ODP logging contractor: LDEO-BRG

Hole: 1252A

Leg: 204

Location: Hydrate Ridge - Cascadia Margin (NE Pacific)

Latitude: 44° 35.1671' N

Longitude: 125° 5.5691' W

Logging date: August 31, 2002

Bottom felt: 1051 mbrf

Total penetration: 259.8 mbsf

Total core recovered: 253.79 m (97.7 %)

Logging Runs

Logging string 1: DIT/APS/HLDT/HNGS/TAP (main and repeat)

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

The wireline heave compensator was used to counter ship heave.

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/APS/HLDT/HNGS: Bottom-hole assembly at ~ 74 mbsf (main and repeat pass)

FMS/DSI/GPIT/SGT: Bottom-hole assembly at ~ 74 mbsf.

Processing

Depth shift: The DIT/APS/HLDT/HNGS/TAP repeat pass was depth matched to the DIT/APS/HLDT/HNGS/TAP main run using the HSGR log. The ECGR log from the FMS/DSI/GPIT/SGT pass 1 was depth matched to the HSGR log from the DIT/APS/HLDT/HNGS/TAP main run, and cross-checked with the caliper logs. The caliper logs from FMS/DSI/GPIT/SGT pass 2 was matched to the shifted caliper logs from pass 1. All logs were then shifted to the sea floor (-1051 m). The sea floor depth was determined by the step in gamma ray values at the sediment-water interface. For Hole 1252A, it is the same as 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 P&S monopole and upper and lower dipole modes. Because of the slow formation, the automatic picking of delay times from sonic waveforms did not provide good results. Processing of the original waveforms was required to extract meaningful compressional and shear velocities. Only the velocities processed by Gilles Guerin are presented in the online database. The original waveforms are also available online.

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

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). Hole conditions were generally excellent with the hole rarely exceeding 12.5 inches, but there are a few rougher areas with washouts reaching 16 inches. Repeatability of the data was good.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 204. For further questions about the logs, please contact:

Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams

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