ODP logging contractor: LDEO-BRG
Hole: 1144A
Leg: 184
Location: Northern Margin of South
China Sea (China Sea)
Latitude: 20° 3.2' N
Longitude: 117° 25.1' E
Logging date: March, 1999
Bottom felt: 2047 mbrf
Total penetration: 452.8 mbsf
Total core recovered: 468.9 m
(103.5 %)
Two passes were recorded with the GHMT tool
string. Processing was performed on the first pass, open-hole section
(109.3-445.2 mbsf). Wireline heave compensator was used to counter ship heave.
Hole conditions
The FMS calipers vary from 9 to 15 inches.
The raw susceptibility has been corrected for hole diameter variations using
the FMS calipers (second pass).
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
(-2050 m). This value corresponds to the mudline as observed on the logs; it
differs 3 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 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 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 and for a linear drift of 45 nT/m.
The magnetic susceptibility data (MAGS) is of good quality.
Lithology
The sediment logged with the GHMT corresponds
to lithostratigraphic unit I (only unit determined at site 1144) which consists
of clay with "iron sulfide" above 283.4 mcd (meters composite depth),
with pyrite below 404.07 mcd, and with siliceous biogenic material between.
Age information
The interval logged by the GHMT corresponds
to the Pleistocene.
Shipboard biostratigraphic studies indicate that there are the following
depth-age relations at Site 1144:
100 mcd: 0.1 Ma
250 mcd: 0.25 Ma
420 mcd: 0.9 Ma
Interpretation of GHMT data
The great number of large spikes present 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 the late Pleistocene, less
than 1 Ma because of a very high sedimentation rate. It corresponds to the
Bruhnes normal magnetic polarity zone, which is in agreement with the result of
the GHMT processing (anticorrelation), even if a short zone of correlation is
present between 230 and 270 mbsf.
Additional information about the logs can be
found in the "Explanatory Notes" and Site Chapter, ODP IR volume 184.
For further questions about the logs, please contact:
Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia