Attenuation Characteristics Of Acoustic Wave Propagation And Its Effect On Rock Acoustic Emission

Acoustic emission is an elastic wave generated by internal damage in a solid.When it propagates in solid media such as rocks,it carries a lot of information related to the properties of the medium.Based on this information,the stress state and structural characteristics of the solid can be inferred,thus solving corresponding engineering geological problems.However,the propagation attenuation of wave will seriously affect the amplitude,frequency and other characteristics of acoustic emission waveform.Therefore,it is necessary to clarify the attenuation law of acoustic wave propagation and clarify the specific impact of propagation attenuation on acoustic emission.Based on the solid internal friction and sound wave propagation theory,this paper first derives the theoretical formulas of physical attenuation and geometric attenuation of sound waves,conducts acoustic excitation attenuation tests using steel,aluminum,copper and sandstone,and verifies and analyzes the theoretical formulas of sound wave attenuation.Secondly,the propagation attenuation characteristics of composite pulse waves were studied through attenuation experiments of composite sound waves with different frequencies and amplitudes.Finally,based on the above research results,combined with the results of rock uniaxial compression acoustic emission monitoring test,the influence of acoustic propagation attenuation on rock acoustic emission spectrum and acoustic emission b-value is discussed in depth,and the following results are obtained:(1)The propagation attenuation of sound waves in solid media includes geometric diffusion attenuation and physical absorption attenuation,with geometric attenuation playing a dominant role.Overall,the total attenuation of sound wave amplitude increases with the increase of propagation distance;The attenuation is minimal when the excitation frequency is the resonant frequency(natural frequency of the medium),and rapidly increases when it is far from the resonant frequency.The experimental results show that the geometric attenuation trend of sound waves is consistent with the total attenuation,and can be characterized by the geometric attenuation coefficient k.The value of k is the highest at resonance frequency,indicating the minimum geometric attenuation;The physical attenuation of sound waves can be characterized by the quality factor Q and the physical attenuation coefficient.The physical attenuation coefficient and quality factor Q decrease with the increase of the elastic modulus of the material.The physical attenuation of sound waves is proportional to the frequency of sound waves and their propagation distance,At high frequencies(300k Hz),the physical attenuation of sound wave amplitude in a waveguide rod with a length of 2000 mm has exceeded 10% and cannot be ignored.(2)The compound acoustic wave propagation attenuation test of multiple frequency components found that the main frequency band in the acoustic emission spectrum is near the natural frequency of the medium.The frequency outside the main frequency band needs more energy to become the main frequency.The low-frequency component can rarely become the main frequency.The cross frequency component needs at least three times the energy of the resonant frequency,while the high-frequency component needs more than five times the energy of the resonant frequency,that is,the cross frequency The emergence of high-frequency and other dominant frequencies means the generation of higher energy.(3)In the analysis of rock acoustic emission spectrum,the influence of rock natural frequency cannot be ignored.The appearance of the main frequency band in the rock acoustic emission spectrum is the result of the attenuation of sound waves.When the frequency component in the sound wave is the natural frequency of the rock,the amplitude and energy decay are small,while other frequency components decay rapidly,highlighting the main frequency band.The acoustic emission spectrum of white sandstone shows two main frequency bands near 41 k Hz and 95.7k Hz,which are the natural frequencies of white sandstone.Other frequencies becoming the dominant frequency in the spectrum often indicate the generation of high-energy fractures within rocks.The b-value error caused by amplitude attenuation can be ignored in the calculation of rock acoustic emission b-value,and the b-value trend calculated after amplitude correction is basically consistent with the traditional b-value trend.However,when calculating the b-value of earthquakes and microseisms,due to the increase in geometric and physical attenuation scales,the amplitude attenuation will inevitably increase,and the resulting b-value error cannot be ignored.