A modified measurement method to observe the electric earth potential difference is proposed in order to reduce the unnecessary signals induced by artificial and natural earth current sources. With this method, the potential difference is measured as the voltage between two electrodes that are buried in different vertical positions beneath the ground. We evaluated this method at four sites under different measurement conditions and we discuss the characteristic features of the three types of observed signals:
(1) Horizontal earth potential difference signals appear according to the current induced by variations in geomagnetic fields. Vertical signals, however, do not appear because there is no induced vertical current. (2) After heavy rainfall the characteristic quasi-periodic pulse train signal occasionally appears. However, no such signal can be found in the horizontal direction. (3) Pulse signals appear with exactly the same period that could be expected to be caused by artificial current sources from structures and buildings. Consequently, in order to use this vertical observation method effectively, we should choose a location where the ground surface is as level as possible and which is as far as possible from current source points.