Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1468B
Expedition: 359
Location: Maldive Archipelago (Central Equatorial N Indian Ocean)
Latitude: 4° 55.9823' N
Longitude: 73° 4.2888' E
Logging date: November 13, 2015
Sea floor depth (driller's): 532.5 m DRF
Sea floor depth (logger's): 532.5 m WRF (MSS/HRLA/APS/HLDS/EDTC-B/HNGS downlog)
Total penetration: 1407.6 m DRF (875.1 m DSF)
Total core recovered: dedicated logging hole, no core recovered
Oldest sediment recovered:
Lithology: Nannofosil planktonic foraminifera-rich packstone and wackestone in Hole U1468A
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 November 2015.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) |
Notes |
1. MSS/HRLA/APS/HLDS/EDTC-B/HNGS
|
Downlog
|
137.5 |
Closed caliper. Invalid HLDS and APS data. Reference. |
||
Repeat
|
recorded open hole
|
APS nuclear source off. Invalid APS. |
|||
Main
|
138.5 |
Hole U1468B was a logging-dedicated hole, therefore no coring was carried out. The hole was swept with high viscosity mud in preparation for logging. Hole conditions were expected to be good, therefore three logging strings were planned: MSS/HRLA/APS/HLDS/EDTC-B/HNGS, FMS/DSI/GPIT/EDTC/HNGS and VSI. The first tool string, consisting of the MSS/HRLA/APS/HLDS/EDTC-B/HNGS tools, was rigged up with nuclear sources. The downlog proceeded without any incident to ab~ 1329 n WRF 796.5 m WSF), where the tool string encountered a blockage. Eventually, however, the tool string reached 1367 m WRF (834.5 m WSF) and a repeat was carried out from that depth to 1263 m WRF (730.5 m WSF). The main pass also started at 1367 m WRF (834.5 m WSF); at about 960 m WRF a significant pick-up in both surface and downhole tension was observed, coupled with a marked decrease in the hole diameter. Since the tool was close to becoming stuck in the hole, the caliper was closed from 960 to about 810 m WRF (427.5-277.5 m WSF). During this time, the minitron source on the porosity tool also switched off, therefore there are no porosity data between 893 and 823 m WRF (360.5-290.5 m WSF). The caliper remained 1-in. open during the rest of the run, due to sand debris trapped behind the arm but fortunately this did not cause any difficulty upon re-entering the pipe. However, because of the degraded hole conditions and the risk of losing a tool on a subsequent run, it was decided to terminate the logging operations.
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 (-
532.5 m). The sea floor depth
was determined by the step in gamma ray values at 532.5 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 thedownlog of the MSS/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 at 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 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.
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 U1468B repeatability among the three passes was good.
The gamma ray logs
recorded through bottom hole assembly (BHA) and drill pipe should be used only
qualitatively due to 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). The caliper shows an irregular hole, with enlarged sections alternated with sudden restrictions, indicating that hole conditions degraded very rapidly after drilling was completed. The caliper had to be closed in the 277-405 m WMSF interval (see details in the Logging Runs section above).
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