The presence of small geological structures is the main cause of coal and gas outbursts,making the precise de-tection of these structures an urgent need.High-precision detection of coal-rock interfaces is essential for identifying small structures and achieving transparent mining faces.The acoustic remote reflection logging technology,with a large detection range,a high resolution,and imaging capability,can accurately identify coal-rock interfaces.In this context,this study proposed a technique for coal-rock interface detection based on acoustic remote reflection logging within crossing boreholes.Specially,by placing acoustic detectors in crossing boreholes,the array waveforms generated by the coal-rock interfaces around the boreholes were collected.Then,the coal-rock interface images were obtained through the inversion of the reflected wave information.This technology,combined with the borehole group in the mining face,al-lows for the overall exploration of the mining face.The steps are as follows:(1)A numerical model of monopole acous-tic remote reflection logging for coal seams was established using the COMSOL Multiphysics software.(2)Through for-ward modeling,the entire spatio-temporal evolutionary laws of full waveform signals and wavefield snapshots were ana-lyzed.(3)The inversion of the acoustic data for acoustic remote reflection logging was performed,enabling the migra-tion imaging of coal-rock interfaces.The forward modeling results indicate that the compressional wave velocity in a coal seam was approximately 1.2 km/s slower than that in its roof and floor.Acoustic waves exhibited faster energy de-cay when propagating in a coal seam and showed dominant frequency shifting when spreading through coal-rock inter-faces.When a measurement point approached the position where an acoustic detector shifted from the rock layer on a coal seam's floor to the coal seam,direct waves exhibited a sharp decrease in the amplitude and an increase in sonic time,while reflected waves from the interfaces displayed changes in the slopes of the inclined events in the time-depth domain.Inversion was completed through four steps:filtering,wavefield separation,reflection wave enhancement,and migration imaging.The imaging results closely resembled the original model,with the coal-rock interface dip angles and coal thickness exhibiting errors of 0.6° and 0.212 m,respectively.Therefore,the acoustic remote reflection logging with crossing boreholes based on the finite element method allowed for the effective inversion of the positions and morpholo-gical characteristics of coal-rock interfaces.This study will provide fundamental theoretical support for the application of acoustic remote reflection logging technology in the coal-rock interface identification through crossing boreholes.
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