Borehole radar investigations for locating ice ring formed by cryogenic condition in an underground cavern

Abstract

A small underground pilot plant was constructed and operated at Korea Institute of Geoscience and Mineral Resources for investigating the feasibility of a new concept of storing liquefied natural gas (LNG) in a lined hard rock cavern. In the real operation of the pilot plant, liquid nitrogen (LN2) was stored instead of LNG for safety purpose. A drainage system controls the development of an impervious ring of ice formed due to storage of LN2 in extremely low temperature. The ice ring formed around the storage cavern is very important because this can act as the final barrier to secure the LNG storage. We carried out borehole radar surveys to study the applicability of the borehole radar reflection method to locating the ice ring formation. Prior to conducting the fieldwork, we performed numerical experiments of borehole radar reflection survey; through the analysis of the numerical modeled data we obtained the most appropriate interpretation strategies for locating the ice rings. Before and after storing the LN2 in the underground cavern, we conducted borehole radar reflection and crosshole level scanning surveys. The data obtained at the two stages of the surveys were compared in order to identify changes occurring in basement rock after storing the LN2. The interpretation of the borehole radar data clearly showed that the ice rings only developed under and in front of the LNG storage cavern of the pilot plant, but not over it. Through the numerical modeling experiments and field monitoring exercises, we were able to know that borehole radar reflection technique is an effective method for locating ice rings formed in basement rock for storing material of extremely low temperature in an underground cavern.