| The semi-airborne transient electromagnetic method(SATEM)is an airborne geophysical exploration method that uses ground-based electrical sources or line sources to emit electromagnetic waves and uses unmanned aerial vehicles(UAV)to carry sensors for fast electromagnetic data acquisition.This method combines the advantages of high signal-to-noise ratio and exploration depth of ground-based transient electromagnetic systems,as well as the fast exploration and non-restriction by surface environment of airborne electromagnetic systems.It has become one of the preferred methods for exploration in complex terrain regions.Considering that the current mainstream semi-airborne transient electromagnetic detection technology generally only collects vertical component data,and the processing methods are mainly one-dimensional inversion and imaging,resulting in low imaging resolution of underground structures in complex terrain areas,this paper proposes a multi-component semi-airborne transient electromagnetic three-dimensional forward and inversion method under complex terrain.The main research contents are as follows:1)Based on the vector finite element method,a three-dimensional forward simulation of semi-airborne transient electromagnetic under complex terrain conditions is implemented.In this study,a fine division of the complex terrain geoelectric model is first performed using unstructured tetrahedral elements.Then,the vector finite element method is used to implement the spatial discretization of the governing equations,and the backward Euler method is used to implement the time discretization.Finally,the direct solver MUMPS is used to quickly solve the final forward equation.2)Aiming at the current situation that the mainstream semi-airborne transient electromagnetic system generally only observes the vertical component response and ignores the horizontal component data,which is more sensitive to lateral structures,this study focuses on the comparative analysis of multi-component semi-airborne transient data for the response characteristics of typical anomalous bodies.Firstly,this paper conducts forward simulation and analysis of horizontal surface models respectively,and finds that the horizontal component data are more sensitive to the horizontal changes of the electrical structure than the vertical component data,and have the potential to improve the horizontal resolution of the inversion results.Then,this paper designs a geoelectric model under complex terrain conditions,and the results show that different component data reflect different information of underground anomalies,and comprehensive multi-component data can more comprehensively reflect the information of underground geological bodies.In addition,this paper analyzes the impact of complex medium parameters such as anisotropy and excitation polarization on semi-airborne transient electromagnetic data through typical examples.3)In terms of inversion,this paper establishes an inversion objective function based on the Tikhonov regularization method,and achieves efficient calculation of the gradient of the objective function through the adjoint forward simulation.Finally,the Limited-memory-quasi-Newton method(L-BFGS)is used to implement the threedimensional inversion of semi-airborne electromagnetic data.The inversion results of the model example under undulating terrain conditions show that the method proposed in this paper can overcome the influence of terrain and accurately restore the position and resistivity information of underground anomalies.Compared with the inversion results of single-component data,the characterization of the electrical structure in the inversion results of three-component data is more accurate.Finally,this paper conducts single-component and multi-component inversion of the confirmed blocky sulfide ore model in Voisey’s Bay,Labrador,Canada.By comparing with published data,it is found that the multi-component inversion algorithm proposed in this paper can effectively overcome the disadvantages of traditional single-component inversion and accurately restore the spatial position and morphology of underground ore bodies.In summary,this paper proposes a three-dimensional forward and inverse modeling method for multi-component airborne transient electromagnetic data in complex terrain.Numerical experiments show that this method has significant advantages in both theoretical calculations and practical mineral body models,and is expected to become a promising direction for the future development of airborne transient electromagnetic technology. |
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