Wireline Standard Data Processing

 

ODP logging contractor: LDEO-BRG

Well name: 752B

Leg: 121

Location: Broken Ridge (tropical SE Indian Ocean)

Latitude: 30° 53.483' S

Longitude: 93° 34.652' E

Logging date: May, 1988

Bottom felt: 1097.2 mbrf (used for depth shift to sea floor)

Total penetration: 435.6 mbsf

Total core recovered: 112.34 m (71 %)

 

Logging Runs

 

Logging string 1: DIT/SDT/AMS/NGT (down and uplog)

Logging string 2: LDT/CNTG/GPIT/NGT (down and uplog)

Logging string 3: ACT/GST/NGT (lower and upper sections)

No information available from shipboard logging report about wireline heave compensator 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 and drill string and/or wireline stretch.

DIT/SDT/AMS/NGT: Bottom-hole assembly at ~ 63 mbsf (downlog)

DIT/SDT/AMS/NGT: Recorded open-hole (uplog)

LDT/CNTG/GPIT/NGT: Bottom-hole assembly at ~131 mbsf

ACT/GST/AMS/NGT: Recorded open-hole (upper and lower sections).

 

Processing

 

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/AMS/NGT downlog and to the sea floor (- 1097.2 m). Correlation of the density data to the rest of the dataset revealed that they were offset with respect to the other data and needed additional shifts ((0.6-1.2 m downward). 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) from the NGT 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 (8-10-10-12') sonic logs have been processed to eliminate some minor 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.

 

Geochemical data: Due to technical problems, no geochemical data from this hole could be migrated into the online database. The results of the processing, however, can be viewed in the paper by Pratson, E. L. et al. (1991), Data report: Geochemical results from wireline logs at Sites 752, 754, and 758. In Weissel, J., Peirce, J., Taylor, E. et al., Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Program), 895-912.

 

Quality Control

 

null value=-999.25. This may replace invalid recorded log values or results (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. The quality of the data collected at this hole is generally very good.

Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal.

The 3-arm mechanical caliper device (MCD) recorded a constant 10" value; which is not to be considered a correct measurement of the hole diameter; in fact, the problems experienced during the recording (the tool string got stuck a few times where the hole tended to collapse) indicate that the hole was far from being in gauge.

All the spurious spikes recorded on the spherically focused resistivity have been edited.

 

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 121. For further questions about the logs, please contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia