Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1095B

Leg: 178

Location: West Antarctic Peninsula Rise (Antarctic Ocean)

Latitude: 66° 59.12659' S

Longitude: 78° 29.26987' W

Logging date: February, 1998

Bottom felt: 3852.6 mbrf

Total penetration: 570.2 mbsf

Total core recovered: 385.8 m (67.7 %)

GHMT Logging Runs

Two passes were recorded. Processing was performed on pass 1, open-hole section (108.3-551.5 mbsf).

Due to the very rough sea conditions during the initial phase of logging the ship heave exceeded the maximum stroke of the wireline heave compensator; for this reason, it was decided not to use it during the first logging run. With sea conditions improving, it was operational during the recording of the GHMT/NGT logs, with the exception of the 372-417 mbsf interval during pass 1, where it stopped working temporarily. The lack of WHC on the first logging run produced depth offsets ranging from 10 to 15 m.

Hole conditions

The hole diameter ranges from about 13 to 18 inches in the logged section. The raw susceptibility is corrected for hole diameter assuming an average diameter of 16 inches.

Depth shift

The GHMT data from pass 1 have been interactively depth shifted with reference to NGT from GHMT/NGT pass 2 and to the sea floor (- 3853 m). This value corresponds to the water depth observed on the GHMT/NGT pass 2 and differs 0.4 m from the drillers' "bottom felt" depth. The depth match of the DIT/APS/HLDS/HNGS logs to the reference run was very difficult because of the poor data repeatability. This was mostly due to the rough sea conditions during the recording and the lack of wireline heave compensator. The program used is an interactive, graphical depth-match program that 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/HNGS 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

Repeatability between passes was good. The total induction signal has been corrected for the presence drill pipe. Numerous spikes are present in the total induction signal (MAGB), which correspond to low values of the NMR outer voltage. The MAGB data are affected by the presence of numerous spikes that create artifacts.

Lithology

The GHMT encompasses lithologic units II and III:

- Unit II (49.3 to 435.5 mbsf): Fine sands and silts and laminated silty clays, interbedded with massive deposits

- Unit III (435.5 to 561.4 mbsf): finely-laminated claystone and sandstone.

Age information

In the logged interval, paleomagnetic chrons C2An.3n (at about 100 mbsf - Pliocene) to C4Ar.1n (top at about 460 mbsf - Late Miocene) have been interpreted from core measurements of the paleo-inclination made on board.

Proposed interpretation of GHMT data

Comparison of the GHMT results with GPTS allows one to determine magnetochrons C3n to C5n (from top to bottom):

- chron C3n from 133 to about 183 mbsf with chron C3n.4n from 165 to 183 mbsf

- chron C3An.1n from 210 to 225 mbsf

- chron C3An.2n from 233 to 265 mbsf

- chron C3Bn from 300 to 315 mbsf

- chron C4n.2n from 335 to 358 mbsf

- chron C4r.2r-1 from 405 to 408 mbsf

- chron C4An and C4Ar.1n from 412 to 493 mbsf

- Top of C5n at about 513 mbsf

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

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 178. For further questions about the logs, please contact:

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