2D Forward Modeling And Inversion Of TEM Data

Transient electromagnetic method(TEM)is called time domain electromagnetic method and has been developing very rapidly in recent years.Because of its low cost,high efficiency,strong ability to penetrate high-resistance layer,The observed field value is a pure secondary field,little influence of random interference,and simple construction,it has been used and played an important role in metal ore resource exploration,oil and gas exploration,environmental and hydrogeological investigations,and groundwater Assessment and monitoring,mine goaf areas and disaster water detection,geological mapping and other areas.At present,most research on TEM is based on circular loops or magnetic dipole sources.In order to be more in line with the actual situation,rectangular line frame should be considered.To reflect the actual geological conditions more accurately,2D forward modeling and inversion of TEM should take the induced polarization effects into consideration.For the TEM forwards modeling considering the induced polarization effect,there is a lot of research on 1D forward mdeling,and there are very few 2D studies,especially the for the rectangular loop source,which needs further study.In the field of TEM inversion,1D research has been relatively mature.However,there are few studies on 2D and 3D inversion of TEM.Based on the above analysis,this paper carried out the study of 2D inversion without considering the induced polarization effect and 2.5D forward problem of TEM considering the effect of the induced polarization.The emission source uses rectangular loop,and each side is regarded as being composed of a myriad of horizontal electric dipoles,and the background electromagnetic field of the 1D medium of the rectangular emission source is integrated with these electric dipoles.Using the finite element method,we carried out a series of work about the 2D TEM forward problem,including discretizing the study area,linearly interpolation,unit analysis,overall synthesis,total variation,solving the linear algebraic equations and so on.Finally,the secondary electromagnetic field was obtained.The background electromagnetic field plus the secondary electromagnetic field results in the total electromagnetic field.By introducing the Cole-Cole model,2.5D forward modeling of TEM considering the induced polarization effect is realized.The Fortran programming language was used to implement the forward modeling considering the induced polarization effect algorithm and the accuracy of the algorithm was verified by comparison with the existing 1D algorithm of predecessors.A typical 2D theoretical model was designed and the above-mentioned 2D forward modeling program of TEM was applied to compute the responses of three different kinds of working devices commonly used in the TEM method(centroid loop device,dipole loop device,and large loop device).These responses were then compared with the 1D results.The response characteristics of the TEM were analyzed and the influence of the 2D induced electric effect on the TEM data was also discussed.2D inversion uses the Occam strategy of the data space.The adjoint equation method is used to first obtain the sensitivity matrix in frequency-wave number domain.Then the required time domain sensitivity matrix is obtained through two inverse transformations in order to realize the final 2D inversion algorithm of TEM without considering induced polarization effect.The Fortran programming language is used to implement the 2D inversion code for TEM without considering the induced polarization effect.The trial calculation of the synthetic data of the theoretical model shows that the 2D inversion results are basically correct.