An Application of the Magnetotelluric Method to the Analysis of the Geoelectric Field Observed by Using Telegraphic Facilities

Abstract

   Impedance tensors for periods of 5.3 to 1920 minutes were estimated by making use of the stable data of the geoelectric field over a long range of time and the geomagnetic variations due to a large-scale magnetic storm. The wavelength of geomagnetic variations used as the inducing field in applying the magnetotelluric method is very long. Multiple coherencies between each component of the geoelectric field and both N-S and E-W components of the geomagnetic field are high; coherencies between the two horizontal components of the geomagnetic field are low. Therefore, estimated impedance tensors are reliable. Since skewnesses are not so high, the subterranean resistivity structure beneath the observation area can be regarded to be two-dimensional and coordinate transformation was performed referring to the principal axes calculated from impedance tensors. Apparent resistivities obtained from transformed data for both H- and E-polarizations are different by a factor of about 100.