Geologic Magnetic Data Processing
ODP logging contractor: LDEO-BRG
Location: Amazon Fan (equatorial NW Atlantic)
Latitude: 5° 5.796' N
Longitude: 46° 48.738' W
Logging date: April, 1994
Bottom felt: 3376.8 mbrf
Total penetration: 254.2 mbsf
Total core recovered: 177.38 m (69.8 %)
GHMT Logging Runs
Two passes were recorded. Processing was performed on the first pass, open-hole section (902.4-209 mbsf). Wireline heave compensator was used to counter ship heave.
The FMS calipers vary from 10 to 12 inches in the logged interval. The raw susceptibility has been corrected for hole diameter variations using the FMS caliper (pass 2).
Depth shift: The depth match was difficult, due to the different gamma ray signature. The second pass of the GHMT logs has been first depth shifted to match the first pass, using the different susceptibility curves for correlation. Both passes have then been depth-shifted with reference to the main DIT/SDT/HLDT/CNTG/NGT pass, using the NGT for correlation. The match, however, is generally poor, due to the different NGT signature. All logs have then been depth shifted to the sea floor (- 3376 m). This amount corresponds to the water depth as observed on the DIT/SDT/HLDT/CNTG/ NGT logs and differs 0.6 m from 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.
The total induction signal (MAGB) has been corrected for the presence of drill pipe. Three large spikes are present on the total induction signal (at 137, 156 and 178 m) and are correlated to low values of the NMRT outer voltage.
The sediment logged with the GHMT corresponds to the lower part of lithologic unit IIB, unit III and to the upper part of lithologic unit IV:
- Unit IIB (14.37-97.62 m) : silty clay with beds of silt and sand
- Unit III (97.62-167.31 m) : thick debris flow deposit
- Unit IV (167.31-249.72 m) : silty clay
The logged section corresponds to late Pleistocene down to 167 m and to middle Pleistocene below. An age of 250 ka has been determined at 180 mbsf from biostratigraphic data.
Proposed interpretation of GHMT data
The three large spikes present in the data recorded in the total magnetic induction data can lead to artifacts in the computation of the correlation slope.
During Pleistocene, the Brunhes/Matuyama boundary between normal to reversed magnetic polarity occured at 780 ka. Because of the low value of the present magnetic inclination at this site, (anti)correlation of the remanent and induced effects signals in the processing steps should correspond to (normal)reversed polarity zones. A change from normal (above) to reversed polarity at 160 m is observed on GHMT data but do not seem to corespond to any magnetic polarity change.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 155. For further questions about the logs, please contact:
E-mail: Cristina Broglia