ODP Technical Note 10

V. CORE ORIENTATION

B. APC Orientation

ODP operates two different magnetic multishot cameras to measure orientation of nonrotary cores taken with the APC. The analog (Eastman-Christensen) magnetic multishot provides a photographic record of a magnetic reference azimuth, the deviation of the core from vertical determined with a pendulum, and the magnetically determined azimuth of that deviation. The analog tool continuously records images of its compass/pendulum "angle unit" on 10-mm movie film at preselected intervals. A digital multishot was recently 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. Tool specifications for the digital multishot are listed in Table V-I. As with other tools used with the APC, the use of a multishot with piston cores is limited to depths at which the force required on pullout is within safe limits, typically 100-300 mbsf.

To fully utilize the magnetic orientation capabilities of either 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 downhole-tools specialist is responsible for all phases of operating the multishot tools. For the analog tool, these tasks include tool preparation for each core, removing and developing the film after the run, interpreting the film record, and recording the relative azimuth angle, deviation from vertical, and direction of deviation on special envelopes in which the developed film is stored. The azimuth orientation angle is recorded as the clockwise angle (when looking down) between magnetic north and the double lines drawn on the core liner. The physical alignment between the double line on the core liner and the reference "lubber" line that is superimposed over the compass image on the multishot pictures is accomplished using a series of mechanical devices built into the equipment. The total error in the azimuth orientation reading introduced by these mechanical devices should not exceed 3°. Running the electronic tool involves checking 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 vertical without any determination of orientation, also known as "drift" of the hole. The analog multishot angle unit determines the verticality using a pendulum suspended over a bull's-eye, while the digital tool uses two accelerometers. The amount of drift can be read from the developed film, although the orientation of deviation from vertical is meaningless when the multishot tool is used in the presence of a magnetic drill collar. The electronic multishot can also be used to determine hole drift without orientation.

To Core Orientation, Section C.
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