Research On Fine Detection Of Water Conducted Karstic Collapse Column Based On Multi-field Joint Inversion

Mine water disasters induced by collapse columns in coal mines generally have the characteristics of strong concealment,high contingency and great damage and often lead to flooding mine or working face accidents,which is major hidden danger of water inrush that threaten the safety of mine production.As collapse columns is often an isolated geological body and the buried depth is generally large,whose developed strata are Ordovician limestone and its top rock formation.With the depth and intensity of coal mining increases,how to realize the fine exploration of the range and water abundance of small-scale collapse columns has gradually become a technical problem to guarantee the safe mining of coal mine effectively by geophysical methods.At present,the commonly used detection methods of disaster geological body such as coal mine collapse columns include drilling and geophysical method.Drilling is the most direct,but it also has the disadvantages of high cost and "a peephole view".Seismic exploration method in geophysical method is the main method to detect collapse column with high accuracy,but the construction period is long and the cost is large,which is difficult to identify the water abundance of geological bodies.Relatively speaking,the electromagnetic method based on the electrical difference of rock strata has the advantages of large exploration depth and sensitivity to low resistivity geological bodies.However,the resolution of electromagnetic method for geological bodies with small scale and transverse discontinuous is relatively low,and it is difficult to detect the development range of collapse columns accurately.In order to further improve the detection accuracy of Integrated geophysical methods on the boundary range and water abundance of water conducted karstic collapse column,this article combined with transient electromagnetic method(TEM),controlled source audiofrequency magnetotelluric method(CASMT)and seismic exploration,the effect of multi-field joint inversion of electromagnetic field and seismic wave field of waterconducting collapse column is studied.Firstly,2.5D and 3D the transient electromagnetic forward and 3D seismic forward are realized by the finite difference method,and 2.5D and 3D controlled source audiofrequency magnetotelluric method forward are realized by the finite element method.Then,the 2.5D TEM and CSAMT forward modeling program is developed.the calculation accuracy of the electromagnetic method forward modeling programs are verified by using the homogeneous half-space model and H-layered model.Secondly,according to the typical stratigraphic structure and hydrogeological characteristics of Ordos coalfield,the 3D geologic-geophysical model of collapse column with different buried depth,different diameter and different water abundance is established.The electromagnetic response characteristics of TEM and CSAMT of geological models with different scale collapse column and seismic wave field response of different diameter collapse column geological model is summarized.Then,taking the 3D geological model of col lapse column with 50 m diameter,250 m buried depth at the top and full water-bearing water as research object,TEM inversion,CSAMT inversion and joint inversion of pseudo-2D TEM and CSAMT are realized based on the least square method respectively.Through the comparative analysis of the inversion results of various methods,it is found that the joint inversion of electromagnetic methods can significantly improve the resolution capability of collapse column.The pseudo-2D joint inversion of TEM and CSAMT based on adaptive depth matching is proposed.Through numerical experiments,it is verified that the inversion method can further improve the resolution capability of collapse column range and water abundance,which provides reasonable initial model parameters for different inversion methods.Then,the 2.5D inversion objective function with single methods is established by joining the ’ Occamlike ’ regularization term.Through the numerical inversion test of 2.5D TEM and 2.5D CSAMT single method with and without regularization term by the particle swarm optimization algorithm(PSO),the analysis results shows that the addition of regularization term can not only improve the continuity of formation transverse electrical property in resistivity profile,but also has the low resistivity anomaly reflection to collapse column.Then,the objective function is developed by adding "Occam-like" regularization term,and the 2.5D joint inversion of TEM and CSAMT by PSO is carried out.Through the analysis of the position error of collapse column boundary,it is found that the joint inversion of 2.5D TEM and CSAMT has not achieved the purpose of fine detection of the boundary and water abundance of collapse column.Therefore,a seismic exploration method with higher resolution for collapse column is introduced.A variety of seismic attributes are extracted from the seismic prestack Kirchhoff depth migration profile.By analyzing the resolution of different seismic attributes on the top and bottom interface of collapse column,the root mean square(RMS)amplitude attribute is selected to study of multi-field joint inversion.PSO is used to cluster the seismic RMS amplitude attribute,which highlights the stratigraphic interface and the top and bottom interface of the collapse column effectively.The multi-field joint inversion objective function is established based on transient electromagnetic field,controllable source audio magnetotelluric field and seismic wave field.Combined with the clustering RMS amplitude attribute data and electromagnetic method data,the numerical experiment of multi-field joint inversion of collapse column is carried out.To discuss the influence of seismic interpretation accuracy on the joint inversion results,the multi-field joint inversion of the waterconducting collapse column under accurate and inaccurate seismic interpretation results is study.The results show that compared with the joint inversion results of electromagnetic method,it is found that the multi-field joint inversion method can achieve fine interpretation of range and water abundance of collapse column through the complementary advantages of electromagnetic field and seismic wave field.At present,many mining areas have drill logging data,which contains accurate formation electrical information.In order to further improve the detection accuracy of joint inversion for water conducted karstic collapse column covered by aquifer,a multifield joint inversion numerical test based on logging constraint is carried out.At the same time,the numerical experiments of different methods joint inversion of waterconducting collapse column under shallow low resistivity cover are carried out.Compared with the joint inversion results of electromagnetic method,the results show that the multi-field joint inversion method can better explain the range and water abundance of collapse column.In particular,there is a marked improvement in the reflection of water abundance of collapse columns.The results show that the multi-field joint inversion method based on logging constraints can better improve the detection accuracy of the boundary position and water abundance of the collapse column,especially for the identification of its water abundance.To a certain extent,it reduces the adverse effect of the regularization term in the objective function on the resolution of the water abundance of the collapse column and realizes the fine detection of the range and water abundance of the collapse column.Finally,combined with the engineering application of water conducted karstic collapse column by integrated geophysics method detection in Huangyuchuan coal mine of Ordos coalfield,the data are processed by using multi-field joint inversion and multifield joint inversion based on logging constraints.Interpretation based on inversion results and geological data to delineate the range of karstic collapse column and the geological results are verified by drilling.The above research on multi-field joint inversion of collapse column by combining numerical experiments and engineering applications shows that the joint inversion method can reflect the electrical difference between the water conducted karstic collapse column and the surrounding rock clearly,which can realize the fine detection of the range and water abundance of small scale collapse column.