Study of the Structure of Metasandstones Using Broadband Acoustic Spectroscopy with a Laser Source of Ultrasound

Abstract

The results of measuring the frequency dependences of the attenuation coefficient and phase velocity of longitudinal ultrasonic waves in metasandstone samples with varying degrees of deformation–metamorphic alteration are presented to study the influence of the sample structure on these acoustic characteristics. The measurements are performed using broadband acoustic spectroscopy with a laser source of ultrasound and piezoelectric detection of nanosecond ultrasonic pulses in the 1–70 MHz operating frequency range. The metasandstones of the zonally metamorphosed Ladoga Group of the Paleoproterozoic Baltic Shield are studied, which exhibit varying degrees of structural and textural changes that occurred in the temperature range of 400–600°С. Cores of two different thicknesses with a similar mineral phase composition, but with some variations in the structural and textural composition of the substrate, were selected at four sampling points of different temperature levels and studied. For metasandstones from three sampling points, the frequency dependences of the attenuation coefficient and phase velocity of longitudinal ultrasonic waves almost completely coincide in three zones of each core and are nearly the same for cores of two different thicknesses. The latter fact confirms the reliability of the results of the ultrasonic studies, and the coincidence of the results for three different zones of each core shows that the structure of these metasandstones is uniform. The attenuation coefficient and phase velocity of ultrasound considerably differ in the studied zones of two metasandstone cores in one of the four sampling points (LV1246) due to the significant nonuniformity of their structure. In addition, for metasandstones from different sampling points, a difference in the absolute values of the attenuation coefficient and phase velocity of ultrasound is found in the entire studied frequency range. The Rayleigh model of ultrasonic scattering is used to estimate the maximum grain sizes in the studied samples. The results correlate with data obtained using optical microscopy of thin sections of all samples. The implemented broadband acoustic spectroscopy method with a laser source of ultrasound can help to reveal the relationship between the frequency dependences of the attenuation coefficient and phase velocity of longitudinal ultrasonic waves, the characteristic features of the structure, and conditions for the formation of metasandstones in each specific deposit.