Geologic Magnetic Data Processing


ODP logging contractor: LDEO-BRG
Great Australian Bight (SW Pacific Ocean)
33° 30.5613' S
128° 3.9844' E
Logging date:
October, 1998
Bottom felt:
795 mbrf (used for depth shift to sea floor)
Total penetration:
463.3 mbsf
Total core recovered:
40.5 m (12.9 %)

GHMT Logging Runs

One pass was recorded with the GHMT tool string. The NMRT component of the GHMT tool did not function during Leg 182, therefore no MAGB data was recorded. The magnetic susceptibility (MAGS) record is of good quality: processing was performed on the open-hole section (118.1-425.4 mbsf). Wireline heave compensator was used to counter ship heave but during the GHMT run it malfunctioned at 165 mbsf.

Hole conditions

The magnetic susceptibility data was corrected for borehole variations using the HLDT caliper from 139 mbsf to the bottom of the hole and a constant caliper value of 16 inches above 139 mbsf.

Depth Shift

The GHMT logs have been interactively depth shifted with reference to HNGS from DIT/HLDS/APS/HNGS run and to the sea floor (795 mbsf). 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 (HSGR or SGR) from the HNGS or 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 182. For further questions about the logs, please contact:

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