Meteorology in Tunnel - Effect of barometric pressure changes on extensometer observations

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One of the main sources of observational noise for 100 meter quartz tube extensometers in Matsushiro Seismological Observatory, Matsushiro, Nagano, Japan is the effect of air temperature changes on the quartz tubes. The temperature changes, in turn, are caused by adiabatic compression/expansion of the inside air due to atmospheric pressure changes. In this study, we performed continuous observation of temperature, humidity and pressure in order to understand exactly what are going on inside the tunnel. The main results are :1. Air pressure and temperature are in very good correlation. Most of the observed temperature changes are caused by pressure changes.2. Temperature changes are approximately proportional to pressure changes, and the coefficient of proportionality varies systematically with frequency. The coefficient is almost constant at periods shorter than 2 hours, whereas it becomes smaller as the period gets longer.3. Relative humidity inside the tunnel is always higher than 90%, and does not change throughout a year. We could not verify whether temperature changes accompany humidity changes or not.4. The coefficient of the proportionality (0.05K/hPa) at the shorter periods is closer to the theoretical value for humid air (0.042K/hPa), rather than that for dry air (0.085K/hPa).5. The frequency dependence of the pressure effect is very well explained by a theoretical model which takes into account adiabatic changes of the air and heat transfer between air and surrounding rocks.These results lead to a conclusion that temperature changes inside the tunnel are predictable from pressure records and that precise observations of pressure as well as temperature enable us to remove observational noise from extensometer records. However, there still remains unexplained facts, including that the observed proportionality favors a humid air model rather than a dry air model.