Standard Wireline Data Processing
IODP logging contractor: USIO/LDEO
Expedition: 341
Hole: U1421A
Location: Gulf of Alaska
Latitude: 59° 30.4399' N
Longitude: 143°2.7395' W
Logging date:
Sea floor depth (driller's): 729.7 m DRF
Sea floor depth (logger's): 729 m WRF (DIT/DSI/HLDS/EDTC-B main run)
Total penetration: 702.7 m DSF (1432.4 m DRF)
Core recovery: 20% of cored interval
Oldest sediment recovered: < 0.5 Ma (biostratigraphy)
Lithologies: mud with and without lonestones, diamict, drilled rock, diatom ooze
The logging data
were recorded by Schlumberger in DLIS format. Data were processed at the
Borehole Research Group of the Lamont-Doherty Earth Observatory in July 2013.
| Tool string | Run
|
Top depth (m WMSF) |
Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
| 1. DIT/DSI/HLDS/EDTC-B | Downlog
|
92 |
|||
Main
|
92 |
Invalid HLDS |
|||
Repeat |
318 |
695 |
Open hole |
Invalid HLDS | |
2. VSI/EDTC-B |
6 good shots |
Hole U1421A was first flushed with 75 bbl of high viscosity mud and then displaced with very heavy mud (11.8 ppg) for logging. Due to the unstable hole conditions, a modified tool string consisting of the DIT/DSI/HLDS/EDTC-B tools was deployed to acquire high priority measurements with one logging deployment (downlog, main and re[pet runs). To minimize risk, the HLDS was run without radioactive source, thus acquiring only caliper measurements. The VSI tool string was then run; it reached total depth (1427 m WRF, 693 m WSF) and acquired six good shots.
The depths in the table are for the processed logs (after depth shift to the sea floor and depth matching between passes). 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.
The sea state was
Depth shift to sea floor and depth match. The original logs were first shifted to the sea floor (-729 m). The sea floor depth was determined by the step in gamma ray values at 729 m WRF. This differs by 0.7 m from the sea floor depth given by the drillers (see above). The depth-shifted logs were then depth-matched to the gamma ray log from the main pass of the tool string.
Depth matching
is typically done in the following way. One log is chosen as reference or 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 are 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.
High-resolution
data. Because the radioactive source was not used during the logging operation, no density logs were obtained from HLDS at this site. Gamma ray data from the EDTC-B tool were recorded at sampling rates of 5.08 and 15.24
cm.
Acoustic data. The dipole shear sonic imager (DSI) was operated in the following modes: low frequency for lower dipole, standard frequency for both upper dipole and P&S monopole. The compressional velocity data from the monopole were excellent, but the shear velocity data from the dipoles were not as good. Waveform processing is recommended to improve the quality of shear velocity data.
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.
Resistivity and sonic compressional data match well with the gamma ray curves, indicating good quality of the log data.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS). A measurement of the hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). The caliper data indicate a small, in-gauge hole (~12 inches) in the lower section (540 to 660 m WMSF) and a slightly large hole (12-17 inches) in the upper section (540 to 100 m WMSF). Accordingly, log data from the enlarged portions of the upper hole section should be used with caution.
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 and Downhole Measurements sections of the expedition reports,
Proceedings of the Integrated Drilling Program, Expedition 341.
For further questions about the logs, please contact:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia