Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1020B

Leg: 167

Location: Baja California Margin (tropical NE Pacific)

Latitude: 41° 0.051' N

Longitude: 126° 26.064' W

Logging date: June, 1996

Bottom felt: 3049.7 mbrf (used for depth shift to sea floor)

Total penetration: 278.8 mbsf

Total core recovered: 265 m (95 %)

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 pass 2, open-hole section (61.6-269 mbsf).

Wireline heave compensator was used to counter ship heave.

Hole Conditions

Magnetic susceptibility data from pass 2 were corrected for borehole variations using an average caliper value of 14 inches from the top of the logged interval to 85 mbsf and the FMS caliper from 85 mbsf to the bottom of the hole.

Depth Shift

The original logs have been interactively depth shifted with reference to HNGS from DIT/HLDT/APS/HNGS pass 2 and to the sea floor (- 3049.7 m). Pass 1 of the GHMT and FMS tool strings have been depth shifted with reference to Pass 2 of the same tool string, by using caliper and magnetic data for correlation. 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 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 encompasses a lower Quaternary to Pliocene sequence where two lithostrtigraphic units are identified from core analyses. Unit I is characterized by siliciclastic clay with small to moderate amounts of siliceous and calcareous biogenic components. The sediments from Unit I are divided into two sub-units based on the relative abundance of biogenic components. From 120 to 230 mbsf the unit is clay mixed with nannofossil clay (I) and then becomes a diatom clay (IB). Contacts between sub-units are not strictly defined because of the gradational nature of changes between the major lithologies. Unit II from 228 mbsf to the bottom of the logged sequence consists of nannofossil chalk mixed with clay.

There is no lithological evidence for a 80 ppm step in the average susceptibility at 163 mbsf. Moreover core susceptibility measurements do not confirm this behavior.

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