The magnetotelluric (MT) method is an essential tool for the exploration of geothermal resources, and threedimensional (3D) inversion is now routinely applied to MT data in geothermal fields in the world. One of the challenges in the 3D MT inversion is the treatment of the topography because the finite-difference method, which is commonly used for 3D inversion codes, may create numerical errors in solving the electromagnetic fields when the mesh discretization is applied to a rough topography variation. In this study, a comparison of different mesh discretization, particularly in the vertical direction, was conducted for the MT dataset obtained at a western flank of northern Hakkoda volcano, Aomori Prefecture, Japan, using the 3D MT inversion code WSINV3DMT (Siripunvaraporn and Egbert, 2009). The horizontal size of each cell in the core zone of the interpretation was 150 m by 150 m, while five different sizes, from 5 m to 100 m, were tested for the vertical cell size for the elevation range where the MT stations were located. The frequency used for the inversion spanned from 0.0067 Hz to 115 Hz targeting a normal depth of geothermal reservoirs. The comparison revealed that the smaller vertical cell settings can reduce the computation time in spite of the larger number of cells, generating a smoother and more reliable resistivity model for the shallow underground in the model. The resultant 3D resistivity models by the smaller vertical cell settings showed good consistency with the borehole logging data obtained by past geothermal exploration projects in the study area. These results suggest that the inclusion of topography in the finite-difference modeling is effective for the 3D inversion of MT data at an area where the topography variation is not neglectable.

The change of a subsurface temperature is smaller than that of an ambient temperature and an extreme value of the subsurface temperature is observed after a certain time lag from the extreme value of the ambient temperatures. Hence, the use of the shallow ground for a heat source has an advantage to the use of atmosphere.

When considering the introduction of a ground source heat pump (GSHP) system by a horizontal ground heat exchanger (HGHE), the understanding of these characteristics is important for effectively using the shallow ground as the heat source for GSHP systems. However, the heat storage surrounding the HGHE during the operation is usually neglected at the design of the GSHP system. The influence on the ecosystem should be considered under the prediction of subsurface temperature changes.

In this study, various thermal characteristics in the shallow ground were considered based on the data obtained at a test site in Tsukuba, Japan during operation of the GSHP system. We also constructed a numerical simulation model that reproduced HGHE and conducted case studies. Constructing a numerical simulation model of GSHP system can be used as a convenient tool for predicting long-term subsurface temperature under optimal design conditions. It is useful for evaluating secular change of subsurface temperature during long-term operations.

At the test site, the subsurface temperature at a depth of 2.0 m changed gently between 14.8 and 22.8 degrees throughout the year, and this seasonal change was smaller than that of the ambient temperature. In addition, the maximum and minimum subsurface temperatures were both observed a few months after the respective peaks observed with ambient temperatures. Therefore, the delay of temperature peak could improve the performance of the GSHP systems. In the cooling and heating seasons of the GSHP system by HGHE, it was shown that the temperature did not completely recover by the start of the next operations after the end of previous operations, which could be an advantage for better annual performance factors. Furthermore, the numerical simulation model which was utilized to estimate the advantages of HGHE under certain conditions as well as the effect on the subsurface ecosystem, indicated the design tool to be highly useful.

Vapor-dominated geothermal resources were the first to be utilized for geothermal power generation. However, since there are few examples, its characteristics, actual conditions and issues are not well known to the general public. This review intends to provide an overview of the current understanding of the vapordominated geothermal resources, the phenomena that have occurred during steam production, the responses to these phenomena, and the issues that remain unresolved. This review is divided into three parts. In the first part, the characteristics of the vapor-dominated geothermal resources are discussed.