| Microseismic monitoring in mines has always been a key technical tool to ensure mine safety production,in which microseismic source localization and microseismic signal characteristics extraction and analysis are the main research directions of microseismic monitoring theory.In this paper,for the complex geological structure of the mine,the research methods of theoretical analysis,model test and field monitoring and application are applied to carry out the research work of refining wave velocity model construction and microseismic signal energy frequency characteristics analysis in the mining collapse area.The main research results are obtained as follows.(1)The geological model of the goaf overburden rock “three zones” is constructed based on the assumption of layered homogeneous media and circular arc interface.Considering the special structures of rock stratum,void area,intra-layer fracture and rock fracture in the goaf overburden rock,the increments of interface and intralayer diffraction paths,as well as the wave velocities reduction for the rock expansion in the fissure zone and caving zone are proposed.Based on Snell’s law,Fermat’s principle and Huygens’ principle,the cross-layer propagation paths,theoretical travel time and equivalent wave velocities calculation equations are deduced,and the propagation mode of seismic waves over the void area via the edge of the void zone is clarified.The wave velocity model is extended from 2-D to 3-D,forming an anisotropic equivalent wave velocity model that can reflect the structural characteristics of the goaf overburden rock “three zones”.(2)According to the mining geological conditions,2-D and 3-D similar material model tests are carried out,and the structures of the goaf overburden rock are gradually formed by step excavation of the simulated coal seam.The pendulum or rotor motor artificial seismic sources are set up in the model to generate vibrations,and a ultrahigh frequency tectonic activity monitor is used to collect the artificial source microseismic signals.Under the 2-D model test,the differences in propagation paths between the simple horizontal layered media wave velocity model and the goaf overburden rock wave velocity model and the matching effect between the theoretical and measured equivalent wave velocities are compared,showing that the goaf overburden rock wave velocity model can better reflect the “ remote point reaching first ”phenomenons in the structure of the goaf overburden rock,and the deviation between the theoretical and measured wave velocities can be controlled within 10%.Based on the 2-D model,the 3-D model test verified that the wave velocity model can adjust the model combination with the excavation work to realize the matching between the geological structure and the wave velocity model,in which the deviations between the theoretical and measured equivalent wave velocity are less than 10% on 90% of the propagation paths.(3)The microseismic signals are processed by the variational modal decomposition(VMD)method to obtain the modal signals at each frequency and extract the signal energy and frequency characteristics.The particle input energy and signal energy are proposed as microseismic energy indicators.Through the 3-D fault model test,the Gaussian distribution relationship between microseismic energy and frequency,and the relationship between the peak center of Gaussian distribution and the change of seismic wave propagation state are found,which clarifies that the geological structure interface has a frequency modulation effect on microseismic wave propagation.A linear relationship exists between the particle input energy and signal energy,and its linear slope can also reflect the change of seismic wave propagation state.The energy frequency characteristics for microseismic signals of the goaf overburden rock model show that the energy frequency distribution characteristics under different propagation states reflected by the shock type seismic sources vary significantly,while the response of the stabilized seismic sources in complex geology is not obvious in terms of frequency,and their frequencies remain relatively stable.(4)Based on the source location principle of the arrival time theory,the objective function is constructed by the difference between the measured arrival time difference and the theoretical travel time difference,and the anisotropic equivalent theoretical wave velocity model of the goaf overburden rock is introduced,then the global minimum of the objective function is solved by the pattern search algorithm to optimize the seismic source location algorithm under complex geological conditions in mines.The location result differences between the location methods of applying the goaf overburden rock wave velocity and the pending wave velocity,average wave velocity and horizontal layered wave velocity are compared through the calculation example,model tests and field monitoring location.Among them,the average wave velocity and horizontal layered wave velocity methods have poor location results in the face of complex geological conditions,while the pending wave velocity method can adapt to complex geology but has unstable location.But the location results of the goaf overburden rock wave velocity model location method remain stable,and the conversion location differences are controlled within 20 m.The on-site location application of the goaf overburden rock wave velocity model shows that the location results are mainly located in the mining area behind the working face and in the overlying rock layer.It is consistent with the results of the on-site investigation and evaluation of large energy microseismic events,in which the "F" type structural breakage and destabilization of the overlying rock profile at the boundary of the mining void area triggered large energy events.(5)For the variability of background noise frequency of the on-site monitoring signals,a joint arrival time pickup method based on time window signal energy and time-frequency power spectrum is proposed,which omits the denoising step before arrival time pickup and prevents the arrival time offset generated by filtering.In the joint method,based on the time window signal energy can better identify the amplitude abrupt signal arrival time,such as shock signal,blast signal,etc.,while based on the time-frequency power spectrum can better distinguish the frequency abrupt signal arrival time,such as mechanical vibration signal.The organic combination of the two principles can enhance the universality and accuracy of the field microseismic signal arrival time pickup method.The research results of this paper propose a new theoretical reference for locating the seismic sources and extracting the energy frequency characteristics of microseismic in coal mine complex geology,and enrich the theory of microseismic monitoring.The paper has 99 figures,64 tables and 168 references. |
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