The area where the geothermal warming rate is higher than the average geothermal warming rate and has significantly increased is called geothermal anomaly area.[1]
The so-called geothermal anomaly area is compared with the normal area.Similarly, in the form of conduction, heat flow varies from place to place on the surface, because different rocks have different thermal conductivity.The surface heat flow is generally between 0.8-2.0 heat flow units, and the thermal conductivity of rock is 4-10 μ cal/cm · s, so the geothermal gradient is generally 8-50 ℃/km.When heat conduction is dominant, the geothermal gradient varies from place to place. At a depth of 3 kilometers underground, the low temperature is only 24 ℃ and the high temperature can reach 150 ℃. However, considering the technology of geothermal power generation, the geothermal anomaly caused by the difference in heat conduction is economically uneconomical for power generation, and should only be directly used for industry, agriculture or life;Therefore, from the perspective of geothermal power generation, it cannot be regarded as thermal anomaly.
The thermal anomaly of geothermal fields discovered in the world is mainly caused by convection.Because the technology mainly uses high-temperature fluid, the fluid brings heat energy to the surface through convection, which is convenient for use.For some dense formations lacking fluid, due to heat conduction, Rong can also form strong thermal anomalies, sometimes with small depth and temperature up to 300 ℃, which is called dry hot rock.It is difficult for dry hot rock to directly use its energy. The method is to use artificial methods to create cracks in hot rock (such as underground conventional blasting, hydraulic pressurization, underground atomic energy blasting, etc.), and then put the water pressure into the well to heat the water in the underground circulation, and return to the ground in the form of hot water or steam for further use.Of course, the technology is complex and the cost is high.Therefore, under the technical conditions, the main goal is still to utilize hot water or steam.When looking for geothermal fields, we mainly look for areas with high temperature steam or hot water.
Geothermal anomaly areas can be divided into three different types.The first is the hot water type dominated by hot water, the second is the steam type dominated by steam, and the third is the dry hot rock type lacking water.Those that have been developed and utilized are limited to the first two.
Geothermal anomaly area of hot water type
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The geothermal anomaly area of hot water type is the most common one.At a certain depth, the water is heated to increase temperature. According to the principle of convection, the water is heated to expand and rise, and the heat energy is transported to the shallow, forming a geothermal anomaly area.
Generally, such abnormal areas have surface indications, such as hot springs and fountains.When the stratum and lithology exposed by hot water can be directly observed when a spring is exposed, it can be determined in which kind of pore system (pore, fissure, solution fissure) it exists.However, it must be noted that sometimes the stratum where the spring outlet is located is not necessarily the stratum where the hot water rising channel is located.For example, the hot springs exposed in the sand layer of the Quaternary sediments are often derived springs. Because they are mixed with the cold water in the sand aquifer, the temperature has decreased and the chemical composition has also changed.This is a case where the outlet of hot water is covered.On the other hand, the hot water is exposed in a certain section due to the barrier of the aquiclude, but the real hot water rising channel is quite far away.Therefore, when working, the exposed position of hot spring should be combined with the local hydrogeological conditions to determine the position of the real ascending channel of hot water.
Generally, rare elements in groundwater are often high in hot water, and most of them are associated with HtwoS and SOtwoEtc.Therefore, there is a view that hot water is the gas and liquid in the later stage of magmatic activity. Due to the release of hot water and steam, some relatively rare elements are concentrated, and even mercury and silver deposits are generated near high-temperature hot springs.According to this view, both hot water and steam come from magma, the so-called primary water, that is, water from the interior of the earth.However, over the years, through the study of hydrogen isotopes in water, a fairly reliable study has been made on the source of hot water and steam, and this view has been denied.
Steam type geothermal anomaly area
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The steam type geothermal anomaly area can be divided into two types: one produces only superheat steam without water, which is called dry steam field; the other produces water with a certain amount of steam, which is called wet steam field.The geothermal anomaly areas of steam fields found in the world are mainly wet steam fields, while there are only three dry steam fields, including La Dreiro in Italy, Geysers in California in the United States, and Matsukawa in Japan.
There are three different theories on the mechanism of steam field.The first is proposed by Facca and Tonani of Italy.They believed that the steam field was originally full of hot water, and the water temperature was close to the boiling point (corresponding to the boiling point under the pressure of the depth of water). The drilling hole was filled with this hot water, which became steam due to decompression boiling, and was ejected upward.If there is sudden change of steam in the borehole, it is a wet steam field; if there is sudden change of steam near the bottom of the borehole, it is a dry steam field.After some water vapor is ejected, the evaporation space near the borehole becomes largerdrill holeWhen the evaporation space of each hole is gradually connected, the upper part of the whole geothermal system is full of steam and the lower part is full of water.[2]
