Standard Wireline Data Processing

 

 

Science operator: Texas A&M University

Hole: U1520C

Expedition: 375

Location: Hikurangi Subduction Margin (SW Pacific Ocean)

Latitude: 38° 58.1532' S

Longitude: 179° 7.9112' E

Logging date: April 9, 2018

Sea floor depth (driller's): 3533 m DRF

Sea floor depth (logger's): 3531 m WRF

Total penetration:  4587.1 m DRF (1054.1 m DSF)

Total core recovered: 235.4 m (57.7 % of cored section)

Oldest sediment recovered: Probably Cretaceous

Lithology:  Cemented sandstone and conglomerate with grains of volcaniclastics and altered basalts

 

 

Data

 

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 April 2018.

 

Logging Runs

Tool string
Pass

Top depth (m WMSF
Bottom depth (m WMSF)

Pipe/Casing depth (m WMSF)

 Remarks
1. HRLA/DSI/HLDS/EDTC-B
Down
0
945
596/642
Reference. No HLDS source. Caliper closed.
Repeat
852
945
Recorded open hole
Main
0
945
596/642

 

 

Processing

 

A modified tool string was employed at hole U1520C, which included the sonic (DSI), resistivity (HRLA), HLDS caliper, and gamma ray (EDTC-B). No HNGS was used and no nuclear source was mounted on the HLDS tool, therefore only caliper data were acquired.

 

The tool string failed to reach the total depth of 4587.1 m DRF after several attempts were carried out to pass a bridge at 4475 m WRF. This depth corresponded to a change in lithology that led to believe that the hole might be collapsed from that depth downward. Three passes were conducted in this order: a downlog, a short repeat and a main pass all the way to the sea floor. The casing shoe was located at 4173 m DRF (642 m DSF) and the drill pipe at about 4127 m DRF (596 m DSF).

 

The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). 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 match and depth shift to sea floor. The original logs were first depth-matched to the gamma ray log from the downlog of the HRLA/DSI/HLDS/EDTC-B 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.

 

The depth-matched logs were then shifted to the sea floor. The sea floor depth was determined by the step in gamma ray values at 3531 m WRF. This differs by 2  m from the sea floor depth given by the drillers (see above).

 

Environmental corrections. The HRLA data were corrected for hole size during the recording.

 

High-resolution data. The Gamma Ray data from the EDTC-B tool were recorded at sampling rates of 5.08 and 15.24 cm.  The HRLA is also acquired every 5.08 cm; in the database it is resampled at 15.24 cm, for ease of comparison with the other logs.

 

Acoustic data. The dipole shear sonic imager (DSI) was operated in the following modes on all passes: P&S monopole, upper dipole, Stoneley (standard frequency), and lower dipole (low frequency). The velocities were computed from the delay times. They are generally of good quality. Post-cruise processing of the sonic waveforms is recommended to get better results, particylarly for the shear velocities.

 

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 sonic velocity log). Correlation between the passes is not always easy due to the locally poor repeatibility. Only the main features can be clearly correlated, while the rest of the logging data are somewhat affected by the poor hole conditions.

 

Gamma ray logs recorded through bottom hole assembly (BHA), drill pipe, and casing interval should be used only qualitatively, because of the attenuation of the incoming signal by the casing and the drill pipe.

 

A wide (>12 inches) and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall. The hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). The hole, ranging in size from around 10 to 17 inches (full extension of the caliper), is very irregular with many washouts.

 

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 375. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of Expedition 375.


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