ODP logging contractor: LDEO-BRG
Well name: 754B
Location: Broken Ridge (tropical SE Indian Ocean)
Latitude: 30° 56.439' S
Longitude: 93° 33.954' E
Logging date: May, 1988
Bottom felt: 1076.4 mbrf (used for depth shift to sea floor)
Total penetration: 354.7 mbsf
Total core recovered: 91.9 m (39 %)
Logging string 1: ACT/GST/GPIT/AMS/NGT
Logging string 2: DIT/SDT/CNTG/AMS/NGT
No information available from shipboard logging report about wireline heave compensator to counter ship heave; according to the operations report sea conditions were rough.
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.
ACT/GST/GPIT/AMS/NGT: Bottom-hole assembly at ~163 mbsf
DIT/SDT/CNTG/AMS/NGT: Bottom-hole assembly at ~163 mbsf.
Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/CNTG/AMS/NGT and to the sea floor (- 1076.4 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) 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. Even after processing, the data in the 187-305 mbsf interval is questionable, due to the poor quality of the original transit times.
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.
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) and a good contact with the borehole wall. Hole conditions at this hole were far from ideal; pebble/gravel layers between 120 and 153 mbsf posed the risk of hole collapse and therefore logging plans were modified to provide maximum coverage at minimum risk to the tools. The geochemical tool string was run first, as it allows logging through bottom-hole assembly; the radioactive sources on this string, however, irradiated the formation, thus limiting the quantitative value of the logs recorded during the DIT/SDT/CNTG/AMS/NGT run.
The hole size was measured by the 3-arm mechanical caliper device (MCD.
The few 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:
E-mail: Cristina Broglia
E-mail: Trevor Williams