Geologic Magnetic Data Processing

ODP logging contractor: LDEO-BRG

Hole: 925C

Leg: 154

Location: Ceara Rise (tropical NW Atlantic)

Latitude: 4° 12.256' N

Longitude: 43° 29.349' W

Logging date: February, 1994

Bottom felt: 3051.5 mbrf

Total penetration: 360.1 mbsf

Total core recovered: 368.78 m (102.4 %)

GHMT Logging Runs

Two passes (main and repeat) were recorded. Processing was performed on both passes in the open-hole section (87-357 and 199-300 mbsf). No meaningful total induction signal (MAGB) could be recorded, thus only the magnetic susceptibility data have been processed.

Wireline heave compensator was used to counter ship heave.

Hole conditions

The FMS caliper varies from 9.5 to 15 inches, with an average value of about 11.5 inches. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper, main pass.

Depth Shift

The GHMT data could not be depth-matched to the reference run using the SGR channel, as no NGT tool was included in the GHMT tool string; however, a rough depth match was possible by correlating the magnetic susceptibility channel (MAGS) with the SGR reference channel (from DIT/SDT/HLDT/NGT). The logs have then been depth to the sea floor (- 3049 m). This amount differs from the "bottom felt" depth given by the drillers because it incorporates some additional depth shift applied by the logging scientists during correlation of the logs with the Multi Sensor Track (MST) data from core. 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.

Log Quality

The magnetic susceptibility signal (MAGS) is of good quality.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 154. For further questions about the logs, please contact:

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