Magnetic Imaging of Ultramafic Bodies on the Site of the Ohi Nuclear Power Station, Central Japan

Abstract

The Ohi nuclear power station is located at the northern Oshima Peninsula in the Wakasa Bay on the coast of Japan Sea, central Japan. The geology of the site of the power station is composed mainly of shales, diabases, gabbros and ultramafic rocks of the Palaeozoic Yakuno Ophiolite. Ultramafic rock is a key geology since fracture zones in the study area can be found only in the ultramafic bodies. To map the distribution of ultramafic bodies, we conducted magnetic surveys on ground and at sea around Daibahama beach. A ground magnetic survey was carried out on a grid and along specified lines on a small peninsula and some reefs by using a proton magnetometer. A seaborne magnetic survey was also conducted by a small rubber boat on which a Cesium magnetometer was mounted. Both measured data were merged and IGRF residual magnetic anomalies were reduced onto a smoothed surface at an altitude of 2.5 m above ground and above sea level at sea assuming equivalent anomalies below the observation surface. 3D magnetic imaging has been applied to the magnetic anomalies and the resultant magnetic structure is generally associated with a dipping-dike by a previous 2D modelling. A reversely magnetized body was imaged with a seaward dip below the surface along the 2D profile but has a horizontal limitation. This means the magnetic imaging is helpful to reveal the three-dimensional subsurface structure of the area.