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Data Processing
IODP-MSP
drilling and logging contractor: ESO
Hole: M0004B
Expedition: 302
Location:
Lomonosov Ridge (
Latitude: 87° 52.018' N
Longitude:
136° 10.475' E
Logging
date: September 4,
2004
Sea floor
depth (driller's):
1289.70 mbrf
Sea
floor depth (logger's): 1291.00 mbrf
Total
penetration: 218.00
mbsf
Total
core recovered:
7.31 m (66.50 % of cored section)
Oldest
sediment recovered:
Middle Eocene
Lithologies: Clays to silty muds
The logging data was recorded by Schlumberger in DLIS
format. Data were processed by the European Petrophysics
Consortium.
Tool string |
Pass |
Top depth (mbsf) |
Bottom depth (mbsf) |
Pipe depth (mbsf)
|
Notes |
|
1 |
45.2
|
217 |
66.5
|
|
FMS/BHC/GPIT/NGT/SGT |
|
0
|
220
|
66.5
|
|
A complete list of tool and log acronyms is available
at
http://brg.ldeo.columbia.edu/data/iodp-eso/exp302/exp_documents/iodp-eso-302-acronyms.html.
After completion of the coring at Hole M0004B, the pipe was pulled to ~66 mbsf and the rig floor prepared for 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 a
'bottom felt' depth in soft sediment.
Depth match and
depth shift to sea floor: The logs were first bulk shifted to sea floor (-1291 m). The sea floor depth was determined
by identifying the step in gamma ray values at the sediment-water interface
from Pass 2. The logging sea-floor depth is within 1.3 m of the 'bottom felt'
depth given by the drillers.
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, but in this case teh caliper log), 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 pass.
The sea floor depth was determined by the step in
gamma ray values at 1291 mbrf. This differs by 1.3 m
from the sea floor depth given by the drillers (see above).
Environmental
corrections: The NGT and SGT data were corrected for hole size during the recording.
High-resolution
data: Gamma Ray data from the SGT tool were recorded at
sampling rates of 5.08 and 15.24 cm.
Acoustic data: The borehole compensated sonic tool (BHC) was run in both pass 1 and pass 2 and records compressional wave velocities only. The BHC applies the 'depth-derived' borehole compensation principle using two transmitter (1x) receiver (2x) groups, one group being inverted. Hole size compensation is achieved by averaging the two compressional wave delay time (ΔT) readings measured across the same interval. The velocity logs are generally of good quality and show no indication for cycle skipping so no corrections were applied.
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 sonic
velocity log).
Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only
qualitatively, because of the attenuation of the incoming signal. The
thick-walled BHA attenuates the signal more than the thinner-walled drill pipe.
A wide (>12") and/or irregular
borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall
(APS, HLDS). Hole diameter was recorded by the FMS
tool (C1 and C2). The
hole was under guage (< 9.5 in) for the most part
and narrows significantly between 75 and 90 mbsf, at
155 mbsf and again between 180-184 mbsf. The caliper logs indicate that nowhere was the
borehole washed out to the degree where it would adversely affect the tool
response. This is supported by a favourable
comparison of parameter magnitudes between passes as well as a good depth match
over much of the logged interval (< +- 1 m). There is an apparant bridge in the borehole at around 150-160 m, clearly observed in the caliper logs. This feature has affected the movement of the tools and the logs in this interval cannot thus be depth matched precisely. Thus, the depths of logs in this around 10 m interval should be treated with caution, with a discrepancy of 3-4 m between pass 1 and pass 2. Above and below this interval the depth matches are good.
A null value of -999.25 may replace invalid log
values.
Additional
information about the drilling and logging operations can be found in the
Operations section of the Site Chapter in IODP Proceedings of Expedition 302.
For further questions about the data, please contact:
Jennifer Inwood
Phone:
011-44-116-252-3327
Fax:
011-44--116-252-3918
E-mail: iodp@le.ac.uk
For any web
site-related problem please contact:
E-mail: logdb@ldeo.columbia.edu