Abstract
Although pH measurements of hot spring water are taken in conventional geothermal resource research, previous studies have seldom created pH distribution maps of stream and spring waters for an entire geothermal field as a technique for geothermal exploration. In this study, a pH distribution map was created by measuring stream and spring water pH at 75 sites in the Mutsu Hiuchi Dake geothermal field, Japan. Areas of abnormally high pH were detected in midstream sections of the Ohaka and Koaka rivers; these matched the location of the Mutsu Hiuchi Dake East Slope Fault, which is believed to have formed a geothermal reservoir. The abnormally high pH zone is attributed to the trapping of rising volcanic gases in a mature geothermal reservoir with neutral geothermal water. This causes the gas to dissolve and prevents it from reaching the surface. Thus, the mapping of stream water pH distribution in a geothermal field could provide a new and effective method for estimating the locations of geothermal reservoirs. As the proposed method does not require laboratory analysis, and is more temporally and economically efficient than conventional methods, it might help to promote geothermal development in inaccessible and remote regions.
Highlights
The installed capacity of geothermal power is increasing each year, and it had reached12,729 MW as of 2015
Since the Great East Japan Earthquake of 11 March 2011, there has been increasing momentum to expand the implementation of geothermal power generation in Japan
The most prospective geothermal field is located near Mutsu Hiuchi Dake volcano [14,15]
Summary
The installed capacity of geothermal power is increasing each year, and it had reached. Classical articles on geothermal exploration reiterate that the goal of geothermal exploration is to find high-temperature subsurface reservoirs composed of near-neutral pH alkali chloride water [7] Such water is called mature water because it undergoes long-lived water-rock interaction processes in deep reservoirs under chemical equilibria [10]. High relief andesite volcanoes result in a scarcity of mature chloride water discharges except at some distance from fumaroles near the summits [7]. This leads to a discrepancy between the high-temperature demand and neutral pH water reservoir demand. This paper focuses primarily on presenting how a geologically identified fault can be geochemically elucidated as a prospective near-neutral pH reservoir by the proposed stream pH mapping method
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