Geological image processing of petrographic thin sections using the rotating polarizer stage

摘要

One of the fundamental problems with image processing of petrographicthin sections is that the appearance (colour I intensity) of a mineral grain will varywith the orientation of the crystal lattice to the preferred direction of the polarizingfilters on a petrographic microscope. This makes it very difficult to determinegrain boundaries, grain orientation and mineral species from a single capturedimage. To overcome this problem, the Rotating Polarizer Stage was used toreplace the fixed polarizer and analyzer on a standard petrographic microscope.The Rotating Polarizer Stage rotates the polarizers while the thin section remainsstationary, allowing for better data gathering possibilities.Instead of capturing a single image of a thin section, six composite datasets are created by rotating the polarizers through 900 (or 1800 if quartz c-axesmeasurements need to be taken) in both plane and cross polarized light. Thecomposite data sets can be viewed as separate images and consist of theaverage intensity image, the maximum intensity image, the minimum intensityimage, the maximum position image, the minimum position image and thegradient image.The overall strategy used by the image processing system is to gather thecomposite data sets, determine the grain boundaries using the gradient image,classify the different mineral species present using the minimum and maximumintensity images and then perform measurements of grain shape and, wherepossible, partial crystallographic orientation using the maximum intensity andmaximum position images.