The Crosshole Gpr Ahead Prospecting Method And Its Application For Adverse Geology In Tunnel Construction

In recent years,with the increasing scale and difficulty of tunnel construction in China,hazards such as water and mud inrush,instability,landslides and so on are easy to occur.This may lead to great casualties and economic losses.Therefore,it is very important to prospect the rock structure and adverse geology ahead in the tunnel construction for the safety of tunnel construction and the prevention and control of hazards mentioned above.At present,the research on ahead prospecting of tunnels is mainly focused on several methods such as seismic wave ahead prospecting method,transient electromagnetic method and resistivity/induced polarization method.However,these methods have low resolution and difficult to satisfy the fine geological prospecting needs of rock mass structure and adverse geology.The cross-hole radar can use only the ahead geological drilling holes or ahead pre-grouting holes to Implementation fine prospecting between the two holes.What is more,it has merits such as of long prospecting distance,high resolution,water sensitive,etc.,to meet the fine geological prospecting needs of rock mass structure.However,to interpert the prospecting data using the cross-hole radar,inversion and migration imaging must be carried out and the traditional inversion and migration imaging methods based on the ray tracing method have some shortcomings in theory so their effect is not ideal.Thus inversion and migration imaging methods for fine prospecting of the rock mass structure and the adverse geology ahead tunnel face are of need.In view of the above problems,it is proposed to use the cross-hole radar in tunnel ahead prospecting,and the full-wave inversion method and inverse time migration imaging method are considered as the breakthrough point of the system research.For the initial model and non-uniqueness of the traditional full-wave inversion,a full-wave inversion method using cross-hole radar based on Laplace domain initial model and inequality constraint is proposed.At the same time,aiming at solving the problem that traditional inverse time migration imaging is limited for cross-hole radar detection,the results of full-wave inversion are used as the initial model,and the cross-hole detection data are used to synthesize the single-hole reflection data by the interference method.Finally,the full-wave inversion method and inverse time migration imaging method using the cross-hole radar in tunnel construction is formed.Through the numerical simulation,the inversion laws and imaging characteristics of several typical adverse geological bodies are revealed,and the results are validated by engineering applications.The main conclusions of this paper are as follows:(1)Aiming at solving the problems existing in ahead prospecting methods,the method using cross-hole radar for ahead prospecting in tunnels is put forward.This method has advantages that its detection distance is far;its resolution is high;it is sensitive to the water bodies;the permittivity and the conductivity distribution of the geological anomaly can be acquired with the inversion method;and by using the migration imaging method the boundary information of the geological anomalies can be obtained,so the need of fine geological detection of the rock mass structure and adverse geology could be met.(2)In view of the existing problems in full-wave inversion using cross-hole radar,a full-wave inversion method for cross-hole radar based on inversion strategy optimization,initial model optimization and inequality constraint is studied.The conjugate gradient iterative method and the parabolic iterative step are used as the inversion strategy.The initial model uses the Laplace domain full-wave inversion results,and in the process of inversion,inequality constraints are applied to the relative permittivity and conductivity based on the parametric vector method.These methods have improved the initial model and the non-uniqueness problems,and finally improved the full-wave inversion result.(3)In order to solve the problem applying the traditional inverse time migration imaging method to the detection using cross-hole radar,an improved inverse time migration imaging method based on full-wave inversion and WIVS is studied.This method uses the results of full-wave inversion as the initial model of inverse time migration imaging,and uses the wave interferometric virtual source algorithm to synthesize the single-hole reflection data from the cross-hole transmission data.This improves the imaging results when the electromagnetic wave reflection signal is not ideal,and ultimately improves the effect of the inverse time migration imaging.(4)A systematic study on the full-wave inversion and inverse time migration imaging of typical rock mass structures and adverse geologies encountered in tunnel construction is carried out for the problem of fine exploration and interpretation of rock mass structures and adverse geologies ahead the tunnel face.This reveals the response characteristics and imaging regularity of typical rock mass structures and adverse geologies,and establishes the interpretation and identification criterion.In this paper,the geological model of typical adverse geological structures such as lithologic interface,fault fracture area,karst pipeline and fracture network is established in numerical simulations.Horizontal and inclined drilling are used for water-bearing and water-free bodies to carry out systematic study on the full-wave inversion and inverse time migration imaging.This has laid the theoretical foundation for the fine geological prospecting in the actual engineering.Finally,an ahead prospecting method using cross-hole radar is developed for the rock mass structures and adverse geoloiesfine prospecting ahead the tunnel face.It has been successfully verified and applied in practical engineering.