ODP logging contractor: LDEO-BRG
Hole: 1148A
Leg: 184
Location: Northern Margin of South
China Sea (China Sea)
Latitude: 18° 50.1672' N
Longitude: 116° 33.9324' E
Logging date: April, 1999
Bottom felt: 3308.7 mbrf
Total penetration: 704 mbsf
Total core recovered: 632.07 m
(89.78 %)
Three passes were recorded with the GHMT tool
string. Processing was performed on the second pass, open-hole section (400-710
mbsf). Wireline heave compensator was used to counter ship heave.
Hole conditions
The FMS or HLDS calipers vary mainly between
10 and 11 inches. The raw susceptibility has been corrected for hole diameter
variations using the FMS caliper (first 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
(- 3305.5 m). This value corresponds to the mudline as observed on the logs; it
differs 3.2 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. The signal is so spiky above 400 mbsf
that no processing is possible. So the processing was done between 400 and 710
mbsf. All the spikes have been manually removed, then the signal was re-sampled
at 0,1524 m. The MAGB signal has not been corrected for the presence of drill
pipe which does not affect the signal at this depth. The magnetic
susceptibility data (MAGS) is of good quality.
Lithology
The sediment analyzed with the GHMT
corresponds to the bottom of lithostratigraphic unit IV, to units V, VI and to
the upper part of unit VII.
- Unit IV (down to 412.22 mcd): brown nannofossil clay
- Unit V (412.22-457.22 mcd): greenish gray nannofossil clay mixed sediment and
nannofossil clay
- Units VI and VII (457.22 mcd to the bottom of the hole): nannofossil clay and
nannofossil clay mixed sediment with slumped and faulted intervals. Hiatus of
2.5 Ma at the base of unit VI.
Age information
The interval logged by the GHMT corresponds
to the early Miocene and Oligocene.
Shipboard biostratigraphic studies indicate that there are the following
depth-age relations at Site 1148:
410.54 mcd -> 21.5 Ma
663.32 mcd -> 30.6 Ma
There is a large hiatus of 2.5 Ma at the base of lithostratigraphic unit VI.
There is 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
Miocene and Oligocene with a hiatus of 2.5 Ma between these periods. 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 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