Geologic Magnetic Data Processing

 

ODP Logging Contractor: LDEO-BRG

Hole: 966F

Leg: 160

Location: Eratosthenes Sea Mount (Eastern Mediterranean)

Latitude: 33° 47.856' N

Longitude: 32° 42.090' E

Logging Date: March, 1995

Bottom Felt: 934 mbrf (used for depth shift to sea floor)

Total Penetration: 356 mbsf

Total Core Recovered: 65.9 m (22 %)

 

GHMT Logging Runs

 

Two passes were recorded. Processing was performed on pass 2, open-hole section (69.7-345.5 mbsf).

Wireline heave compensator was used to counter ship heave resulting from the mild sea conditions.

 

Hole conditions

 

The FMS calipers show that the hole is circular with variations from 12 to 18 inches. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper.

 

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 (- 934 m). The program used is an interactive, graphical depth-match program that 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 repeatability of both magnetic susceptibility and total induction is excellent in the high frequencies, despite a shift of 2 m between pass 1 and 2. The pull up of the pipe during pass 1 caused a double " drift " of the total induction signal, difficult to remove. Thus, the processing has been made using the second pass where the pipe effect is visible on the total induction signal from the top of the logged section to about 160 mbsf. The total induction signal (MAGB) has been corrected for the presence of drill pipe. Also, the drop of the drill bit at the bottom of the hole affects the total induction signal from 325 mbsf to the bottom of the hole.

 

Lithology

 

The GHMT was recorded in a homogeneous, bioclastic carbonate formation.

 

 

Age information

 

Shipboard biostratigraphic data obtained on cores show the contact between Miocene and Pliocene at 120 mbsf; due to the absence of upper Eocene, Oligocene, and lower Miocene, the contact between middle Eocene and middle Miocene is at 388 mbsf.

 

Interpretation of GHMT data

 

No shipboard magnetostratigraphic result is available from core analysis for the depth interval logged with the GHMT. Many reversals should be present in this interval (Miocene times). The interpretation of GHMT data should be performed jointly with detailed paleomagnetic and/or biostratigraphic studies.

 

 

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 160. For further questions about the logs, please contact:

 

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