Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1103A

Leg: 178

Location: Antarctic Peninsula (Antarctic Ocean)

Latitude: 63° 59.97002' S

Longitude: 65° 27.91901' W

Logging date: March, 1998

Bottom felt: 505 mbrf

Total penetration: 362.7 mbsf

Total core recovered: 44.5 m (12.3 %)

GHMT Logging Runs

Two passes (downlog and uplog) were recorded. Processing was performed on the uplog, open-hole section (87.8-243.5 mbsf).

Wireline heave compensator was used to counter ship heave resulting from the mild sea conditions. The WHC was turned off at 98 mbsf during the uplog.

Hole conditions

The hole diameter ranges from about 12 to 15 inches in the logged section. The raw susceptibility is corrected for hole size assuming an average diameter of 13 inches.

Depth shift

The GHMT data from the downlog have been interactively depth shifted with reference to NGT from GHMT/NGT uplog and to the sea floor (- 502 m). This value corresponds to the water depth observed on the logs and differs 3 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.

Log quality

The repeatability of both passes is not very good but the general trend is similar The logging data have been corrected for the drill pipe effect visible on the total induction signal.

Numerous spikes are present in the total induction signal (MAGB), which correspond to low values of the NMR outer voltage. The 'MAGS' data are saturated and not usable. The low resolution magnetic susceptibility channel, RMGS, is used instead.

Lithology

The core recovery in the logged section is only 2.3 %, so no stratigraphic subdivision was possible. The recovered sediment consists of diamictites, poorly sorted sandstones and mudstones.

Age information

Because of the very poor core recovery, no paleomagnetic or biostratigraphic age constraint was given on board. The age of the sediment corresponding to the logged interval is supposed to be late Pliocene - Pleistocene.

Proposed interpretation of GHMT data

Because of the numerous spikes in the total induction signal, any interpretation of the results is difficult. Moreover no age constraint is known so it is not possible to interpret the results before any biostratigraphic results are obtained post-cruise.

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:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia