Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG
Hole: 1168A
Leg: 189
Location: West Tasmania Slope (Tasman Sea)
Latitude: 42° 36.58' S
Longitude: 144° 24.76' E
Logging date: March, 2000
Bottom felt: 2474 mbrf
Total penetration: 883.5 mbsf
Total core recovered: 837.1 m (94.7%)

GHMT Logging Runs

Two passes of the GHMT/DSI/NGT combination were recorded openhole: the main pass from 101 to 723 mbsf and the repeat pass from 242 to 373 mbsf Processing was performed on the main pass.

The wireline heave compensator was used to counter ship heave (typically 3 m).

Hole conditions

The HLDS caliper shows strong variations, with maximum readings of 19 inches.

The raw susceptibility has bee corrected for hole diameter variations using an average value of 12.8 inches.

Depth Shift

Since the depth match between the total gamma of the two logging runs is somewhat ambiguous, the conductivity from the GHMT (MAGC) was matched to the DIT from the DIT/APS/HLDS/HNGS main pass. This led to an improved depth match. All the logs were then shifted to the sea floor (-2475 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. It differs 1 m from the "bottom felt" depth given by the drillers (see above).

Depth matching is typically done in the following way. One log is chosen as reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent), and then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed automatically, and the result checked and adjusted as necessary. The depth adjustments that were required to bring the match log in line with the base log are then applied to all the other logs from the same tool string.

The depth match between the main passes of the two tool strings is fairly robust for most of the hole, but a lack of correlateable features in the intervals 305-350 and 425-555 mbsf makes the matches there more questionable.

Depth matching was also made difficult by the variation in tool speed caused by the tool strings dragging on the bridges while logging up the hole. While the cable speed (CS) at the surface remains fairly constant, the actual tool speed is slowed, as indicated by increased tension at the head of the tool (DF, down hole force). For example, density and porosity measurements are offset from each other in intervals of high DF, for example from 410-440 and 720-755 mbsf.

Log Quality

The total magnetic induction (MAGB) is of good quality, with the exception of about 20 spiky zones, which were removed manually. The pipe effect is visible on the total induction signal down to about 250 mbsf; it is corrected as well as a linear drift of -0.035 nT/m. A variation of MAGB off the theoretical pipe effect line is observed at 140 mbsf because of pipe movement during the recording.

The magnetic susceptibility signal is of good quality except at 303 mbsf, where a spiky zone has been removed manually.

Lithology

The GHMT crosses lithostratigraphic units IB to IIIB, which consist of a succession of ooze, chalk, claystone, and siltstone.

Age information

The processed section corresponds to Late Miocene (top of the processed GHMT section: 135 mbsf) to Early Oligocene (bottom of the processed GHMT section: 725 mbsf) periods, as inferred onboard from biostratigraphic data.

Despite the good core recovery, the shipboard magnetostratigraphy obtained from core is very poor, mainly because of the weak intensity of magnetization of the sediments. In the studied depth section, magnetochrons C5Cn to C6n normal polarity intervals are observed on core between 250 and 350 mbsf.

Proposed interpretation of GHMT data

Because of the weakness of the sediments magnetization intensity, it is very hard to propose any magnetostratigraphic interpretation from GHMT data at this site. The local magnetic field (remanent + induced) seems to mainly consist of the field due to the susceptibility.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 189. For further questions about the logs, please contact:

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