Comparative Analysis Of Forward Numerical Simulation And Inversion Effect Of Magnetotelluric

Magnetotelluric sounding is a geophysical exploration method.It uses natural alternating electromagnetic fields to study the earth's electrical structure.Because it does not need to manually establish a field source,it has low cost,is convenient to work,and is not shielded by high-resistance layers.The layer resolution is high,and the depth of exploration is only related to the frequency of the electric field.The shallow part can be detected within tens of meters,and the deep part can detect hundreds of kilometers.Therefore,it is widely used in both prospecting and water exploration and engineering survey.However,there is often no ideal condition for data collection in the field.The electrical inhomogeneity of the underground formation and the fluctuation of the terrain of the work area will have an impact on the detected data.Therefore,it is important to understand the different structures and the influence of terrain on the magnetotelluric data,and the respective suitable inversion methods to improve the efficiency of our actual work.This paper is based on the theoretical basis of magnetotelluric and two-dimensional forward modeling theory and four common inversion methods,including Bostick inversion,OCCAM inversion,RRI inversion and NLCG inversion,and summarizes the four inversion methods.Advantages and disadvantages.By establishing different models of faults,folds and topography,the obtained forward results are analyzed,and then the forward results are Bostick inversion,OCCAM inversion,RRI inversion and NLCG inversion.The effect is compared and the inversion result closest to the model is obtained.According to the analysis results,for the forward simulation results,the stratification of the impedance phase is better than the resistivity,the TE polarization mode has higher resolution for the vertical direction,and the TM polarization mode has higher resolution for the lateral direction;For the terrain,both polarization modes are affected,but the terrain has little effect on the TE polarization mode,while the polarization mode of the TM is very large,and the low frequency part has a relatively large influence on the resistivity.Where the change occurs,there will be a sudden change in the resistivity contour,and the place where the terrain rises is more affected than the terrain.The one-dimensional inversion result of OCCAM inversion is far from the actual situation.Especially,OCCAM is often smoothed out due to its smooth constraint,which is often inconsistent with the actual situation.Therefore,this method is suitable as the initial model of NLCG;Bstick The inversion results are similar to the RRI inversion results,but Bostick is much faster than the RRI inversion as a one-dimensional inversion method;for the model built,it is the best choice to get NLCG,but different initial models will have some The effect,but the difference is not big,and the choice of polarization mode is very important.Since the TM polarization mode is affected by the terrain,the data of the TE polarization mode is basically closer to the real model.Therefore,in the process of daily processing of actual data,Bostick one-dimensional inversion can be used as a reference,and the inversion results of NLCG are mainly used to analyze the underground situation.Finally,an example is used to compare the four inversion methods.The example is mainly to study the deep structure for the later geothermal exploration.The comparison of the inversion results of the example is better.The effect of the inversion result of the initial model is the Bostick inversion result..