Geologic Magnetic Data Processing


ODP logging contractor: LDEO-BRG

Hole: 1081A

Leg: 175

Location: Walvis Ridge (tropical SE Atlantic)

Latitude: 19° 37.1818' S

Longitude: 11° 19.1598' E

Logging date: September, 1997

Bottom felt: 805.5 mbrf

Total penetration: 452.7 m

Total core recovered: 393.5 m (91.4 %)




GHMT Logging Runs


Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (72.9-448.1 mbsf).

Wireline heave compensator was used to counter ship heave.


Hole conditions


The hole diameter ranges from about 10 to 12 inches in the logged section. It is very regular below 300 mbsf. The raw susceptibility is corrected for hole size assuming an average diameter of 11 inches.


Depth shift


The GHMT logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT run and to the sea floor (- 804 m). This amount corresponds to the sea floor depth observed on the logs and differs 1.5 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) 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 logging data have been corrected for the drill pipe effect visible on the total induction signal.

Numerous spikes are present in the total induction signal (MAGB) below 403 mbsf, which correspond to low values of the NMR outer voltage. Any correlation analysis is meaningless in the 403-451 mbsf interval because the spikes create artifacts.




The GHMT encompasses lithologic units I and II:

- Unit I (72.9-390 mbsf): clay with fossils

- Unit II (390-448.1 mbsf): clayey nannofossil ooze.


Age information


The logged section corresponds to biozones CN13a to CN9a, 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 C2n to C4n (from top to bottom). Two intervals are undetermined because of the presence of spikes in the total induction measurement (MAGB) from 125 to 145 and from 290 to 303 mbsf. Using the ten windows of correlation analysis (SLOPE1 to SLOPE10) the following chrons could be determined:


- chron C2n from 100 to 114 mbsf

- chron C2r1n from 147 to 153 mbsf

- chron C2An from 168 to 223 mbsf with chron C2An.3n between 212 and 223 mbsf

- chron C3n from 250 to 275 mbsf

- chron C3An.1n at about 313 mbsf

- chron C3An.2n at about 330 mbsf

- chron C3Bn at about 362 mbsf

- chron C4n from 375 to 395 mbsf


These chrons correlate well with the age-depth relationship determined from the shipboard biostratigraphic results.


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