3D Numerical Modelingand Inversion For Transient Electromagnetic Field Excited By Source Loop Based On Vector Finite Element Method

Transient electromagnetic method(TEM)is a geophysical electromagnetic(EM)exploration method.TEM uses a transmitter to motivate EM fields and receives induced magnetic fields.The transmitter source can be a loop or a long wire.A long wire or big loop is used for detecting deep target body,and a small loop is used for detecting shallow target body.TEM measures time-domain EM fields,and the vertrical component is the commonest signal in the method.TEM inverse use the measured fields to deduce geo-electrical structure and is the most used method for interpretation.TEM is widely implemented in the fields of hydrogeophysics and engineering geology,the requirement of detailed detection is increasing rapidly.1D TEM inverse can not be satisfied the requirement as a simple imterpreation method.Therefore,implementing a feasible 3D TEM inverse flow is at all imminent.At present,the 3D TEM inverse method is unstable and inefficient.The method has not been implemented in the application of geophysical EM exploration.To obtain a stable and fast 3D TEM inverse method,this study have carried out TEM forword modling with vector finite element method for rugged topography,and have developed a 3D TEM inverse program.Then,this study used the 3D TEM inverse program to survey 3D structure groundwater systems in a debris landslide.The main contents are as follows:(1)3D TEM forward modling for rugged topographyFirst,this study used vector finit element method with arbitraryhexahedral mesh to simulate the vector wave equation with electric fields.Isoparametric transfer is used to compute the integration of shape function in a vector element.The system equation is built with vector finit element method.Second,hankel numerical filtter is used to compute Bessel function integration in frequency-domain fields and transform the frequency-domain fields to time-domain.Third,a fast correction has been discussed for the problem of ramp waveform.Last,referring to the parameters of time-domain EM data acquisition system Spectrem 2000,3D TEM forward modling is simulated and the result is verified with comparing with the result of LokiAir program.(2)Solving the EM system equationFirst,referring to the parameters of frequency-domain EM data acquisition system Dighem,regarding complexity of geo-electrical model and position of transmitter as variables,the influence of geo-electrical structure and the layout of transmitter and receiver with three reistivity models were analised.It shows that,the complex geo-electrical model have heavy influence on the solving process and can change convergence in the iterative soling method.Different attitude angle and postion of transmitter only have limited influcen on the solving process.Second,this study have putted forward an induced dimension reduction with s dimension(IDR(s))which is a krylov short recursive iterative method for the solving EM finite element system eqaution.How to set the parameters in IDR(s)has been discussed detailedly.It shows that,the IDR(s)has same ability to get a good convergence result with comparing BiCGSTAB,and adjusting the dimimension s and initial shadow vector p can improve the convergence effect.(3)Numerical simulating the 3D TEM inverseFirst,this study has specificated how to compute the roughness matrix,inverse of Hessian matrix and sensitivity matrix in updating predictive model.Second,quisi-3D TEM inverse has beenanalied with seafloor massive sulfide(SMS)model generated by Trans-Atlantic geotraverse(TAG)hydrothermal fluid system.The strategy of TEM inverse is verified.Third,six geo-electrical model were used to present the ability which TEM inverse can image the presuppose model exactly in 2D profiles.Last,referring to the parameters of frequency-domain EM data acquisition system Dighem and time-domain EM data acquisition system VTEM,the advantage of TEM 3D inverse have been summarizedby comparing the frequency-domain 3D inverse with the time-domain 3D inverse.TEM responses which contains richer information about geo-electrical structure than the frequency EM method containscan be used to obtain accurate geo-electrical structure.(4)Surveying structure of groudwater in debris landslide with TEMA TEM survey was carried out on a debris landslide.This study has confirmed deep drainage fissuresinferred by previous studies,3D seepage drainage systemwhich has significant influence on the stability of the landslide in the gravel soil has been delineated,and the treatment effects of a drainage ditch were evaluated by the hydrogeological structure deduced by this method.