The Jeju dataset: Three-dimensional interpretation of MT data from mid-mountain area of Jeju Island, Korea

Magnetotelluric (MT) surveys have been performed along five lines in the Jeju volcanic island located at South Sea of Korea. The purposes of the MT surveys are to see if there still remains thermal regime and if there exist deeply extended fractures or an aquifer system beneath the mid-mountain region of Mt. Halla in Jeju. By performing Audio-frequency MT (AMT) surveys together with MT surveys at each site and by operating far-remote reference site of approximately 480 km apart, we could get a broadband (10− 4-104 Hz) MT dataset of totally 108 sites with very good quality. 3D inversion of MT data from 88 measurement sites gives a reasonable estimation of the subsurface structures beneath the island. An induction vector analysis and 3D inversion results commonly indicate the existence of a conductive anomaly extended down to a few-kilometer depth beneath the central part of the island, which can be related to ancient volcanic activities. Because the island is surrounded by electrically conductive sea and has steep topographic variation, measured MT data in terms of impedance is seriously biased by distortion of electric fields and is suffered from the static shifts. The Jeju MT dataset is valuable for the research purpose in the sense that the dataset contains so many problems, some of which can frequently be observed in every field data, and that the geology of Jeju Island is relatively simple.