Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG
Hole: 1170D
Leg: 189
Location: South Tasman Rise (Tasman Sea)
Latitude: 47° 9.04'S
Longitude: 146° 2.991'E
Logging date: April, 2000
Bottom felt: 2715.8 mbrf
Total penetration: 779.8 mbsf
Total core recovered: 287.69 m (81.1 %)

GHMT Logging Runs

Two passes of the GHMT/DSI/NGT combination were recorded open hole: pass one from 535 to 780 mbsf and pass two from 533 to 631 mbsf. Processing was performed on pass one.

The wireline heave compensator was used to counter ship heave.

Hole conditions

The HLDS and FMS calipers show a good borehole, except around 648 mbsf. The raw susceptibility has been corrected for hole diameter using a mean value of 10.5 inches.

Depth Shift

Since the depth match between the total gamma of the DIT/APS/HLDS/HNGS and GHMT/DSI/NGT logging runs is somewhat ambiguous, the conductivity from the GHMT (MAGC) was matched to the DIT from the DIT/APS/HLDS/HNGS second pass. This led to an improved depth match. The GHMT logs were then shifted to the sea floor (-2711.8 m). The mudline was not logged; according to the logs, however, the end of the bottom hole assembly was 4 m shallower than according to the drillers. Therefore the logger's sea floor depth is also taken to be 4 m shallower.

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 wireline heave compensator stopped at certain intervals during logging, because of excessive ship heave. Depth control is degraded in these intervals, and consequently the match between different runs, and even between different tools on the same tool string will be poor. These intervals are 712-747 mbsf (pass 1) and 584-610 mbsf (pass 2).

Log Quality

The total magnetic induction (MAGB) and magnetic susceptibility signals are of good quality. The pipe effect is visible on the total induction signal and is corrected as well as a weak linear drift of -0.07 nT/m. Above 600 mbsf, the MAGB data do not seem to be valid, possibly due to the strong heave.

Lithology

The GHMT data correspond to lithostratigraphic unit VB, which consists of silty claystone.

Age information

The processed section corresponds to Middle Eocene, as inferred onboard from biostratigraphic data.

Below 450 mbsf, fossil preservation is poor. The dynocists indicate an age of 39.5 Ma around 570 mbsf and an age of 43 Ma around 770 mbsf. The magnetostratigraphy was obtained down to about 400 mbsf. The deepest magnetochron of this interval is the top of C18n.1n (25.82 Ma) at 404.5 mbsf (Hole 1170A). Below, a 400 m-long magnetochron of normal polarity is inferred (C20n or C21n).

Proposed interpretation of GHMT data

Contrarily to what observed on core, the GHMT data indicate mainly a reverse polarity chron (600-780 mbsf). A possible interpretation (in agreement with the biostratigraphic - dynocists - age) could be :

- C18n from 610 to 620 mbsf
- C18r from 620 to about 695 mbsf
- C19n from 695 to 705 mbsf
- C19r below 705 mbsf.

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