Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1265A

Leg: 208

Location: Walvis Ridge (tropical SE Atlantic)

Latitude: 28° 50.101' S

Longitude: 2° 38.360' E

Logging date: April 1, 2003

Bottom felt: 3071 mbrf

Total penetration: 321 mbsf

Total core recovered: 300 m (93.5 %)

Logging Runs

Logging string 1: DIT/APS/HLDS/HNGS/TAP/MGT (main and repeat passes followed by one MGT pass)

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

Logging string 3: WST3

All passes reached the base of the hole. The APS gave bad measurements shallower than 136 mbsf in the main pass and for the whole of the repeat pass; below 136 mbsf in the main pass, the APS logs are questionable. The wireline heave compensator was used to counter ship heave.

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 and/or wireline stretch.

DIT/APS/HLDS/HNGS/TAP/MGT: bottom hole assembly at ~69 mbsf (main pass)

DIT/APS/HLDS/HNGS/TAP/MGT: recorded open hole (repeat pass)

FMS/DSI/GPIT/SGT: bottom hole assembly at ~70 mbsf (main pass)

FMS/DSI/GPIT/SGT: recorded open hole (repeat pass)

Processing

Depth shift: The original logs were depth matched to the HSGR log from the main pass of the DIT/APS/HLDS/HNGS/TAP/MGT tool string and were then shifted to the sea floor (-3075 m). The sea floor depth was not determined directly because there is no clear step in gamma radiation values; also, no mud line was recovered in this hole. The amount of depth shift to the sea floor was defined so that the base of the bottom hole assembly on the logs (69 mbsf) would match the depth given by the drillers. As a consequence, the sea-floor depth differs by 4 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 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.

Sonic data: The DSI tool was run in P&S monopole, lower dipole, and first motion detection modes. The two compressional wave velocities from the monopole and FMD modes logs logs match over some intervals, especially in the lower section. The repeat logs match the main logs quite well. Some unrealistic values are present, especially in the upper section, where the hole is wide; better velocity logs can potentially be derived from the full sonic waveform data.

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 casing 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 becomes wider from the bottom (~13 inches) to 160 mbsf (18 inches). Above this depth, the hole was wider than the maximum extent of the caliper arms (18.5 inches). Below 245 mbsf, the hole is smooth and in good condition.

Additional information about the logs can be found in the “Explanatory Notes” and Site Chapter, ODP IR volume 208. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia