This paper discusses a method of borehole core orientation based on the observation that when a rock sample is progressively demagnetized in an a-c field, the magnetization vector moves away from the present field direction toward the ancient field direction because the viscous component parallel to the present field is demagnetized by very low a-c fields. In contrast, the component parallel to the ancient field direction is likely to be either TRM or CRM, both of which are extremely stable over long periods of time and against a-c demagnetization. An example of the procedure is given and some problems discussed.

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