Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 931B

Leg: 155

Location: Amazon Fan (equatorial NW Atlantic)

Latitude: 5° 8.520' N

Longitude: 46° 37.980' W

Logging date: April, 1994

Bottom felt: 3486.7 mbrf

Total penetration: 421.3 mbsf

Total core recovered: 296.05 m (70.3 %)

GHMT Logging Runs

Two passes were recorded. Processing was performed on the second pass, open-hole section (859-243 mbsf). Wireline heave compensator was used to counter ship heave.

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 (repeat pass).

Depth Shift

Depth shift: Original logs have been interactively depth shifted with reference to NGT from ACT/GST/NGT run and to the sea floor (- 3483.5 m). This value does not correspond to the drillers' water depth ("bottom felt") but to the depth of the sea floor as seen on the logs. 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 total induction signal (MAGB) has been corrected for the presence of drill pipe. Six spikes were present on the total induction signal (at about 160, 162, 195, 210, 224, and 243 m) and have been removed. They are correlated to low values of the NMRT outer voltage.

Lithology

The sediment logged with the GHMT corresponds to the lower part of lithologic unit IIC and to the upper part of the lithologic unit III:

- Unit IIC (27.5-199.8 mbsf: silty clay

- Unit III (199.8-349.25 mbsf): muddy sand, silty clay, and clay

Age information

The logged section corresponds to late Pleistocene as inferred onboard from biostratigraphic data. The bottom of the logged section corresponds to an age of about 40 ky.

Proposed interpretation of GHMT data

During late Pleistocene, there is no magnetic polarity reversal. The entire logged interval corresponds to the Brunhes normal polarity zone. Because of the low value of the present magnetic inclination at this site, anti-correlation of the signals called remanent effect and induced effect in the processing steps should correspond to a normal polarity zone. The general trend resulting from the processing indicates a normal polarity interval, even if a few zones of unexplained correlation of signals are present.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 155. For further questions about the logs, please contact:

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