Wireline Standard Data Processing


ODP logging contractor: LDEO-BRG

Hole: 1084A

Leg: 175

Location: Cape Basin (tropical SE Atlantic)

Latitude: 25° 30.8345' S

Longitude: 13° 1.6668' E

Logging date: September, 1997

Bottom felt: 2003.5 mbrf (used for depth shift to sea floor)

Total penetration: 605 mbsf

Total core recovered: 511.6 m (91.4 %)


Logging Runs


Logging string 1: DIT/SDT/NGT

Logging string 2: APS/HLDS/HNGS

Logging string 3: FMS/GPIT/NGT

Logging string 4: GHMT/NGT (2 passes)


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/SDT/NGT: Bottom-hole assembly at ~58 mbsf

APS/HLDS/HNGS: Bottom-hole assembly at ~58 mbsf

FMS/GPIT/NGT: Bottom-hole assembly at ~58 mbsf

GHMT/NGT: Bottom-hole assembly at ~58 mbsf.




Depth shift: Original logs have been interactively depth shifted with reference to NGT from APS/HLDS/HNGS run and to the sea floor (- 2003.5 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 (SGR or HSGR) from the NGT or HNGS tools 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 and HNGS data have been processed to correct for borehole size and type of drilling fluid. The NGT logs recorded by the DIT/SDT/NGT tool string through the bottom hole assembly are not included in the database as after processing they become largely negative (original API units were less than 10).


Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') logs. The sonic logs are of excellent quality and do not need processing.


High-resolution data: Neutron porosity data (APS) were recorded at a sampling rate of 5.08 cm (2 in) in addition to the standard sampling rate of 15.24 cm (0.5 ft).


Geological Magnetic Tool: The Geological Magnetic Tool collected data at two different sampling rates, 0.1524 and 0.0508 m. Both data sets have been depth shifted to the reference run and to the sea floor.


Quality Control


null value=-999.25. This value may replace recorded log values or results which are considered invalid.


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, such as the NGT and HNGS data above 58 mbsf, 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 string (C1 and C2).



Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 175. For further questions about the logs, please contact:


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