Standard Wireline Data Processing

IODP -USIO logging contractor: LDEO-BRG

Hole: U1301B

Expedition: 301

Location: Juan de Fuca Ridge (NE Pacific)

Latitude: 47º 45.229' N

Longitude: 127º 45.826' W

Logging date: August 2004

Sea floor depth 2667.84 mbrf

Total penetration: 582.80 mbsf

Total core recovered: 69.05 m (29.81% of cored section)

Oldest sediment cored: no core taken in sediments.

Lithologies: basalt-hyaloclastite breccia, massive and pillow basalts

 

 

Data

 

The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty  Earth Observatory in August 2004.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Casing depth (mbsf) Bit depth (mbsf) Depth shift (m) Remarks
1. QAIT/APS/HLDS/HNGS downlog
0
552
352
265
-2666
Invalid caliper and density
main
300
579
352
-2666
repeat
0
455
352
265
-2666
2. UBI/GPIT/SGT pass 1
325
425
352
-2673
Poor quality images
pass 2
335
425
352
-2673
Poor quality images
casing
0
337
352
246
-2671
Poor quality images
3. FMS/DSI/GPIT/SGT pass 1
302
423
352
-2667
pass 2
312
424
352
-2667
casing
197
342
352
236
-2667
4. WST
359
413
352
-2668

 

 

Logging at Hole U1301B was eventful, but most of the standard-resolution logs provide good data. A wiper trip was done and approximately 50 bbls of sepiolite were circulated (not displaced) prior to dropping the bit on the seafloor and beginning the logging operations. A new wireline heave compensator was used for the first time in this hole. It appears not to have been functioning correctly, because the downhole tool motions are jumpy; to some extent they affect all the logs, but particularly the high-resolution image logs of the FMS and UBI tools. The QAIT/APS/HLDS/HNGS tool string reached the bottom of the hole without problems, however subsequent tool strings could not pass below an obstruction at about 425 mbsf.

 

Both the UBI/GPIT/SGT and FMS/DSI/GPIT/SGT tool strings logged the cased part of the hole to find the origin of a tight spot observed when re-entering the hole with the drill string. There appears to be a gap in the casing from 249-259 mbsf. At some stage during pass 2 of the FMS/DSI/GPIT/SGT tool string, one of the caliper arms became bent, (probably caused by the uncompensated tool motion) and this caused difficulties on re-entering the pipe, where the arm became detached.

Spontaneous potential (SP) logs were recorded on the first three tool strings.

The final tool string was the WST, which was used after the WST3 tool failed to power up. Stations were taken at 359, 364, 382, 402, and 413 mbsf.

 

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 the 'bottom felt' depth in soft sediment.

 

Processing

 

Depth match and depth shift to sea floor: Depth matching between passes was difficult because of the low gamma radiation levels and general lack of common features in the gamma ray logs. Therefore differential depth matching (stretching and squeezing the depth scale to line-up common features) was not done and only block shifts were applied. Some common features could be identified in the caliper logs, for example the base of a zone where the hole was widened (404 mbsf) and the base of the casing (352 mbsf). Depth shifts (listed in the table above) were applied to bring these log features into line with each other, to match the step in gamma ray in the casing logs to the sediment/basement interface at 2933 mbrf (268 mbsf), and to fit the drillers' sea floor depth of 2668 mbrf. The sea floor could not be clearly determined from the logs themselves.

 

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.

 

Acoustic data: The dipole shear sonic imager (DSI) was operated in the following modes for pass 1 and pass 2: P&S monopole, cross-dipole, and Stoneley; for the casing section, it was operated in P&S monopole, upper and lower dipole, and Stoneley modes. The velocity logs are generally of low quality, with limited repeatability between passes, due to the rapid changes in borehole diameter and the tool motion.

 

High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm, respectively. 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. QAIT Resistivity data were sampled every 7.62 cm.

 

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).

 

Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only qualitatively, because of the attenuation on 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, APS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the calipers on the FMS tool (C1 and C2). Between 352 and 464 mbsf the diameter of the hole fluctuates widely, reaching >18 inches between 395-405 mbsf.

 

The sonic velocity logs contain minor intervals of bad data, but are mostly reliable.

 

The lack of heave compensation is likely to have distorted the apparent depths of features in the logs, perhaps by up to as much as +/- 2m. This is more apparent in the high resolution FMS and UBI image logs than the standard-resolution logs. The UBI tool string recovered data open hole only in the 350-428.2 mbsf interval, due to an abstruction at 428.2 mbsf. The image data, however, is of such poor quality that no processing could be performed. Only the gamma ray, spontaneous potential and some engineering channels (CS, TENS, DF) are available online.

 

A null value of -999.25 may replace invalid log values.

 

Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in IODP Initial Reports volume 301. For further questions about the logs, please contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia