ODP logging contractor: LDEO-BRG
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 %)
Logging string 1: DIT/APS/HLDS/HNGS
Logging string 2: FMS/SDT/GPIT/NGT (2 passes)
Logging string 3: GHMT/GPIT/NGT (2 passes)
The wireline heave compensator was used to counter ship heave.
The following bottom-hole assembly and pipe
depths are as they appear on the logs after differential depth shift (see
"Depth shift" section) and depth shift to the sea floor. As such,
there might be a discrepancy with the original depths given by the drillers
onboard. Possible reasons for depth discrepancies are ship heave, use of
wireline heave compensator, and drill string and/or wireline stretch.
DIT/APS/HLDS/HNGS: Bottom-hole assembly at ~85 mbsf
DIT/APS/HLDS/HNGS: Drill pipe at ~14 mbsf
FMS/SDT/GPIT/NGT: Recorded open-hole (pass 1)
FMS/SDT/GPIT/NGT: Bottom-hole assembly at ~86 mbsf (pass 2)
GHMT/GPIT/NGT: Recorded open-hole (pass 1)
GHMT/GPIT/NGT: Bottom-hole assembly at ~86 mbsf (pass 2)
Depth shift: Original 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.
Gamma-ray processing: NGT data from FMS/SDT/GPIT/NGT and GHMT/GPIT/NGT runs have been processed to correct for borehole size and type of drilling fluid. HNGS data have been corrected for hole size during the recording.
Acoustic data processing: The SDT data, recorded in depth-derived, borehole compensated, long-spacing (8-10-10-12 ft) sonic mode were of very good quality and did not require any additional processing. DTLN adn DTLF have been converted into velocity.
High-resolution data: Neutron porosity data were recorded at a sampling rate 5.08 cm.
Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate and 0.0508 m. Both data sets have been depth shifted to the reference run and to the sea floor.
null value=-999.25. This
value may replace invalid log values or results.
During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS, HLDS) and a good contact with the borehole wall. Hole deviation can also affect the data negatively; the FMS, for example, is not designed to be run in holes deviated more than 10 degrees, as the tool weight might cause the caliper to close.
Data recorded through bottom-hole assembly, such as the NGT data above 86 mbsf, should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the caliper on the HLDS tool (LCAL) and FMS string (C1 and C2). The caliper measurements show moderate rugosity in the upper part of the hole; this increases considerably below 255 mbsf, thus degrading the quality of the porosity and density readings.
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:
E-mail: Trevor Williams
E-mail: Cristina Broglia