LWD Standard Data Processing
ODP logging contractor: LDEO-BRG
Location: Hydrate Ridge - Cascadia Margin (NE Pacific)
Latitude: 44° 35.1865' N
Longitude: 125° 7.1719' W
Logging date: July 16-17, 2002
Water Depth: 906 mbrf (as seen on the logs)
Total penetration: 380 mbsf
The logs were recorded using the LWD (Logging-While-Drilling) technique, which allows 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 free of washouts.
During Leg 204, the following LWD tools were employed:
RAB = Resistivity-at-the-bit (resistivity, gamma ray, borehole images)
ADN = Azimuthal Density-Neutron (density, porosity, differential caliper)
NMR = Nuclear Magnetic Resonance (free-fluid volume, bound-fluid volume, porosity).
In Hole 1244D, the penetration rate was approximately 25 m/hr. A MWD (Measurement While Drilling) "Power-pulse" unit transmitted a sub-set of the down-hole data (weight-on-bit, resistivity, etc) to the ship as a signal on a 6 Hz fluid pressure wave.
Depth shift: Original logs have been depth shifted to the sea floor (-906 m). The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface.
Neutron porosity data: The neutron porosity measurements have been corrected for bit size, 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 resistivity curves are sampled at a 0.0304 m (1.2 in) sampling rate.
Nuclear Magnetic Resonance Data: The NMR tool measures the nuclear magnetic resonance properties of hydrogen in the formation. Initially, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). The hydrogen atoms are then tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The precession of the hydrogen atoms induces a signal in the tool's antenna, and the decay of this signal is measured as the transverse relaxation time, T2. Because the formation contains hydrogen in different forms (in water in large pores, small pores, and bound in clay minerals, and in methane hydrate), there is a distribution of T2 times, here given from 3 ms to 3 s. The T2 distribution is the basic output of NMR measurement. It is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2, such as the bound and free fluid volumes.
The Anadrill NMR tool used during Leg 204 was an experimental tool; the processing was performed onshore by Anadrill.
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 204. For further questions about the logs, please contact:
E-mail: Cristina Broglia