Electrical Structure Of Geothermal Area In Northeast Of Datong Basin

Datong area is an important coal producing area in China.Due to the depletion of coal resources and environmental pollution,the adjustment of energy structure in Datong area is in urgent need of solution.The study found that Datong Basin is rich in geothermal resources,and geothermal resources,as a renewable and clean energy,have great potential in energy structure adjustment and new energy development and utilization because of its large reserves,wide distribution,renewable,pollution-free and climate-free characteristics.Therefore,the scientific research,development and utilization of geothermal resources in Datong Basin plays a decisive role in the development of local economy and society.In the past,the research on geothermal resources in Datong Basin is carce,and mainly concentrated in the shallow layer.The geothermal geological characteristics of the geothermal area in the northeastern part of Datong Basin have an obvious response.In order to scientifically and sustainably use the geothermal resources in this area,it is urgent to explore the distribution law and formation mechanism of the geothermal resources in the geothermal area in the northeastern Datong Basin,and understand the geothermal system key elements(including heat source,reservoir,cap rock,etc.).Therefore,this study first selects the Tianzhen and Yanggao areas in the northeast of Datong Basin as the key research areas for geothermal resources.Ten magnetotelluric survey lines,two magnetotelluric encryption zones,and two audio-frequency magnetotelluric survey lines are arranged in the study area,and the time series of the magnetotelluric and audiofrequency magnetotelluric raw data are subjected to Fourier transform,Robust estimation,and distance measurement.Reference and other preprocessing to obtain high-quality data information.Then,Swift decomposition,GB decomposition and phase tensor decomposition were performed on ten MT survey lines and two AMT survey lines,and the two-dimensionality of the underground medium of the survey lines was determined to be better.The GB multi-frequency and multi-point decomposition method was used to statistically analyze MT and the electrical principal axis information of the underground medium of the AMT survey line determines the electrical principal axis direction of the MT and AMT survey lines.In addition,the Niblett-Bostick method is used to calculate the skin depth of the above survey lines,and finally the inversion depths of the MT and AMT survey lines are determined to be 15 km and 1km,respectively.At the same time,the phase tensor decomposition method is used to analyze the dimensionality of the magnetotelluric data of the two encryption areas,and it is determined that the two encryption areas have three dimensional characteristics.Subsequently,the two dimensional inversion of MT and AMT survey lines was carried out,and the three dimensional inversion of MT survey lines in the dense area was carried out to comprehensively analyze and study the electrical structure characteristics of the geothermal area in the northeast of Datong Basin.Combined with the existing geothermal geology,geochemistry and comprehensive geophysical data,the location of GR1 high temperature geothermal well is delineated,and the geothermal structure model,key elements of geothermal system and the formation mechanism of geothermal resources are determined;finally,according to the electrical structure characteristics of the study area and the geophysical characteristics of hot dry rock(HDR),combined with the geothermal drilling GR1 and D1 Determine the possibility of the existence of hot dry rock geothermal resources in the area.Based on the above research,the following understandings are obtained:(1)According to the analysis of existing data,it is determined that the Datong Basin is rich in geothermal resources and its northeast is a key research area for geothermal resources,and Tianzhen and Yanggao dense areas are delineated as refined research areas.(2)The inversion results show that there are two different geoelectric structure models in the study area.One is the surface low resistivity layer,and the lower one is the relatively low resistivity area.The deep low resistivity body is connected with the shallow low resistivity layer through the relatively high resistivity region;in another geoelectric model,the resistivity is distributed in layers,from top to bottom,in the order of relatively low resistivity,high resistivity,relatively low resistivity and low resistivity.(3)The three dimensional geothermal geological model in the study area was established,and the geothermal system in the study area was determined as high temperature convection conduction geothermal system,and the key elements of the geothermal system in the study area are shown.There are two types of geothermal system in the study area: conduction geothermal system and convection geothermal system.The conductive geothermal system is mainly distributed in the south of the study area.The heat source,heat reservoir and cap rock are partial melting body,d3 electric layer and d2 electric layer respectively.The convective geothermal system is distributed in the northern part of the study area.The geothermal resources are mainly geothermal fluid.The heat source,heat reservoir and cap rock are deep partial melting body,Archean metamorphic rock and clay layer.The heat source,reservoir and cap rock are connected by deep faults.(4)The partial melting body as the heat source in the study area comes from two forms: after the eruption of Datong Volcano Group,the magma not ejected from the surface migrates to NE direction of the basin along the crustal fissure and forms partial melting body;The craton failure of the Datong Basin corresponds to the western part of North China,and the asthenosphere appears upwelling.Mantle activity transfers heat to the crust,which thickens and decompresses the crust under the action of plate compression,and the crust is dehydrated and extruded to form partial melt.(5)Through MT & AMT detection results,the scope of geothermal resources is delineated,and the location of geothermal borehole in well GR1 is determined.After drilling well GR1,a very high temperature is obtained.By analyzing the geochemical data of geothermal borehole GR1,it is determined that the hot water of geothermal resources mainly comes from the infiltration and recharge of atmospheric precipitation in the northern mountainous area.At the same time,it can be inferred that there are hot dry rock type geothermal resources in the study area,and they are mainly distributed in the central and southern parts of the study area.The southern hot dry rock show structural deformation dominated by thermodynamics,and the deep heat source diapir uplift is controlled by vertical heat conduction.The formation of shallow geothermal resources,while the central hot dry rock is mainly magma intrusion,through deep and large faults,high temperature anomalies are formed in the shallow.