GFeologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1085A

Leg: 175

Location: Cape Basin (tropical SE Atlantic)

Latitude: 29° 22.4665' S

Longitude: 13° 59.4064' E

Logging date: September, 1997

Bottom felt: 1724.8 mbrf (used for depth shift to sea floor)

Total penetration: 604 mbsf

Total core recovered: 594.4 m (98.4 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. The total induction (MAGB) measurement was out of range for the tool in this location, so no reliable MAGB data was recorded. The magnetic susceptibility (MAGS) record is of good quality: data is from the main pass, open-hole section (60.9-593.5 mbsf).

Wireline heave compensator was used to counter ship heave.

Depth shift

The GHMT logs have been interactively depth shifted with reference to NGT from APS/HLDS/HNGS run and to the sea floor (- 2003.5 m). The program used is an interactive, graphical depth-match program, which 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 or HNGS tools 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.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 175.

For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.