Research on inversion and application of failure depth of coal seam roof and floor based on triangular network acoustic CT tomography

Acoustic tomography is an important test method in engineering geology and underground engineering, however, the problems of model parameterization, forward modeling and inversion in the process of tomographic imaging still need further study. In this work, the triangulation method of two-dimensional complex area was studied and the global algorithm of triangular ray tracing in two-dimensional complex area was formulated. Numerical simulation results of triangular acoustic ray tomography show that with the increase of iteration times, the ray tracing becomes increasingly close to the ray-tracing forward from the velocity model; through seven iterations, the low-speed plate with a thickness of only 0.56 m can also be clearly reflected. Besides, based on triangular ray-tracing tomography of the two-dimensional complex structure, the deep acoustic detection of coal seam floor failure zone is carried out. After the initial velocity model is determined by the single-hole acoustic detection results, the velocity distribution of the floor between holes is obtained by triangular network acoustic tomography from the premining, mid-mining and postmining acoustic penetration detection. Field measurements show that the 17-m-span acoustic wave penetration is successfully realized in sandstone and mudstone strata of coal seam floor, with accurate delineation of the development range of water-conducting fissures in floor failure. The triangular acoustic tomography method makes full use of the characteristics of flexible model division and makes the imaging results appear closer to the actual structural morphological characteristics.