As concentration of carbon dioxide in the atmosphere due to consuming fossil fuels has increased, global warming is accelerated in last decades. In addition, increases in consuming fossil fuels have led to their depletion in recent years. One of the practical measures to those two challenges is the conversion of energy resources to natural gas having less environmental impact. Gas hydrates trapping natural gas have been discovered in the sea around Japan. They are expected to serve as a new non-conventional natural gas resource. To understand the mechanism of gas hydrate accumulation, the amount of free gas in sediments should be known. However, without affecting other properties, its non-destructive measurement is difficult. Therefore, we examined a new technique for measuring the amount of free gas using Time Domain Reflectometry (TDR). TDR has been popular to measure moisture content of soil. In this study, we estimated a gas ratio in the sea-bottom sediment obtained from the Eastern Margin of Japan Sea using TDR. We found that the gas phase exist up to about 7%.
