1997 年 49 巻 11-12 号 p. 1697-1715
MT and AMT surveys were conducted in the Minami-Kayabe area, southern Hokkaido, over a period of about two months in the fall of 1988. In this survey, 161 sites were arranged in a square lattice shape. FFT techniques were used in the initial data processing in 1998. Here we describe improved data processing using the cascade decimation technique. Data were processed efficiently by constructing a system using coherency and remote-reference. More than 10 sets of threshold values for the coherencies of (Ex, Hy) and (Ey, Hx) were then determined. Power spectra were sorted by these threshold values and then stacked. Satisfactory results were obtained for the frequency range from 3.4 Hz to 250 Hz. At frequencies below 3.4 Hz, however, the result of cascade decimation was not satisfactory because a sufficient number of stacks could not be obtained due to high local noise and limited measurement time.