AN INVESTIGATION OF THE RESISTIVITY STRUCTURE BENEATH OKI-DOGO ISLAND

Extremely Low Frequency (ELF) and Very Low Frequency (VLF) Magnetotell uric soundings were conducted at fourteen sites on the Oki-Dogo Island in the Japan sea. The Oki-Dogo Island which was formed during the Mio cene volcanic activity, mainly consists of volcanic and metamorphic ro cks. Electrical resistivity structures revealed by the magnetotelluric (MT) method well coincide with the classifications of the surface geo logy. The magnetotelluric investigation of the island also suggests th at a clear resistivity contrast exists between the eastern and western parts of the island within a few kilometers in depth. In the western part of the island, a relatively high conductive layer of less than 20 ohm-m is detected at a depth of 200 m. This layer is interpreted as b eing a ''water-aquifer''. In the eastern part of the island, a relativ ely resistive layer exists from the surface to 2 km in depth and this resistive layer is thought to relate to past volcanic activities. The resistivity contrast is also confirmed by the results of Schlumberger resistivity soundings.