| Many coal mine inundations would happen when pillars protecting water were broken through to active mine due to absence of necessary geological data or using inaccurate map. According to statistic, 60% of mine inundations were caused by water inrush from goafs. Thus, it is necessary to develop an effective method that can detect void or uncertain boundary of pillar between two adjacent coal or active sites and goafs. In preceding researches, because mechanics of channel waves have not been developed fully and methods of processing channel wave have been limited to traditional seismic geological exploration to indentify reflecting interfaces, the in-seam seismic (ISS) technology has been jammed since 1990s.In order to solve the problem of void detection in a coal seam, the Mine Safety and Health Administration (MSHA) has funded 8 organizations to study the feasibility of void detection using all kinds of methods, including Pennsylvania State University. The research team of Pennsylvania State University is going to use the in-seam seismic method to detect voids in a coal seam. Author has attended two researches in Black King Mine and Cumberland Mine as a member of research team. When the distinguishable Airy phase method was used, the result of research showed that the detecting range and accuracy are better than ones of other technologies using in void detection.According to the result of research, the effectiveness of dispersion is stronger when the coal seam becomes thinner. The velocity of channel wave depends on the frequency, so it is difficult to get exact speed of channel wave in a thin coal seam. In addition, the range of propagation of channel wave decreases because of high frequency. As a result, it will be difficult to indentify the reflected signal from the reflection survey in a thin coal seam.The dissertation focused on the feasibility of void detection in a 0.9m-thin coal seam of Black King Mine, analyzed systematically the dispersion characteristics of channel wave in a thin coal seam, and explained the reason of high and low frequency domain in theory. Based on the deduced equation of phase velocity and group velocity, the dispersion curves of fundemental order, first higher order and second higher order were plotted using numerical method.According to study the seismic signal recorded in the 0.9m-thin coal seam in the condition of dynamite as seismic sources, two frequency bands are shown in energy spectrum. After using wavelet transform, the two frequency bands almost started on the same moment. Furthermore, the wavelet coefficients distribution shows a certain similar behavior in a thin coal, which is outstanding feature that is different from channel waves recorded in a 2.1m-thin coal seamBecause the seismic source set in the middle of coal seam, the vibration of roof and floor caused by dynamite has played an important role to form a second higher order of symmetrical wave shape. The velocity of channel wave in second higher mode is the same as the one of S wave in roof and floor, which is important to confirm the velocity of S wave in rock.The dissertation has detailed the method of void detection using ISS in a thin coal seam, and the results are positive. The research has verified the effectiveness of the distinguishable Airy method, and proved the theory of existence of second higher mode in channel wave. Because channel waves in a coal seam have a distinctive characteristic of dispersion and a wide frequency domain, uncertain boundary of pillar or reflecting interface could be decided using the method of analogy in practical engineering case.The dissertation is part of void detection using ISS technology. If combing with the result studied in a thick coal seam, it will be formed to a systematical method. Author will integrate this technology to the Digital Mine System in future, and this technology will be developed quickly in practical engineering case. |
PDF Full Text Request