The deep geological structure of Jeju Island deduced from 2-D interpretation of AMT and MT data

Abstract

Two-dimensional audio-frequency magnetotelluric (AMT) and magnetotelluric (MT) surveys have been carried out at the mid-mountain area of the Jeju volcanic island to figure out any possible structures or evidences for remnant deep geothermal energy. Comparison of the two-dimensional inversion results using MT band only and those using both AMT and MT bands said that high frequency information is useful especially when the formation is so resistive that the skin depth at highest frequency of MT band readily reaches to a few hundred meters as in case of Korea. Two-dimensional inversion of MT data along four survey lines surrounding Mt. Halla showed very clear layered structure, which showed very good consistency with the general stratigraphy of the island deduced from numerous drilling results. Topmost basalt layer has resistivity of several hundred ohm-m, and a thick conductive layer having around 10 ohm-m in the depth of a few hundred meters is overlain by the top layer, which can be the unconsolidated sedimentary layer and the marine originated sedimentary layer, and then the basement with welded tuff and granite is followed with resistivity of order of thousands ohm-m. The results also showed a conductive anomaly extending to more than 2 km depth at the central part of all the four survey line, which can possibly be related with old volcanic activities during the formation of Mt. Halla or can be deeply connected fractures. Further geological/ geophysical investigations will be followed.