LWD Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 808I

Leg: 196

Location: Nankai Trough (NW Pacifc Ocean)

Latitude: 32° 21.2145' N

Longitude: 134° 56.7003' E

Logging date: May 22-27, 2001

Bottom felt: 4686 mbrf (used for depth shift to sea floor)

Total penetration: 1058 mbsf

Logging Tools

The logs were recorded using the LWD (Logging-While-Drilling) technique, which allows obtaining 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.

During leg 196 the LWD employed the following tool combinations:

RAB=Resistivity-at-the-bit (resistivity-gamma ray-borehole images)

ADN=Azimuthal Density-Neutron (density-porosity-differential caliper)

ISONIC=Ideal Sonic-while-drilling (sonic velocity)

Casing

The casing shoe was set at 156 mbsf.

Processing

Depth shift: Original logs have been depth shifted to the sea floor (- 4801.9 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.

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 resistivity curves are sampled at a 0.0304 m (1.2 in) sampling rate.

Isonic data: The following is an excerpt from: D. Goldberg, et al., 2005 "Velocity analysis of LWD sonic data in turbidites and hemipelagic sediments offshore Japan, ODP Sites 1173 and 808I", in Mikada, H., et al., Proceedings of the Ocean Drilling Program, Scientific Results Volume 190-196. The acoustic data were analyzed to extract compressional velocity using high-resolution dispersion analysis and appropriate tool and borehole modeling. The sonic waveform data show an overall increase in velocity with decreasing frequency. Numerical modeling indicates that velocity computations should be made in the frequency band of highest waveform energy. Estimates of velocity using this approach compare well with core data and wireline sonic logs at the reference site (Site 1173), whereas conventional semblance analysis and dispersion corrections using a look-up table tend to underestimate the velocity by as much as 10%. Synthetic seismograms computed from the LWD sonic data using this approach correlate well to seismic reflection profiles at Site 1173. At Site 808, correlation of synthetic seismograms from LWD data analysis suggests that transverse velocity anisotropy is present in the prism sediments. High-resolution dispersion analysis is recommended for velocity analysis from LWD sonic data in similar low-velocity marine sediments.

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; 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 196. For further questions about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia