Geophysical well logs for evaluating seawater intrusion of western coast in Jeju island, Korea

Jeju, known as one of the best resort areas in Korea, is a volcanic island, and consists of extensive basaltic lava flows, hundreds of scoria cones, and hydrovolcanic eruptions. Since Jeju has mainly supplied the demand for groundwater itself, Jeju Province has enacted a law for the development and management sustainable groundwater resources. There are many monitoring wells operated for seawater intrusion problems in coastal area. Various geophysical well loggings, including radioactive logs, electrical logs (or induction logs), caliper logs, and fluid temperature/conductivity logs, have been conducted in 13 monitoring wells to interpret geological sequences and seawater intrusion in western coasts.
We concluded that seawater intrusion of the western coasts is mainly influenced by geological heterogeneity. And this phenomenon is distinguished from the characteristics of eastern area where seawater intrusion happens over the side area. The typical results for each region locating boreholes are as follows. To boreholes, Hanrim #1 and #2 located in the northwest of Jeju, the change of temperature/conductivity log according to the depth, that is a good indicator of fluid flow within borehole, is mainly involved in boundaries of lava flow unit within Trachybasalt more than stratigraphic boundaries. Note that the change of temperature/conductivity log in the eastern area is generally taking place in the lithologic boundaries. As the results of well logs data in the open borehole, Youngsu #1, the range of natural gamma intensity is 26 —30 API. The average density for Trachybasalt is about 2.60 g/cm³, porosity is about 31 LPU, and resistivity is about 308 ohm-m, while in Hyalocastite the average density is about 2.25 g/cm³, porosity is about 49 LPU, and resistivity is about 53 ohm-m. The change of temperature/conductivity log is associated with the layer boundaries in this borehole, Mureung #1. Furthermore, about 120 µS/cm freshwater is measured although this area is 279 meters from the coast. This distribution of fresh main aquifer is resulted from non-permeable volcanic tuff, sedimentary rocks, or Seogwipo-formation existing in the lower depth of Mureung area.