Geologica Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1014A

Leg: 167

Location: Baja California Margin (tropical NE Pacific Ocean)

Latitude: 32° 49.994' N

Longitude: 119°58.903' W

Logging date: May, 1996

Bottom felt: 1175.8 mbrf (used for depth shift to sea floor)

Total penetration: 449 mbsf

Total core recovered: 404.4 m (90 %)

GHMT Logging Runs

Two passes were recorded. Due to malfunction of the magnetic induction tool (NMRS) only magnetic susceptibility data were recorded. The magnetic susceptibility (MAGS) record is of good quality: data is from the pass 1, open-hole section (75.6-439.5 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole Conditions

Magnetic susceptibility data from pass 1 were corrected for borehole variations using an average caliper value of 12 inches from the top of the logged interval to 101 mbsf and the FMS caliper from 101 mbsf to the bottom of the hole.

Depth Shift

The GHMT logs have been interactively depth shifted with reference to NGT from DIT/HLDT/APS/HNGS pass 1 and to the sea floor (- 1175.8 m). 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 from the NGT tool and/or 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.

Lithology

The logged section is within a Pliocene-Quaternary sedimentary sequence dominated by biogenic assemblages and clastic siliceous clays. The tool was run through lithologic unit IA (56-140 mbsf) and IB (140-438 mbsf). Unit IA consists of clay interbedded with foraminifers and nannofossil ooze with foraminifers and clay. The dominant lithology of unit IB is a nannofossil ooze and chalk alternating with clay and clays with foraminifers intervals.

The transition between units IA and IB is marked by a gradual increase in nannofossils that explains the slow decrease of the average susceptibility in the interval between 130 and 150 mbsf.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 167. For further questions about the logs, please contact:

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