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 %)

Logging Runs

Logging string 1: DIT/APS/HLDS/HNGS
Logging string 2: FMS/SDT/GPIT/NGT (2 passes)
Logging string 3: GHMT/GPIT/NGT (3 passes)
The wireline heave compensator was used to counter ship heave.

Bottom-hole Assembly/Pipe

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 ~111 mbsf
DIT/APS/HLDS/HNGS: Drill pipe at ~40 mbsf
FMS/SDT/GPIT/NGT: Bottom-hole assembly at ~199 mbsf (pass 1)
FMS/SDT/GPIT/NGT: Bottom-hole assembly at ~199 mbsf (pass 2)
GHMT/NGT: Recorded open-hole (pass 1)
GHMT/NGT: Recorded open-hole (pass 2)
GHMT/NGT: Bottom-hole assembly at ~199 mbsf (pass 3)

Processing

Depth shift: Original 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.

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, apart from an anomalous spike at ~395 mbsf, which was removed. DTLN adn DTLF have been converted into velocity.

High-resolution data: Neutron porosity data were recorded at a 5.08 cm sampling rate.

Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, the standard 0.1524 m rate and 0.0508 m. All three data sets have been depth shifted to the reference run and to the sea floor. Erroneous spikes are present in the MABG (magnetic field) logs; the second and third passes are less affected than the first.

Quality Control

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 HNGS data above 111 mbsf, and the NGT data above 199 mbsf, should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the calipers on the HLDS tool (LCAL) and FMS string (C1 and C2). Between the base of the BHA and 181 mbsf, the hole alternates between very wide (>18") and (<9"); between 181 and 400 mbsf, the hole is rugose, but generally less than 17" wide, with a small bridge from 289-291 mbsf; the remainder of the hole, below 400 mbsf, was in gauge and of very good quality. After the first toolstring run, the BHA was lowered to 199 mbsf. Shallow penetration logs above 181 mbsf are not reliable.

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