Standard LWD Data Processing
IODP-USIO logging contractor: LDEO-BRG
Location: Ursa Basin (Gulf of Mexico)
Latitude: 28° 4.786' N
Longitude: 89° 8.357' W
Logging date: June 17-19, 2005
Water Depth (as seen on logs): 1066.5 mbrf
Total penetration: 612 mbsf
The logs were recorded using the LWD (Logging-While-Drilling) technique, which allows the 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 largely free of wash-outs.
The following LWD tools were employed in Hole U1324A:
ADN = Azimuthal Density-Neutron (density, porosity, differential caliper)
ARC-6A = Resistivity (resistivity derived from phase shift and attenuation, gamma ray).
RAB = Resistivity-at-the-bit (resistivity, gamma ray, borehole images)
In Hole U1324A, the rate of penetration was approximately 30 m/hr, slowing to approximately 20 m/hr in the lowermost part of the hole. A subset of the data was transmitted back to the ship during drilling. Barite mud (10 ppg) was pumped below 440 mbsf in order to control flow into the hole. The barite mud causes attenuation of the gamma ray signal and increased PEF.
Depth shift: Original logs have been depth shifted to the sea floor (-1066.5m). 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): the GR from the RAB tool has been corrected for hole size (bit size), collar size, and type of drilling fluid.
Neutron porosity data processing: The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature, and weight).
Density data processing: Density data have been processed to correct for the irregular borehole using a technique called "rotational processing", which is particularly useful in deviated or enlarged borehole with irregular or elliptical shape. This statistical method measures the density variation while the tool rotates in the borehole, estimates the standoff (distance between the tool and the borehole wall), and corrects the density reading (a more detailed description of this technique is available upon request).
Resistivity data: The RAB resistivity is sampled with a 0.03048 m (1.2 in) sampling rate. The ARC resistivity is sampled with a 0.1524 m (0.5 ft) sampling rate.
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; this is particularly true for the density tool, which uses clamp-on stabilizers to eliminate mud standoff and to ensure proper contact with the borehole wall. A data quality indicator is given by the density caliper (DCAV) measurement of hole diameter. Another quality indicator is represented by the density correction (DRHO).
Additional information about the logs can be found in the 'Explanatory Notes' and Site Chapter, IODP Expedition Reports volume 308. For further questions about the logs, please contact:
E-mail: Cristina Broglia