Study Of Three-dimensional Forward Modeling And Fast Imaging Of Semi-airborne Frequency Tipper Sounding Method

The semi-airborne frequency domain electromagnetic method measure high-precision magnetic field signal which excited by high-power electrical or magnetic source on the ground using the flight platform.The method has the advantages of large depth measurement on the ground and rapid measurement in the air,and can be applied to large depth exploration in complex terrain conditions such as mountains,deserts and forests.The semi-airborne frequency domain electromagnetic method can obtain more abundant and effective underground information with the maturity and popularization of airborne three-component magnetic field technology,thus further improve the resolution and application effect.Therefore,a semi-airborne frequency tipper sounding method(SAFTEM)is proposed,which is defined by refer to the magnetotelluric tipper.Based on one-dimensional and dimensional forward response,the effectiveness and feasibility of the method are discussed,and a fast imaging algorithm is developed,laying foundation for the application and promotion of this method.Firstly,the principle and working semi-airborne frequency tipper sounding method are systematically described.The expressions of semi-airborne frequency tipper for uniform half space and layered media models are derived from the electric dipole electromagnetic field.Based on this,the one-dimensional forward algorithm of semi-airborne frequency tipper sounding method is implemented.The effect of frequency,offset and flight height under excitation of grounded long wire source is summarized of semi-airborne frequency tipper sounding method by one-dimensional forward of layered model.After that,the finite volume algorithm is used to solve the coupled symmetric equations of magnetic vector potential A and scalar potential φ under Lorenz norm conditions,and the Jacobi iterative QMR algorithm is used to solve large linear equations,the three-dimensional forward modeling of semiairborne frequency tipper sounding mehod is realized.The accuracy of the three-dimensional forward algorithm is verified by compared with the one-dimensional forward modeling results of semi-airborne frequency domain electromagnetic method.The forward modeling results of typical three-dimensional geoelectric model show that the frequency tipper can effectively reflect the transverse electrical properties of underground media.Compared with the three-component response anomaly of magnetic field,the results show that the tipper response is less affected by the shadow effect than the magnetic field component,and can accurately identify the boundary position of the three-dimensional anomaly.It is proved by the forward modeling of different source morphology and orientation that the reasonable arrangement of multiple sources can enhance the signal strength and highlight the response caused by the electrical change of abnormal boundary.In order to improve the practicability of the semi-airborne frequency tipper sounding method.The full-zone apparent resistivity of the semi-airborne frequency tipper sounding method is defined,and calculation full-zone apparent resistivity of different tipper components is realized by dichotomy iterative technique.The fast imaging is realized by combining with the deduced formula for calculating effective skin depth of the semi-airborne frequency tipper sounding method.The imaging results of geoelectric model are presented,and the accuracy and effectiveness of the imaging algorithm are verified by one-dimensional and three-dimensional abnormal body model tests.Therefore,the algorithm and results provide a basis for the design of working method and data processing and interpretation of semi-airborne frequency tipper sounding method.This method can effectively improve the exploration accuracy and effect of the semi-airborne frequency domain electromagnetic method.