Logging-While-Drilling Data Processing


IODP logging contractor: CDEX

Hole: C0002A

Expedition: 314

Location: Nankai Trough (NW Pacific Ocean)

Latitude: 33° 18.0192'

Longitude: 136° 38.1810' E

Logging-while-drilling date: October 13, 2007

Sea floor depth (as seen on logs): 1964.5 mLRF

Total penetration:  1401.5 mLSF (3366 mLRF)



Logging Tools


The downhole logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) system, which employs 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 during drilling, so that it is in good condition and largely free of wash-outs. The data is recorded in the tool's memory and downloaded when the drill string returns to the rig floor. A small subset of the data is transmitted up to the ship during drilling.


The following LWD/MWD services were employed in Hole C0002A:


geoVISION (RAB resistivity and gamma ray)

adnVISION: (density, porosity, caliper)

sonicVISION (velocity)

MWD-APWD = Measurement-While-Drilling (azimuth, shock, fluid pressure and temperature,fluid circulating density)


In Hole C0002A, the rate of penetration is mostly in the 30-50 m/hr range. 





Depth shift: The original logs have been depth shifted to the sea floor (- 1964.5 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 onboard by Schlumberger personnel after downloading of the data from the tool. Gamma Ray data is measured as Natural Gamma Ray (GR): the GR from the geoVISIONtool 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.


Resistivity data: The geoVISION resistivity is sampled with a 0.03048 m (1.2 in) and 0.1524 m (6 in) sampling rate.


Acoustic velocity: The sonicVISION data for Hole C0002A were processed by a Schlumberger Data Consulting Services (DCS) specialist on board the Chikyu. A detailed description of the processing performed is provided in the Site Chapter of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 314. The compressional velocity has been computed from the the processed delay time (DTCO).


Quality Control


During the processing, quality control of the data is performed by inter-comparison of all logging data to ensure that reasonable values are returned for expected lithology types and features on the logs reflect true formation characteristics and are not artifacts. 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 (IDDR).


Additional information about the drilling and logging operations can be found in the Site Chapter of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 314 . For further questions about the logs, please contact:


Yoshinori Sanada
IODP Department, CDEX
Japan Agency for Marine Science Technology(JAMSTEC)
3137-25, Showama-machi, Kanazawa-ku
Yokohama, 236-0001, Japan
tel: +81-45-778-5649

fax: +81-45-778-5704
e-mail: sanada@jamstec.go.jp


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