Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1253A

Leg: 205

Location: Costa Rica Convergent Margin (equatorial NE Pacific)

Latitude: 9° 38.8583' N

Longitude: 86° 11.4337' W

Logging date: October 7-8, 2002

Bottom felt: 4387 mbrf

Total penetration: 600 mbsf

Total core recovered: 148.6 m (64.6 %)

Logging Runs

Logging string 1: DIT/APS/HLDT/HNGS/TAP

Logging string 2: FMS/GPIT/DSI/SGT (2 passes)

The hole was logged after casing was set at 4800 mbrf (413 mbsf driller's depth). A previous attempt at logging the uncased hole failed because of an obstruction in the hole not far below the pipe. There was also a first attempt at logging with the DIT/APS/HLDS/HNGS/TAP tool string, but the HLDS failed and had to be replaced with the HLDT. The interval logged in open hole consists of two gabbro sills separated by 30 m of sediments.

The wireline heave compensator was used to counter ship heave.

Casing

The depth of the casing shoe (414 mbsf) is as it appears on the logs after differential depth shift (see "Depth shift" section) and depth shift to the sea floor. There is a 1 m discrepancy with the depth given by the drillers onboard. This is the result of the different bottom depth used to shift the logs to the sea floor (see below).

Processing

Depth shift: The FMS/GPIT/DSI/SGT pass 1 was depth matched to the DIT/APS/HLDT/HNGS run using the ECGR log and then it was checked with the caliper logs. The ECGR log from the F FMS/GPIT/DSI/SGT pass 2 was depth matched to the ECGR log from the FMS/GPIT/DSI/SGT pass 1. All logs were then shifted to the sea floor (-4390 m). The sea floor depth was determined by the step in gamma ray values at the sediment-water interface. For Hole 1253A, it differs by 3 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 HLDT have been corrected for standoff and hole diameter respectively during the recording.

Acoustic data processing: The dipole shear sonic imager (DSI) was operated in the following modes for pass 2: P&S monopole (standard frequency), lower dipole (low frequency), upper dipole (standard frequency), and Stoneley (standard frequency). The modes were the same for pass 1 except no upper dipole logs were taken. The compressional velocities from the monopole mode generally repeat quite well between the two passes. The lower dipole shear velocities are generally good. The upper dipole shear velocities are poor. Stoneley wave velocities are well determined in the gabbro, but not in the sediments.

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

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 cross-correlation of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (APS, HLDT) 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 HLDT tool (CALI) and on the FMS string (C1 and C2). The hole was in excellent condition in the gabbro sections (414-428 and 458-558 mbsf), mostly reading 12 inches except for isolated rougher sections where the diameter can reach 18 inches. The hole exceeds 18 inches in diameter in the entire sediment section between the gabbros (428-458 mbsf).

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 205. For further questions about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia