Standard LWD Data Processing
IODP-USIO logging contractor: LDEO-BRG
Location: Cascadia Margin (NE Pacific)
Latitude: 48° 37.6350' N
Longitude: 127° 3.0289' W
Logging date: September 22-23, 2005
Water Depth (as seen on logs): 1838 mbrf
Total penetration: 300 mbsf
The logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-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/MWD services were employed in Hole U1326A:
GeoVision (RAB resistivity and gamma ray)
EcoScope (ARC resistivity, density, porosity, geochemistry, gamma ray, temperature, and pressure)
TeleScope (real time transmittal of data to the ship (MWD))
ADN Vision (ADN density, porosity, caliper)
ProVision (NMR magnetic resonance, porosity, bound fluid volume)
Unfortunately, postcruise processing indicatd that a tool malfunction had prevented the recording of any reliable data from the ProVision tool during the expedition.
In Hole U1326A, the rate of penetration was approximately 30 m/hr in the top 120 mbsf, and 50 m/hr thereafter. The drilling fluid was sea water.
Depth shift: The original logs have been depth shifted to the sea floor (-1838 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 of the data is performed in real-time onboard by Schlumberger personnel. Gamma Ray data is measured as Natural Gamma Ray (GR): the GR is normally corrected for hole size (bit size), collar size, and type of drilling fluid. Comparison between gamma ray data collected in LWD and wireline holes during Expedition 311 shows that the former have a much higher value range than the latter. Because the GR value range from the wireline holes appears to be the one expected for these lithologies and is in agreement with the data acquired in ODP Leg 146, the LWD GR is currently under investigation, to assess the cause of such discrepancy. Caution is therefore suggested in interpreting the LWD GR data.
Neutron porosity data: The neutron porosity measurements have been corrected for standoff, temperature, mud salinity, and mud hydrogen index (mud pressure, temperature, and weight).
Density data: 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.
Acoustic data: During LWD operations, the SonicVISION processing parameters were configured to isolate the fluid wave in order to monitor the possible occurrence of free gas in the borehole fluid. The sonic waveforms were later reprocessed by Schlumberger to extract the compressional velocity. Because of the slow formation and of the dispersive energy of the leaky P-mode, compressional velocity values above 70 mbsf are not usable.
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 Proceedings volume 311. For further questions about the logs, please contact:
E-mail: Cristina Broglia
E-mail: Gilles Guerin