Image analysis and circular statistics for shape-fabric analysis: applications to lithified ignimbrites

Abstract

Computer-assisted image analysis can be successfully used to derive quantitative textural data on pyroclastic rock samples. This method provides a large number of different measurements such as grain size, particle shape and 2D orientation of particle main axes (directional-or shape-fabric) automatically and in a relatively short time. Orientation data reduction requires specific statistical tests, mainly devoted to defining the kind of particle distribution pattern, the possible occurrence of preferred particle orientation, the confidence interval of the mean direction and the degree of randomness with respect to pre-assigned theoretical frequency distributions. Data obtained from image analysis of seven lithified ignimbrite samples from the Vulsini Volcanic District (Central Italy) are used to test different statistics and to provide insight about directional fabrics. First, the possible occurrence of a significant deviation from a theoretical circular uniform distribution was evaluated by using the Rayleigh and Tukey x 2 tests. Then, the Kuiper test was performed to evaluate whether or not the observation fits with a unimodal, Von Mises-like theoretical frequency distribution. Finally, the confidence interval of mean direction was calculated. With the exception of one sample (FPD10), which showed a well-developed bimodality, all the analysed samples display significant anisotropic and unimodal distributions. The minimum number of measurements necessary to obtain reasonable variabilities of the calculated statistics and mean directions was evaluated by repeating random collections of the measured particles at increments of 100 particles for each sample. Although the observed variabilities depend largely on the pattern of distribution and an absolute minimum number cannot be stated, approximately 1500-2000 measurements are required in order to get meaningful mean directions for the analysed samples.

An integrated paleomagnetic and magnetic anisotropy study of the Oligocene flysch from the Dukla nappe, Outer Western Carpathians, Poland

“…This method has been used previously to determine sedimentary transport direction in ignimbrites (Capaccioni & Sarocchi 1996;Capaccioni et al 1997;Biró et al 2015). The x and z directions were marked on the thin sections, so the comparison between the anisotropy principal directions and any grain fabric orientation direction was possible.…”

Vent area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (Campi Flegrei, Italy): new insights from directional fabric through image analysis

“…χ 2 values exceeding 5.99 (at 5% of significance level) proves the existence of anisotropy which, under the hypothesis of unimodality, also proves the occurrence of a preferred orientation of particles. The equivalent information, not-dependent on the sample size but only by the "strength" of particle isoorientation, is accurately provided by the normalized Tukey index V 2 % (Capaccioni et al 1997).…”

“…Directional fabric data were obtained using a computerassisted image analysis method, already described by Capaccioni et al (1997). The output data, consisting in the azimuths of the particle long axes and their areas, were processed to identify any preferred particle orientations and their strengths.…”

Estudio de flujos granulares de tipo geológico por medio del simulador multisensor GRANFLOW-SIM

“…Por medio de las fotografías de alta resolución se realizan estudios granulométricos ópticos a detalle (Sarocchi et al, 2005) y de la micro-fabrica (Capaccioni et al, 1997), también se analizan la distribución del material depositado, el ángulo de apertura del depósito y su alcance.…”