Long Wire Source Transient Electromagnetic Constrained Inversion

Transient electromagnetic sounding method is commonly used for dipole source or return line source.It is difficult to arrange the launch device due to the influence of supply current and field observation conditions.At the same time,the signal generated by the electric dipole source is weak,which is not conducive to the signal acquisition.Therefore,it is thought that the electric dipole is integrated into the line source along the vertical observation direction,which not only enhances the signal-to-noise ratio of the observed signal,but also Solving the problem of terrain factors causing difficulty in the placement of the launching device.We call the device that emits a long wire as a long wire source transient electromagnetic method.The use of long electric wires as a source of emission can overcome the influence of terrain and facilitate the development of work.The longwire source transient electromagnetic attenuation process is generally divided into two stages,early and late.The early electromagnetic field is equivalent to the high-frequency component in the frequency domain,and the attenuation is fast and the skin depth is small.The late component is equivalent to the frequency domain.The low frequency component has a slow decay and a large skin depth.Starting from the Maxwell wave equation,the forward calculation formula of the horizontal electric dipole source is obtained by combining the boundary conditions and the vector bits.On this basis,the outer layer integration of the dipole source forward calculation formula is obtained,and the long wire source transient electromagnetic method is obtained.The numerical calculation formula of forward modeling,and then using the Hankel transformation proposed by Guptasarma to solve the numerical calculation results of long-wire source transient electromagnetic.Based on the calculation formula of forward response,the variation trend of geoelectric parameters and device parameters is analyzed.The influence of forward-time wire source transient electromagnetic method forward on the sensitivity,the influence of surface resistivity change on sensitivity is calculated,which provides a theoretical basis for the discussion of constrained inversion..A transient electromagnetic one-dimensional inversion method based on shallow electrical parameter constraints is proposed.In the Lagrangian multiplier search process of the inversion algorithm,the one-dimensional linear search method is used to update the step size,which effectively improves the computational complexity of the OCCAM inversion method.In terms of the constraints of the inversion model,the shallow electrical parameters of the model are hard-bound based on the prior geological information,and the correctness of the deep electrical structure correction during the iterative process is ensured.Inversion trial calculations were carried out on several sets of theoretical models,and the constrained inversion method was applied to the actual data processing of a graphite ore exploration,and a good inversion effect was obtained.The results of the above-mentioned inversion trial calculations show that the imaging effect of the deep stratum structure in the inversion model can be effectively improved by constraining the shallow ground layer electrical parameters.This is of great theoretical significance and practical application value for improving the resolution of the long-wire source transient electromagnetic method to the target layer(especially the low-resistance thin layer).