The geological structure of Yaimagahara (Narusawa village in Prefecture Yamanashi, northern foot of Mt. Fuji) was investigated by means of VLF-MT method. This area is characterized by volcanic ejecta (basaltic lava flow, tuff breccia, scoria and so on) resulted from the eruption of Mt. Fuji volcano.
VLF-MT measurements were made along 17 lines using 50 m station interval. Apparent resistivity and phase angle between magnetic and electric field were measured at each station. When interpreting the structure by two-layer model, evaluation of the resistivity of the top layer p1 must be done. This evaluation was done in following manner:(1) Value for apparent resistivity at station where value for phase angle is between 41-49 degree is regarded as value for p1, and this value is used as pi near this station.(2) In the area where no station that have a value for phase angle 41-49 degree is present, based on the known geology (boring data, outcrop and so on), the value for pi is estimated from the geological column.
The result of survey indicated that the VLF-MT method is useful for estimation of structure in volcanic area where no outcrops are present.