Acoustic properties of the rocks

ACOUSTIC PROPERTIES OF THE ROCKS (EN: acoustic properties of rock; DE: akustische Eigenschaften von Gesteinen; FR: proprietes acoustiques des roches; ES: propiedades acusticas de las rocas; RU: акустические свойства горных пород) determine the nature of the propagation of the elastic waves in the rocks.

The irreversible losses of the energy during the propagation of the elastic waves are associated with the acoustic absorption of the rocks, which is caused mainly by the internal friction and the thermal conductivity. The contribution of these mechanisms to the total absorption is different in the different frequency ranges, because their parameters depend on the frequency of the wave. The acoustic absorption is one of the causes for the dispersion of the velocities of the elastic waves, and for the distortion of the impulsive signals, which are propagating in the rocks. They evaluate the rate of the damping of the oscillations with the help of the "g" coefficient of the losses, or of the "Q" (factor) coefficient of the quality. The change of the amplitude of the wave during the propagation in the rocks determines the "a" [metres ^-1] coefficient of the damping, which is the value, which is inverse to the distance, at which the amplitude of the wave decreases by the 2.718 times.

The velocity of the propagation of the elastic (longitudinal, transverse, and surface) waves also belongs to the major indicators of the acoustic properties. The velocity of the propagation of the longitudinal waves is approximately by 1.7-1.9 times more than of the transverse ones, and by 2 times more than of the surface ones.

The acoustic properties are closely associated with the physical-mechanical properties, the thermodynamical state, and the structural peculiarities of the medium. For example, the coefficient of the damping for the hard rocks is approximately proportional to the first power of the frequency, and for the loose rocks to the square of the frequency. The velocities of the propagation of the elastic waves increase with the increase of the moduli of the elasticity of the rocks, of the density of the rocks, and of the pressure (of the depth of the laying); the coefficient of the damping decreases with the increase of the depth. The acoustic properties also depend on the temperature: during the increase of the temperature, the velocity of the elastic waves decreases, and the coefficient of the damping increases.

There are adopted in the practice of the mining craft the resonance (at the natural frequency of the oscillation of the sample) and the impulsive (using the periodic impulsive explosive or striking source of the elastic oscillations) methods of the measurement of the characteristics of the acoustic properties.

The coefficient of the damping: for the undisturbed hard rocks in the frequency range of 1-100 Hz is from 10^-6 to 10^-3 metres ^-1, for the frequencies of 1-10 kHz is from 5 • 10^2 to 1 metres ^-1; for the loose rocks in the frequency range of 1-100 Hz is from 10^-2 to 10^-1 metres ^-1. The velocities of the propagation of the longitudinal waves for the igneous rocks (granite, diabase, and gabbro) are 4500-6800 metres per second, for the metamorphic ones (crystalline schists) are 4000-5600 metres per second, for the sedimentary ones (limestone, dolomite) are 3200-5500 metres per second, in the grounds are 300-1900 metres per second.

They study the acoustic properties for the determination of the elastic, strength, and visco-plastic characteristics of the rocks during the research of the geological structure, during the evaluation of the stress state and the fracturedness of the massif, of the effectiveness of the striking or explosive impact on the rocks, during the choice of the sound insulating materials, which are made of the natural stone.