Geologic Magnetic Data Processing


ODP logging contractor: LDEO-BRG
East Tasman Plateau (Tasman Sea)
43° 57.5545' S
149° 55.7169' E
Logging date:
2-3 May 2000
Bottom felt:
2633 mbrf (used for depth shift to sea floor)
Total penetration:
766.5 mbsf
Total core recovered:
237.09 m (79.6 %)

GHMT Logging Runs

Two passes of the GHMT/DSI/NGT combination were recorded open hole: a main pass from 165 to 759 mbsf and a repeat pass from 259 to 379 mbsf. Processing was performed on the main pass.

Hole 1172D was logged under conditions of high ship heave; it was not used for most of the GHMT runs.

Hole conditions

The HLDS caliper shows a borehole varying in size from 11 to 19 inches. The raw susceptibility has been corrected for hole diameter using a mean value of 12 inches.

Depth Shift

The SGR log from the GHMT/NGT/DSI run was used as the reference for depth matching. The logs were then shifted to the sea floor (-2633 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. In this case it is the same as 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.

Log Quality

The total magnetic induction (MAGB) is of good quality except the presence of about 5 spiky zones, which were removed manually. The pipe effect is visible on the total induction signal and is corrected as well as a weak linear drift of -0.03 nT/m.

The magnetic susceptibility signal is of good quality.


The GHMT crosses lithostratigraphic units IB to IVB, which consist of a succession of nannofossil ooze and claystone.

Age information

The processed section corresponds to Late Miocene (top of the processed GHMT section) to Cretaceous (below 700 mbsf), as inferred onboard from biostratigraphic data. The deepest recognized magnetochron is C18n with its top at 383 mbsf (Hole 1172A).

Proposed interpretation of GHMT data

Because of the weakness of the intensity of magnetization of sediments, it is very hard to propose any magnetostratigraphic interpretation from GHMT data. 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