Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 634A

Leg: 101

Location: Bahamas (tropical NW Atlantic)

Latitude: 25° 23.02' N

Longitude: 77° 18.88' W

Logging date: March, 1985

Bottom felt: 2867 mbrf (used for depth shift to sea floor)

Total penetration: 479.4 mbsf

Total core recovered: 27.3 m (8.5 %)

Logging Runs

Logging string 1: CNL/GST/GR (5 passes)

No wireline heave compensator was used to counter ship heave. The data was acquired through bottom hole assembly up to 314 mbsf and then through 5" drill pipe. Drill pipe collars are spaced 10-m apart and are clearly visible on the logs.

Processing

Depth shift: Original logs from passes 2 and 5 have been interactively depth shifted with reference to GR from passes 3 and 4 and to the sea floor (- 2867 m). Even though drill pipe collars are present at 10-m spacing, the general GR trend allows for a decent correlation among 4 of the five passes. No good correlation could be performed using pass 1. 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.

Geochemical data: The elemental yields recorded by the GST tool represent the relative contribution of only some of the rock-forming elements (iron, calcium, chlorine, silicon, sulfur, hydrogen, gadolinium, and titanium - the last two ones usually computed during geochemical processing) to the total spectrum. Because other rock-forming elements are present in the formation (such as aluminum, potassium, etc.), caution is recommended in using the yields to infer lithologic changes. Instead, ratios (see acronyms.doc) are more appropriate to determine changes in the macroscopic properties of the formation.

No geochemical processing is usually performed when the data is acquired through drill pipe or bottom-hole assembly, as the results are usually very poor. The data should therefore be used qualitatively only. Hole 634A was the first ODP hole in which the GST tool was recorded.

Quality Control

Because the data was recorded through bottom-hole assembly and drill pipe, it should be used qualitatively only because of the attenuation on the incoming signal. Moreover, after the first pass the formation remained radioactively activated, thus resulting in increasingly higher readings on each subsequent pass. This is particularly evident on the GR data, which range from 0-15 API units in the first pass to 40-130 API units in the last pass.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 101. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia