Geomagnetic Transfer Functions in Japan Obtained by First Order Geomagnetic Survey.

Abstract

Repeated measurements of the Earth's time varying magnetic field have been made across Japan by the Geographical Survey Institute. Such measurements are known as a first order geomagnetic survey. There are 105 geomagnetic stations located uniformly across the whole of Japan. Since 1987, triaxial fluxgate magnetometers have been used to obtain one-minute data at almost all stations. From the geomagnetic data, transfer functions were determined for each station. Traditional vertical field transfer functions of wide period range (from 4 min to 128 min) were obtained, making use of the interstation method. Additionally, we calculated transfer functions for the horizontal field. The distribution of the transfer functions can be used to form a reference map of geomagnetic induction in Japan. Moreover, the distribution of the horizontal transfer functions in Japan has been examined for the first time. After eliminating the effect of the sea water using thin-sheet models, large regional anomalies are found to remain. These anomalies are diagnostic of the resistivity structure and show patterns of current channeling in Japan.