Wireline Standard Data Processing

 

ODP logging contractor: LDEO-BRG

Hole: 905A

Leg: 150

Location: New Jersey Offshore (NW Atlantic)

Latitude: 38° 36.828' N

Longitude: 72° 17.024' W

Logging date: July, 1993

Bottom felt: 2709 mbrf (used for depth shift to sea floor)

Total penetration: 740.1 mbsf

Total core recovered: 834.8 m (93.7%)

 

Logging Runs

 

Logging string 1: DIT/DSI/NGT (upper and lower sections)

Logging string 2: HLDT/CNTG/NGT (upper and lower sections)

 

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 ~104 mbsf

HLDT/CNTG/NGT: Bottom-hole assembly at ~106 mbsf.

 

Processing

 

Depth shift: Original logs have been interactively depth shifted with reference to NGT from spliced HLDT/CNTG/NGT runs and to the sea floor (- 2709 m). While the upper section showed a good correlation with the reference run, however, the lower section correlated quite poorly, making depth shifting difficult. 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: The NGT data have been processed to correct for borehole size and type of drilling fluid.

 

Acoustic data processing: DSI data were reprocessed on the shipboard MAXIS system.

 

Quality Control

 

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, HLDT) and a good contact with the borehole wall.

     

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 and by the hydraulic caliper on the HLDT tool (CALI). Only the latter, however, is reliable; note that the tool saturates at 16 inches and therefore real-time correction for larger hole diameter are not performed. Also, because the caliper closed at 140 mbsf, the density readings above this depth up to the bottom of bottom hole assembly (106 mbsf) could not be properly corrected for hole size.

 

Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 150. For further information about the logs, please contact:

 

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