Geologic Magnetic Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 1000B
Leg: 165
Location: Pedro Channel-N Nicaragua Basin (Caribbean Sea)
Latitude: 16° 33.219' N
Longitude: 79° '52.046' W
Logging date: January, 1996
Bottom felt: 927.2 mbrf
Total penetration: 695.9 mbsf
Total core recovered: 142.97 m (67.6 %)
GHMT
Logging Runs
Two passes were recorded. Only the magnetic susceptibility (MAGS) record is of good quality: data is from pass 1, open-hole section (276.3- 675.6 mbsf).
Wireline heave compensator was not used due to a major system failure.
Hole
conditions
The raw susceptibility has been corrected for hole diameter variations borehole variations using the HLDT caliper from 304 mbsf down to the bottom of the logged section and an average caliper value of 11 inches above.
Depth
Shift
The GHMT logs have been interactively depth shifted with reference to NGT from DIT/SDT/HLDT/CNTG/NGT run and to the sea floor (- 924 m). This value corresponds to the bottom of the sea floor as seen on the logs and is 3.2 m shallower than the "bottom felt" depth given by the drillers. 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) from the 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.
Both susceptibility and total field signals are very weak as it can be expected through a calcareous chalk. Despite the very low concentration in magnetic material, the repeatability of the susceptibility log between the two passes is excellent, except between 508 and 513 mbsf and at 522 and 526 mbsf where spikes occur on one of the two passes for an unknown reason.
The poor quality of the total induction logs prevents any processing to detect reversals.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 165. For further questions about the logs, please contact:
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia