Standard Wireline Data Processing

 

IODP logging contractor: USIO/LDEO

Hole: U1382A

Expedition: 336

Location: Mid Atlantic Ridge (central tropical N Atlantic)

Latitude: 22° 45.3531' N
Longitude: 46° 4.8911' W

Logging date: 9 October, 2011

Sea floor depth (driller's): 4494 m DRF

Sea floor depth (logger's): 4497 m WRF

Casing depth: 4596 m DRF

Total penetration:  4704 m DRF (210 m DSF)

Total core recovered: 33.29 m (33 % of cored section)
Oldest sediment cored: Nannofossil ooze (Miocene)
Lithologies: Nannofossil ooze, mixed breccia, phyric and aphyric basalts

 

 

Data

 

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 October 2011.

 

Logging Runs

Tool string Run/Pass Top depth (m WMSF) Bottom depth (m WMSF) Pipe depth (m WMSF) Notes
DEBI-T/HRLA/HLDS/EDTC-B
 Downlog
73
203
58

HLDS not valid
DEBI-T/HRLA/HLDS/EDTC-B
 Uplog
122
205
FMS/GPIT/HNGS
 Downlog
0
205
58
FMS/GPIT/HNGS
 Pass 1
90
205
FMS/GPIT/HNGS
 Pass 2
0
205
58

Reference

 

 

The hole was circulated/displaced with seawater after five wiper trips before logging. A power arc failure occurred at 122 m WMSF while logging up with the DEBI-T tool, which prematurely ended the recording with this tool. No major incidents occurred during the FMS tool deployment except for some troubles encountered during the tool string's re-entry into the drill pipe.

 

The sea state was relatively calm with a peak-to-peak heave of ~ 0.6 m. The wireline heave compensator was used during the logging operation with the DEBI-T tool string, but was turned off during the FMS logging operation due to some erratic behavior.

 

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.

 

 

Processing

 

Depth shift to sea floor and depth match. The sea floor depth was determined by the step in gamma ray values at 4497 m WRF. This differs by 3 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 FMS/GPIT/HNGS pass 2 (reference).

 

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.

After depth shifting and depth matching the casing shoe is at 97 m WMSF.

 

Environmental corrections. The HNGS and HRLA data were corrected for hole size during the recording. The HLDS data were corrected for standoff and hole size respectively during the recording.

 

High-resolution data. Bulk density (HLDS) and EDTC-B data were recorded with 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.

 

Quality Control

 

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 density 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 (HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and also on the FMS tool (C1 and C2). The caliper logs indicate that the borehole sections at 100-108 m, 142-146 m, 156-159 m WMSF were largely washed out to the degree (>15-19") where they have adversely affected the tool response. Thus, density and probably resistivity logs in these depth intervals 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 336. 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