Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 987E

Leg: 162

Location: Greenland Margin (Greenland Sea)

Latitude: 70° 29.787' N

Longitude: 17° 56.188' W

Logging date: August, 1995

Bottom felt: 1684.2 mbrf

Total penetration: 363.3 mbsf

Total core recovered: 308.8 m (62.3 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (94-483 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole conditions

The HLDT caliper shows variations from 11 to 18 inches. The raw susceptibility is corrected for hole size.

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 (- 1682 m). Note that the depth of the sea floor as seen on the logs differs from the "bottom felt" depth given by the drillers (-2.2 m). The program used is an interactive, graphical depth-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 is excellent. The magnetic susceptibility record shows sharp changes in its amplitude, due to strong lithologic contrasts. The presence of drill pipe is visible on the total induction signal from the top of the logged section to about 140 mbsf; the total induction signal (MAGB) has been corrected for the presence of drill pipe.

Lithology

Three different lithologic units have been logged with the GHMT. The uppermost unit (unit I) is a silty shale similar to unit IIIA. Unit II, between 300 and 370 mbsf looks like a debris flow, with a high magnetic. At the bottom of the hole, lithologic unit IIIA consists of a silty shale.

Age information

The age of unit I is upper Pliocene to Pleistocene, while the age of the bottom of the logged section is upper Pliocene. A biostratigraphic tie point (nannofossils) gives an age of 0.45 Ma at 46 mbsf.

Proposed interpretation of GHMT data

With the exception of the debris flow intervals, the magnetochrons can be well determined. A comparison of the GHMT result and the GPTS indicates the following normal magnetochron:

- onset of chron C1n.1n at 101 mbsf

- chron C2n from 193 to 222 mbsf

- chron C2r.1n from 257 to 268 mbsf

- onset of chron C2An.1n at 409 mbsf

- top of chrons C2An.2n and C2An.3n at 420 mbsf

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