Geoelectrical structure of Ulleung Island, Korea: interpretation of three-dimensional magnetotelluric data

A three-dimensional (3D) magnetotelluric (MT) survey was carried out to delineate subsurface structures and possible fractures for geothermal development around Ulleung Island, Korea. Good-quality MT data were obtained throughout the survey region with respect to a remote reference located in Sawauchi, Japan, similar to 900 km from the centre of the field site. 3D modelling and inversion were then performed, considering the sea effect on MT measurements near the coastline. The modelling results showed that seawater surrounding Ulleung Island affected MT data at frequencies below 10 Hz, depending on the distance from the shore. Based on this information, two types of 3D MT data inversion were performed: ordinary 3D inversion and 3D inversion using sea effects as a constraint. 3D inversion using the sea-effect constraint gave more reasonable and reliable results. In addition, the inversion that accounted for sea effects indicated a clear east-west conductive anomaly along the line connecting test boreholes GH-1 and GH-2; a high geothermal gradient of similar to 100 degrees C/km was recorded. The geothermal gradients were lower, < 80 degrees C/km, at boreholes located in the southern (GH-3) and northern (GH-4) regions of the study area. These results suggest that deep circulation of fluid through deeply connected fractures, e.g. the anomaly observed in the east-west direction, carries heat from the bottom depths to near-surface layers. This would explain the large differences in gradients among sites in the study area.