The applicability of ungrouted ground heat exchangers (GHEs) in thermal response tests (TRTs) has not been confirmed since TRTs are generally carried out in grouted GHEs. In this work, two TRTs were carried out in a 102 m GHE of 100 mm casing ID under ungrouted and grouted conditions. The TRT results were interpreted through the conventional graphical method and more advanced approach using the cylindrical source function and optical fiber thermometers. The interpretation results of thermal conductivity in the ungrouted and grouted GHE showed reasonably good matching in both interpretation methods, indicating the applicability of the TRTs to ungrouted GHEs for the optimum design and the development of potential maps of ground source heat pump systems.

This paper compares the geothermal heat source of the Hachimantai, Japan, and the Geysers, U.S.A., by investigating the intrusion age of the underlying plutonic rocks. The zircon U-Pb age of the Quaternary Kakkonda Granite, the heat source of the Kakkonda geothermal field in the Hachimantai geothermal area, was further constrained to be 0.09 ± 0.01 Ma (error shown as 95% confidence level) using a high resolution type of ICP-MS (sector-field ICP-MS). Therefore it is reconfirmed that this granite acts as a heat source at Kakkonda. On the contrary, the Geysers Plutonic complex or GPC, the presumed heat source at the Geysers, was dated as 1.8–1.1 Ma by the zircon U-Pb method (Schmitt et al., 2003). Since this intrusion age of the GPC is too old to maintain the present Geysers geothermal system, undiscovered younger and smaller intrusions are assumed at the Geysers. Considering whether a large shallow pluton young enough to serve as the present geothermal system exists or not, it is plausible to assume that the Kakkonda has a greater geothermal potential than the Geysers. Moreover, it is assumed that there are unidentified young plutons in the Hachimantai geothermal area such as the Kakkonda Granite at Kakkonda, which further enhances its geothermal potential in the Hachimantai geothermal area.