Optimization And Application Of Seismic Computerized Tomography In Coal Mines

Rockburst is one of main threats to safe coal mining,and mining pressure will be more serious and complicated with the increase of mining depth,therefore,it is of great significance to accurately and rapidly obtain high-resolution coal-rock stress imaging for predicting and preventing rockburst hazards.The thesis focuses on the application and problems of seismic CT technique in coal mines.First of all,influence factors of inversion results are deeply analyzed,including the error of P-wave arrival time in active CT and unreasonable grid division in passive CT model,and then corresponding optimization measures are put forward.At last,the optimized methods are applied to the engineering practice and field application.After establishing active CT model,the optimal inversion algorithm is selected based on the accuracy and time consumption,and two delay-time experiments are carried out using this optimal inversion algorithm.Experimental results show that if all detonators have same delay time,the longer the delay time is,the grater the regional velocity will decline and the higher the errors will be according to inversion results,although changes of high and low velocity areas are not obvious.However,velocity variations and errors are very large in the study area if different delay combinations are used in an active CT operation,and areas with high and low velocities will change unpredictably.According to relationship between P-wave travel time and propagation distance,a detonator delay elimination(namely,P-wave arrival time modification)method is proposed adopting least squares linear fitting method.The actual P-wave propagation time was obtained using this method in an active CT operation in No.3310 working face of Xingcun Coal Mine.After that,P-wave arrival time with delay eliminated was applied to inversion calculation.Inversion results show that there were four high velocity areas in 3310 working face,and these areas were certified to be high stress areas though theoretical analysis and field observations,which proves the reliability of CT technique and accuracy of delay elimination method.Based on principle of passive CT technique and characteristics of mine tremor distributions during coal mining period,an adaptive unequal-spacing grid division method is proposed which can improve the contradiction between the resolution and efficiency existed in the equal-spacing model.This grid-dividing method adopts PCA to calculate principal directions of mine tremors,and uses concepts of Chi-square distribution,Confidence interval and Mahalanobis distance to determine tremor concentration regions,thereby obtaining confidence ellipsoid.Grids will be divided densely within the confidence ellipsoid projection,while grids will be divided sparsely outside the confidence ellipsoid projection.Unequal-spacing model was formed using mine tremor data during mining period of No.5302 working face in Jisan Coal Mine.Practice indicates that this grid-dividing method could automatically establish inversion model,which could increase grid number within the tremor concentration ranges and reduce grid number out the tremor concentration ranges.Compared with equal-spacing inversion model,this unequal-spacing grid-dividing method could greatly decrease grid number and improve inversion efficiency.Apart from that,this method could also increase inversion resolution in the key research area.The future monitoring results showed that tremors with high energy mostly occurred in the high velocity areas predicted by inversion result with unequal-spacing model,proving that reliability will be much higher if passive CT using this unequal-spacing model compared with equal-spacing model.