Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1241B

Leg: 202

Location: Cocos Rise (equatorial NE Pacific)

Latitude: 05°50.5699'N

Longitude: 86°26.810'W

Logging date: May 24, 2002

Bottom felt: 2037.1 mbrf (used for depth shift to sea floor)

Total penetration: 395 mbsf

Total core recovered: 302.23m (99.55%)

Logging Runs

Logging string 1: DIT/HLDT/APS/HNGS/TAP/MGT. This tool string was run twice; on the first pass the MGT tool was switched off. On the second pass the Schlumberger and TAP tools were switched off and a pass with the MGT tool was successfully completed.

Logging string 2: FMS/GPIT/DSI/SGT (1 pass)

No problems were encountered during the recording. All passes reached the base of the hole. 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/TAP/MGT: Bottom hole assembly at ~81.5 mbsf

FMS/GPIT/DSI/SGT: Bottom hole assembly at ~81.5 mbsf

Processing

Depth shift: The original logs were depth matched to the HSGR log from the DIT/HLDT/APS/HNGS pass and were then shifted to the sea floor (-2037.1 m). The sea floor depth is determined by the step in gamma ray values at the sediment-water interface. For Hole 1241B, it is the same as 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.

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: P&S monopole (mid-frequency), lower dipole (low frequency), and FMD (first motion detection) modes. Two compressional wave velocity logs are available, from the P&S mode and from the FMD. Both have problems: they show only modest similarity in curve shape to each other and to the resistivity logs. The FMD has generally higher velocities than the P&S monopole mode velocities, and has improbably high values between 251-281 mbsf. The P&S monopole mode velocities have stepped decreases to low values from the pipe to 256 mbsf, and from 278-300 mbsf. The shear wave velocity logs are good, apart from some occasional small spikes to higher values.

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.

Quality Control

Null value=-999.25. This value may replace invalid 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) and by the caliper on the FMS tool string (C1 and C2). The borehole had only small variations in diameter, ranging from 11.5" to 14.5" 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