ANALYSES OF MAGNETOTELLURIC FIELDS FOR A 3-DIMENSIONAL EARTH ON THE BASIS OF TRANSFER-FUNCTIONS

In three-dimensional geologic circumstances, MT information in terms of the electric and magnetic fields themselves are more useful for subsurface conductivity imaging rather than impedance tensors. We propose a method in which MT fields at all stations can be treated, as if they were measured simultaneously. We applied the code developed for this algorithm to field data and examined its validity. It is preferable that electric and magnetic fields are measured simultaneously at all MT sites. In practice this is almost impossible. If the magnetic field is constant everywhere at the Earth's surface, as in the case of any layered (one-dimensional) structures, or the TM-mode in a two-dimensional structure, the electric field can be obtained by simply multiplying the MT impedance by the constant magnetic field. However, the magnetic field for the TE-mode is not constant. In a more general three-dimensional structure, we can no longer expect a constant magnetic field. A practical means of obtaining the fields which we need is to set up a reference point and derive magnetic field transfer functions between the reference and each individual MT site. The magnetic fields at all the sites can then be simulated through the transfer functions thus derived. The electric field can be finally computed by multiplying the MT impedance at respective sites by the derived magnetic field. We made both the model and case studies and found that the present method works as we presumed.