Comprehensive geophysical exploration system for high-temperature geothermal field, and geothermal sweet spot area evaluation method

Abstract

A comprehensive geophysical exploration system for a high-temperature geothermal field, and a geothermal sweet spot area evaluation method. A large-area infrared remote sensing measurement and a radio

Description

Technical Field The present invention belongs to the technical field of geophysical exploration, and particularly relates to a comprehensive geophysical exploration system for high temperature geothermal fields and an evaluation method for geothermal sweet spot areas. Background Geothermal resource is a very valuable comprehensive mineral resource, which has many functions and uses. Geothermal resource is not only a clean energy resource that can be used for power generation and heating, but also a hot brine resource and natural fertilizer resource that can be used for extracting industrial raw materials such as bromine, iodide, borax, sylvite and ammonium salt. At the same time, geothermal resource is also a valuable resource for medical thermal mineral water and drinking mineral water and a water source for living water supply. A number of hypotheses about the source of geothermal heat are proposed. It is generally believed that geothermal heat comes mainly from the exothermic energy of radioactive element disintegration inside the earth, and secondly from the rotational energy produced by earth rotation as well as the heat energy released by gravity differentiation, chemical reaction, and rock and ore crystallization. During the formation of the earth, the total amount of the above heat energy exceeds the heat energy escaping from the earth, forming huge heat reserves and causing partial melting of the earth crust to induce magmatism and metamorphism. It has been basically calculated that the temperature of the earth core is 6000°C, the temperature of the bottom earth crust is 900-1000°C, and the ground temperature is increased with the depth within about 15 kilometers below the constant zone of subsurface temperature (about 15 meters below the ground surface). The average geothermal gradient is about 3°C / 100 m. The geothermal gradient is different in different areas, an area with a geothermal gradient close to the average value is called a normal temperature area, and an area with a geothermal gradient higher than the average value is called a geothermal anomalous area. Geothermal anomalous areas are the main objects of geothermal resource research and exploitation. Crustal plate edges, abyssal faults and volcanic distribution zones are obvious geothermal anomalous areas. The generation of geothermal resource has close internal relations with the formation, development and evolution of lithospheric plates of the earth as well as the accompanying crustal thermal conditions and thermal history, and particularly has direct relations with the tectonic stress field and thermal dynamic field since Pleistocene. From the perspective of global geological structure, a high-temperature geothermal resource zone above 150 °C mainly appears at the edge of each major plate in the veneer of crust, such as a plate collision zone, a plate cracking site and a modern rift zone; and medium and low temperature geothermal resource below 150 °C is distributed in an active fault zone, a rift valley and a depression basin within the plate. According to the distribution characteristics of geothermal resource, although the underground temperature is increased gradually with the depth, if the geothermal resource is buried too deep, and no sufficient groundwater recharge channel is provided, the cost of exploiting and utilizing geothermal energy resource by pumping high temperature underground hot water or high temperature water vapor to the ground surface is too high, and it is not possible to realize economic and efficient exploitation of geothermal energy resource in a large scale. Therefore, it is necessary to comprehensively utilize a variety of different geophysical exploration techniques to explore and discover high temperature geothermal field areas with high average geothermal gradient (>3 °C / 100 m), shallow burial, fault and fracture development, and adequate ambient groundwater recharge, so as to exploit geothermal resource at a low cost and a high efficiency. Summary To achieve the purpose of comprehensively utilize a variety of different geophysical exploration techniques to explore and discover excellent high temperature geothermal field areas, the present invention proposes a comprehensive geophysical exploration system for high temperature geothermal fields and an evaluation method for geothermal resource sweet spots. The present invention has the following specific technical solution: A comprehensive geophysical exploration system for high temperature geothermal fields, comprising an anomalous area detection system and a comprehensive geophysical data acquisition system; The anomalous area detection system is an airborne or ground infrared remote sensing measurement and radioactive measurement system arranged in a large area; The comprehensive geophysical data acquisition system comprises a ground data acquisition subsystem, or is a well-ground joint data acquisition system composed of a ground data acquisition subsystem and a downhole data acquisition subsystem; The ground data acquisition subsystem comprises a ground armored optical cable and a plurality of ground three-component geophones, and a single-mode optical fiber is arranged in the ground armored optical cable; an extinction device is installed at the tail end of the single-mode optical fiber; the single-mode optical fiber is connected with a DAS / DTS composite modulation and demodulation instrument; a shallow gully is dug along a ground measuring line of a three-dimensional survey grid, the ground armored optical cable is buried in the shallow gully, and the plurality of ground three-component geophones are arranged at equal intervals along the ground measuring line; The downhole data acquisition subsystem comprises a downhole armored optical cable and a plurality of downhole three-component geophones, and a single-mode optical fiber and a multimode optical fiber are arranged in the downhole armored optical cable; an extinction device is installed at the tail end of the single-mode optical fiber; the single-mode optical fiber of the downhole armored optical cable is connected with a DAS signal input end of a DAS / DTS composite modulation and demodulation instrument, and the multimode optical fiber of the downhole armored optical cable is connected with a DTS signal input end of the DAS / DTS composite modulation and demodulation instrument; the downhole armored optical cable is arranged outside or inside a casing pipe of a drilling hole or outside a pipe string in a cased well, and the plurality of downhole three-component geophones are arranged at equal intervals in the casing pipe; The comprehensive geophysical data acquisition system also comprises a ground broadband magnetotelluric data acquisition system, a seismic source and a ground high-precision gravity measuring instrument; the ground broadband magnetotelluric data acquisition system is arranged point by point at equal intervals according to the three-dimensional survey grid, and the ground high-precision gravity measuring instrument is arranged point by point at equal intervals according to the three-dimensional survey grid; the seismic source is arranged point by point at equal intervals according to a three-dimensional seismic source excitation grid. Specifically, the seismic source comprises a ground P-wave source and a ground S-wave source, the ground P-wave source is one of an explosive source, a vibroseis, an air gun source, a weight drop seismic source and a spark source, and the ground S-wave source is a S-wave vibrator; the seismic source has a point interval of 12.5 meters, 25 meters or 50 meters and a line interval of 25 meters, 50 meters or 100 meters. The ground three-component geophones have a point interval of 6.25 meters, 12.5 meters or 25 meters and a line interval of 12.5 meters, 25 meters or 50 meters. The ground broadband magnetotelluric data acquisition system comprises an audio magnetotelluric AMT and a magnetotelluric MT data acquisition module, and the ground broadband magnetotelluric data acquisition system has a point interval of 250 meters or 500 meters and a measuring line interval of 250 meters or 500 meters. The ground high-precision gravity measuring instrument has a point interval of 100 meters or 200 meters and a measuring line interval of 100 meters or 200 meters. Further, the single-mode optical fiber of the ground armored optical cable is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated single-mode optical fiber with high sensitivity and hydrogen loss resistance; The single-mode optical fiber of the downhole armored optical cable is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the multimode optical fiber is a high temperature resistant multimode optical fiber for well temperature measurement; the multimode optical fiber is a hybrid core or pure silicon core carbon-coated multimode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated singlemode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance. An evaluation method for geothermal sweet spot areas adopting the above comprehensive geophysical exploration system for high temperature geothermal fields, comprising the following steps: SI. Arranging the anomalous area detection system in a potential high temperature geothermal field exploration area, processing the infrared remote sensing measurement and radioactive measurement data of the anomalous area detection system, and delineating an anomalous area with high surface temperature and high radioactivity; S2. Arranging the comprehensive geophysical data acquisition system in the anomalous area; (A) If the comprehensive geophysical data acquisition system is the ground data acquisition subsystem: Respectively using the P-wave source and the S-wave source to perform excitation at each prearranged position of the seismic source, and using the ground data acquisition subsystem to acquire ground three-dimensional three-component P-wave data and S-wave data; Respectively processing the ground three-dimensional three-component P-wave data and S-wave data, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in an underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting fine reflected P-wave and reflected S-wave migration imaging of an underground geological structure; (B) If the comprehensive geophysical data acquisition system is the well-ground joint data acquisition system: Acquiring well-ground joint three-dimensional three-component P-wave data and S-wave data, using the DAS / DTS composite modulation and demodulation instrument to acquire three-component P-wave data and three-component S-wave data recorded by the downhole armored optical cable, using the ground three-component geophones and the downhole three-component geophones to synchronously acquire seismic signals excited by the seismic source, and using the DAS / DTS composite modulation and demodulation instrument and the multimode optical fiber in the downhole armored optical cable to simultaneously measure the well temperature and ground temperature gradient data of a whole well; For the downhole three-component P-wave data and S-wave data in well-ground combined mining, firstly, extracting the downhole well flooding parameters (including P-wave velocity distribution and S-wave velocity distribution) of the downhole P-wave data and S-wave data respectively, obtaining a TAR compensation factor and an attenuation coefficient Q, calculating an anisotropy coefficient of the underground three-dimensional space, then conducting well flooding processing with improved resolution and high precision respectively to the ground three-dimensional three-component P-wave data and S-wave data in well-ground combined mining, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in the underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting high-resolution fine reflected P-wave and reflected S-wave depth domain migration imaging of an underground geological structure; Collecting mud logging data, logging-while-drilling data and wireline logging data of all wells in a three-dimensional work area, processing and comprehensively interpreting all the mud logging and well logging data, emphatically analyzing and interpreting various mud logging and well logging data measured by the multimode optical fiber in the downhole armored optical cable, searching for single or composite geophysical parameters sensitive to fluid in high temperature stratigraphic rocks and high temperature strata, and quantitatively calibrating all the sensitive geophysical parameters based on well temperature according to the measured downhole temperature and temperature gradient; S3. Using the ground broadband magnetotelluric data acquisition system to acquire ground three-dimensional broadband magnetotelluric data point by point along the prearranged three-dimensional survey grid; Processing the ground three-dimensional broadband magnetotelluric data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground three-dimensional broadband magnetotelluric data based on the three-dimensional geological structure model to obtain a three-dimensional resistivity distribution model with high resolution; S4. Using the ground high-precision gravity measuring instrument to acquire ground high-density three-dimensional gravity data point by point along the prearranged three-dimensional survey grid; Processing the ground high-density three-dimensional gravity data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground high-density three-dimensional gravity data based on the three-dimensional geological structure model to obtain a three-dimensional density distribution model with high resolution; S5. Searching for an anomalous area with three-dimensional geothermal sensitive parameters of the same burial depth area or an anomalous area with the same three-dimensional geothermal sensitive parameters as those of a high temperature anomalous section of a well in the high temperature geothermal field exploration area; S6. Fusing the data of the anomalous area with three-dimensional geothermal sensitive parameters in the underground three-dimensional space, delineating a common anomalous area with low P-wave velocity, low velocity ratio of P-wave to S-wave, low density and low resistivity, interpreting the result in combination with an underground geological structure, a fault development zone, a fracture development zone and a volcanic rock channel, and judging and evaluating whether the common anomalous area is a target area of a high temperature geothermal field; S7. Evaluating an area with shallow underground burial, high average geothermal gradient and temperature, fault and fracture development, and adequate groundwater recharge as a sweet spot area of the high temperature geothermal field according to the comprehensive geophysical data processing and interpretation of steps S2 to S6. The present invention proposes a comprehensive geophysical exploration system for high temperature geothermal fields and an evaluation method for geothermal sweet spot areas, comprising ground or airborne infrared remote sensing measurement and radioactive measurement arranged in a large area. Ground three-dimensional or well-ground joint three-component P-wave and S-wave data acquisition, ground three-dimensional broadband magnetotelluric data acquisition and ground high-density three-dimensional gravity data acquisition are arranged in an area with high radioactivity. Well logging and DAS-VSP data of wells in a high temperature geothermal field exploration area is used to search for geophysical parameters which are sensitive to high temperature geothermal field anomalies. The data bodies of P-wave velocity field, S-wave velocity field, velocity ratio of P-wave to S-wave, Poisson's ratio, resistivity and density in underground three-dimensional spaces at the same depth are compared and calibrated. Then, a geothermal sweet spot area with high temperature, fault and fracture development, and abundant groundwater recharge is delineated according to the selected geophysical parameters which are sensitive to high temperature geothermal field anomalies. Finally, detailed investigation and comprehensive evaluation are performed to the predicted geothermal sweet spot area, thus to explore and discover a high temperature geothermal field rapidly and efficiently with a low cost. Description of Drawings Fig. 1 is an arrangement diagram of a comprehensive geophysical exploration system for high temperature geothermal fields in an embodiment of the present invention; Fig. 2 is an arrangement diagram of a well-ground joint data acquisition system in an embodiment of the present invention; Fig. 3 is a work flow diagram of an evaluation method for geothermal sweet spot areas in an embodiment of the present invention. Detailed Description For the convenience of understanding the present invention, the present invention will be described below in detail in combination with the drawings and specific embodiments. Preferred embodiments of the present invention are given in the drawings. However, the present invention may be achieved in many different forms and will not be limited to the embodiments described in the Description. On the contrary, the purpose of providing the embodiments is to understand the disclosure in the present invention more thoroughly and comprehensively. The embodiments do not constitute a limitation to the present invention and are merely examples. At the same time, the advantages of the present invention will become clearer and easier to understand by description. In the present invention, the proposed comprehensive geophysical exploration system for high temperature geothermal fields and evaluation method for geothermal sweet spot areas comprises: arranging large-area infrared remote sensing measurement and ground or airborne radioactive measurement in a potential high temperature geothermal field target area in advance; interpreting the result based on the processing of the infrared remote sensing measurement and radioactive measurement data to delineate an anomalous area with high average geothermal gradient (>3°C / 100 m), high surface temperature and high radioactivity; arranging ground three-dimensional or well-ground joint three-component P-wave and S-wave data acquisition, ground three-dimensional broadband magnetotelluric data acquisition and ground high-density three-dimensional gravity data acquisition in the delineate anomalous area. As the groundwater in rock pores of a high temperature geothermal field is turned into water vapor under the action of an underground high temperature, fl-wave velocity is decreased due to the density decrease of the fluid in the high temperature rock pores, but the S-wave velocity will not be changed because of not being affected by the fluid in the rock pores, therefore, the velocity ratio of P-wave to S-wave in the high temperature geothermal field will also be decreased; in addition, due to the evaporation of the groundwater in the high temperature rock pores, a large amount of groundwater and water vapor with low density relative to intact rocks are stored in fault and fracture development zones within and around the high temperature geothermal field, resulting in a relatively low density anomaly in the high temperature geothermal field, and a relatively low resistivity anomaly will also appear due to the large amount of water contained. Well logging and DAS-VSP data of wells in an exploration area with high average geothermal gradient (>3°C / 100 m), high surface temperature and high radioactivity is used to search for geophysical parameters which are sensitive to high temperature geothermal field anomalies. The data bodies of P-wave velocity field, S-wave velocity field, velocity ratio of P-wave to S-wave, Poisson's ratio, resistivity and density in underground three-dimensional spaces at the same depth are compared and calibrated. Then, a geothermal sweet spot area with high temperature, fault and fracture development, and abundant groundwater recharge is delineated according to the selected geophysical parameters which are sensitive to high temperature geothermal field anomalies. Finally, detailed investigation and comprehensive evaluation are performed to the predicted geothermal sweet spot area, thus to explore and discover a high temperature geothermal field rapidly and efficiently with a low cost. An arrangement diagram of the comprehensive geophysical exploration system for high temperature geothermal fields of the present invention is shown Fig. 1, and a well-ground joint data acquisition system is used in Fig. 1. Fig. 2 is an arrangement diagram of a well-ground joint data acquisition system in an embodiment of the present invention. A comprehensive geophysical exploration system for high temperature geothermal fields, comprising an anomalous area detection system 1 and a comprehensive geophysical data acquisition system; The anomalous area detection system 1 is an airborne or ground infrared remote sensing measurement and radioactive measurement system arranged in a large area; The comprehensive geophysical data acquisition system comprises a ground data acquisition subsystem, or is a well-ground joint data acquisition system composed of a ground data acquisition subsystem and a downhole data acquisition subsystem; The ground data acquisition subsystem comprises a ground armored optical cable 21 and a plurality of ground three-component geophones 51, and a single-mode optical fiber is arranged in the ground armored optical cable 21; an extinction device 4 is installed at the tail end of the singlemode optical fiber; the single-mode optical fiber is connected with a DAS / DTS composite modulation and demodulation instrument 7; a shallow gully is dug along a ground measuring line of a three-dimensional survey grid, the ground armored optical cable 21 is buried in the shallow gully, and the plurality of ground three-component geophones 51 are arranged at equal intervals along the ground measuring line; The downhole data acquisition subsystem comprises a downhole armored optical cable 22 and a plurality of downhole three-component geophones 52, and a single-mode optical fiber and a multimode optical fiber are arranged in the downhole armored optical cable 22; an extinction device 4 is installed at the tail end of the single-mode optical fiber; the single-mode optical fiber of the downhole armored optical cable 22 is connected with a DAS signal input end of a DAS / DTS composite modulation and demodulation instrument 7, and the multimode optical fiber of the downhole armored optical cable is connected with a DTS signal input end of the DAS / DTS composite modulation and demodulation instrument 7; the downhole armored optical cable 22 is arranged outside or inside a casing pipe of a drilling hole or outside a pipe string in a cased well, and the plurality of downhole three-component geophones 52 are arranged at equal intervals in the casing pipe; The comprehensive geophysical data acquisition system also comprises a ground broadband magnetotelluric data acquisition system 3, a seismic source 6 and a ground high-precision gravity measuring instrument 8; the ground broadband magnetotelluric data acquisition system 3 is arranged point by point at equal intervals according to the three-dimensional survey grid, and the ground high-precision gravity measuring instrument 8 is arranged point by point at equal intervals according to the three-dimensional survey grid; the seismic source 6 is arranged point by point at equal intervals according to a three-dimensional seismic source excitation grid. The seismic source 6 comprises a ground P-wave source and a ground S-wave source, the ground P-wave source is one of an explosive source, a vibroseis, an air gun source, a weight drop seismic source and a spark source, and the ground S-wave source is a S-wave vibrator; the seismic source 6 has a point interval of 12.5 meters, 25 meters or 50 meters and a line interval of 25 meters, 50 meters or 100 meters. The ground three-component geophones 51 have a point interval of 6.25 meters, 12.5 meters or 25 meters and a line interval of 12.5 meters, 25 meters or 50 meters. The ground broadband magnetotelluric data acquisition system 3 comprises an audio magnetotelluric AMT and a magnetotelluric MT data acquisition module, and the ground broadband magnetotelluric data acquisition system 3 has a point interval of 250 meters or 500 meters and a measuring line interval of 250 meters or 500 meters. The ground high-precision gravity measuring instrument 8 has a point interval of 100 meters or 200 meters and a measuring line interval of 100 meters or 200 meters. The single-mode optical fiber of the ground armored optical cable 21 is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated single-mode optical fiber with high sensitivity and hydrogen loss resistance; The single-mode optical fiber of the downhole armored optical cable 22 is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the multimode optical fiber is a high temperature resistant multimode optical fiber for well temperature measurement; the multimode optical fiber is a hybrid core or pure silicon core carbon-coated multimode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated singlemode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance. The work flow diagram of the evaluation method for geothermal sweet spot areas, as shown in Fig. 3, comprises the following steps: SI. Arranging the anomalous area detection system 1 in a potential high temperature geothermal field exploration area, processing the infrared remote sensing measurement and radioactive measurement data of the anomalous area detection system 1, and delineating an anomalous area with high surface temperature and high radioactivity; S2. Arranging the comprehensive geophysical data acquisition system in the anomalous area; (A) If the comprehensive geophysical data acquisition system is the ground data acquisition subsystem: Respectively using the P-wave source and the S-wave source to perform excitation at each prearranged position of the seismic source 6, and using the ground data acquisition subsystem to acquire ground three-dimensional three-component P-wave data and S-wave data; Respectively processing the ground three-dimensional three-component P-wave data and S-wave data, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in an underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting fine reflected P-wave and reflected S-wave migration imaging of an underground geological structure; (B) If the comprehensive geophysical data acquisition system is the well-ground joint data acquisition system: Acquiring well-ground joint three-dimensional three-component P-wave data and S-wave data, using the DAS / DTS composite modulation and demodulation instrument 7 to acquire three-component P-wave data and three-component S-wave data recorded by the downhole armored optical cable 22, using the ground three-component geophones 51 and the downhole three-component geophones 52 to synchronously acquire seismic signals excited by the seismic source 6, and using the DAS / DTS composite modulation and demodulation instrument 7 and the multimode optical fiber in the downhole armored optical cable 22 to simultaneously measure the well temperature and ground temperature gradient data of a whole well; For the downhole three-component P-wave data and S-wave data in well-ground combined mining, firstly, extracting the downhole well flooding parameters (including P-wave velocity distribution and S-wave velocity distribution) of the downhole P-wave data and S-wave data respectively, obtaining a TAR compensation factor and an attenuation coefficient Q, calculating an anisotropy coefficient of the underground three-dimensional space, then conducting well flooding processing with improved resolution and high precision respectively to the ground three-dimensional three-component P-wave data and S-wave data in well-ground combined mining, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in the underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting high-resolution fine reflected P-wave and reflected S-wave depth domain migration imaging of an underground geological structure, Collecting mud logging data, logging-while-drilling data and wireline logging data of all wells in a three-dimensional work area, processing and comprehensively interpreting all the mud logging and well logging data, emphatically analyzing and interpreting various mud logging and well logging data measured by the multimode optical fiber in the downhole armored optical cable 22, searching for single or composite geophysical parameters sensitive to fluid m high temperature stratigraphic rocks and high temperature strata, and quantitatively calibrating all the sensitive geophysical parameters based on well temperature according to the measured downhole temperature and temperature gradient; S3. Using the ground broadband magnetotelluric data acquisition system 3 to acquire ground three-dimensional broadband magnetotelluric data point by point along the prearranged three-dimensional survey grid; Processing the ground three-dimensional broadband magnetotelluric data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground three-dimensional broadband magnetotelluric data based on the three-dimensional geological structure model to obtain a three-dimensional resistivity distribution model with high resolution; S4. Using the ground high-precision gravity measuring instrument 8 to acquire ground high-density three-dimensional gravity data point by point along the prearranged three-dimensional survey grid; Processing the ground high-density three-dimensional gravity data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground high-density three-dimensional gravity data based on the three-dimensional geological structure model to obtain a three-dimensional density distribution model with high resolution; S5. Searching for an anomalous area with three-dimensional geothermal sensitive parameters of the same burial depth area or an anomalous area with the same three-dimensional geothermal sensitive parameters as those of a high temperature anomalous section of a well in the high temperature geothermal field exploration area, for example, a low P-wave velocity area, a low velocity ratio of P-wave to S-wave area, a low density area, a low resistivity area, etc.; S6. fusing the data of the anomalous area with three-dimensional geothermal sensitive parameters in the underground three-dimensional space, delineating a common anomalous area with low P-wave velocity, low velocity ratio of P-wave to S-wave, low density and low resistivity, interpreting the result in combination with an underground geological structure, a fault development zone, a fracture development zone and a volcanic rock channel, and judging and evaluating whether the common anomalous area is a target area of a high temperature geothermal field; S7. Evaluating an area with shallow underground burial, high average geothermal gradient and temperature (low P-wave velocity area and low velocity ratio of P-wave to S-wave area), fault and fracture development (low density area), and adequate groundwater recharge (low resistivity area) as a sweet spot area of the high temperature geothermal field according to the comprehensive geophysical data processing and interpretation of steps S2 to S6.

Claims

1. A comprehensive geophysical exploration system for high temperature geothermal fields, comprising an anomalous area detection system (1) and a comprehensive geophysical data acquisition system;the anomalous area detection system (1) is an airborne or ground infrared remote sensing measurement and radioactive measurement system arranged in a large area;the comprehensive geophysical data acquisition system comprises a ground data acquisition subsystem, or is a well-ground joint data acquisition system composed of a ground data acquisition subsystem and a downhole data acquisition subsystem;the ground data acquisition subsystem comprises a ground armored optical cable (21) and a plurality of ground three-component geophones (51), and a single-mode optical fiber is arranged in the ground armored optical cable (21); an extinction device (4) is installed at the tail end of the single-mode optical fiber; the single-mode optical fiber is connected with a DAS / DTS composite modulation and demodulation instrument (7); a shallow gully is dug along a ground measuring line of a three-dimensional survey grid, the ground armored optical cable (21) is buried in the shallow gully, and the plurality of ground three-component geophones (51) are arranged at equal intervals along the ground measuring line;the downhole data acquisition subsystem comprises a downhole armored optical cable (22) and a plurality of downhole three-component geophones (52), and a single-mode optical fiber and a multimode optical fiber are arranged in the downhole armored optical cable (22); an extinction device (4) is installed at the tail end of the single-mode optical fiber; the single-mode optical fiber of the downhole armored optical cable (22) is connected with a DAS signal input end of a DAS / DTS composite modulation and demodulation instrument (7), and the multimode optical fiber of the downhole armored optical cable (22) is connected with a DTS signal input end of the DAS / DTS composite modulation and demodulation instrument (7); the downhole armored optical cable (22) is arranged outside or inside a casing pipe of a drilling hole or outside a pipe string in a cased well, and the plurality of downhole three-component geophones (52) are arranged at equal intervals in the casing pipe;the comprehensive geophysical data acquisition system also comprises a ground broadband magnetotelluric data acquisition system (3), a seismic source (6) and a ground high-precision gravity measuring instrument (8); the ground broadband magnetotelluric data acquisition system (3) isarranged point by point at equal intervals according to the three-dimensional survey grid, and the ground high-precision gravity measuring instrument (8) is arranged point by point at equal intervals according to the three-dimensional survey grid; the seismic source (6) is arranged point by point at equal intervals according to a three-dimensional seismic source excitation grid.

2. The comprehensive geophysical exploration system for high temperature geothermal fields according to claim 1, wherein the seismic source (6) comprises a ground P-wave source and a ground S-wave source, the ground P-wave source is one of an explosive source, a vibroseis, an air gun source, a weight drop seismic source and a spark source, and the ground S-wave source is a S-wave vibrator; the seismic source (6) has a point interval of 12.5 meters, 25 meters or 50 meters and a line interval of 25 meters, 50 meters or 100 meters.

3. The comprehensive geophysical exploration system for high temperature geothermal fields according to claim 1, wherein the ground three-component geophones (51) have a point interval of 6.25 meters, 12.5 meters or 25 meters and a line interval of 12.5 meters, 25 meters or 50 meters.

4. The comprehensive geophysical exploration system for high temperature geothermal fields according to claim 1, wherein the ground broadband magnetotellune data acquisition system (3) comprises an audio magnetotelluric AMT and a magnetotelluric MT data acquisition module, and the ground broadband magnetotelluric data acquisition system (3) has a point interval of 250 meters or 500 meters and a measuring line interval of 250 meters or 500 meters.

5. The comprehensive geophysical exploration system for high temperature geothermal fields according to claim 1, wherein the ground high-precision gravity measuring instrument (8) has a point interval of 100 meters or 200 meters and a measuring line interval of 100 meters or 200 meters.

6. The comprehensive geophysical exploration system for high temperature geothermal fields according to claim 1, wherein:the single-mode optical fiber of the ground armored optical cable (21) is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated single-mode optical fiber with high sensitivity and hydrogen loss resistance;the single-mode optical fiber of the downhole armored optical cable (22) is a three-component distributed optical fiber acoustic sensing optical fiber arranged on a cylindrical elastomer, or a spiral optical fiber wound on the cylindrical elastomer in the manner of a spiral pipe; the multimodeoptical fiber is a high temperature resistant multimode optical fiber for well temperature measurement; the multimode optical fiber is a hybrid core or pure silicon core carbon-coated multimode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance; the single-mode optical fiber is a hybrid core or pure silicon core carbon-coated singlemode optical fiber with high temperature resistance, high sensitivity and hydrogen loss resistance.

7. An evaluation method for geothermal sweet spot areas, adopting the comprehensive geophysical exploration system for high temperature geothermal fields according to any one of claims 1 to 6, and comprising the following steps:SI. arranging the anomalous area detection system (1) in a potential high temperature geothermal field exploration area, processing the infrared remote sensing measurement and radioactive measurement data of the anomalous area detection system (1), and delineating an anomalous area with high surface temperature and high radioactivity;S2. arranging the comprehensive geophysical data acquisition system in the anomalous area;(A) if the comprehensive geophysical data acquisition system is the ground data acquisition subsystem:respectively using the P-wave source and the S-wave source to perform excitation at each prearranged position of the seismic source (6), and using the ground data acquisition subsystem to acquire ground three-dimensional three-component P-wave data and S-wave data;respectively processing the ground three-dimensional three-component P-wave data and S-wave data, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in an underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting fine reflected P-wave and reflected S-wave migration imaging of an underground geological structure;(B) if the comprehensive geophysical data acquisition system is the well-ground joint data acquisition system:acquiring well-ground joint three-dimensional three-component P-wave data and S-wave data, using the DAS / DTS composite modulation and demodulation instrument (7) to acquire three-component P-wave data and three-component S-wave data recorded by the downhole armored optical cable (22), using the ground three-component geophones (51) and the downhole three-component geophones (52) to synchronously acquire seismic signals excited by the seismic source (6), and using the DAS / DTS composite modulation and demodulation instrument (7) and the multimode optical fiber in the downhole armored optical cable (22) to simultaneously measure the well temperature and ground temperature gradient data of a whole well;for the downhole three-component P-wave data and S-wave data in well-ground combined mining, firstly, extracting the downhole well flooding parameters (including P-wave velocity distribution and S-wave velocity distribution) of the downhole P-wave data and S-wave data respectively, obtaining a TAR compensation factor and an attenuation coefficient Q, calculating an anisotropy coefficient of the underground three-dimensional space, then conducting well flooding processing with improved resolution and high precision respectively to the ground three-dimensional three-component P-wave data and S-wave data in well-ground combined mining, obtaining three-dimensional P-wave velocity distribution and three-dimensional S-wave velocity distribution of underground rock strata, extracting three-dimensional P-wave impedance, extracting three-dimensional S-wave impedance, calculating the velocity ratio of P-wave to S-wave in the underground three-dimensional space, calculating the Poisson's ratio in the underground three-dimensional space, and respectively conducting high-resolution fine reflected P-wave and reflected S-wave depth domain migration imaging of an underground geological structure;collecting mud logging data, logging-while-drilling data and wireline logging data of all wells in a three-dimensional work area, processing and comprehensively interpreting all the mud logging and well logging data, emphatically analyzing and interpreting various mud logging and well logging data measured by the multimode optical fiber in the downhole armored optical cable (22), searching for single or composite geophysical parameters sensitive to fluid in high temperature stratigraphic rocks and high temperature strata, and quantitatively calibrating all the sensitive geophysical parameters based on well temperature according to the measured downhole temperature and temperature gradient;S3, using the ground broadband magnetotelluric data acquisition system (3) to acquire ground three-dimensional broadband magnetotelluric data point by point along the prearranged three-dimensional survey grid;processing the ground three-dimensional broadband magnetotelluric data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground threedimensional broadband magnetotelluric data based on the three-dimensional geological structure model to obtain a three-dimensional resistivity distribution model with high resolution;S4. using the ground high-precision gravity measuring instrument (8) to acquire ground high-density three-dimensional gravity data point by point along the prearranged three-dimensional survey grid;processing the ground high-density three-dimensional gravity data, constructing an underground three-dimensional geological structure model based on the three-dimensional depth domain migration imaging result, and conducting constrained inversion to the ground high-density three-dimensional gravity data based on the three-dimensional geological structure model to obtain a three-dimensional density distribution model with high resolution;S5. searching for an anomalous area with three-dimensional geothermal sensitive parameters of the same burial depth area or an anomalous area with the same three-dimensional geothermal sensitive parameters as those of a high temperature anomalous section of a well in the high temperature geothermal field exploration area;S6. fusing the data of the anomalous area with three-dimensional geothermal sensitive parameters in the underground three-dimensional space, delineating a common anomalous area with low P-wave velocity, low velocity ratio of P-wave to S-wave, low density and low resistivity, interpreting the result in combination with an underground geological structure, a fault development zone, a fracture development zone and a volcanic rock channel, and judging and evaluating whether the common anomalous area is a target area of a high temperature geothermal field;S7. evaluating an area with shallow underground burial, high average geothermal gradient and temperature, fault and fracture development, and adequate groundwater recharge as a sweet spot area of the high temperature geothermal field according to the comprehensive geophysical data processing and interpretation of steps S2 to S6.

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