Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1473A
Expedition: 360
Location: SW Indian Ridge (SW Indian Ocean)
Latitude: 32° 42.3622' S
Longitude: 57° 16.688' E
Logging date: January 23-24, 2016
Sea floor depth (driller's): 721 m DRF
Sea floor depth (logger's): 721 m WRF (FMS/DSI/GPIT/EDTC/HNGS Pass 3)
Total penetration: 1510.2 m DRF (789.2 m DSF)
Total core recovered: 469.15 m (63.2 % of cored section)
Oldest sediment recovered:
Lithology: Olivine gabbro
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 January 2016.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. MSS/HRLA/HLDS/EDTC-B/HNGS
|
Downlog
|
~45 |
Caliper closed. Invalid HLDS. |
||
Repeat
|
Open hole |
||||
Main
|
~45 |
||||
2. FMS/DSI/GPIT/EDTC-B/HNGS
|
Downlog
|
~30 |
Caliper closed. Invalid FMS. |
||
Pass 1
|
Open hole |
||||
Pass 2
|
Open hole |
||||
Pass 3
|
~30 |
Depth reference |
|||
3. UBI/GPIT/EDTC-B/HNGS |
Downlog
|
~30 |
|||
Uplog
|
~30 |
In praparation for logging, the shipboard scientists requested to displace the hole with fresh water instead of salt water, hoping to thus reduce the resistivity contrast for the FMS images in the gabbro formation, which is characterized by resistivities well above 20,000 ohmm. The drill pipe was initially set at 766 m WRF (45 m WSF) but was pushed upward to 751 m WRF (30 m WMSF) during the FMS and UBI runs, in order to optimize the amount of data acquired in open hole.
Three logging runs were planned (see table above) and were carried out smoothly. Only the second pass of the FMS/DSI/GPIT/EDTC-B/HNGS had to be cut short, as the calipers were not fully open.
The average ship's heave was 1.5 m. with a maximum of 3 m prior to logging. The Wireline Heave compensator was used during the logging operations.
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.
.
Depth shift to sea floor and depth match.
The original logs were first shifted to the sea floor (-
721 m). Usually, the HNGS gamma ray curve shows a clear step at the sea floor. At this site, this step is not clearly visible on any of the passes, due to the very low signature of the gamma ray logs in the olivine gabbros. For the sake of consistency with the driller's water depth, a value of 721 m has been used to depth shift the data to the sea floor. Subsequently, pass 3 of the FMS/DSI/GPIT/EDTC-B/HNGS has been used as reference to depth match all the passes.
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 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.
Environmental corrections. The HLDS, HNGS, and HRLA data were corrected for hole size during the recording.
High-resolution
data. Bulk density
(HLDS) was recorded with a sampling rate of 2.54 cm, in addition to the standard sampling rate of 15.24 cm.
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 are 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.
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: P&S monopole, upper and lower dipole and
Stoneley (downlog) and P&S monopole, Stoneley and cross-dipole (passes 1, 2, 3). The velocities were computed from the DTCO (compressional), DT4S (shear) and DTST (Stoneley) delay times. They are
generally of good quality. Post-cruise processing is required to obtain accurate shear velocities from the cross dipole 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. As expected in an olivine gabbro formation, the gamma ray signature is extremely weak (< 20 API units), which makes it difficult to accurately depth match the logging passes.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the FMS tool (C1 and C2). The calipers indicate a hole of 10.5"-12.5" in diameter form total depth to about 579 m WMSF. Above this depth it ranges from 11" to 17" up to the drill pipe at ~ 45 m WMSF.
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 report, Proceedings of the International Ocean Discovery Program, Expedition 360. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition.
After the moratorium period you may direct your questions to:
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