Logging-While-Drilling Data Processing
ODP logging contractor: LDEO-BRG
Well name: 1275C
Leg: 209
Location: Mid-Atlantic Ridge (tropical NW Atlantic)
Latitude: 15° 44.4408' N
Longitude: 46° 54.2234' W
Logging date: June 25, 2003
Water Depth (ass seen on logs): 1565 mbrf
Total penetration: 20 mbsf
The logs were recorded using the LWD (Logging-While-Drilling) technique, which allows acquisition of open-hole logs using instruments that are part of the drill string itself. The advantages of this technique include being able to log in formations that would not provide a stable hole for wireline logging (e.g. the upper section of sedimentary formations), and logging a hole immediately after it is drilled, so that it is in good condition and free of wash-outs.
The RAB tool used on Leg 209 was the RAB-8 (RAB-while-coring) system, used with a standard 9.875 in. ODP RCB bit.
In Hole 1275C the penetration rate generally ranged between 1 and 3 m/hr.
Processing
Depth shift: Original logs have been depth shifted to the sea floor (-1565 m). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface.
Gamma Ray data processing: Processing of the data is performed in real-time onboard by Schlumberger personnel. Gamma Ray data is measured as Natural Gamma Ray (GR): it has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Resistivity data: The resistivity curves are sampled at a 0.0304 m (1.2 in) sampling rate.
Quality Control
During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. The best data are acquired in a circular borehole.
Additional information about the logs can be found in the Explanatory Notes and Site Chapter, ODP IR volume 209.
For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.