Standard Wireline Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 15A (proposed site GDGH11)
Expedition: NGHP-1
Location: Krishna-Godavari Basin, Eastern India (Bay of
Bengal)
Latitude: 16°
5.6983' N
Longitude: 82° 9.7467' E
Logging date: July 14, 2006
Sea floor depth (drillers'): 937.9 mbrf
Sea floor depth (loggers'): 939 mbrf
Total penetration: 1137.9 mbrf (200 mbsf)
Total
core recovered: 180/06 m (90 %
of cored section)
Oldest sediment cored: n/a
Lithologiy: Nannofossil and foraminifer-bearing clay
The logging data was recorded by
Schlumberger in DLIS format. Data were processed at the Borehole Research Group
of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. DIT/HLDS/APS/HNGS |
Downlog |
27 |
187 |
63 |
|
| Main |
0 |
199 |
54 |
Reference |
|
| 2. FMS/DSI/GPIT/SGT |
Pass 1 |
84 |
200 |
||
| Pass 2 |
61 |
200 |
Prior to logging the
hole was conditioned with a wiper trip and sepiolite mud sweeps, and then
displaced with 60 bbl of 10.5 ppg barite mud. Both tool strings reached the
bottom of the hole. The LDEO Wireline Heave Compensator compensated ship heave
during logging.
The depths in
the table are for the processed logs (after depth matching between passes and
depth shift to the sea floor). Generally, discrepancies may exist between the
sea floor depths determined from the downhole logs and those determined by the
drillers from the pipe length. Typical reasons for depth discrepancies are ship
heave, wireline and pipe stretch, tides, and the difficulty of getting an
accurate sea floor from the 'bottom felt' depth in soft sediment.
Depth match and depth shift to
sea floor: The DIT/HLDS/APS/HNGS Main pass
was used as the depth reference, and the other passes were matched to it using
the gamma ray and caliper logs. All passes were then shifted to the sea floor
(-939 m), based on the step in gamma radiation at the sea floor in the
DIT/HLDS/APS/HNGS Main pass.
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 no sudden
changes in cable speed), and then the features in the equivalent logs from the
other runs are matched to it in turn. This matching is performed manually. 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 sea floor
depth was determined by the step in gamma ray values in the DIT/HLDS/APS/HNGS
Main pass at 939 mbrf. This differs by 1.1 m from the sea floor depth given by
the drillers (see above).
Environmental
corrections: The HNGS and SGT data were
corrected for hole size during recording. The APS and HLDS have been corrected
for standoff and hole diameter respectively during the recording.
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. SGT gamma
ray was recorded at 15.24 and 5.08 cm sampling rates.
Acoustic data:
The dipole shear sonic imager (DSI) was
run in the following modes:
Pass 1: Low
frequency monopole, low frequency upper dipole, standard (high) frequency lower
dipole, and Stoneley modes.
Pass 2: Low
frequency monopole, and low frequency crossed dipole modes.
Because
of the slow formation, the automatic picking of wave arrivals in the sonic
waveforms did not provide reliable results. Reprocessing of the original
waveforms was required to extract meaningful compressional and shear
velocities. The most reliable shear velocity value is the one derived from the
upper dipole (VS2) during the first pass, where the lower source frequency used generated more coherent waveforms.
The quality of the data is
assessed by checking against reasonable values for the logged lithologies, by
repeatability between different passes of the same tool, and by correspondence
between logs affected by the same formation property (e.g. the resistivity log
should show similar features to the acoustic log). There is a bridge between
154 and 156 mbsf, and washouts occur between 89-132 mbsf.
Gamma ray logs recorded through
bottom hole assembly (BHA) and drill pipe should be used only qualitatively,
because of the attenuation on the incoming signal. The thick-walled BHA
attenuates the signal more than the thinner-walled drill pipe.
A null value of -999.25 may
replace invalid log values.
For further questions about the processing,
please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Crisitna Broglia