Figure 2: Locations of the electromagnetic field observation sites (⚫) in central Japan utilizing borehole antenna 150–1200 m long as the principal sensor. Those sites have been constructed during 1989~1996. Every site measures the vertical component of the electric field. Additionally mutually perpendicular short dipoles of 20~60 m long were used for measurement of horizontal electric component at five sites (Hasaki, Chikura, Koufu, Nagaoka and Sagara) by 2002. And the induction type magnetometers were used at two sites (Nagaoka, and Sagara) by 2002 [25, 28].