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

日本长野松城地震观测站100米石英管延伸计观测噪声的主要来源之一是空气温度变化对石英管的影响。温度的变化，反过来，是由大气压力变化引起的内部空气的绝热压缩/膨胀引起的。在这项研究中，我们对温度、湿度和压力进行了连续的观察，以便准确地了解隧道内的情况。主要结果是：1。气压和温度有很好的相关性。大多数观测到的温度变化是由压力变化引起的。温度变化与压力变化近似成正比，比例系数随频率系统变化。当周期短于2小时时，该系数几乎是恒定的，而当周期变长时，该系数则变小。隧道内相对湿度始终保持在90%以上，全年不发生变化。我们无法证实温度的变化是否伴随着湿度的变化。较短时段的比例系数（0.05K/hPa）更接近于湿润空气的理论值（0.042K/hPa），而不是干燥空气的理论值（0.085K/hPa）。一个考虑了空气的绝热变化和空气与围岩之间的热传递的理论模型很好地解释了压力效应的频率依赖性。这些结果得出的结论是，隧道内的温度变化可以从压力记录中预测出来，并且对压力和温度的精确观测使我们能够从延伸计记录中消除观测噪声。然而，仍然存在一些无法解释的事实，包括观测到的比例倾向于湿润空气模式而不是干燥空气模式。

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.