Electrical resistivity and MT imaging in the northern-middle part of Eumsung Basin (Cretaceous), Korea

A geophysical survey was undertaken in the Eumsung Basin to delineate the basin geometry. The basin boundaries are identified in electrical dipole-dipole resistivity section as high-resistivity contrast bands. The high resistivities most likely originate from the basement of Jurassic granite and Precambrian gneiss, contrasting with the lower resistivity of the sedimentary sequence (Cretaceous). The boundaries are also identified in the segments of magnetotelluric (MT) resistivity section, which are qualitatively correlated with the results of vertical electrical sounding, very-low-frequency electromagnetic survey, and magnetic field measurement. Three-dimensional geometry of the basin is delineated using the composite resistivity-structure information from three MT lines (two cross-basin E-W lines and an intersecting N-S line). The maximum depth of the basin varies from about 1 km in the northern part to 3 km or more in the middle part. This variation supports the view that the basin experienced pull-apart opening with rapid subsidence of the central blocks and asymmetric cross-basinal extension.