Standard Wireline Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 19B (proposed site MNGH Gap)
Expedition: NGHP-1
Location: Manahadi Basin, Eastern India (Bay of Bengal)
Latitude: 18°
58.6532' N
Longitude: 85° 39.5160' E
Logging date: August 8, 2006
Sea floor depth (drillers'): 1433 mbrf
Sea floor depth (loggers'): 1436 mbrf
Total penetration: 1713 mbrf (280 mbsf )
Total core recovered: 271.28 m (90.4 % of cored section, from Hole NGHP-19A)
Oldest sediment cored: n/a
Lithologies: Nanofossil-rich clay (from Hole NGHP-19A)
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 |
Main |
0 |
280 |
50.5 |
|
| 2. VSI |
6 good stations | ||||
| 3. FMS/DSI/GPIT/SGT |
Pass 1 |
69 |
263.4 |
||
| Pass 2 |
46 |
265.4 |
Hole 19B was a logging-dedicated
hole. Prior to logging the hole was conditioned with a 40 bbl sepiolite sweep
and then displaced with 100 bbl of 10.5 ppg barite. The DIT/HLDS/APS/HNGS run
went smoothly and reached the bottom of the hole. Due to daylight requirements,
the next run was the VSI; after some initial difficulties in reaching the
open-hole interval, the tool was finally deployed to about 17 m above TD. Of
the 10 stations attempted, six delivered reasonable results. The
FMS/DSI/GPIT/SGT string was run last. Again, after some difficulty in passing
an obstruction, the string reached about 13 m above TD on both passes.
The LDEO Wireline Heave
Compensator compensated ship heave throughout the logging operations.
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 pass was
used as the depth reference, and the other passes were matched to it using
resistivity and acoustic logs. All passes were then shifted to the sea floor
(-1436 m), based on the step in gamma radiation at the sea floor in the
DIT/HLDS/APS/HNGS pass. The DIT/HLDS/APS/HNGS pass was chosen as the reference
run because it was the only one to reach the sea floor and to cover most of the
open hole interval.
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
pass at 1436 mbrf (after depth matching). This differs by 3 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: Medium
frequency monopole, and cross 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).
The hole is rough and irregular,
with most readings above 12 inches. Despite the poor hole conditions, most of
the log data of Hole 19B is of acceptable quality; the gamma ray logs from the
three passes, however, correlate poorly, and the SFLU reads about 0.2 ohmm
lower than the deep and medium resistivity (with the exception of the uppermost
20 m). The cause of this discrepancy could not be determined.
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: Cristina Broglia