Standard Wireline Data Processing (re-entry, Phase 1)
IODP-USIO logging contractor: LDEO-BRG
Hole: 1256D
Expedition: 309
Location: Guatemala Basin (NE equatorial Pacific)
Latitude: 6° 44.163' N
Longitude: 91° 56.061' W
Logging date: July 17-18, 2005
Sea floor depth (driller's): 3645.4 mbrf
Sea floor depth (logger's): 3643.5 mbrf
Total penetration: 752 mbsf (Leg 206)
Total core recovered: 227m (47.8%) (Leg 206)
Oldest sediment recovered: Calcareous nannofossil ooze (Middle Miocene) at Hole 1256B
Lithologies: Clay-rich sediments and calcareous nannofossil ooze (sediments); basalt (basement)
The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Casing depth (mbsf) | Notes |
| 1.DLL/HLDS/APS//HNGS/TAP |
main
|
252
|
719
|
269
|
Bit at 262
mbsf
|
|
repeat
|
627
|
722.5
|
|
|
|
| 2. DSI/FMS/GPIT/SGT |
uplog
|
264
|
713
|
269
|
|
Hole 1256D, originally drilled during Leg 206, was logged at the start of Expedition 309, with the aim of acquiring a temperature profile down the hole under equilibrated conditions. The tools reached to within 27 m of the base of the hole, and the hole was successfully logged from this point. The wireline heave compensator was used to counter minor ship heave.
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.
Depth shift: The original logs were shifted to the sea floor (-3643.5 m), and were then depth matched first to the LLS log and then to the ECGR and caliper from the Leg 206 first pass of the FMS/SGT tool string. During Leg 206, the sea floor depth was not determined directly; the sea floor depth was set so that the bottom of the casing in the logs appeared at the same depth (269 m) as given by the drillers. For Hole 1256D, it differs by 1.9 m from the "bottom felt" depth given by the drillers (see above).
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 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 SGT data were corrected for hole size during the recording. The APS and HLDS have been corrected for standoff and hole diameter respectively during the recording.
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. SGT gamma ray was recorded at 15.24 and 5.08 cm sampling rates.
Acoustic data: The dipole shear sonic imager (DSI) was run in P&S, upper dipole, lower dipole, and Stonely modes.
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 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 on the FMS tool (C1 and C2). The hole was in very good condition, generally reading between 10-12 inches in diameter. Slightly wider sections with thin washouts occur at 348-403, 418-435, 450-473, 530-605, and 678-694 mbsf. The hole had not deteriorated significantly since it was drilled 3 years previously on Leg 206.
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 Expedition Reports Volume 308. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia