Geologic Magnetic Tool Data Processing

ODP logging contractor: LDEO-BRG

Hole: 984-B

Leg: 162

Location: Bjorn Drift (Central N Atlantic)

Latitude: 61° 25.527' N

Longitude: 24° 04.949' W

Logging date: July, 1995

Bottom felt: 1659 mbrf

Total penetration: 503.7 mbsf

Total core recovered: 458.3 m (91 %)

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. Processing was performed on the main pass, open-hole section (86.7-496.2 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The FMS calipers show a slightly elliptical hole from the top of the logged section to 300 mbsf. The raw susceptibility from the main pass is corrected for hole diameter variations using the FMS caliper (pass 1) from 106 to 489 mbsf and a constant caliper value of 12 and 10 inches respectively above and below this interval.

Depth shift

The GHMT logs have been interactively depth shifted with reference to NGT from ACT/GST/NGT run and to the sea floor (- 1660.5 m). Note that the depth of the sea floor as seen on the logs differs from the "bottom felt" depth given by the drillers (+1.5 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) from the NGT 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.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 162. For further questions about the logs, please contact:

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