Experimental Study On Dispersion Characteristics Of Elastic Wave Parameters With Frequency In Coal Body Baseed On Stress-Strain Method

The elastic wave parameters with frequency have dispersion characteristics,and the ultrasonic parameters measured in the laboratory have errors when applied to the acoustic seismic exploration.Based on the design principle of Batzle et al.’s low frequency stress and strain measurement system,the low frequency seismic coal and rock physics testing system is built,and the elastic wave parameters of coal body are measured by stress-strain method,and the dispersion response characteristics of elastic wave of coal body are discussed.The aim is to establish a method for measuring the elastic wave parameters of coal by stress-strain method and reveal the dispersion characteristics of elastic wave in coal.The main results and conclusions are as follows:(1)A test system of elastic wave parameters of coal sample at low frequency,has been built in the laboratory,and the main controlling factors that affect the reproduction and error of the measured results by stress-strain method are revealed.The selection and assembly of components,functional debugging and measurement of the system are completed.The system consists of computer and software control,Wheatstone bridge,signal generator,power amplifier,shaker,sample gripper,pressure-temperature monitoring system and signal acquisition system.The factors that affect the reproducibility and error of the stress-strain method include the selection of reference sample,sample excitation eccentricity,excitation power and the connection of signal channel lines.Among them,the selection of reference sample,sample excitation eccentricity and excitation power have less influence on the measurement results of elastic parameters.(2)The stress-strain method is realized to measure the elastic wave parameters of coal body.We measured the elastic parameters in the low frequency band[4～128Hz]and the vertical and horizontal wave velocity in the ultrasonic frequency band of 500k Hz and1000k Hz for the coal samples of 7 metamorphic degrees from Urumqi Nanshan,Pingdingshan,Hebi and Jiaozuo,and calculated the elastic parameters such as Young’s modulus and Poisson’s ratio at different frequencies.And elastic parameters such as velocity and attenuation coefficient of horizontal and horizontal waves.(3)The frequency response and dispersion characteristics of elastic wave in coal body are revealed.From the elastic parameters of coal and the elastic wave parameter inversion results and analysis,it is found that:Young’s modulus,poisson’s ratio,and vertical and horizontal wave velocity has a certain degree of dispersion phenomenon,low frequency dispersion degree than ultrasonic frequencies,young’s modulus and longitudinal wave velocity all showed a trend of rising with increasing frequency,the longitudinal wave velocity and shear velocity dispersion phenomenon more obvious,the velocity of the seismic exploration frequency band(10~1-10~2Hz)is smaller than that of the ultrasonic wave;In addition,the measured results of compressional wave velocity of coal samples are consistent with the calculated results of BISQ theoretical model,which proves that the stress-strain method is feasible to measure the elastic wave parameters of coal.This study is conducive to extending the single frequency band coal and rock physical measurement parameters to the full frequency band,so that the geophysical measurement data of different frequency bands can be connected,and the comprehensive application can be realized in the same scale,so that the laboratory coal sample physical measurement results can be better used to guide the seismic exploration.