Interpretation method of coal seam thickness based on dispersion characteristics of Love channel waves

The inversion of coal seam thickness using the dispersion characteristics of channel waves is one of the main methods for the detection of coal seam thickness.However,the dispersion curve caused by the changes in coal seam thickness along the wave propagation path is different from the theoretical curve calculated by constant coal seam thickness.The tomography of a single frequency or narrow band filtering method is difficult in taking into account the change of group velocities and the energy of channel waves received in different locations,especially in the case of large coal seam thickness changes.In this case, the dispersion characteristics of Love channel waves caused by coal seam thickness changes were obtained by three dimensional numerical simulation and field test.The results show that once the channel wave forms, both the frequency and the energy of Love channel waves change weakly when the coal seam thickness increases. However,when the coal seam thickness decreases,the energy of Love channel waves attenuates rapidly.The main frequency moves to the high frequency direction, and the proportion of the high frequency components of the channel waves increases.The dispersion of Love channel waves in thick coal areas is compressed in frequency. With the decrease of the coal seam thickness,the dispersion of Love channel waves is stretched in frequency. The frequency of Airy phase moves to high frequency, and the group velocity of Airy phase increases significantly. The working face with coal thickness varying from 1.55 m to 8.09 m is selected for channel wave survey test. The results show that the change of coal seam thickness greatly influences the channel wave velocity in the low frequency range. With the increase of the channel wave frequency, the difference of channel wave velocity in different coal thicknesses decreases.Broadband filtering channel waves is carried out using four different frequency bands with a bandwidth of 50 Hz. Love channel waves in the frequency band from 120 Hz to 170 Hz can not only reflect the large response range of group velocity, but also take into account the energy of channel waves received in different locations.The dominant frequency band for Love channel waves are selected for group velocity tomography.The third order polynomial is used to fit the correlation between the exposed coal thickness and the group velocity, so as to inverse the coal thickness distribution of the working face. However, the group velocity in this frequency band changes slightly when the thickness of coal seam is 4-6 m. The wider the filtering frequency band is, the weaker the resolution of coal seam thickness is. Therefore, it is necessary to carry out research on the most effective frequency band and the best bandwidth to improve the interpretation accuracy.