CORE ORIENTATION

Paleomagnetism and many studies would be greatly enhanced if the azimuthal orientation of the core was known. While the principle of placing the core into proper geographic coordinates is simple, in practice such orientation is difficult and, when accomplished, is fairly crude. Currently, the only ODP drilling tool that can provide core orientation is the Tensor tool (Fig. F24; note that "Tensor" is a brand name of Tensor Inc., Austin, TX), which can only be used for APC coring. The Tensor tool has replaced the Eastman-Whipstock multishot tool (frequently referred to simply as the "multishot" tool). Scribing core catches have been attempted with rotary core barrel (RCB) coring as discussed in Technical Note 10 (Fisher and Becker, 1993), but these systems were never developed fully. Currently, ODP does not maintain any form of orientation tool for extended core barrel (XCB) or RCB cores. In addition to coring tools, other indirect methods can be used to estimate orientation on APC, XCB, and RCB cores as discussed below.

Equipment and Operation

The Tensor tool (see Storms et al., 2006) consists of a three-component fluxgate magnetometer and a three-component accelerometer rigidly attached to the core barrel to record the azimuth and dip (drift) of the hole and magnetic tool-face (MTF) angle, which is the angle between magnetic north and the double-line orientation mark on the core liner (Fig. F25). The instrument is divided into four modules: (1) the magnetometer triad including accelerometers, (2) the data acquisition module, (3) the control-power regulator-memory module, and (4) the battery pack. The battery pack is detachable for easy replacement.

The ODP downhole tools Marine Specialist and the paleomagnetism technician are responsible for all phases of operating the Tensor tool, including tool preparation, data download, and data interpretation. This tool requires a special nonmagnetic drill collar (the Monel collar) as part of the bottom-hole assembly, as well as batteries and additional setup time (10–15 min per core) on the part of the Marine Specialists. The decision to orient cores at a given site should therefore be made well in advance of the cruise. It is also important to provide the technical/logistics staff with an estimate of the number of cores to be oriented.

Before deployment with the APC core barrel, the probe is programmed via computer interface to determine the holdoff time and the interval between "shots" (sets of sensor readings). The instrument is designed to store a maximum of 1023 sensor readings. The tool stores data, which are uploaded to a computer and analyzed once the tool is back on deck. Orientation is not usually attempted for the top two or three cores (~20–30 mbsf) until the BHA is sufficiently stabilized in the sediment.

The output from the Tensor tool, which contains a variety of angles, including the inclination angle of the hole and the MTF angle, is archived in the Janus database (Figs. F7, F8). The dip or inclination of the hole is a measure of the deviation of the hole from vertical, which is typically <5° for ODP holes. The MTF angle is the angle between magnetic north and the double-line orientation mark on the core liner. The core liner is always cut so that the double lines are at the bottom of the working half. Using the ODP coordinate system for the archive and working halves or for samples taken from them (Fig. F2), the measured remanent declination can then be corrected to magnetic north by adding the MTF angle, and it can be further corrected to true north by adding the deviation of magnetic north from true north, the latter of which can be estimated from the International Geomagnetic Reference Field (IGRF) coefficients. The equation is

D_True = D_Observed + MTF + MIGRF,

where

D_True = the Tensor tool corrected or true declination,

D_Observed = the observed declination output from the cryogenic magnetometer,

MTF = the MTF angle, and

MIGRF = the deviation of magnetic north from true north.

Standard Queries

Standard queries through the Web (Fig. F8; www-odp.tamu.edu/database) allow downloads of core orientation data that can be used to reorient APC cores into the geographical reference frame.

Currently, there is no hard rock or XCB and RCB orientation system in place. However, there are three methods that have been successfully applied to orient the azimuths of APC, XCB, and RCB cores:

1. Use the declination of intervals with known polarity (e.g., McLeod et al., 1996). This technique works well for relatively young sediments or rocks (<5 Ma) that have not been affected by tectonic rotations. The time-averaged direction for normal or reversed polarity intervals in such a case would be 0° or 180°, respectively. The method could also be used for older rocks, but the effect of past plate motion would need to be removed.
2. Compare borehole images obtained during the logging program with the FormationMicroScanner (FMS), which have known azimuthal orientation, with the core or images of the exterior of the core (e.g., Haggas et al., 2001).
3. Use the orientation of the recent (Brunhes) geomagnetic field, which may partially overprint the ChRM magnetization of the core (e.g., Shibuya et al. 1991). This technique is typically not applicable because present-day overprints often do not exist or are too small to be accurately estimated. Moreover, Brunhes overprints typically would be acquired by magnetic minerals with the low coercivity (short relaxation times), which are likely to be masked by drilling overprints.