Study on influence of collapse columns on refracted waves between coal seam and rocks and corresponding tomography method

After activated in a coal seam， incident waves will produce channel waves and refracted waves when the incident angles are equal to or greater than the critical angel.Refracted waves propagate along interfaces between the coal seam and rocks and frequently re-enter the coal seam to be received by geophones.Different from the refraction waves excited and received on the ground， the refraction waves excited and received in the coal seam have high periodicity and dispersion and are important components of underground seismic wave field， but in-depth research into its capability of detecting abnormal structures is largely absent.In order to test the effectiveness that refracted waves have on detecting collapse columns， we analyzed the production and propagation of the refracted wave， and the changes on velocity and amplitude of the refracted wave when it encounters a collapse column.After simulating the propagation of refracted waves using a model involving collapse columns， we compared and analyzed the different effects from collapse column on the amplitudes of refracted waves and channel waves，then applyed the method of attenuation tomography to refracted waves.We achieved the goal of detecting hidden collapse columns in the work-plane using refracted waves and processed the data from the No.2101 working face from Niangou Mine.Results from processed simulating data and actual data indicates: Both channel waves and refracted waves are recorded in underground seismic data.The velocity of the refracted wave is close to that of surrounding rocks.The amplitude of refracted wave has a smaller ampli-tude compared to channel wave， but it can reverse under some special geological conditions； the anomalous structures such as collapse column which penetrate coal seams and surrounding rocks，will have greater impact on the amplitude of refracted wave than that of channel wave.The tomography of refraction amplitude attenuation coefficient can effectively reflect the actual positions and boundaries of collapse columns.The fact that precision of the tomography of refracted wave is lower than that of channel wave， makes the tomography of refracted wave an effective supplement for the channel wave detecting methods， especially when the channel wave energy could be very low under the special geological condition， the refracted wave can be a substitute for channel wave to detect collapse columns.