Inversion of thermal conductivity and heat flow from borehole temperature data affected by recent variation in ground surface temperature

We inverted the thermal conductivity of earth material, heat flow, and ground surface temperature (GST) history from borehole temperature data obtained at the carbon dioxide capture and storage demonstration project site in Tomakomai, Japan, by Bayesian inverse analysis. Past GST change that had disturbed the underground temperature was treated as a heat source in inversion analysis to determine thermal conductivity of the earth material from borehole temperature and underground structure data. The inverted past GST change suggested an increase of GST by 3 K from 1800 to the present. The inverted thermal conductivity of the earth material appeared to correspond to lithology in the borehole. The inverted heat flow was consistent with those measured around the site. To invert these model parameters from borehole temperature data, underground structure data are required. For inverse analysis of borehole temperature data without underground structure data, we propose a simple method for inferring underground structure related to thermal conductivity of the earth material and report the effectiveness of this method for the inversion of model parameters from borehole temperature data only.