Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG
Well name: 418A
Leg: 102
Location: Bermuda Rise (tropical NW Atlantic)
Latitude: 25° 2.1' N
Longitude: 68° 3.44' W
Logging date: April, 1985
Bottom felt: 5511 mbrf (used for depth shift to sea floor)
Total penetration: 878 mbsf

Logging string 1: DIT/LSS/GR
Logging string 2: LDT/CNTG/NGT
Logging string 3: DLL/GR
Logging string 4: Multichannel Sonic

No wireline heave compensator was used to counter ship heave resulting from the mild sea conditions. Ship heave 0.3-0.6 meters.

The following bottom-hole assembly and casing 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, lack of wireline heave compensator, and drill string and/or wireline stretch.

DIT/LSS/GR: Bottom-hole assembly and casing at ~324 mbsf
LDT/CNTG/NGT: Bottom-hole assembly at ~488 mbsf
DLL/GR: Bottom-hole assembly at ~488 mbsf.

Depth shift: Original logs have been interactively depth shifted with reference to NGT from LDT/CNTG/NGT run and to the sea floor (- 5511 m). The program used is an interactive, graphical depth-match program which allows to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma ray curve (GR or SGR) from the NGT or GR tool run on each logging string is used to correlate the logging runs most often. In general, the reference curve is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics is obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request.

Gamma-ray processing: NGT data have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The long-spacing sonic logs have been processed to eliminate some of the noise and cycle skipping experienced during the recording. Using two sets of the four transit time measurements and proper depth justification, four independent measurements over a -2ft interval centered on the depth of interest are determined, each based on the difference between a pair of transmitters and receivers. The program discards any transit time that is negative or falls outside a range of meaningful values selected by the processor. In addition, full sonic waveform processing from the LSS and Multichannel Sonic tool has been performed. The results are not included in this database but can be obtained upon request.

null value=-999.25. This value generally appears in discrete core measurement files and also it may replace recorded log values or results which are considered invalid (ex. processed sonic data).

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 (CNTG, LDT) and a good contact with the borehole wall.

For some reason, the depth shift of ILD and SFLU introduced many spikes. The program has been run several times without any substantial change.

Due to electronics problems, no good LLD (deep laterolog) was recorded. Also, the medium resistivity curve (ILM) is not included here, because of its poor reliability.

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 3-arm mechanical caliper on the DIT/LSS/GR string.

Additional information about the logs can be found in the Explanatory Notes and Site Chapter, ODP volume 102. For further questions about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: chris@ldeo.columbia.edu