本論文では，ケニアにおける未開発地熱地帯について，温泉に特化して述べる。地熱資源探査を地化学試料のサンプリングとその分析から始め，貯留層の地化学温度計として化学種の濃度を用いた。さらに，本論文ではケニアにおける地熱マップを更新し，また6つの温泉の地化学分析を行った。分析を行った温泉の温度は30～75℃，pH値は7～8，導電率は18.4-2,370 m-s/m であった。本論文は潜在的な地熱地帯のマッピング，温泉水の分類，そしてその地域における資源の潜在的な利用方法を見つけ出すことを目的としている。主要な陽イオン，陰イオン，二酸化ケイ素の分析を温泉水サンプルに対して行い，二酸化ケイ素と陽イオンの地化学温度計により，貯留層温度は80～247℃であることが推定された。Homa Hills では硫酸と塩化物濃度の高いNa-HCO₃ 型の特徴が示された。パイパーダイアグラムでは全ての温泉水は希釈された炭酸水として分類されているが，Kakdhimu温泉以外の温泉は地下水によってかなり希釈されているため，化学成分の濃度が低く，その起源を知るための情報はないに等しい。 Mulot温泉水はHCO₃^-1 の濃度が高く，ソーダ味を示す。これらの温泉は低温のため，養殖，スパ，ビニールハウスなどの直接利用に適しているといえる。

Little is known about the impact of vapor emitted from cooling towers on vegetation around geothermal power plants. In this study, the growth status of Fagus crenata saplings, placed at the site with different concentration levels of hydrogen sulfide (H₂S) emitted from volcanic gas fumaroles, was confirmed by visual observation of leaves and branches and by a vegetation index (NDVI) calculated from multispectral images. The results showed that during the four-month experimental period from late May to mid-September, the onset of leaf necrosis and a decrease in the vegetation index was observed at the site with the highest H₂S concentration. This was considered an effect of H₂S absorbed by the leaves of the saplings, due to the high sulfur content in the leaves. On the other hand, no detrimental signs on the leaves were observed at sites with similar H₂S exposure conditions to those around geothermal power plants, suggesting that the concentration levels of H₂S measured around geothermal power plants have no impact on vegetation. No detrimental signs on the leaves were also observed for the trees naturally grown around fumaroles, even at the sites with higher H₂S concentrations than those around geothermal power plants. This result suggests that the trees naturally grown in the local environment may be more tolerant to H₂S than the saplings used in the experiment because they might have already obtained the form suited to the higher H₂S concentrations.

In this study, for the purpose of demonstrating the effectiveness of thermal conductivity profiling using heating cable and multi-point temperature sensor (TCP using heating cable) inserted into the geological boreholes in designing ground-source heat pump (GSHP) systems, TCPs using heating cable at a depth of 50 m were conducted at 47 sites in Fukushima Prefecture, Japan. After conduction of TCPs using heating cable at 27 sites, a 50 m-deep borehole heat exchanger (BHE) was installed at the same locations and conventional TRT and TCP using optical fiber sensors inserted into the U-tubes of BHEs (TCP using heat transfer medium) were conducted.

The temperature profiles and apparent thermal conductivity (λ) values of both TCPs were consistent each other at many sites. On the other hand, at a few sites, several range of λ values of the TCP using heat transfer medium were extremely higher than that of the TCP using heating cable. The same section of temperature profiles of the TCP using heat transfer medium showed that the temperature recovered rapidly, suggesting that crossflow may be occurring in the annulus area in the BHEs. Since the TCP using heating cable can be tested in a borehole with a smaller annulus area than the BHEs, it was also confirmed that the risk (e.g., crossflow) of ground modification on the estimation of λ value can be minimized.

The 2,187 λ values by depth for TCPs using heating cable obtained at 47 sites were classified by geology, and their statistics for each geology were obtained. Comparison of the median λ values by geology with representative values of effective thermal conductivity in the literature was consistent for many geologies.

In order to use the λ values estimated from TCP using heating cable for the design of GSHP systems, the averaging method of λ values by depth for TCP using heating cable was investigated: the arithmetic mean calculated after correcting λ value over 4 W/(m·K) to 4, and the arithmetic mean values calculated after excluding λ values above 5 W/(m·K) were consistent with the λ value of the conventional TRT at the same site, respectively. However, since the section of borehole with large λ value has high heat exchange capacity, it is considered more suitable to use the arithmetic mean λ value correcting large λ values for the design rather than excluding the values.

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 third part, we will review, phenomena related to long-term steam production and remaining questions on the vapordominated geothermal resources.