Three-dimensional magnetotelluric surveys for geothermal development in Pohang, Korea

A three-dimensional (3D) magnetotelluric (MT) survey has been carried out to delineate subsurface structures and possible fractures, for development of low-temperature geothermal resources in Pohang, Korea. Quite good quality MT data could be obtained throughout the survey region by locating the remote reference in Kyushu, Japan, which is ~480 km from the centre of the field site.3D modelling and inversion are performed taking into account the sea effect in MT measurements near the seashore. The nearby sea in the Pohang area affects MT data at frequencies below 1 Hz ~0.2 Hz, depending on the distance from the seashore. The most severe sea effects were observed in the south-east parts of the survey area, closer to Youngil Bay. 3D inversion with and without the seawater constraint showed very similar results at shallow depths, roughly down to 2 km. At greater depths, however, a strong sea effect seems to form a fictitious conductive structure in ordinary 3D inversion, especially in the south-eastern part of the survey region.Comparison between drilling results and the resistivity profiles from inversions showed that five layered structures can be distinguished the subsurface beneath the target area. They are: (a) semi-consolidated mudstones with resistivity less than 10 Ω m, which are ~300 m thick in the northern part and ~600 m thick in the southern part of the survey area; (b) occasional occurrence of trachybasalt and lapilli tuff within the mudstone layer has resistivity of a few tens of Ω m, (c) intrusive rhyolite ~400 m thick has resistivity of several hundreds of Ω m, (d) alternating sandstone and mudstone down to 1.5 km depth shows resistivity of ~100 Ω m, (e) a conductive structure was found at a depth of ~3 km, but more geological and geophysical study should be carried out to identify this structure.