Paleomagnetic core orientation requires that a number of 1-in.-diameter plug samples be collected from the core at the surface ( Figure 3). In specially equipped laboratories, the signals in rock samples can be separated, interpreted, and used to orient core to present-day geographic north. A primary signal is imprinted near the time of deposition over time, one or more secondary signals may be overlain on the primary signal. These minerals act as miniature compasses and lock-in the earth's ambient magnetic field. The paleomagnetic technique is based on the fact that nearly all rocks, including quartzite, chert, and chalk, contain at least trace amounts of magnetic minerals such as magnetite and hematite. Another approach to core-based orientation involves analysis of naturally existing paleomagnetic signals in rock to determine their relationship to present-day geographic north. If bedding planes and fractures can be discerned in the core, it may be possible to use dipmeter logs or data from borehole televiewers to determine the orientation of the core. This system has the advantage that it is not necessary to shut down pumps and rotation to record data.Ĭore-based techniques rely at least in part on data derived from the core for orientation. The recorded data are retrieved from the unit when the barrel is tripped out. The unit is powered by a battery pack and records azimuth data continuously. A recent refinement of this system uses a modified measurement while drilling (MWD) unit to replace the camera and timer within the nonmagnetic collar. Results can be available at the wellsite within hours of core retrieval. The azimuth of the compass lug, and hence of the reference groove, is then determined for the depths at which vibration-free shots were taken. At the surface, the film is retrieved and developed.
The process is repeated at intervals until the core barrel is tripped out. Such an arrangement allows up-down directions on the core to be distinguished easily.Īt agreed upon depth intervals, pumps and rotation are shut down for a period of several minutes to allow for a vibration-free film frame to be snapped. A preferred arrangement is an asymmetric scribe shoe in which the angles from the reference scribe to each of the secondary scribes differs ( Figure 2). Angles between the scribe knives vary between coring companies, but the reference scribe is usually offset from the secondary knives by oblique angles on the order of 130° to 150°. As the core is cut and enters the mouth of the barrel, grooves are cut in the surface of the core by the scribe knives. As the coring assembly is made up, a lug on the compass is aligned with a reference scribe (one of usually three knives set in a scribe shoe at the base of the inner core barrel) ( Figure 2). Each frame records the compass and the position of a reference mark.
The battery-driven timer is set prior to running in the hole to expose a frame of film automatically at intervals of 1 to 8 min. The compass, camera, and timer system is mounted at the top of the inner core barrel, inside a nonmagnetic collar. The industry standard for many years has been a mechanical orientation technique utilizing a specially designed core barrel and nonmagnetic drill collar with a compass, camera, battery pack, and timer.
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