Geothermal Exploration in Ulleung Island, Korea

Abstract

An integrated geothermal exploration has been performed in a volcanic island, Ulleung Island, to investigate geothermal potential. It includes a 3D MT survey to delineate the subsurface electrical structures, magnetic data analysis, and drilling two 1-km deep coring boreholes from north and south part of the island to measure the temperature and physical properties of rock cores at each depths and formations. Various well logs including temperature, caliper, gamma ray, and resistivity log have been acquired at the two boreholes to get the geothermal gradients from temperature profiles and physical properties of the formation at depths. A 3D MT survey has also been carried out to delineate subsurface structures and possible deep-seated fractures in the island. A 3-D inversion with sea-effect constraint gave reliable results and showed clear E-W trend of conductive anomaly along the connection line between the test boreholes GH-1 and GH-2, where it showed very high geothermal gradients of about 100 °C/km. Comparing with the gradients of the other two boreholes in northern and southern part of the island being lower than 80 °C/km, one can deduce that the deep circulation of convective fluid through the deeply connected fractures, say E-W trends conductive anomaly, carries the heat from the deep to near the surface. This can explain the big gradient differences from site to site in this small island.