A Study Of Forward And Inversion For Frequency-domain Airborne Electromagnetic Method In Terms Of Permeability And Resistivity

In recent years, Frequency-domain Airborne Electromagnetic method (AEM) has already been widely used in electrical work at home and abroad. However, there are always some controversies on Frequency-domain AEM for the mapping of apparent resistivity in magnetic regions. The influence of magnetosphere is conventionally negligible. Moreover, permeability of vacuum is applied in the model instead of dielectric permeability. Although the method does not seriously influence apparent resistivity mapping in low magnetic regions, overlarge permeability give rises to high calculated resistivity and thus affect the precision of apparent resistivity mapping due to overlapping of displacement current and conduction current. As a result, it is very necessary to study the Frequency-domain AEM under magnetic condition.The article aims to realize the forward calculation of Frequency-domain AEM in terms of ferromagnetism in magnetic regions and magnetic variables of targets. In addition, a new method will be provided for apparent resistivity mapping in ferromagnetic regions, in which the permeability of target mediums is to be calculated through approximate inversion method when the resistivity inversion will be finished for multi-frequency airborne system under magnetic conditions.The typical research work and findings are covered.Firstly, it introduces recent development of Frequency-domain AEM, briefly describes its fundamentals and further studies basic magnetic theory.Secondly, the magnetic response of orthogonal dipoles, finite dipole sources and horizontal large-loop line source are discussed on the premises of magnetic response of finite elements on the laminar semi-space derived by M.N.Nabighian. Moreover the equation on the frequency-domain airborne magnetic field above the circular large-loop and laminar ground is further derived.Thirdly, the analytic solutions on the orthogonal field above such a ground are further discussed. The kernel function is forced to convergent by adding one item and subtracting another item, after which the characteristics of the reflection coefficient is analyzed. In addition, the numerical integration method and linear filter method to calculate the Hankel transform are compared and carried out respectively in the homogenous semi-space model.Fourthly, the forward calculation is systemically investigated by discussing magnetic responses in different frequency, resistivity, permeability, flight height, sending and receiving distance.It is found that the relative permeability seriously affects real component of normalized secondary field in significant relativity. However imaginary component is almost no related to relative permeability. The magnetic response in variable resistivity and frequency is the same once the induction constant is the same in the condition that other parameters are stable. In addition, the real and imaginary components are exponentially decreased along with an increase in flight height. Moreover, an increase in the sending and receiving distances gives rises to the strengthening of magnetic response.Fifthly, the three-dimensional vector graphics is drawn out and analyzed by involving the calculations of permeability variables into conventional two-dimensional vectorgraph.Sixthly, it provides a new method to calculate the apparent resistivity and permeability of homogenous magnetic half-space. After neighboring low frequency pair is randomly selected, the apparent permeability is estimated firstly under low induction from the real and imaginary components of magnetic response in relatively low frequency. Subsequently the apparent resistivity is obtained if imaginary component is given. The calculation method has removed the influences of high magnetic field on the transformation of apparent resistivity.Seventhly, calculation about the actual AEM data of a test area shows that the permeability-resistivity approximate inversion method is fast, reliable, and this algorithm is an effective exploration method for AEM apparent resistivity mapping at the strong magnetic regions. .In conclusion, the article has performed the calculation of orthogonal magnetic field theory in terms of Frequency-domain AEM for the laminar loop-line ground. Forward and inversion of permeability and resistivity is carried out under magnetic conditions. It provides a more accurate method for resistivity mapping in magnetic regions.