Characterization of Coastal Aquifers in Jeju Volcanic Island Using Geophysical Well Logging and Borehole Temperature Monitoring

Abstract

As part of an effort to characterize coastal aquifers, geophysical well logging and borehole temperature monitoring were applied systematically to the coastal area of Jeju Island, Korea. A variety of well loggings were performed to understand the subsurface geological features and groundwater flow in the local area. The geophysical well log responses were sensitive to several stratigraphic characteristics in basalt, and statistical analysis of natural gamma logs provided useful data for classifying geological characteristics. The geophysical well log responses and physical properties allowed for characterization of the subsurface strata, and in particular, the spatial distribution of the porous and permeable layer was delineated to estimate the main pathways of groundwater flow. In addition, the temperature and fluid electrical conductivity logs provided more specific results for interpreting the groundwater flow patterns. For the purpose of understanding the characteristics of coastal aquifers, geophysical well logging and borehole temperature monitoring were undertaken on the eastern coast of Jeju Island. Geophysical well logging, especially electrical conductivity and temperature logs, determined spatial variations in the distributions of fresh water and salt water. Borehole temperature monitoring enabled us to characterize the variability of coastal aquifers at a high temporal and spatial resolution, and furthermore, heterogeneous aquifer systems and the behavior of fresh water and salt water could be estimated. A conceptual model of seawater intrusion in the eastern part of Jeju Island was proposed based on interpretation of the geophysical well logging and borehole temperature monitoring data. Freshwater-saltwater interfaces and main aquifers primarily formed the geological boundaries, implying that the coastal aquifers flow through the boundary between each volcanic formation. Consequently, geophysical well logging and borehole temperature monitoring could enhance knowledge of the subsurface structure of basalt and interactions of fresh water and salt water in coastal areas. This approach is expected to increase our understanding of and ability to manage groundwater systems of a volcanic island.