Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 903A

Leg: 150

Location: New Jersey Offshore (NW Atlantic)

Latitude: 38° 56.298' N

Longitude: 72° 49.032' W

Logging date: June, 1993

Bottom felt: 455.5 mbrf

Total penetration: 702.8 mbsf

Total core recovered: 637.77m (90.6%)

Logging Runs

Logging string 1: DIT/SDT/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 ~77 mbsf

HLDT/CNTG/NGT: Bottom-hole assembly at ~72.5 mbsf

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from spliced DIT/SDT/NGT runs and to the sea floor (- 457 m); this amount corresponds to the sea floor depth observed on the logs and differs 1.5 m from the "bottom felt" depth given by the drillers. 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: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') logs. The sonic logs have been processed to eliminate some of the noise and 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.

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. Invalid gamma ray readings from 61 to 71 mbsf (DIT/SDT/NGT run) and from 64 to 68 mbsf (HLDT/CNTG/NGT run).

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI).

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