Downhole Measurements

Drill String Tools

Core Orientation

Core orientation involves two primary components, rotation about an axis parallel to the core barrel, and translation along this axis. Tools are routinely available aboard the JOIDES Resolution to assist with orientation of APC cores for paleomagnetic studies of sediments.

Orientation of APC cores is facilitated by several factors. First, APC cores are collected rapidly in soft sediments. Recovery is often close to 100%, and it is likely that the APC core barrel maintains it's rotational orientation during the few seconds that a core is collected. In addition, each APC core retains it's orientation relative to the core barrel after the core is collected. Thus only one downhole tool is needed for APC orientation: a "multishot" compass which records the rotational orientation of the core barrel before, during, and after the barrel is fired into the sediments.

Orientation of RCB cores requires that several additional data types be collected and combined, and that individual pieces of core be oriented separately. Because of this orientation of RCB cores has met with difficulty and is not routinely available.

A digital multishot was developed for ODP use by Tensor, Inc. The Tensor tool uses two accelerometers and three orthogonal magnetometers to provide core-orientation information. The tool records hole inclination (drift), azimuth, and magnetic tool face (core alignment) direction, either in single-shot or continuous modes. As with other tools used with the APC, the use of a multishot with piston cores is limited to depths at which the force on pullout is within safe limits, typically 100-300 mbsf.

To fully utilize the magnetic orientation capabilities of the multishot tool, a special, 30-ft, nonmagnetic drill collar must be included in the BHA just above the lowermost drill collar (also known as the outer core barrel). Therefore the decision to obtain oriented cores at a given site must be made before the BHA is made up and the pipe is run into the hole. The multishot is housed in a nonmagnetic pressure case that is installed between two nonmagnetic sinker bars in the assembly used to connect the coring line to the APC core barrel. When coring commences, handling the multishot will add 5-10 minutes per core to normal coring times.

The ODP Paleomagnetic Specialist is responsible for all phases of operating the multishot tool: checking the battery power, arming the tool, handing it over to the core techs for deployment, retrieving the data after the station and archiving raw data in the shipboard database.

Even if the nonmagnetic drill collar is not installed in the BHA, the multishot can be used to measure the deviation from verticle without any determination of orientation, also known as "drift" of the hole.