Reorientation of Cores Using the Formation Microscanner and Borehole Televiewer: Application to Structural and Paleomagnetic Studies with the Ocean Drilling Program

Abstract

Structural and paleomagnetic studies of Ocean Drilling Program (ODP) cores are severely hampered by the fact that the recovered cores can rarely be reoriented with any degree of confidence. Although several techniques for orienting cores have been used, none is without limitations. Core orientation devices used during drilling are expensive to operate and are usually unreliable. A common alternative, used when no core orientation tools are available, is to assume that the horizontal component of the magnetization direction of the core pointed north, and thus that features may be oriented relative to this direction. However, this neglects the effects of secular variations in the Earth's magnetic field and of later tectonic rotation on the sample. In this paper we outline a technique for core reorientation that matches distinctive inclined planar features measured on the core with their images on Formation MicroScanner and Borehole Televiewer wireline logs. It allows reorientation of structural and magnetic data, and hence the possibility of detecting vertical axis tectonic rotations using Paleomagnetism. Its methodology is described and an example, from ODP Leg 135, presented to illustrate its application.