ODP logging contractor: LDEO-BRG

Hole: 1258C

Leg: 207

Location: Demerara Rise (equatorial NW Atlantic)

Latitude: 9° 26.00' N

Longitude: 54° 43.966' W

Logging date: January 31 - February 1, 2003

Bottom felt: 3203 mbrf (mudline not recovered)

Total penetration: 485 mbsf

Total core recovered: 161.08 m (74.85%)

Logging Runs

Logging string 1: DIT/HLDS/APS/HNGS/TAP/MGT (two passes without MGT, followed by 2 passes with MGT only)

Logging string 2: FMS/LSS/SGT (two passes)

All of the passes reached close to the hole total depth without problems from bridges. The wireline heave compensator was used throughout.

Bottom Hole Assembly

The following bottom hole assembly (BHA) 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. Typical reasons for depth discrepancies are ship heave, use of wireline heave compensator, and drill string or wireline stretch.

DIT/HLDS/APS/HNGS/TAP/MGT: bottom hole assembly at ~86 mbsf (both passes)

FMS/LSS/SGT: bottom hole assembly at ~86 mbsf (both passes)

Processing

Depth shift: The original logs were depth matched to the HSGR log from the first pass of the DIT/HLDS/APS/HNGS/TAP/MGT tool string, and were then shifted to the sea floor (-3195 m). The DIT/HLDS/APS/HNGS/TAP/MGT pass 1 was chosen as the reference run because it was the only run to cross the seafloor. The DIT/HLDS/APS/HNGS/TAP/MGT pass 2 and FMS/LSS/SGT pass 2 were matched to the reference run by total gamma ray, with cross checking using the caliper logs. The FMS/LSS/SGT pass 1 was matched to the shifted FMS/LSS/SGT pass 2 by the caliper logs.

The sea floor depth was determined by a combination of the step in gamma ray values and the HLDS logs to be at 3195 mbrf. For Hole 1258C, this differs by 8 m from the sea-floor depth given by the drillers (see above). The sea-floor value of 3195 mbrf gives a good depth correlation between logs and cores. However, it also leads to an offset of about 3 m between the logger's and driller's total depths (488 vs. 485 mbsf). The absence of a good mudline/sea-floor value makes resolution of this problem difficult.

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 four transit times from the Long Spacing Sonic tool were processed using an in-house program that compares the slownesses derived from the 8 different transmitter-receiver pairs at each depth, and discards those times that are significantly different from the majority as bad data. This leads to improved compressional wave velocity logs that are free of the artifacts present in the velocities derived directly from the recorded delay times (DTLN and DTLF).

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, 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 on the FMS tool (C1 and C2). The hole was in excellent condition, with the caliper generally reading about 10 inches in diameter down to 455 mbsf. Below this depth, the hole is locally wider than 18 inches.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 207. 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