Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Well name: 986C

Leg: 162

Location: Svalbard Margin (Greenland Sea)

Latitude: 77° 20.431' N

Longitude: 9° 4.664' W

Logging date: August, 1995

Bottom felt: 2062.5 mbrf (used for depth shift to sea floor)

Total penetration: 408 mbsf

Total core recovered: 229.8 m (56.3 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (96.6-367.3 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The FMS calipers show a circular hole, with diameter ranging from 12 to 16 inches. The raw susceptibility has been corrected for hole size using the FMS caliper.

Depth shift

The GHMT logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT main run, and to the sea floor (- 2062.5 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.

Log Quality

The repeatability of both magnetic susceptibility and total induction measurements is good. The presence of drill pipe is visible on the total induction signal (MAGB) from the top of the logged section to about 140 mbsf: the total induction signal has been corrected for the presence of drill pipe.

Lithology

The GHMT was recorded through lithologic unit II; this unit consists of silty shale, with intercalated three debris flows at 153-171, 228-253, and 288-328 mbsf, which are all characterized by low susceptibility values. Because these formations are disturbed, this must be taken into account in the magnetostratigraphic interpretation.

Age information

Shipboard biostratigraphic data (nannofossils and foraminifers) obtained on cores show that the age of the logged interval is Pleistocene. Two datums are available: 0.5 Ma at 118 mbsf and 1.8 Ma at 548 mbsf.

Proposed interpretation of GHMT data

The presence of large spikes in the total field data at about 140, 227, 236, 252, 257, 262, 286, 300, 340, and 342 mbsf lead to artifacts in the computation. Moreover, the presence of the three debris flows mentioned above prevents any interpretation in these sections.

Using four windows of correlation analysis, the following magnetochrons can be proposed:

- chron C1n (Brunhes) from the top of the logged section to 235 mbsf

- chrons C1r.1r and C1r.2r from 235 to 350 mbsf

The boundaries of these magnetochrons are located in the debris flows and cannot be precisely determined.

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