Geologic Magnetic Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 1051A
Leg: 171B
Location: Blake Nose (NW Atlantic)
Latitude: 30° 03.174' N
Longitude: 76°21.458 ' W
Logging date: January, 1997
Bottom felt: 1993.3 mbrf
Total penetration: 644.6 mbsf
Total core recovered: 599.9 m (94.3 %)
GHMT
Logging Runs
Two passes (main and repeat) were recorded. Processing was performed on the main pass, open-hole section (125.1-638.4 mbsf).
Wireline heave compensator was not available on Leg 171B. Sea conditions were moderate, with sea swells in the order of 2.5 m.
The FMS calipers show a slightly elliptical hole in the repeat section. Because of an apparent re-calibration of the FMS calipers during the main pass, the HLDT caliper has been used instead to correct the raw susceptibility for hole diameter variations.
The original logs have been interactively depth shifted with reference to NGT from DIT/HLDT/APS/HNGS run and to the sea floor (- 1992 m). The amount of depth shift to the sea floor corresponds to the water depth as seen on the logs and differs 1.3 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 or HSGR) from the HNGS/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 total induction signal (MAGB) has been corrected for the presence of drill pipe. Numerous spikes are present on the total induction signal, which are correlated to low values of the NMRT outer voltage.
Lithology
The GHMT was recorded through lithologic units ID (middle Eocene) to III (Late Paleocene):
- Unit ID (top of the logged section to 379.9 mbsf): Siliceous nannofossil chalk to nannofossil chalk with siliceous microfossils.
- Unit II (379.3-389.9 mbsf): Porcellanized foraminifer packstone, olive green claystone, and clay-rich firm ground.
- Unit III (389.9 mbsf to bottom of the logged section): Siliceous nannofossil chalk to siliceous nannofossil chalk with clay.
Age
information
The logged section corresponds to biozones CP14a to Cp3, as inferred onboard from nannofossil data.
Proposed
interpretation of GHMT data
Paleomagnetic chrons C19r to C26r have been determined from comparison with core measurements of the paleo-inclination made on board.
The numerous spikes present in the data recorded in the lower part of the hole lead to artifacts in the computation of the correlation slope.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 171B. For further questions about the logs, please contact:
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia