Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1082A

Leg: 175

Location: Walvis Basin (tropical SE Atlantic)

Latitude: 21° 5.6373' S

Longitude: 11° 49.2361' E

Logging date: September, 1997

Bottom felt: 1290.7 mbrf

Total penetration: 600.6 mbsf

Total core recovered: 502 m (91.33 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (64.0- 593.9 mbsf).

Wireline heave compensator was used to counter ship heave.

Depth Shift

The GHMT logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT run and to the sea floor (- 1291.5 m). This amount corresponds to the sea floor depth observed on the logs and differs 0.8 m from the drillers' "bottom felt" 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 or HSGR) from the NGT or HNGS tools 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.

Hole conditions

The hole diameter ranges from about 10 to 12 inches in the logged section. The raw susceptibility is corrected for hole size assuming an average diameter of 11 inches.

Log quality

The logging data have been corrected for the drill pipe effect visible on the total induction signal as well as for a linear drift of 0.061 nT/m.

Spikes are present on the total induction signal (MAGB) at 233, 276, 296, 320, 325, 352, and 426 mbsf, which correspond to low values of the NMR outer voltage. Any correlation analysis is meaningless at these depths because the spikes create artifacts.

Lithology

The GHMT encompasses lithologic units I and II:

- Unit IA (80-112 mbsf): intervals of nannofossil and foraminifer-rich clay

- Unit IB (112-369 mbsf): intervals of nannofossil- and diatom-rich clay and nannofossil-rich diatomaceous clay

- Unit IC (369-475 mbsf): nannofossil clay

- Unit II (475-590 mbsf): nannofossil ooze.

Age information

The logged section corresponds to biozones CN13a to CN10a, as inferred onboard from nannofossil data.

Proposed interpretation of GHMT data

Comparison of the GHMT results with core paleomagnetic measurements allows one to determine chrons C1r to C2An:

- chron C1r from 80 to 158 mbsf

- chron C2n from 158 to 181 mbsf

- chron C2r from 181 to 263 mbsf

- chron C2An from 263 to 380 mbsf

Below this interval, no clear normal or reversed polarity zone can be determined from the GHMT data.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 175. For further questions about the logs, please contact:

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