| The Changbai Mountain volcanic region is located at the subduction front of the Pacific Ocean,with active geological and tectonic movements.It is considered to be a dormant volcano with potential eruption,posing a significant threat to human life and property.Meanwhile,the region is fully fractured and there are several hot spring clusters in the area,which are rich in geothermal resources.However,the Changbai Mountain volcanic region is covered by a large area of Cenozoic igneous rocks,which makes it difficult to understand the internal tectonic features and causes difficulties in geothermal resource development and volcanic hazard prediction.Geophysical survey methods,such as gravity survey,magnetic survey,electric survey and seismic survey,are important tools for geothermal resource exploration.However,the topography of Changbai Mountain volcanic area is complex and the vegetation covers a large area,which makes the construction difficult and the economic cost high when using electric and seismic exploration methods.The gravity and magnetic exploration method has the advantages of low detection cost,wide coverage and large detection depth,and plays an important role in identifying fracture structures and tracing hidden rock and ore bodies.In this paper,we use this method to invert and interpret Changbai Mountain volcanic area,thus revealing the distribution of Moho surfaces and Curie surface,detecting the fracture structures related to thermal anomalies,and characterizing the spatial distribution of density and magnetic in the deep subsurface.At the same time,the distribution of regional geothermal heat flow and the cooling model of tectonic magma capsule are calculated by deep learning methods to study the spatial and temporal evolution of the geothermal field,which could improve the geothermal genesis mechanism and provide theoretical support for the development of geothermal resources in the region.The geological overview of the Changbai Mountain volcanic region is studied,and regional tectonic features,magmatic activity,stratigraphic lithology and hot spring distribution are analyzed.Several large fracture zones,mainly in the north-west,north-east and north-northeast directions,have formed the main tectonic pattern of Changbai Mountain and its surrounding area since the Cenozoic.The Yalu River-Zengfeng Mountain rift and the Ma'anshan-Sandaobaihe rift,which pass through the Tianchi volcano,provide magma transport channels.At the same time,the study area has many hot springs with high water temperature and large water gush,reflecting that the area has good prospects for geothermal resource exploration.The satellite gravity and magnetic data are processed and interpreted to understand the deep tectonic structure of the Changbai Mountain volcanic region.The classical Parker-Oldenburg interface inversion method is investigated,and the Moho surface and the interior of the Curie surface are inferred from the regional gravity and magnetic data.The results show that the maximum depth of Moho in this region is about 44 km,which is located in the region of Korean.At the same time,there is a secondary Moho depth center about 25 km southeast of Tianchi,which is more than 40 km deep and gradually becomes shallow to the surrounding.The burial depth of Curie is shallow in the study area,and the Curie uplift area is located in Tianchi,Changbai and Linjiang areas,which has a good corresponding relationship with hot spring points and faults.The theory of three-dimensional gravity and magnetic inversion method is studied.A fast forward algorithm based on sensitivity matrix of regular grid is studied to construct pseudo sensitivity matrix,which can reduce the calculation time and improve the calculation speed.Through theoretical analysis and model test,the influence of different weighting functions,such as depth weight function,model constraint function,geometric constraint condition and physical property upper and lower limit constraint,on inversion algorithm is studied.An inversion algorithm based on adaptive quadtree data compression technology is proposed.This method can realize fine sampling where the data gradient is large and sparse sampling where the data gradient is small,so as to reduce the amount and weight of inefficient information data,reduce the required computer memory,and improve the inversion speed and efficiency.The data from the Changbai Mountain volcanic region are separated by combining upward extension method,wavelet multiscale analysis and radial logarithmic power spectrum.Meanwhile,the information separated by different methods is fused by data fusion technology based on wavelet transform,so that the local information is optimized and enhanced.The 3D gravity inversion algorithm is used to invert the separated local fields and finely characterize the subsurface density and magnetic structure.The gravity inversion results show that there are three important deep faults,including Dunhua-Mishan fault,Fuerhe-Gudonghe fault and Ma'anshan-Sandaobaihe fault.In addition,there are several NW,NNW and NS faults passing through the Tianchi area.The gravity inversion results also show that there are large-scale and low-density bodies under the Changbai Mountain,which are distributed from 5 km to 40 km depth in the shallow,and are considered to be the main part of the magma.In addition,there are low-density bodies which diverge from the main body of the magma and have relatively high density,which are considered to be the channels for the deep magma to migrate to the shallow.The results of magnetic inversion show that there are dense,discontinuous and staggered distribution of high and low magnetic anomalies in the shallow part,so it is inferred that it is a fracture zone,which is conducive to the enrichment and migration of hot water.The distribution characteristics of geothermal field in Changbai Mountain volcanic region are studied.Based on the statistical laws of 17 kinds of geological parameters such as crustal thickness,topography,bouguer gravity anomaly,magnetic anomaly and geothermal flow,a deep neural network training model is constructed.The reliability of the training method is verified by the measured heat flux in the nearby area,and then the heat flux in the study area is successfully predicted.The results show that the isoline of the geothermal flow value looks like an elliptical closed curve,and the major axis of the ellipse is near the northeast direction.In Tianchi,Changbai and their surrounding areas,the high value of geothermal flow is abnormal.The highest geothermal flow value is in the south of Tianchi,which is higher than 84 m W/m~2,and gradually decreases to the surrounding areas.It is speculated that there is a huge heat source material in the deep.At the same time,by constructing the cooling model of high temperature magma,the influence of heat flux and magma temperature on the temporal and spatial evolution characteristics of temperature field is studied.The results show that the high temperature magma continuously transfer heat to the surrounding strata,which promotes the rise of geothermal gradient in the region.In addition,the temperature of magma has a great influence on the temperature field,and the temperature of magma is basically cooled within 1 Ma without heat supplement.On the basis of the study,a model of the regional thermal genesis of Changbai Mountain volcanic is given.The geothermal resources of Changbai Mountain are deep magma type geothermal,and the heat is mainly from the huge magma capsule in the crust below the crater.The deep magma is transported upward along the fracture channel,which leads to the increase of geothermal heat flow value,while the existence of mantle heat channel makes the heat from deep supply the magma capsule in crust continuously.In summary,through the investigation of geothermal geological conditions in Changbai Mountain volcanic region,the processing and inversion interpretation of gravity and magnetic data,and the study of temperature field distribution characteristics,we have a clear understanding of the geothermal mechanism in Changbai Mountain volcanic region,which is of great significance to geothermal resource exploration,evaluation and development. |
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