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During the epidemic period, when we go in and out of crowded places, we are always blocked by a temperature measuring gun, and our body temperature can be accurately obtained with one drop. Having a normal body temperature means that our bodies produce heat normally and do not have to be dragged for examination. So does the earth have a body temperature? Is the temperature of the earth normal? Today we are going to talk about the temperature of the earth.

First of all, everyone should already have a basic understanding of the earth, including stratification and plate tectonics. The basic rule is that the deeper the earth goes, the hotter the layers of the earth should be. For example, the temperature in the deep crust can reach 1000 ℃, and the temperature in the deep mantle may reach 1000-2000 ℃. The hottest is our core, and the core center is even as high as 5000 ℃ or more.

Of course, we usually take a person's body temperature is not likely to measure the body temperature of internal organs, basically only measure the temperature on the skin. For the earth, it is too small for us to measure the temperature inside it directly. What we can do, or what we need is the shallow ground temperature, that is, the part of the earth's crust. Even if we measure the ground temperature in the shallow layer, there are very few direct measurement methods that can be used to measure the ground temperature in the deeper layer. At present, the only way to directly measure the ground temperature is to drill a geotemperature measurement well. due to the current limitations of funds and technical means, the deepest well in the world has only been drilled to about 12km (Kola ultra-deep drilling in the former Soviet Union). Therefore, it is impossible to measure the geotemperature directly in the deeper layer, but we can calculate the geothermal gradient through the shallow geothermal measurement, so we can infer the temperature in the deep. However, the shallow geothermal gradient is limited by many factors, such as time influence, climate influence and so on, so it is impossible to measure simultaneously in the world, so it is very important to choose the appropriate parameter to represent the geothermal characteristic, which is the geothermal flow value (Heat Flux).

The geodetic heat flow value is defined as the heat flow value passing through the unit area of the earth's surface in a unit time. This parameter eliminates the effect of time and is the best parameter to characterize the surface temperature of the earth at present. The formula is as follows:

Q =-100 Kr (dT/dz)

Kr is the rock thermal conductivity and dT / dz is the geothermal gradient, so the heat flow value includes the thermal characteristics of the rocks in this area. Using the geothermal gradient and other data obtained from various places, and combined with the geological and lithology of various places, the heat flow values of various places can be calculated, thus the geothermal data set is synthesized. Many scientists and researchers use these data sets to simulate the global surface geothermal characteristics, which is like taking the temperature of the earth. We can see the global geothermal characteristics given by Davies in 2010, and the heat source is basically distributed in the new oceanic crust, that is, the plate boundary. However, due to the sparsity of marine data and bipolar data, the estimation error will also be too large.

Global heat flow distribution map (Davies et al.,2010) occasionally the earth will develop acne or fever, such as volcanic eruptions. Sometimes, we go to natural hot springs. Have you ever wondered what hot springs are all about? In fact, this is caused by geothermal anomalies. This is the second topic we are going to talk about today, geothermal energy.

After double carbon was proposed in 2020, it has become the consensus of the society to use renewable energy to replace traditional energy. Among them, geothermal energy, as one of the main renewable energy, has the characteristics of low cost, sustainable utilization and environmental protection, so it has been paid more and more attention by the society.

According to thermal fluid transfer and development mode, geothermal resources can be divided into three types: shallow geothermal energy, hydrothermal geothermal energy and dry hot rock geothermal energy (enhanced geothermal system) (Zhou Zongying et al., 2015). Among them, the dry and hot rock has the characteristics of large energy, wide distribution, extremely high utilization rate, good safety, no pollution, no need for tail water recharge, continental marginal basin with high quality resources, high efficiency of power station construction, stable and sustainable thermal energy, strong controllability of power generation, good effect of disaster reduction and emission reduction, and so on. it is an important goal of geothermal energy development. At present, the accepted definition of dry-hot rock is a kind of compact hot rock body without water or vapor, buried depth of 3-10km and temperature of 150-650℃ (Li Dewei et al., 2015). At present, most of the dry and hot rocks found in many areas are granite. By comparison: Kola ultra-deep drilling 12km depth, the highest ground temperature is only 200℃, while dry-hot rock drilling 4km may reach about 300C. This is the temperature of hot and dry rock!

Dry and hot rocks are distributed almost all over the world, and there are dry and hot rock resources under each continent. The places with the greatest potential for the development and utilization of dry and hot rocks are some new volcanic active areas, or areas with thin crust, which are mainly located on the edge of global plates or tectonic terranes (Lin Wenjing et al., 2012). In 2011, the China Geological Survey carried out a potential assessment of dry and hot rock resources in China's continental areas. the results show that the dry and hot rock resources in China are huge, widely distributed, account for a large proportion of recoverable resources, and have great potential for development (Wang Ji et al., 2011).

In 2018, the Land and Resources system carried out geological exploration of dry and hot rock resources in high heat flow areas such as Qinghai, Xizang, Sichuan, Fujian, Guangdong, Hunan, Songliao Basin and Hainan, and carried out preliminary drilling of dry and hot rocks in Guide and Gonghe in Qinghai, Lijin in Shandong, Huizhou in Guangdong and Kangding in Sichuan (Xu Tianfu et al., 2014). China is rich in dry and hot rock resources, but the exploration and development of dry and hot rock is still in the preliminary stage and needs further research. In addition, the follow-up development of dry and hot rocks will still face many challenges, such as the possibility of inducing earthquakes and social impacts, which require more and more in-depth research. There is still a long way to go for the development and utilization of dry and hot rocks in China.

The global heat flow value of the dry hot rock development system map (source: international New Energy Network) can be applied to the study of geodynamics and promote the study of the earth's thermal mechanism. The local geothermal anomaly can be developed and utilized as green energy. Have to sigh, our earth is a treasure!

reference

Davies J H, Davies D R. Earth's surface heat flux [J]. Solid Earth Discussions, 2010. 1 (1): 5-24.

Zhou Zongying, Liu Shiliang, Liu Jinxia. Characteristics and development countermeasures of geothermal resources in China [J]. Journal of Natural Resources, 2015. 30 (07): 1210-1221.

Li Dewei, Wang Yanxin. Some major problems in the research and development of geothermal energy in dry and hot rocks [J]. Geosciences (Journal of China University of Geosciences), 2015 Jing 40 (11): 1858-1869.

Wang Jiyi, Hu Shengbiao, Pang Zhonghe, he Lijuan, Zhao Ping, Zhu Chuanqing, Rao Song, Tang Xiaoyin, Kong Yanlong, Luo Lu, Li Weiwei. Evaluation of geothermal resource potential of dry and hot rocks in Chinese mainland [J]. Science and Technology Bulletin, 2012, 30 (32): 25-31.

Xu Tianfu, Hu Zixu, Li Shengtao, Jiang Zhenjiao, Hou Zhaoyun, Li Fengyu, Liang Xu, Feng Bo. Enhanced geothermal system: international research progress and current research status in China [J]. Acta Geologica Sinica, 2018. 92 (09): 1936-1947.

Lin Wenjing, Liu Zhiming, Ma Feng, etc. Estimation of dry and hot rock resource potential in continental area of China [J]. Journal of Earth, 2012, 33 (6): 1-5.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), written by Xingyu, edited by Liu Yuling

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