Evaluating Clay Microfabric Using Scanning Electron Microscopy and Digital Information Processing

Several aspects related to the qualitative and quantitative evaluation of clay microfabric are presented. A digital image processing technique called “intensity gradient analysis” is used in evaluating clay microfabric from scanning electron micrographs. Details are given of three computer programs (histogram.c, average.c and ellipse.c) that provide for the quantitative analysis of the orientation of clay particles. The important aspects related to sample and surface preparation, digital image acquisition, and analysis are explained. Two natural clays, Pennsylvanian fire clay and Pleistocene varved glacial lake clay, considered in the analysis show well-defined preferred and random fabric. The polar histograms of the intensity gradient analysis for the two clays are compared at varying magnifications, and conclusions were drawn. The output data was conveniently reduced to a few parameters that indicate the strength and magnitude of a given orientation. The concept of fabric index is used to quantitatively describe the preferential or randomness of clay microfabric. A simple scheme is proposed by which clay microfabric is classified as preferred when the fabric index is less than a value of 0.75 and random otherwise. The advantages of the signal intensity gradient method are that it is based on intensity changes rather than absolute values, the rosette diagrams of several micrographs can simply be added to get an overall idea, and it provides a common frame of reference for description of microfabric. The disadvantages are that it requires careful surface preparation, and the technique is valid for linear features. With the advent of inexpensive and powerful image processing tools, the technique has the potential to become a routine tool for the orientation analysis of clay micrographs seen in scanning electron microscopy.