oa Subsurface imaging with EM migration of magnetic fields

In the magnetotelluric (MT) method, underground impedance profile is estimated through the observation and the processing of both electric and magnetic components. However, there are mainly two problems in the observation of electric component. One is that the effect of the static shift biases the apparent resistivity at the surface, and the other is the difficulty in the observation of electric field at topographically planar surface suitable for the assumption of place wave incidence. Coupled with these known observation problems, the analysis of electromagnetic data is costly in the computation, especially in 3D studies. In this study, we propose a new method to use magnetic components at the surface in order to reduce the burden of computation that may help to cope with these problems. We first adopted a migration method which is usually used in seismic data processing and applied this method to the magnetic field observed at the surface. If magnetic field is used for the analysis, we could control the static shift that appears in electric field. Second, the mobility of magnetometers makes it possible to obtain field data much easier even at non-planar surface. Also, a migration method leads us to reduce the computational costs. Through the numerical calculation analyses, it was confirmed that the application of the proposed migration method to magnetic component is effective and efficient to process data. In this migration method, the larger value of reflection intensity is estimated around correct area if the proper resistivity structure is assigned to the migration. This means that the resistivity structures in the subsurface have an important role in the migration results. The combination of this electromagnetic migration method and inversion has the possibility to reveal more detailed subsurface resistivity structure.