Method And Application Of Acoustic And Electromagnetic Wave Tomography For Complex Structures

As an important technique applied in engineering geological exploration, the study rock mass engineering stability, non-destructive engineering quality inspection, engineering safety evaluation and other aspects, acoustic wave and electromagnetic wave detecting techniques play important roles in hydroelectricity, railway, coal, petroleum, architectural and other fields. The large-scale engineering geological detecting requires that the acoustic wave detector and data processing method can be adaptable to the high resolution detection of large-scale complex structure mass. Hence, sonic detector, acoustic wave tomography forward and inverse algorithm and their applications, and the attenuation time-frequency character of electromagnetic wave were systematically investigated in the present work.In chapter 1, the general development of tomography and several common image reconstruc-tion algorithms are reviewed. The current status in acoustic wave raytracing algorithms are analyzed. The importance of studying the triangle grid raytracing algorithm is explained. For problems in the coal mine electromagnetic wave penetration, the study on electromagnetic wave attenuation characters in frequency-domain is significant.In chapter 2, firstly, the system components, performance indeies of the large-scale sonic detector and the structure and features of the superpower GMM acoustic transducer are listed. Secondly, functions, like signal collecting, presenting, signal processing are described in detail. Finally, two application exmples in the detection of roof coal thickness of mine working faces and the non-destructive inspection of large span concrete structures are given.In chapter 3, according to the topology relationship of the node, line and face in subdivision area for the triangle subdivision of 2-demension complex structure, triangle subdivision algorithm that meets the characteristic of complex 2-dimensional area and geophysical forward and inversion is realized based on the the optimization rule of Delaunay triangle subdivision. The density control function is considered while the nodes and grid are formed and conformation function by itself is realized.In chapter 4, complex structure triangle grid raytracing global algorithm is put forward and reseached. In section 4.2, the definitions on the topological relations and related ideas of triangle cells raytracing are given. The approximate global algorithm of secondary sources of nodes is discussed in three aspects: calculation of the least travel time of wave front neighborhood points and choosing of their secondary sources, the wave level expansion and the regressive retrieving of wave ray paths. Hyperbola approximate global algorithm is used in the true secondary sources retrieve. In section 4.3, the results of the raytracing for three kinds of models are given. the results of theory calculation and the numerical simulation for the two level stratified model are compared.In chapter 5, the basic theory of acoustic wave tomography is described and the image reconstruction techniques of the ray tomography based on triangle grid are discussed. The tomography inversion numerical simulation on complex structure model of irregularity area with thin slab low speed subfield and circle low speed subfield is performed. The program also gives the raytracing results and the model velocity distributions iterative process of tomography inversion for 7 times. The results of numerical simulation indicate that the triangle grid tomography method has some obvious characteristics, including high adaptability in complex area, flexible description of velocity interface, high resolution, and the imaging results closer to the real structure shape feature.In chapter 6, the results of detection test on the destruction depth in the floor of the coal seam with large-scale acoustic wave tomography and the results on the engineering quality inspection of large span concrete structures are given. In section 6.1, the movement and failure regularity of stratum in the floor of working faces and the wave field response of floor's failure are introduced. The schemes and the results of acoustic wave tomography for detecting the failure depth in the floor of the coal seam are given. The dynamic changing process about P-wave velocities of floor stratum with the working face impelling is analyzed in detail, and the exact depth of destruction band at the floor of coal seam exploited is determined. In section 6.2, the P-wave and S-wave velocities tomography results of the concrete structure with a span of 12m are obtained.In chapter 7, the physical model is established and the abnormal geological objects with high or low resistivities in the working face are taken as the practical background. Two-dimensional forward numerical simulation for electromagnetic wave equation is implemented with the finite-difference method. According to the simulation results, the time-frequency domain attenuation features are studied respectively. The tomography results of field intensity and centroidal frequency shift are compared.In conclusion, the research work of this dissertation is summarized, and the future developing directions are indicated.