text: Hande Unlu
The University of Tokyo, Institute of Industrial Science
published on April 26th, 2021
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motivation

In 2019, November our journey of re-reading the characteristics of Memu had led us to dig under the ground. Our motivation of trying to understand the ecosystem that is surrounding Memu could not be completed without bringing the underground resources under the daylight. As widely known underground resources could offer great potentials to generate a stable supply of energy and water (1-3). Due to its thermal isolation properties, underground spaces have also been particularly utilized today for the housing and storage of foods for centuries (4, 5). Additionally, the structure, the chemical composition of subsurface soil would tell us stories about its past, explain the present, and even helps us know what to expect in the future from the soil and its ecosystem.

reading geology

In September, we visited Hokkaido University and Hokkaido Geological Survey where we were given access to the maps and technical information about previously performed geological surveys in the south Tokachi region close to Memu. Talking to the experts on their comprehensive geothermal research in Hokkaido (6-8) and reviewing the literature on energy potentials in the south Tokachi region (9-11), provided us a valuable perspective structuring our survey.

In summary, based on literature and expert opinions, the Memu area due to its proximity to water sources, has the potential of being a stable geothermal energy source. Yet, as Figure 1 illustrates the geological formation is not as volcanic as inner Hokkaido where there is a 6-10°C rise of underground temperature at every 100 meters below the earth (11). It is only estimated about 1.5 °C of temperature rise at every 100 meters for southern Tokachi, the region of Taiki where Memu stands (see Figure 2).

Figure 1. geothermal gradient map of Hokkaido (ref. 11)

Figure 2. relationship between temperatures of underground water and depths in Hokkaido (ref. 12)

digging under memu

For an accurate reading of the underground potentials at Memu, in November 2019 we realized a shallow subsurface survey at a 20 meters borehole. In addition, we realized a Standard Penetration Test at every 1 meter. The survey provided us the information to analyze and understand the soil structure and the water depth, as discussed below.

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Video 1: breaking the ground for digging out the borehole

Following the shallow subsurface survey, we installed temperature sensors outside and inside the borehole at every 2.5 meters under the ground until -20 meters. This allowed us to map the soil subsurface structure together with the temperature readings as shown in Figure 3. Additionally, we installed a water level sensor at 15m deep inside the hole for a deeper understanding of water, soil substructure, and underground temperature effects on each other. #Water

Figure 3. soil substructure data and its temperature around the year

0.00 – 1.00m: The surface soil was about 1m in depth, and consisted of silty-sand and small gravels.

1.00 – 3.30m: The next level soil structure had less silty-sand and contained gravels with a maximum diameter of 100mm.

3.30 – 9.00m: After 3.30m the soil structure became harder to penetrate and showed resistance above 50 strikes during the Standard Penetration Test as plotted in Figure 4 and observed in Video 2.

Figure 4. standard penetration test

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Video 2: the standard penetration test

The gravel diameters at this level were mainly between 2-40mm. However, 60-70% of the soil structure was composed of smaller pebbles sizing between 6.2 – 7.00mm.

Until 7.5m under the ground the water consistency of the soil was low. However, on the 22nd of November 2019, during the survey water was found at 7.60m below the ground. Later, the water depth at this level was measured to be 7.35m (until 14.95m below the ground).

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Video 3: taking out samples from the water level

9.00- 15.00m: Gravels in this layer showed diameters between 2-30mm. Around the mid-level, they reach 60mm in diameter. 50% of the soil was composed of round pebbles and irregular-shaped gravels. The amount of the water in soil was high and consists of silty-sand.

15.00- 20.00m: Gravels in this layer occupied 50-60% of the soil structure and showed the same size as the above layer (2-30mm). At the center level, the diameter of the gravels was mainly about 70mm and a maximum of 210mm. These gravels are to be classified as boulders of eroded rock fragments. Sand at this level had silty property too.

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future of reading the shallow subsurface…

Thermal conductivity properties of soil are defined by several factors and among those its density, and water amount are one of the most important factors (13). During our survey, we found that the soil density has largely increased after 3.30m. In other words, the underground temperature after 3.30m could be expected to be more stable compared to the surface soil due to its relatively higher thermal resistance. The thermal conductivity properties of the soil were found to be in favor of providing stable temperature, especially where the soil contained relatively larger gravels.

Later, in 2020 our all-year-round underground temperature readings validified our thermal property interpretations based on the soil substructure. It was found correct that the temperature below 7.60m (the water level during the survey), was stable (see Figure 1). More complete interpretations about the temperature readings could be found under #shallow subsurface – temperature.

Once the underground temperature is stable as observed in our survey, geothermal energy generation and the creation of habitable space below the ground for varied activities could become an exciting opportunity to utilize what nature generously provides us.

special Thanks to…
– Hokkaido University, Department of Socio-Environmental Engineering
– Hokkaido Research Organization, Hokkaido Geological Survey

references

(1) Grab T., Storch T., Gross U., Performance of a geothermal heat pipe using propane, ASME 2011 International Mechanical Engineering Congress & Exposition, Colorado u.s.a., Nov.11-17 (2011)

(2) Florides G., Kalogirou, S., Ground heat exchangers—A review of systems, models and applications, Renewable Energy, 2 (2007) 2461–2478

(3) Kemper K.E., Groundwater—from development to management, Hydrogeology Journal (2004) 12:3–5

(4) Aydana Ö., Ulusay R., Geotechnical and geoenvironmental characteristics of man-made underground structures in Cappadocia, Turkey, Engineering Geology, Volume 69, Issues 3–4, June 2003, Pages 245-272

(5) Meir, I. A., I. Gilead, Underground dwellings and their microclimate under arid conditions, Design With the Environment, Proc. 19th PLEA 2002 Int. Conf. Toulouse, France, Vol.2, pp. 618-624.

(6) Sakata Y., Katsura T., Nagano K. and Ishizuka M., Field Analysis of Stepwise Effective Thermal Conductivity along a Borehole Heat Exchanger under Artificial Conditions of Groundwater Flow, Hydrology, March 2017

(7) Nagano K., The progress of GSHP in Japan. IEA Heat Pump Cent. Newsl. 2015, 33, 21–25.

(8) Sakata Y., Ikeda R., Regional Mapping of Vertical Hydraulic Gradient Using Uncertain Well Data: A Case Study of the Toyohira River Alluvial Fan, Japan, Journal of Water Resource and Protection, Vol.5 No.8(2013), Article ID:35842,12 pages(2)

(9) 十勝平野南部地域地質図および説明書、監修：北海道立地質研究所、発行：十勝支庁農業振興部、平成13年3月　(Geologic Map and Explanatory Text of the Southern Tokachi Plain, supervision by Geological Survey of Hokkaido, Issued by Tokachi Branch Office Agriculture Promotion Department, 2001)

(10) 大樹地域の地質、地質調査所、昭和49年 (Geology of Taiki Region, Regional Geological Research Report, 1973), Geological Survey of Japan online archives https://www.gsj.jp/data/50KGM/PDF/GSJ_MAP_G050_02064_1974_D.pdf

(11) 北海道地温配図、北海道立地下資源調査所1995 (Geothermal Gradient Map of Hokkaido, Geological Survey of Hokkaido, 1995)

(12) Wakahma H., Akita F. and Matsunami T., Explanatory Text of the Geothermal Gradient Map of Hokkaido, Geological Survey of Hokkaido, 1995

(13) Márquez, J.M.A., Martínez Bohórquez, M.A. and Gómez Melgar, S., Ground Thermal Diffusivity Calculation by Direct Soil Temperature Measurement. Application to very Low Enthalpy Geothermal Energy Systems, Sensors 2016, 16, 306; doi:10.3390/s16030306

text: Hande Unlu
The University of Tokyo, Institute of Industrial Science
published on April 26th, 2021
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potential that lies in the underground temperature

Underground heat has always been a great potential of energy resource and habitat to varied activities for a long time in the world history¹. Therefore, reading the underground soil structure and looking for possible effectors like nearby aquifers and water well, which all might affect the underground temperature, was necessary to understand the nature lying underneath Memu.

reading the underground temperature for a year

Following the underground survey (#shallow subsurface – geology), we installed 8 temperature sensors inside the borehole at every 2.5m under the ground until -20 meters. Additionally, we installed a sensor at 1m high above the ground to read the outdoor temperature.

After temperature sensors started their operation, we also installed a water level sensor at 15m deep inside the hole.

In 2020, temperature and water level sensors provided us to monitor around-the-year natural phenomenon occurring underneath Memu.

what we found underneath Memu…

In 2020, all year-long hourly recorded temperatures showed us that at Memu, regardless of the above-ground being extremely cold or hot, the temperature is quite stable after 2.5m under the ground, unlike the outdoor conditions (see Figure 1). Particularly, after 10 meters of depth, the ground has a stable 9 °C temperature all year round.

Figure 1. monthly average underground temperature

The impact of outdoor conditions was found majorly on the surface of the soil and gradually fades until 10 meters. The earlier boring survey showed us that the soil substructure until 3.30 meters was silty-sand (which has poor thermal conductivity) and contained very little amount of water. This finding could explain the large temperature changes observed between the subsurface’s 0- 2.5m depth.  However, with the increased amount of water from 7.6m and below and changed soil density, thermal conductivity properties have improved and provided a more stable temperature underneath Memu (1).

During the coldest day of the year (see Table 1), while the outdoor temperature was -22.5°C, at 5:00 am in the morning, under the ground at Memu was between 6.9°C to 9°C, which was about 32°C warmer than outdoors. On the other side, during the hottest day of the year with a 34.2°C temperature outside, the underground was only between 9°C to 10.3°C, which was about 26°C cooler than the outdoor temperature (see Table 1).

Table 1. coldest and the hottest underground temperature readings in 2020; (𝚫t: temperature difference, t(air): air temperature)

Another fact analyzed from the readings is that the outdoor temperature was reflected upon underground with a 1 to 3 months delay in time (see Figure 2 and Table 2). For example, the coldest time of the year in the air was January 2020. However, exactly this time of the year showed the warmest temperature at -7.5m under Memu. The average temperature difference (μ𝚫t) between the coldest month and the hottest month of 2020 was 26℃ in air, whilst this difference was as little as 7℃ at -2.5m and only 0-1℃ after -10m.

Figure 2. monthly average underground temperature in 2020

Table 2. coldest and the hottest underground temperature readings in 2020 by the depth

Our literature review and expert interviews about the geological properties of the South-Tokachi region showed us that the underground temperature rises about 1.5℃ at every 100 meters below the earth’s surface (2). Therefore, based on this given range of temperature rise, one could expect 10.5℃ to 11.5℃ of heat about 100m below Memu.

Details about the shallow subsurface’s geological and thermal conductivity properties at Memu are discussed under #shallow subsurface – geology.

potentials driven by the underground temperature

In summary, reading the underground temperature at Memu has surprised us by being vastly different than the air temperature for the given same time of the year. Particularly, underground offering a cool ambient during hot summer days and offering warm ambient during the coldest time of the year makes us see it as a favorable space for varied activities and a good source of energy.

Given that the rural conditions of the location ( infrastructural limitations, such as electricity distribution of the grid and heating demand during winter) by the explored potential lying in underground temperature could create self-sustaining solutions to overcome the limitations.

footnotes

1. Geothermal resources and underground heat have provided comfortable habitats for living and storing food for centuries (3). There are written records of geothermal usage in China which are over two thousand years old (4). Shallow ground geothermal heat has been effectively used on the household level for heating, hot water provision, and cooling (5, 6). Previous research showed that under the same climatic condition, the geothermal energy (Ground Sourced Heat Pump) system could save energy by 26–60% when working in heating mode, and 19–55% of energy could be saved when the system was working in cooling mode (7). Another benefit of utilizing underground heat is that it is independent of weather conditions and outdoor climate. Under the surface soil, the earth has a stable and comfortable temperature (8). Overall, the possibilities of effective use of geothermal heat are specific to the location. Currently, geothermal power has been left behind wind and solar in both growth and installed capacity. The main reasons may be a high initial investment, long payback time and construction time, difficulty to assess resources, and difficulty to modularize. Nevertheless, geothermal power has the potential to grow exponentially in the future (9).

references

(1) Márquez, J.M.A., Martínez Bohórquez, M.A. and Gómez Melgar, S., Ground Thermal Diffusivity Calculation by Direct Soil Temperature Measurement. Application to very Low Enthalpy Geothermal Energy Systems, Sensors 2016, 16, 306; doi:10.3390/s16030306

(2) Hiroshi Wakahma, Fujio Akita and Takeo Matsunami, Explanatory Text of the Geothermal Gradient Map of Hokkaido, Geological Survey of Hokkaido, 1995

(3) Alkaff, Saqaff A., Sim. S.C., Efzan, M.N., A review of underground building towards thermal energy efficiency and sustainable development, Renewable and Sustainable Energy Reviews, Volume 60, July 2016, Pages 692-713

(4) Fridleifsson Ingvar B., Geothermal energy for the benefit of the people, Renewable and Sustainable Energy Reviews, Volume 5, Issue 3, September 2001, Pages 299-312

(5) Dingfeng Caoa, and et.al., Investigation of the influence of soil moisture on thermal response tests using active distributed temperature sensing (A–DTS) technology, Energy & Buildings 173 (2018) 239-251

(6) Grab, T., Storch T., Gross, U., Performance of a geothermal heat pipe using propane, ASME 2011 International Mechanical Engineering Congress & Exposition, ColoradoU.S.A. Nov.11-17 (2011)

(7) Urchueguia J.F., Zacares M., Corberan J.M., Montero A., Martos J., Witte H., Comparison between the energy performance of a ground-coupled water to water heat pump system and an air to water heat pump system for heating and cooling in typical conditions of the European Mediterranean coast, Energy Conversation Management, 49 (2008), pp. 2917-2923

(8) Florides G., Kalogirou S., Ground heat exchangers—A review of systems, models and applications, Renewable Energy 32 (2007) 2461–247

(9) Keween Li., Bian, H., Liu C., Zhang D., Yanga Y., Comparison of geothermal with solar and wind power generation systems, Renewable and Sustainable Energy Reviews, Volume 42, February 2015, Pages 1464-1474

text: Hande Unlu
The University of Tokyo, Institute of Industrial Science
published on April 26th, 2021
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dynamism of water

The nature lying underneath the earth often doesn’t sound loud to us to turn our eyes and listen. Yet, the moment we lift its blanket and read deeper inside, we see how dynamic, diverse and harmonious at the same time.

Nature never remains still. What we read above the ground is under constant change. It changes its form, often flows, and penetrates the earth. During our survey, we were thrilled to record this very circle of nature: the water.

Like how water brings life under the earth, underground water can also significantly contribute to creating social and economic benefits to its local area¹. Therefore, for an accurate understanding of the underground potentials at Memu, we realized a shallow subsurface survey and analyzed the dynamism of the water.

the readings from the survey

In November 2019 we realized a shallow subsurface survey by digging a 20 meters borehole in the ground. The data collected through this survey provided us the information to analyze the soil structure, underground temperature, and the dynamism of the water level around the year.

During the soil survey on 22 November 2019, we found the water at the level of 7.60m below the ground. The readings later showed us that the water elevation at this date was 7.35m. Details about the survey and its findings are discussed at #shallow subsurface – geology.

Following the soil substructure and temperature surveys, in September 2020, we installed a water-level-transmitter type of sensor at 15 meters under the ground inside the borehole. The underground water level was recorded hourly between September 2020 until April 2021 (ongoing) to analyze the impact of the outdoor winter.

The winter recordings have shown us how dynamic the water was right below us while the outdoors were in a deep sleep under the white elegant blanket of nature; the snow of Hokkaido. Figure 1 illustrates the monthly average underground water level recorded during the mentioned period of the year.

Figure1. monthly average underground water level (meter)

Table 1 shows that the highest water level was observed on the 27^th of March 2021, whilst the lowest level was recorded just a month earlier, on the 15^th of February 2021. During this period, the difference between the water levels (Max. 𝚫L) was 2.22 meters, which is the maximum recorded level change between September 2020 – April 2021.

Table 1. comparison of the highest and the lowest water level

Moreover, according to the readings during the winter season, the average water level was  7.78m below the ground. Again, during the same period, the average depth of the water was about 7.22m.

Later, we overlapped the underground water level data with the outdoor and underground temperature data illustrated in Figure 2. Analyzing the overlapped data sets, we found a sudden rise in the water level between February and March when the air temperature had risen from -5℃ to +2℃ (recorded monthly average temperature). In March 2021, the average air temperature reached above 0℃ for the first time since it dropped below 0℃ in November 2020. This recovery from subzero air temperature could be interpreted as the cause of the melting snow and ice. Consequently, the melted snow and ice resulted in rising the water level below the ground at Memu.

Figure 2. underground water level and temperature by months

Later, April 2020 was found to be the coldest time of the year at -2.5m which could be the time that the melting ice and snow penetrated the subsurface and reached this level. Following the underground water flow, the coldest underground temperature at -5m and -7.5m was recorded respectively in May and June.

future for the underground water @ memu

Our survey showed us that the impacts of the winter season at Memu were in the form of melted water, penetrating the shallow subsurface along the spring season. The rise of underground water caused by the melting ice and snow started in the middle of February and peaked by the end of March.  The underground water flow certainly is not the only factor cooling the subsurface temperature, but one of the most effective causes due to its high thermal conductivity (1).

Briefly,  starting from the end of February throughout the spring season, one can expect an abundance of water resources under the ground of Memu. This underground water resource could be used for natural heating, cooling, and ventilation either for above the ground purposes or for purposes built down below the ground (2-4).

….and this is where our readings hope to give a sparkle to creative ideas.

* the further readings from the shallow subsurface survey series can be found under #shallow subsurface – geology and #shallow subsurface – temperature.

footnotes

 1. Groundwater is vital to many nations, irrespective of their stage of economic development. Over the past few decades, accelerated groundwater development has resulted in great social and economic benefits by providing low-cost, drought-reliable, and (mainly) high-quality water supplies for urban areas, for the rural population, and for irrigation of (potentially high-value) crops. (5)

references

(1) Li B., Han Z., Hu H., Bai C., Study on the effect of groundwater flow on the identification of thermal properties of soils, Renewable Energy, Volume 147, Part 2, March 2020, Pages 2688-2695

(2) Meir, I. A., Gilead I., Underground dwellings and their microclimate under arid conditions, Design With the Environment, Proc. 19th PLEA 2002 Int. Conf. Toulouse, France, Vol.2, pp. 618-624.

(3) Unver, B., Agan, C., Application of heat transfer analysis for frozen food storage caverns, Tunnelling and Underground Space Technology 18(2003)7–17

(4)Fujio Tsuchiya, Development of Large Storage Facility for Practical Use under Low Temperature using Heat Pipes in Yachiyo Region in Obihiro, Hokkaido, journal of natural energy research Hokkaido, no.6, 15-22, 2009

(5) Kemper, E.K.,  Groundwater—from development to management, Hydrogeology Journal (2004) 12:3–5

 (6)大樹地域の地質、地質調査所、昭和49年 (Geology of Taiki Region, Regional Geological Research Report, 1973), Geological Survey of Japan online archives https://www.gsj.jp/data/50KGM/PDF/GSJ_MAP_G050_02064_1974_D.pdf

(7)営農用深層地下水調査 南十勝地区, 北海道開発局,昭和46年 (Groundwater Survey for Farming, South Tokachi Region, issued by Hokkaido Regional Development Bureau of Agriculture and Fisheries Department, 1971), Hokkaido Research Organization Archives

(8) 十勝南部地域の地形と地質,北海道立地下資源調査所,平成2年 (Topography and Geology of South Tokachi Region,1990, Hokkaido Research Organization