Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1207B

Leg: 198

Location: Shatsky Rise North (NW Pacific Ocean)

Latitude: 37°47.437' N

Longitude: 162°45.053' E

Logging date: 12-13 September, 2001

Bottom felt: 3111.7 mbrf

Total penetration: 622.8 mbsf

Total core recovered: 60.2 m (12.9 %)

GHMT Logging Runs

Two passes were recorded. Full processing was not preformed due to the bad quality of the data (see Log Quality section).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The principal control on log quality is hole diameter. Hole diameter was measured by the hydraulic caliper on the HLDS (LCAL) and on the FMS string (C1 and C2). Hole diameter varies between 10 inches at the base of the hole to 17 inches at 155 mbsf.

Depth shift

The GHMT logs have been interactively depth shifted with reference to the HNGS from the DIT/HLDS/APS/HNGS/MGT run (pass 1) and then shifted to the sea floor. 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 sea floor depth was then determined from the step in gamma radiation at the sediment-water interface to be 3110.5 mbrf. This depth differs by 1.2 m from the water depth determined from the mud-line core (see "bottom-felt" depth above). Depth differences occur because of wireline slip stretch, drill pipe stretch, tides, heave, etc.

Log quality

The GHMT data are of very poor quality because of tool malfunction. The total field log (MAGB) alternates between 32000 and 42000 nT, whereas small variations about 42000 nT are expected. The susceptibility logs (MAGS, RMGS) show large negative values near the base of the hole, and a high baseline of about 500 ppm in the upper part of the hole. A baseline close to zero is expected in these sediments. In the interval of overlap with the core measurements, the log and core susceptibility data show similar patterns and therefore the logs might still provide some information, though they should be used with caution. No processing, however, has been performed, due to the overall poor quality of the data.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 198. For further questions about the logs, please contact:

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