Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1467C
Expedition: 359
Location: Maldives (Central Equatorial N Indian Ocean)
Latitude: 4° 51.0362' N
Longitude: 73°17.0198' E
Logging date: November 3, 2015
Sea floor depth (driller's): 498.2 m DRF
Sea floor depth (logger's):498.2 m WRF (downlog)
Total penetration: 1212.2 m DRF (714 m DSF)
Total core recovered: 266.15 m (80.1 % of cored section)
Oldest sediment recovered:
Lithology: Unlithified/partially lithified and lithified planktonic foraminfera-rich wackestone to packstone
The logging data
was recorded by Schlumberger in DLIS format. Data were processed at the
Borehole Research Group of the Lamont-Doherty Earth Observatory in November 2015.
| Tool string | Run
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. MSS/HRLA/APS/HLDS/EDTC-B/HNGS
|
Downlog
|
101 |
Closed caliper. Invalid HLDS and APS data. |
||
Repeat
|
open hole |
APS nuclear source off. Invalid APS data. |
|||
Main
|
101 |
Hole conditions were reported to be good, therefore the first tool string, consisting of the MSS/HRLA/APS/HLDS/EDTC-B/HNGS tools, was rigged up with nuclear sources. The hole remained filled with sea water, as there had been no sticking during drilling. The downlog proceeded without any incident to a depth of 1211 m WRF.
A repeat pass was carried out from total depth to 1085 m WRF, with an open caliper on the HLDS tool but with the APS nuclear cource shut off, to avoid activating the formation before the main pass. During the repeat pass it was noticed that the HLDS caliper was indicating a very large hole. Initially a problem with the caliper was suspected, however there was no indication of any failure or damage. Furthermore, a lack of significant variation in the tension valeus seemed to indicate that the tool was not making contact with the borehole wall. A main pass was conducted from total depth to 485 m WRF. As in the case of the repeat pass, the caliper remained fully open for most of the open hole interval, reading lesser values down to 14" only over a small portion of the hole. Since the caliper tool did not seem to be malfunctioning and indicated an enlarged hole it was decided to cancel any further logging operations and instead use the time to drill a dedicated loggiong hole. Upon retrieval of the tool string it was found that the caliper was indeed intact and functional.
The sea state was calm, therefore no Wireline Heave Compensator (WHC) was used.
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 (-
498.2 m). The sea floor depth
was determined by the step in gamma ray values at 498.2 m WRF on the MSS/HRLA/APS/HLDS/EDTC-B/HNGS downlog. This corresponds to the sea floor depth given by the drillers (see
above). The depth-shifted logs have then been depth-matched to the gamma ray log from the downlog of theMSS/HRLA/APS/HLDS/EDTC-B/HNGS tool string.
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.
Environmental corrections. The HNGS and HRLA data were corrected for hole size during the recording. The APS and HLDS data were corrected for standoff and hole size respectively during the recording.
High-resolution
data. Bulk density (HLDS) and neutron porosity (APS) data were recorded sampling rates of 2.54 and 5.08 cm, respectively, 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 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. Gamma Ray data from the EDTC-B tool were recorded at sampling rates of 5.08 and 15.24 cm.
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). In Hole U1467C repeatability among the tree passes was excellent.
The 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.
Hole diameter was recorded by the caliper on the HLDS
tool (LCAL). As described in detail in the Logging Runs section above, the caliper provided very high readings in most of the hole, with the exception of a short interval from
370 to 450 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 359. 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:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu