Research And Application Of Seismic Imaging Methods For Mine Goaf Detection

There have been a large number of gob areas in different mines in China due to the exploitation of underground mineral resources.The formation of gob areas can lead to changes in the original geological structures and in-situ stress fields.If not handled properly in time,it may cause geological disasters such as ground collapse and ground fissures.Therefore,detecting and imaging the location,buried depth and shape of existing goaf areas and studying the likely mechanisms of surface collapse caused by goaf can provide valuable information for the restoration of gob areas and prevention of the goaf collapse.As the goaf destroys the original stratum and the surrounding geological environment is complex,it is challenging for the application of geophysical exploration methods in the gob areas.Conventional detection methods have high requirements for stie conditions and have poor detection resolutions.In recent years,seismic imaging technology has been rapidly developed and widely used in underground structure imaging.Surface wave method based on active seismic sources and microtremor method based on ambient noise sources have been attempted in the goaf detection.Howeveer,seismic imaging technologies such as ambient noise tomography and microseismic travel time tomography are rarely used in the goaf detection and imaging.Therefore,it is of great significance to systematically study the applications of seismic imaging methods in the goaf detection for different mines.This thesis mainly studies the application of seismic imaging technologies in goaf detection,such as microtremor imaging based on the ESPAC method of linear array,ambient noise tomography and microseismic travel time tomography based on a dense array,active seismic source surface wave imaging based on phase shift method and Radon transform method.In order to test the effectiveness of seismic imaging methods in the goaf exploration,we have systematically applied various seismic imaging methods in three different mines including Zhongjiu iron mine in Ma’anshan City,Dongxing salt mine in Dingyuan City and Fengtai Phosphate Mine in Fengtai City.Zhongjiu iron mine in the Ma’anshan city is a typical metal mine.The surrounding rock of the goaf is magmatic rock.For the first time,both the extended spatial autocorrelation(ESPAC)imaging technology and station autocorrelation method along the linear dense array have been jointly applied to detect the abandoned tunnels.The results show that the ESPAC method and station autocorrelation method using the linear array is effective in the shallow goaf detection.Dongxing salt mine in Dingyuan is a typical non-metallic mine.Although the surrounding rock of the goaf is layered sedimentary rocks,the structure is complex.We have deployed about 120 stations covering an area of 3 km by 3 km for one month for imaging cavities formed by solution salt mining.We first carried out two-station ambient noise tomography.The results show that the ambient noise tomography based on this dense array can detect cavities due to solution salt mining.At the same time,we also make the best use of the microseismic events induced by the development of Dingyuan salt mine recorded by the dense array.These recorded data are processed by deep learning based methods including the CNNDetector for detecting seismic events,PhaseNet for picking the first arrivals,and Rapid Earthquake Association and Location(REAL)for the phase association and initial event locations.Finally,double-difference seismic tomography(tomoDD)is used for event relocation and Vp tomography.The velocity models around the Rt2 solution cavity from ambient noise tomography and microseismic travel time tomography are compared and analyzed with three-dimensional seismic exploration results.Fengtai phosphate mine is also a non-metallic mine,and the surrounding rock of the goaf is massive sedimentary rocks.By making full use of the shot data collected during the previous two-dimensional seismic exploration,the active source surface wave imaging technology based on phase shift method and Radon transform method is studied for detecting the goaf.At the same time,we also deploy a linear dense array along the same active source survey line in order to compare the active and passive seismic imaging methods.In addition,we also compare and analyze seismic imaging results with the high-density electrical method.It is found that passive seismic imaging method is preferred for the imaging the goaf in the Fengtai phosphate mine.The research of three typical goaf detection shows that with abundant highfrequency ambient noise,by deploying a linear array and collecting several hours of ambient noise data,the imaging based on ESPAC method and station autocorrelation method can be used to detect the goaf at the depth of 0-200 m.For mines in production with complex geological conditions,a dense array can be arranged to collect ambient noise data and microseismic data for about one month,make full use of the data,and carry out ambient noise surface wave travel time tomography,microseismic travel time tomography,microtremor imaging and other work at the same time.The imaging results of dense array show that the surface wave travel time tomography results of ambient noise are better than those of microseismic travel time tomography and microtremor imaging.The surface wave imaging results based on active and passive sources show that in the case of abundant high-frequency noise sources,the surface wave imaging results of passive sources are generally better than those of active sources.In the case of poor high-frequency noise sources,the goaf detection at the depth of 0-100 m can be carried out through active source surface wave imaging based on phase-shift method and Radon transform method.From the applications of various seismic imaging methods in these different mines,it can be seen that seismic imaging based on ambient noise is an effective method to detect the mine goaf.Finally,the likely mechanisms for surface subsidence induced by mining in Dingyuan salt mine and Fengtai phosphate mine are also proposed.According to the multi-stage three-dimensional seismic exploration data,microseismic monitoring data,surface subsidence and passive seismic imaging data for the Dingyuan salt mine,the development stages for the solution cavity responsible for the surface collapse are described and analyzed in detail.For different stages,the rising rate of cavity top,characteristics of microseismity and surface deformation are analyzed,which can provide valuable information for the early warning of ground collapse and mine safety production.For the Fengtai phosphate mine,according to the geological,geophysical and drilling results,we also analyze the causes of surface collapse induced by mining,which can provide support for the restoration and treatment of the gob areas.