LWD Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 1249A

Leg: 204

Location: Hydrate Ridge - Cascadia Margin (NE Pacific)

Latitude: 44° 34.2246' N

Longitude: 125° 8.8423' W

Logging date: July 21, 2002

Water Depth: 787 mbrf (as seen on the logs)

Total penetration: 91.5 mbsf

Logging Tools

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 1249A, the penetration rate was approximately 25 m/hr, except for the top 30m, where a lower rate (15 m/hr) and fluid pressure were used to moderate formation washout at shallow depths. Below 30 mbsf, 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.

The first use in ODP of "Logging While Coring" technology took place in Hole 1249B. This consisted of a modified 8-inch RAB tool, which permits cores to be taken through it. The test was successful: RAB images were acquired and 8 cores were recovered (32.9% recovery).

Processing

Depth shift: Original logs have been depth shifted to the sea floor (-787m). 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: 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; processing was performed onshore by Anadrill.

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 204. For further questions about the logs, please contact:

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