Quantitative Estimation of the Characteristic Grain Sizes of Laboratory Rock Samples by the Broadband Optoacoustic Spectroscopy Method

A technique for quantitative estimation of characteristic grain sizes in laboratory rock samples using the relationship between the frequency and attenuation of longitudinal ultrasonic waves in the samples is proposed and implemented experimentally. This relationship is quantified using broadband optoacoustic spectroscopy with a laser source of ultrasound and piezoelectric registration of nanosecond ultrasonic pulses in the operating frequency range of 1-70 MHz. The application of the theoretical model of ultrasound scattering in single-phase polycrystalline materials to quantitative estimation of the maximum and average grain sizes in multiphase rocks is shown using five samples of metasandstones of zonally metamorphosed Ladoga series of the Paleoproterozoic of the Baltic Shield, which underwent different degrees of structural and textural transformations during ancient metamorphic events. The reliability of the data obtained using broadband optoacoustic spectroscopy was for the first time confirmed by independent scanning electron microscopy of the polished surfaces of all samples. The average and maximum grain sizes were estimated separately using the conventional method of line crossing from optical micrographs of thin sections performed for two selected samples, which also showed good agreement with the acoustic spectroscopy data. The proposed method of broadband optoacoustic spectroscopy for estimation of characteristic grain sizes of laboratory rock samples can be used to analyze the possible relationship between their structural features and thermobaric conditions of formation.