9:15 AM - 9:30 AM
[S24-2-04] Long-term measurement of 1m-depth geo-temperature and its relationship with ambient temperature change
It was in 1951 that a group of geophysicists working on hotsprings completed a very good compilation of 1m-depth subsurface temperature data through four seasons at many locations in Japan. After a long time of lack of such kind of data, a new project has been put forward by Ehara in 2012 to restart systematic measurements of geo-temperature at 1m-depths with a standardized simple thermometer system, intending to observe the long-term change of earth's surface temperature and to look into the evidence of global warming as well as that of heat-island phenomena in the Japanese territory. Within the Kanto Plain area where the latter effect is considered the strongest in the world, we have currently 8 stations for long-term measurement.
The focus of this paper is to examine the change of geo-temperature at 1m-depth as time-series, trying to estimate the thermal property of each measurement site from the relationship between the geo-temperature change at 1m-depth and its major thermal driving force, i.e., ambient atmospheric temperature. Its mathematical formulation has been given in textbooks on geothermics. We can see a clear correspondence between the time-series of 1m-depth geo-temperature which we really observed and that of daily averaged air temperature at the same location, with a certain delay-time. Based on the delay-time and also the amplitude attenuation, we estimate the Effective Thermal Diffusivity (ETD) of the materials beneath the ground surface using the Angstrom method. By picking up variation components having periods of 10 to 20 days, the estimated ETD values range from 3.5 to 7.0 E-7 m**2/s. For longer-period components, such as the annual variation, our observation duration up to now might not be long enough yet. The obtained values of ETD, as well as those of annual mean 1m-depth geo-temperature derived from our work, would also be useful in other studies of geothermics, like the inversion of past climatic change based on deep borehole observation.
The focus of this paper is to examine the change of geo-temperature at 1m-depth as time-series, trying to estimate the thermal property of each measurement site from the relationship between the geo-temperature change at 1m-depth and its major thermal driving force, i.e., ambient atmospheric temperature. Its mathematical formulation has been given in textbooks on geothermics. We can see a clear correspondence between the time-series of 1m-depth geo-temperature which we really observed and that of daily averaged air temperature at the same location, with a certain delay-time. Based on the delay-time and also the amplitude attenuation, we estimate the Effective Thermal Diffusivity (ETD) of the materials beneath the ground surface using the Angstrom method. By picking up variation components having periods of 10 to 20 days, the estimated ETD values range from 3.5 to 7.0 E-7 m**2/s. For longer-period components, such as the annual variation, our observation duration up to now might not be long enough yet. The obtained values of ETD, as well as those of annual mean 1m-depth geo-temperature derived from our work, would also be useful in other studies of geothermics, like the inversion of past climatic change based on deep borehole observation.