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Core Orientation and Susceptibility Logging |
Department of Geological Sciences, The University, Plymouth PL4 8AA, UK
All rocks have petrofabrics that can be determined rapidly and precisely by magnetic anisotropy as this depends on the net alignment of the crystallographic properties and the shape orientations of all grains within the rock, i.e. the grains themselves act as fabric markers. When ferromagnetic minerals are present in quantities >0.0001%, their properties dominate the observed fabric. As most rocks contain at least this level of ferromagnetic content, most techniques reflect the petrofabric of the ferromagnetic minerals, although rocks where these are absent still have petrofabrics that can be measured quickly and precisely by magnetic methods. Such magnetic fabrics can be used to define the direction of flow and velocities of the currents when the sediment was being deposited. The original fabric may be modified during shallow burial, enabling estimation of the degree of bioturbation. Subsequent fluid migration (gases, water, hydrocarbons, etc.) through the sediments superimposes magnetic fabrics than can be used to define the three-dimensional nature of such secondary channels. The very high sensitivity of the technique also enables it to detect the onset of anchimetamorphic changes at lower levels than detectable by standard techniques.
