Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG
Well name: 1166A
Leg: 188
Location: Prydz Bay (Antarctic Ocean)
Latitude: 67° 41.8' S
Longitude: 74° 47.2' E
Logging date: February 20-21, 2000
Bottom felt: 480 mbrf
Total penetration: 381.3 mbsf
Total core recovered: 18.6%

GHMT Logging Runs

Two passes of the GHMT were recorded open hole. Processing was performed on pass two, from 40 to 385 mbsf.

The Wireline Heave Compensator was used to counter ship heave.

Hole conditions

According to the FMS calipers, the hole size varies from 11 to 19 inches. The raw susceptibility has been corrected for hole diameter using an average value of 12 inches.

Depth Shift

The GHMT logs were depth matched to the HNGS from the DIT/APS/HLDS/HNGS run and then shifted to the sea floor (-480 m).

Log Quality

The total magnetic induction (MAGB) is generally of good quality except the presence of a few very locally spiky zones that were removed manually. The pipe effect is visible on the total induction signal down to about 120 m and is corrected as well as a linear drift of 0.13 nT/m.

The magnetic susceptibility signal is of good quality except at 277 mbsf, where a spiky zone has been removed manually. Due to local saturation of the MAGS signal, the RMGS signal was used instead.

Lithology

The GHMT crosses lithologic units Ib to V, which consist of a succession of poorly sorted sands, fine-grained sediments, and diamict.

Age information

The processed section corresponds to Quaternary (top of the section) to Early Cretaceous (bottom of the section) periods, as inferred onboard from biostratigraphic data. There is a major unconformity observed on cores at 135 mbsf, separating upper Pliocene sediment above from lower Oligocene sediments below.

Because of the very poor core recovery, the shipboard magnetostratigraphy obtained from core is very poor. Normal polarity intervals are observed at 115 mbsf (C2An.1n), and between 126 and 160 mbsf (C12n / C13n ?).

Proposed interpretation of GHMT data

Because of the very poor core recovery and the intervals barren of fossils, it is not possible to propose any precise magnetostratigraphic interpretation from GHMT data. Nevertheless, the few normal zones observed on cores are also observed on the GHMT data. The GHMT data also indicates the presence of normal polarity zones between about 270 and 350 mbsf.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 188. For further questions about the logs, please contact:

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