Imaging small geological structure in coal mines based on reflected acoustic logging of underground cross-seam boreholes

Abstract

Accurate identification of small geological structures is the key and difficult point in coal mine geological survey. The reflected acoustic logging technology has the advantages of large detection range and high imaging resolution, and can realize the accurate investigation of small structural interfaces. Therefore, the idea of identifying small geological structures in coal mines by using cross-seam boreholes reflected acoustic logging is proposed. By placing acoustic logging tools in underground cross-seam boreholes, waveforms generated at coal-rock interfaces around the borehole are collected. The reflection wave information is then inverted to obtain interface images of small structures. First, COMSOL Multiphysics software is utilized to build finite element models for reflected acoustic logging in coal-bearing formations containing coal seam fractures, small normal faults, and coal seam thinning zones, respectively. Then forward modeling is conducted to analyze the spatiotemporal variation patterns of the full waveform signals and velocity snapshots. Finally, the reflected acoustic logging data is inverted to realize the offset imaging of small structures in coal mines. The forward modeling results show that: the arrival times of reflected waves varying with the source longitudinal coordinates exhibit good consistency with the interface morphology of borehole vicinity small structures. The inversion results show that: when fractures exist in the coal seam, the imaged dip angle of the fracture is 54.14°, with an error rate of 2.07% compared to the analytical model, and the imaged fracture length is 15.32 m, with an error rate of 4.57%. When small faults exist in the coal seam, the imaged dip angle of the fault plane is 70.63°, with an error rate of 1.41% compared to the analytical model, and the imaged fault plane length is 13.75 m, with an error rate of 3.03%. When thinning zones exist in the coal seam, the imaged dip angles of the upper and lower interfaces of the thinning zone are 44.61° and 57.80°, with error rates of 0.31% and 0.40% respectively compared to the analytical model. The imaged lengths of the upper and lower interfaces are 9.32 m and 18.73 m respectively, with error rates of 0.32% and 1.08% respectively. Acoustic reflection logging technology can effectively invert the locations and morphological characteristics of small structures in coal mines. This study can provide fundamental theoretical support for the application of acoustic reflection logging technology in cross-seam boreholes for identification of small structures in coal seams.