LWD Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: North Barbados Ridge (tropical NW Atlantic)
Latitude: 15° 31.477' N
Longitude: 58° 44.875' W
Logging date: May, 1994
Bottom felt: 4891 mbrf
Total penetration: 583.7 mbsf
The logs were recorded using the LWD (Logging-While-Drilling) technique, which allows to obtain open-hole logs during drilling operations. The advantages of this technique are many: real-time analysis can accelerate drilling speed, avoid stuck pipe, and reduce borehole problems. LWD can also collect open-hole data in the uppermost part of the hole; this cannot be accomplished with wireline tools as the drill string is usually kept in the upper part of the borehole where hole conditions are generally bad.
The LWD employs the following tool combinations:
CDR = Compensated Dual Resistivity (resistivity-gamma ray)
CDN = Compensated Density-Neutron (density-porosity-caliper)
Depth shift: Original logs have been depth shifted to the sea floor (- 4897.5 m).
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) and Spectral Gamma Ray (NGT). Penetration rates during Leg 156 of greater than 45/m hr degraded the resolution of the spectral data components, while the total gamma ray (SGR) values are generally comparable with the natural gamma ray curve (GR).
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). Intermittent problems with one of the detectors were experienced in the upper 80 m of the hole; the neutron porosity data are usually higher than the density-derived porosity.
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).
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 differential caliper (DCAL) channel which measures the tool standoff during the recording. 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, ODP IR volume 156. For further questions about the logs, please contact:
E-mail: Cristina Broglia