Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 8A
Leg: 2
Location: Hatteras-Sohm Ridge (Gulf of Mexico)
Latitude: 35° 25.00' N
Longitude: 67° 33.20' W
Logging date: October 1968
Sea floor depth: 5183.5 mbrf
Sea floor depth (from logs): logs did not reach the mudline
Total penetration: 314.2 mbsf
Total core recovered: 3.04 m (8.7 % of cored section)
Oldest sediment cored: Early Eocene
Lithologies: siliceous oozes, muds, chert.
The logging data were recorded by Schlumberger in analog format only. Data were digitized by Centerline Data and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. GNT |
Uplog
|
183.2
|
196.5
|
Logged
thru BHA
|
|
Logging was performed through BHA (bottom hole assembly) using the Gamma-Gamma Neutron Tool, which provides a measurement of the natural radioactivity of the formation as well as a qualitative response to its hydrogen content. It is inferred that the drill pipe was located above the logged interval because neither log shows the presence of drill pipe collars.
The depths in the table are for the processed logs (after depth shift to the sea floor). Generally, discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length or from echo-sounding. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor from the "bottom felt" depth in soft sediment.
Depth shift to sea floor: The gamma ray log was depth matched to the neutron log, because according to the log header it had been recorded 3.23 m too deep. There is no clear definition of the water depth (referred to the rig floor) in the DSDP Reports. Two values are given: 5169 and 5183.5 m, possibly referring to the sea level and rig floor, respectively. Because the logs did not reach the mudline and the latter value was used to shift the cores, 5183.5 m was used to depth shift the logs as well.
In Hole 8A the logs were recorded through bottom hole assembly (BHA); because of the attenuation on the incoming signal they should be used only qualitatively. The thick-walled BHA attenuates the signal more than the thinner-walled drill pipe. (The neutron log can sometimes be used qualitatively through BHA and drill pipe, but most of the other logs will not give usable data.) Though at different depths, both gamma ray and neutron logs show a step at the bottom of the hole that might correspond to the top of the drilled chert layer.
A null value of -999.25 may replace invalid log values.
Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in DSDP Initial Reports volume 2. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia