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics is obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request.
Data repeatability in the two passes is not very, however the general trend is similar.
The logging data have been corrected for the drill pipe effect visible on the total induction signal.
The GHMT was recorded through lithologic unit III.
Unit III (173 mbsf to bottom of logged interval): fine-grained, thinly laminated and generally non-bioturbated dark gray and dark greenish gray silty clays; homogenous, intensely bioturbated dark greenish gray and greenish gray silty clays.
In the depth interval logged by the GHMT, the paleomagnetic chron C2An.3n has been interpreted from core measurements of the paleo-inclination made on board between 367 and 405 mbsf.
Proposed interpretation of GHMT data
From the comparison of the GHMT results with the GPTS and the shipboard paleomagnetic measurements, only two magnetochrons can be determined in the logged section: chrons C2An.3n and C2Ar (Pliocene). The boundary between these chrons is at 408 mbsf.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 178. For further questions about the logs, please contact:
E-mail: Cristina Broglia