LWD Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: N Barbados Ridge (tropical NW Atlantic)
Latitude: 15° 32.0284' N
Longitude: 58° 40.580' W
Logging date: January, 1997
Bottom felt: 5064 mbrf (used for depth shift to sea floor)
Total penetration: 337 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 data open-hole 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 (- 5064 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); during leg 171A only the former has been corrected for hole size (bit size), collar size, and type of drilling fluid. Due to a bug in the acquisition software, the NGT total and computed gamma ray (SGR and CGR) could not be environmentally corrected and converted into API units. For this reason, they are not included in the database.
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 processing: A deconvolution technique called "qualitative resistivity overlay" aimed at providing enhanced vertical resolutions is used for both shallow and deep resistivity measurements to compute output with 1-2-3-4-5 ft vertical resolution (documentation on this technique is also available upon requests). The ouputs are sampled at a 0.0762 m (3 in) sampling rate and are included in the database along with the standard 0.1524 m (0.5 ft) channels.
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 171A. For further questions about the logs, please contact:
E-mail: Cristina Broglia