GHMT Processing - ODP Leg 184, Hole 1146A
ODP logging contractor: LDEO-BRG
Hole: 1146A
Leg: 184
Location: Northern Margin of South China Sea (China Sea)
Latitude: 19° 27.4014' N
Longitude: 116° 16.13636' E
Logging date: March, 1999
Bottom felt: 2102.6 mbrf
Total penetration: 607.0 mbsf
Total core recovered: 603.85 m (99.48 %)
Three passes were recorded with the GHMT tool string. Processing was performed on the third pass, open-hole section (249.9-590.1 mbsf). Wireline heave compensator was used to counter ship heave.
Hole conditions
The FMS and HLDS calipers vary from 9 to 14 inches. The raw susceptibility has been corrected for hole diameter variations using the HLDS caliper.
Depth shift
The GHMT logs have been interactively depth shifted with reference to HNGS from DIT/APS/HLDS/HNGS run and to the sea floor (-2106.5 m). This value corresponds to the mudline as observed on the logs; it differs 3.9 m from the "bottom felt" depth given by the drillers (see above). 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 NGT/HNGS 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 are 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 many spikes over the total length of the record. This very high noise strongly affects the quality of the MAGB record. All the spikes have been manually removed, then the signal was re-sampled at 0.1524 m. The MAGB signal has been corrected for the presence of drill pipe. The magnetic susceptibility data (MAGS) is of good quality.
Lithology
The sediment logged with the GHMT corresponds to lithostratigraphic unit II (242.6-553.0 mcd: meter composite depth) and the top of lithostratigraphic unit III:
Unit II: clayey foraminifer and nannofossil ooze
Unit III: nannofossil clay.
Age information
The interval logged by the GHMT corresponds to the early Pliocene and Miocene.
Shipboard biostratigraphic studies indicate that there are the following depth-age relations at Site 1146:
250 mcd: 3.5 Ma
470 mcd: 11.2 Ma
590 mcd: 16.5 Ma
There are no shipboard magnetostratigraphic results for the depth interval logged with the GHMT.
Interpretation of GHMT data
The great number of large spikes present in the data recorded in the total magnetic induction data can lead to artifacts in the computation of the correlation slope. 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. The studied interval corresponds to Early Pliocene and Miocene. These periods present many reversals of the Earth's magnetic field.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 184.
For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.