Determination of fabric and strain ellipsoids from measured sectional ellipses—implementation and applications

Geologists examine fabrics to constrain models of formation or of deformation of rocks, and it is often convenient to summarise the results by a fabric ellipsoid. As fabric data are commonly collected on planar sections through the rock, estimating a fabric ellipsoid from sectional ellipses, often with arbitrary orientations, is an important task. An algebraic method to calculate such an ellipsoid, presented in an earlier paper, has been implemented with the program ellipsoid. It is used here on examples that illustrate questions and issues that arise when collecting, selecting and processing sectional fabric data, and when assessing the results. The quality of fit of the ellipsoid to the data is assessed in all cases. Examples include a case in which the average sizes of markers on individual sections can be used in the determination of the ellipsoid, and other cases in which such sizes are not useful; a case in which sectional ellipses are not obtained from closed markers; and a case in which data scatter and insufficient coverage of section orientations lead to a hyperboloid instead of an ellipsoid.