Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Well name: 792E
Location: Izu Bonin Forearc (NW Pacific)
Latitude: 32° 23.96' N
Longitude: 140° 22.79' E
Logging date: May, 1989
Bottom felt: 1798.2 mbrf (used for depth shift to sea floor)
Total penetration: 885.9 mbsf
Total core recovered: 361.9 m (48.2 %)
Logging String 1: DIT/SDT/NGT
Logging String 2: FMS/GPIT/NGT (upper and lower runs)
Logging String 3: ACT/GST/NGT
Logging String 4: HLDT/SDT/NGT
Logging String 5: WST
The wireline heave compensator was used to counter ship heave during the first two runs but failed at the beginning of the third and was discontinued.
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 ~290 mbsf
FMS/GPIT/NGT: recorded open-hole
ACT/GST/NGT: bottom hole assembly at ~290 mbsf
HLDT/SDT/NGT: bottom hole assembly at ~290 mbsf.
Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT run and to the sea floor (- 1798.2 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.
NOTE: the data was depth shifted using as reference the DIT/SDT/NGT run indicated by the logging scientists of Leg 126. In the original geochemical data processing whose results were published in the ODP SR volume 126 the FMS/GPIT/NGT runs were used as reference. In this processing, the FMS/GPIT/NGT has thus been depth shifted as well.
Gamma ray processing: NGT data from the ACT/GST/NGT run have been processed to correct for borehole size, type of drilling fluid and bottom-hole assembly.
Acoustic data processing: The SDT sonic logs have been processed to eliminate some of the noise and cycle skipping experienced during the recording. The data from the HLDT/SDT/NGT run rather than those from the DIT/SDT/NGT run have been used in the processing, as they were less affected by noise and cycle skipping. 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.
null value=-999.25. This value may replace invalid recorded values or results.
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 (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.
The acoustic data are of overall good quality after the processing; the data is suspicious in the lower 100 m, as shown by the lack of correlation with other logging data.
The short-spacing detector of the HLDT tool failed while going into the hole. Therefore only the long-spacing detector data are valid. The data are of good quality in the 410-561 mbsf interval.
Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI) 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 126. For further questions about the logs, please contact:
E-mail: Cristina Broglia