基于穿层钻孔声波远探测有限元方法的煤岩界面成像

Coal-rock interface imaging based on acoustic remote reflection logging within crossing boreholes using the finite element method

小型地质构造是造成煤与瓦斯突出事故的主要因素,小构造的精准探测是亟需解决的关键问题。煤岩界面高精度探测是查明小构造,实现透明工作面的基础。声波远探测技术具有探测范围大、分辨率高、可成像等优点,能够实现对煤岩界面的精准识别。为此,提出了基于穿层钻孔声波远探测的煤岩界面探测技术,通过在井下穿层钻孔内布置声波远探测仪,采集孔周煤岩分界面产生的阵列波形,并利用反射波信息反演获得煤岩界面成像图,进一步结合钻孔群,实现工作面的整体勘察。首先,利用COMSOL Multiphysics软件构建出煤系单极远探测数值模型;然后通过模型正演分析全波信号与波场快照的全时空变化规律;最后对远探测声波数据进行反演实现煤岩界面的偏移成像。正演结果表明:模型中煤层的纵波波速比顶底板岩层慢1.2 km/s左右,声波在煤层中传播时能量衰减得更快,同时声波穿过煤岩界面时会出现主频的漂移;当测点趋近于仪器从底板岩层向煤层过渡的位置时,直达波的变化特征为幅度的骤降与声时的增大,而界面反射波的特征为时间-深度域中倾斜同相轴的斜率改变。对采集到的波形数据进行滤波、波场分离、反射波增强、偏移成像四个步骤完成模型反演,成像结果与原始模型相似度高,煤岩界面倾角误差0.6°、煤厚误差0.212 m,穿层钻孔远探测声波有限元方法可以有效地反演出煤岩界面的位置和形态特征。该研究可为声波远探测技术应用于穿层钻孔煤岩界面识别提供基础理论支撑。

The presence of small geological structures is the main cause of coal and gas outbursts,making the precise detection 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,allows for the overall exploration of the mining face.The steps are as follows:(1)A numerical model of monopole acoustic remote reflection logging for coal seams was established using the COMSOL Multiphysics software.(2)Through forward modeling,the entire spatio-temporal evolutionary laws of full waveform signals and wavefield snapshots were analyzed.(3)The inversion of the acoustic data for acoustic remote reflection logging was performed,enabling the migration 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 decay when propagating in a coal seam and showed dominant frequency shifting when spreading through coal-rock interfaces.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 morphological 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.