Standard Wireline Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 5E (proposed site KGGH02-A)
Expedition: NGHP-1
Location:
Krishna-Godavari Basin, Eastern India (Bay of Bengal)
Latitude: 16¡ 1.72' N
Longitude: 81¡ 2.67' E
Logging date: June 14-15, 2006
Sea floor depth (drillers'): 955 mbrf
Sea floor depth (loggers'): 956.5 mbrf
Total penetration: 200 mbsf
Total core recovered: none
Oldest sediment cored: n/a
Lithology: Nannofossil-rich clay with occasional thin silt beds
and abundant carbonatic nodules (from Hole 5C)
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. DSI/SGT |
Pass 1 |
46 |
193 |
55.5 |
|
Pass 2 |
46 |
193 |
55.5 |
||
Pass 3 |
106 |
120 |
55.5 |
||
2. DIT/APS/HLDS/HNGS |
Downlog |
0 |
168 |
65 |
|
Main |
0 |
197 |
56 |
Reference |
|
3. FMS/GPIT/HNGS |
Pass 1 | 75 |
197 |
open hole |
|
Pass 2 |
37 |
186 |
54 |
||
4. VSI |
20 stations of good data |
Prior to logging the hole was
displaced with 10.5 ppg (barite) mud. The DIT/HLDS/APS/HNGS and FMS/ DSI/ GPIT/SGT
runs went smoothly, and reached the bottom of the hole. On the next run, there
were initial software problems communicating with the VSI tool and after the
tool was pumped out of the pipe its progress downhole was blocked by a bridge.
At that time it was dark, and it was decided to terminate logging Hole 1A. The LDEO Wireline Heave Compensator
compensated ship heave throughout the operation.
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 FMS/DSI/GPIT/SGT 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
(-2667 m), based on the step in gamma radiation at the sea floor in the
DIT/HLDS/APS/HNGS Main Pass. The FMS/DSI/GPIT/SGT Pass 2 was chosen as the
reference run because it covered the whole 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
Main pass at 2667 mbrf (after depth matching). This differs by 7.2 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
P&S monopole, upper and lower dipole, and Stoneley modes on both main and
repeat pass. 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 log data of Hole 1A is
of good quality. The hole is typically around 12 inches in diameter.
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