Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 884E

Leg: 145

Location: Detroit Sea Mount (NW Pacific)

Latitude: 51° 27.034' N

Longitude: 168° 20.216' E

Logging date: August, 1992

Bottom felt: 3836 mbrf (used for depth shift to sea floor)

Total penetration: 929.8 mbsf

Total core recovered: 66.8 m (76.6 %)



SUMT/NMRS Logging Runs


The SUMT and NMRS were run separately. Two passes were recorded for each tool. Processing was performed on pass one of each tool, open-hole section (70-682 mbsf).

The wireline heave compensator was not used on any of the logging runs due to electronic failure. Ship heave was mild, up to 1 m.


Hole conditions


The FMS calipers vary from 10 to 15 inches. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper.


Depth Shift


The magnetic logs could not be interactively depth-shifted with reference to NGT from the DIT/SDT/HLDT/CNT/NGT run because no gamma ray was run on the magnetometer and susceptibility tool strings. They were first depth shifted to the sea floor (- 3836 m) and then pass 2 was shifted with reference to pass 1. A list of the amount of differential depth shifts applied at this hole is available upon request.


Log Quality


The total magnetic induction (SUMT) and magnetic susceptibility (NMRS) signals are of good quality. The pipe effect is visible on the total induction signal down to about 175 m and is corrected as well as a linear drift of 0.317 nT/m.




The processed SUMT/NMRS logs cross lithologic unit I and subunit IIA:

- Unit I (0-604.8 mbsf) : clay and claystone, clayey diatom ooze, clayey diatomite, and diatomite

- Subunit IIA (604.8-694.7 mbsf): claystone, chalk


Age information


The logged section is Pleistocene to Oligocene in age, as inferred onboard from nannofossil data and paleomagnetic analysis.


Proposed interpretation of NMRS data


Paleomagnetic chrons C2n to C5n.2n have been determined directly from the magnetic induction signal (BTcorf in the joined file) by Dubuisson et al. (ODP S. Volume 145). This was possible because of the high-latitude of the logged site and because of the low susceptibility values. Because of the low susceptibility and mainly because of the uncertain depth-match between the NMRT and the SUMS for our method, Dubuisson et al's method worked better and more precisely than the traditionnl method of processing used here.

The following normal polarity zones have been determined in the depth intervals listed below:


Above 120 mbsf: chrons C2n and C2r.1n

120-180 mbsf: chrons C2An.1n, C2An.2n, C2An.3n

180-310 mbsf: 4 normal intervals of chron C3n

310- 365 mbsf: 2 normal intervals of chron C3A

365-372 mbsf: chrons C3Bn and C3Br.1n

376-405 mbsf: 3 normal intervals of chron C4n

420-450 mbsf: 3 normal intervals of chron C4An and chron C4Ar.1n

410-510 mbsf: probably chron C5n.2n with uncertain limits

Below, both methods do not show clear magnetic polarity events.



Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 145. For further questions about the logs, please contact:


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