The formation of lava tubes is one of the most significant factors controlling the emplacement of lava flows. However, extents and structures of lava tubes are typically not precisely known due to the difficulty in finding lava tubes in the field. We developed a new stepped‐frequency ground penetrating radar (GPR) system with shielded antennas, which allows measurements that have both high spatial resolution and large penetration depth. We performed two types of measurements over an inactive lava flow and show that this method can easily detect the existence of a lava tube. Importantly, phase reversals of the reflection signals can help identify reflections from a lava tube. Using these reflection patterns, we estimate the vertical dimension and the depth of a lava tube at Fuji volcano, which are validated by survey measurements. The presented method may be the most practical way to map terrestrial and perhaps extraterrestrial lava tubes.

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To establish sealing performance suitable for geological environmental conditions of Japan, it is necessary to obtain basic data related for the sealing function through laboratory and in-situ experiments. These data are to be used for practical design and to establish the method for analysis of the sealing performance.Japan Nuclear Cycle Development Institute (JNC) has joined the international project, the Tunnel Sealing Experiment (TSX), to demonstrate the sealing performance of full scale plugs in-situ, and to develop analytical method of the sealing performance of the plugs at Underground Research Laboratory (URL) of Atomic Energy of Canada Limited (AECL) in Canada [1]. This experiment is in the final stage and fundamental data for the sealing function of clay and concrete plugs have been obtained.This paper presents the total data handling and interpretation of the sealing experiment, as well as the numerical analysis and interpretation for clay plug performance based on tracer test results. As a result of the numerical analysis, the sealing performance of the clay plug was confirmed and it was shown that the excavation disturbed zones (EDZ) around the clay plug and the tunnel could be a dominant transport pathway for radionuclides.

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