Dielectric anisotropy as a petrofabric indicator |
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| Authors: | Gary J Tuck Frank D Stacey |
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| Institution: | Physics Department, University of Queensland, Brisbane 4067 Australia |
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| Abstract: | A sensitive instrument for measurement of the angular dependence of dielectric constant in a rotating rock specimen is described and the precautions necessary to obtain meaningful results are summarized. An important problem is the choice of length/diameter ratio for cylindrical specimens, so that complete three-dimensional information can be obtained from a single specimen; this ratio averages 0.86, but is found to be a function of dielectric constant and of electrode geometry. The measurements reported here have been concerned with weakly anisotropic rocks which lack visible fabrics, especially sandstones and basalts. A sandstone was used in a study of the statistical “noise” or scatter in anisotropy axes of a set of 52 similar specimens. The anisotropy was found to be much greater than expected from the finite numbers of grains in a sample. It is concluded that, in spite of careful drying of specimens, the anisotropy is dominated by thin grain coatings which have very high dielectric constants or may even be conducting. A basalt has been found in which the dielectric and magnetic anisotropies have different axes, apparently reflecting different grain alignment events in the history of the rock. However, this is the only example that has been found in which all of the information obtainable from dielectric anisotropy could not have been obtained more easily from magnetic anisotropy. |
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