Wireline Standard Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 1238A
Leg: 202
Location: Carnegie Ridge (equatorial SE Pacific)
Latitude: 1°52.31' S
Longitude: 82°46.94' W
Logging date: May 8, 2002
Bottom felt: 2213 mbrf (logger's sea floor depth = 2213.5 mbrf)
Total penetration: 430.6 mbsf
Total core recovered: 409.77 m (95.2%)
Logging
Runs
Logging string 1: DIT/HLDT/APS/HNGS/TAP/MGT. This tool string was run three times; on the first pass the MGT tool was switched off and the TAP tool failed to work. On the second and third pass the Schlumberger and TAP tools were switched off and main and repeat passes were made with the MGT tool.
Logging string 2: FMS/GPIT/DSI/SGT (2 passes)
No problems were encountered while logging Hole 1238A; all passes reached the base of the hole and good logs were obtained. The wireline heave compensator was used to counter ship heave resulting from the calm sea conditions.
Bottom-hole
Assembly
The following bottom-hole assembly depths are as they appear on the logs after differential depth shift (see "Depth shift" section) and depth shift to the sea floor. As such, there might be a discrepancy with the original depths given by the drillers onboard. Possible reasons for depth discrepancies are ship heave, use of wireline heave compensator, and drill string and/or wireline stretch.
DIT/HLDT/APS/HNGS/MGT/TAP: Bottom hole assembly at ~ 84 mbsf
FMS/GPIT/DSI/SGT : Bottom hole assembly at ~ 84 mbsf (main pass)
FMS/GPIT/DSI/SGT: recorded open hole (repeat pass)
Processing
Depth shift: The original logs were depth matched to the ECGR (environmentally-corrected gamma ray, from the SGT tool) from the main FMS/GPIT/DSI/SGT pass and were then shifted to the sea floor (-2213.5 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. For Hole 1238A, it differs by 0.5 m from the "bottom felt" depth given by the drillers (see above).
Depth matching is 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 automatically, and the result checked and adjusted as necessary. 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.
For Hole 1238A, the ECGR from the main FMS/GPIT/DSI/SGT pass was preferred to the HSGR from the DIT/HLDT/APS/HNGS/MGT/TAP pass because the HSGR had to be stretched by about 3 m over a 10 m interval centered on 180 mbsf in order to match the ECGR logs from the two FMS/GPIT/DSI/SGT passes. The origin of this depth discrepancy is unclear; the depths of the DIT/HLDT/APS/HNGS logs should be regarded with caution in the interval 175-185 mbsf.
Gamma-ray processing: The HNGS and SGT data were corrected for hole size during data recording.
Acoustic data: The DSI tool was operated in the following modes for both passes: P&S monopole (mid-frequency), lower dipole (low frequency), and FMD (first motion detection) modes. The compressional wave velocity data from the P&S monopole mode are generally good and repeat between the main and repeat passes, apart from the interval 387-410 mbsf. The FMD compressional wave velocities generally match, apart from spikes and elevated values between 300-385 mbsf in the repeat pass; the main pass should be used. In the 180-342 mbsf interval the FMD compressional wave velocities are generally about 0.1 km/sec higher than the P&S monopole mode velocities. Shear wave velocity data match very well between the main and repeat passes; the data is bad only in the upper section, above 168 mbsf in the main pass, and 148 mbsf in the repeat pass.
High-resolution data: High-resolution bulk density and neutron porosity data were not recorded at this site, due to an oversight when trouble-shooting some telemetry problems before logging. SGT gamma ray was recorded at 15.24 and 5.08 cm sampling rates.
Quality
Control
Null value=-999.25. This value may replace invalid recorded log values or results.
During the processing, quality control of the data is mainly performed by inter-comparison of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS, HLDS) and a good contact with the borehole wall. Hole deviation can also affect the data negatively; the FMS, for example, is not designed to be run in holes deviated more than 10 degrees, as the tool weight might cause the caliper to close.
Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal.
Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). The borehole had only small variations in diameter, ranging from 11.5 to 15 in.
Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP Leg 202 IR volume. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia