Characterization of the luminance and shape of ash particles at Sakurajima volcano, Japan, using CCD camera images

Abstract

We develop a new method for characterizing the properties of volcanic ash at the Sakurajima volcano, Japan, based on automatic processing of CCD camera images. Volcanic ash is studied in terms of both luminance and particle shape. A monochromatic CCD camera coupled with a stereomicroscope is used to acquire digital images through three filters that pass red, green, or blue light. On single ash particles, we measure the apparent luminance, corresponding to 256 tones for each color (red, green, and blue) for each pixel occupied by ash particles in the image, and the average and standard deviation of the luminance. The outline of each ash particle is captured from a digital image taken under transmitted light through a polarizing plate. Also, we define a new quasi-fractal dimension (D qf ) to quantify the complexity of the ash particle outlines. We examine two ash samples, each including about 1000 particles, which were erupted from the Showa crater of the Sakurajima volcano, Japan, on February 09, 2009 and January 13, 2010. The apparent luminance of each ash particle shows a lognormal distribution. The average luminance of the ash particles erupted in 2009 is higher than that of those erupted in 2010, which is in good agreement with the results obtained from component analysis under a binocular microscope (i.e., the number fraction of dark juvenile particles is lower for the 2009 sample). The standard deviations of apparent luminance have two peaks in the histogram, and the quasi-fractal dimensions show different frequency distributions between the two samples. These features are not recognized in the results of conventional qualitative classification criteria or the sphericity of the particle outlines. Our method can characterize and distinguish ash samples, even for ash particles that have gradual property changes, and is complementary to component analysis. This method also enables the relatively fast and systematic analysis of ash samples that is required for petrologic monitoring of ongoing activity, such as at the Sakurajima volcano.