ODP logging contractor: LDEO-BRG
Well name: 1166A
Leg: 188
Location: Prydz Bay (Antarctic Ocean)
Latitude: 67° 41.8' S
Longitude: 74° 47.2' E
Logging date: February 20-21, 2000
Bottom felt: 480 mbrf
Total penetration: 381.3 mbsf
Total core recovered: 18.6%
Logging
Runs
Logging string 1: DIT/APS/HLDS/HNGS + TAP
tool
Logging string 2: GHMT/DSI/NGT (2 passes)
Logging string 3: FMS/GPIT/NGT (2 passes)
Hole 1166A was a very good hole for logging.
All logs reached total depth and were of very good quality. The hard diamict
formation that made up the top 120 m of the cored interval enabled the BHA to
be set at 30 mbsf. With the exception of some washed out and irregular
intervals, the hole was quite circular and in gauge. The DSI (Dipole Sonic
Imager) was combined with the GHMT for the first time in ODP.
The Wireline Heave Compensator was used to
counter ship heave.
Bottom-hole
Assembly
The following bottom-hole assembly 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, drill string and/or wireline
stretch, and tides.
DIT/APS/HLDS/HNGS: Bottom-hole assembly at 30
mbsf
GHMT/DSI/NGT: Bottom-hole assembly at 30 mbsf
FMS/GPIT/NGT: Bottom-hole assembly at 30 mbsf.
Processing
Depth shift: The original logs were depth matched to the HNGS from
the DIT/APS/HLDS/HNGS run and then shifted to the sea floor (-480 m). The sea
floor depth is determined by the step in gamma ray values at the sediment-water
interface. This depth is the same as the driller's sea-floor depth, which was
determined by TV camera pictures while re-entering the hole after a storm. The
initial driller's mudline estimate of the sea-floor depth had been 486.7 mbrf;
mudlines tend to be unreliable when recovered by RCB coring.
Depth matching is typically done in the
following way. One log is chosen as reference (base) log (usually the total
gamma ray log from the run with the greatest vertical extent), and then the
features in the equivalent logs from the other runs are matched to it in turn.
This matching is performed automatically, and the result checked and adjusted
as necessary. The depth adjustments that were required to bring the match log
in line with the base log are then applied to all the other logs from the same
tool string.
The depth match between the different passes
of the tool strings at 1166A is generally robust.
Gamma-ray processing: NGT data from GHMT/DSI/NGT and FMS/GPIT/NGT runs have
been processed to correct for borehole size. The HNGS data from
DIT/HLDS/APS/HNGS was corrected for hole size during the recording. The HNGS
data are statistically better than the NGT data. While total gamma ray counts
(HSGR and SGR) are reproducible and show very similar features, the potassium,
thorium, and uranium logs are less robust, due to the statistical nature of the
gamma ray measurement.
Acoustic data processing: The DSI tool was operated in P&S and lower dipole
modes for pass 1 and just in the P&S mode for pass 2. The waveform data
were processed onboard ship to give DTCO (compressional wave slowness) for both
pass 1 and pass 2, and DTSM (shear wave slowness) for pass 1 only. Some minor
editing of anomalous spikes was performed for the DTCO, based on whether peaks
were seen in the resistivity log at the same depth or not, and a lack of
correspondence between pass 1 and pass 2 in the same intervals. The DTSM log
was left unedited, despite probable bad values between 160-191 mbsf. The
compressional and shear slownesses were then converted to velocities. The
unedited DTCO logs are included in the data files.
High-resolution data: Bulk density and neutron porosity data were recorded
at a sampling rate of 2.54 and 5.08 cm, respectively. The enhanced bulk density
curve is the result of Schlumberger enhanced processing technique performed on
the MAXIS system onboard. While in normal processing short-spacing data is
smoothed to match the long-spacing one, in enhanced processing this is
reversed. In a situation where there is good contact between the HLDS pad and
the borehole wall (low density correction) the results are improved, because
the short-spacing has better vertical resolution.
Geological Magnetic Tool: The Geological Magnetic Tool collected data at a
sample interval of 5.08 cm. There are a handful of isolated spikes in the MAGB
logs that do not repeat between the two passes, and are therefore artifacts.
See the associated document for details of the processing of the GHMT data.
Quality
Control
Null value=-999.25. This value may replace
invalid log values or results.
Large (>12") and/or irregular
borehole affects most recordings, particularly those that require
eccentralization (APS, HLDS) and a good contact with the borehole wall. In
general. Hole 1166A was smooth, varying between 13 and 16 inches in diameter,
with more variable intervals between 72-143 and 162-198 mbsf, where the caliper
saturates at a couple of levels. APS and HLDS data quality is reduced in the
rough and/or wide intervals. Hole diameter was recorded by the hydraulic
caliper on the HLDS tool (LCAL) and on the FMS string (C1 and C2).
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia