Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1218A

Leg: 199

Location: Paleogene Equatorial Transect (Equatorial E Pacific Ocean)

Latitude: 8° 53.3667' N

Longitude: 135° 22.0002' W

Logging date: 16-17 November, 2001

Bottom felt: 4837 mbrf (used to shift to sea floor)

Total penetration: 276.8 mbsf

Total core recovered: 276.8 m (100 %)

Logging Runs

Logging string 1: DIT/HLDT/APS/HNGS/MGT (3 passes, MGT only on Pass 3)

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

No major problems were encountered while logging Hole 1218A. The logs recorded were of good quality. The new logging cable was used for the first time in this hole; hence descent speed through the pipe was limited to 5000 ft/hr.

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/DSI/GPIT/NGT: Bottom hole assembly at ~56 mbsf.

Processing

Depth shift: The original logs were depth matched to the HSGR from DIT/HLDT/APS/HNGS pass 1 and then shifted to the sea floor (-4837m). The sea floor depth was determined by identifying the step in gamma ray values at the sediment-water interface. The logging sea-floor depth 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 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. In Hole 1218A, the gamma radiation values are quite low (<30 API units), and common features are not always easy to identify in all passes. For the two FMS/DSI/GPIT/NGT passes, the caliper logs were used for depth matching, because they gave clearer results than the gamma logs and are more likely to lead to well-matched FMS images.

Gamma-ray processing: NGT data have been processed to correct for borehole size and type of drilling fluid. The HNGS data were corrected for hole size during the recording.

Acoustic data: The DSI tool was operated in two modes for both passes: P&S and upper dipole mode. The results are generally bad, due to the slow formation velocities. Both the original travel time data and the edited velocity data (derived from the travel time data) are included in the data files. Compressional wave data from pass 1 between 153 and 212 mbsf are generally reliable.

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 HLDT pad and the borehole wall (low-density correction) the results are improved, because the short spacing has better vertical resolution.

Quality Control

Null value=-999.25. This value generally replaces invalid log values or results (ex. processed sonic data).

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, 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 13-14 inches below 187 mbsf, and wider than the maximum extent (18 inches) for about half of the interval between 187 mbsf and the pipe.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP Leg 199 IR volume. For further questions about the logs, please contact:

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