Geoelectric Potential Observation
Within the surface layer of the earth, weak electric current, called telluric current, is flowing. It is also called geoelectric potential difference as it is actually measured by electric potential difference between electrodes at different sites.
VAN Method
It is known in laboratory experiments that electric current is generated before fracture in rocks under load. To apply this phenomenon to the "earth" was the motivation of the VAN-method.
Main point that made the VAN method unique compared with conventional methods is their efforts for noise rejection. To discriminate signal from noise, it is necessary to install at each station at least two measuring short dipoles (30~200 m) with different length in two orthogonal orientations (total of at least four dipoles) and at least two long dipoles (~a few to several km). By this dipole arrangement, noise from electrode instability, rain fall and nearby sources can be eliminated. VAN method can be said as a telluric current monitoring method in which noise rejection techniques are systematically employed. Even so, there is still much to be improved for the rejection of noise from far away sources.
Telluric Current (Geoelectric Potential Difference) Measurement
Pb-PbCl2 pipe non-polarizing electrodes (40 cm in length and 3 cm in outer diameter) are buried at 2 m depth. Data are sampled at 10 or 1 sec interval and telemetered once a day to RIKEN IFREQ through public telephone. For long dipole observation, NTT telephone cable facilities are used.