Standard Wireline Data Processing
Operator and logging contractor: LDEO-BRG
Hole: 14A (proposed site GDGH14-A)
Expedition: NGHP-1
Location: Krishna-Godavari Basin, Eastern India (Bay of
Bengal)
Latitude: 16° 3.5577' N
Longitude: 82° 5.6218' E
Logging
date: July 12, 2006
Sea
floor depth (drillers'): 906.6 mbrf
Sea
floor depth (loggers'): 909 mbrf
Total
penetration: 1086.6 mbrf (180 mbsf)
Total core recovered: 152.77 m (84.9 % of
cored section)
Oldest
sediment cored: n/a
Lithology:
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 | Main |
0 |
179 |
66 |
Reference |
2. FMS/DSI/GPIT/SGT |
Pass 1 |
71 |
179 |
||
Pass 2 |
60 |
171 |
62 |
||
3. VSI |
91 |
177 |
6 stations of good data |
Prior
to logging the hole was conditioned with a wiper trip and a 40 bbl sepiolite
sweep, and then displaced with 60 bbl of 10.5 ppg barite mud. The
DIT/HLDS/APS/HNGS tool string initially could not exit the pipe and one joint
of pipe (9.5m) was added. All tool strings reached the bottom of the hole. Only
6 out of 17 attempted VSI stations recorded good waveforms because of
difficulty coupling in the soft formation. 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 (-909 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 909 mbrf (after depth
matching). This differs by 2.4 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).
The hole is typically around 11-12 inches in diameter, but there is a bridge in
the FMS caliper data at 150-154, and washouts occur between 129-149 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