Three-dimensional electrical structure model of the Yangbajain geothermal field in Tibet:Evidence obtained from magnetotelluric data

The Yangbajain Geothermal Field in Tibet is located in the fault subsidence basin of the central Yadong-Gulu Rift Valley.The spatial distribution of the field is controlled by mountain-front fault zones on the northwestern and southeastern sides of the basin.Geothermal power has been generated in Yangbajain for more than 40 years.However,owing to the lack of threedimensional(3D) geophysical exploration data,key geological issues related to the partial melt body of the Yangbajain Geothermal Field,such as its location,burial depth,and geometric form,as well as the ascending channel of the geothermal fluid,have for a long time been controversial.In this study,3D inversion was performed using measured geo-electromagnetic total impedance tensor data from 47 survey points.The extracted horizontal sections at different depths and profiles,and at different lines,reflect the 3D electrical structure model of the geothermal field in the study area.Subsequently,three findings were obtained.First,the partial melt body,located below the China-Nepal Highway extending along the northeast direction,is the heat source of the Yangbajain Geothermal Field.The burial depth range of the molten body was determined to range between approximately 6.2 and 14 km.Moreover,the geothermal fluid ascended a horn-shaped circulation channel with an up-facing opening,located in the northern section of the sulfur ditch area.The study results revealed that deep rock fissures(>2 km) were not well developed and had poor permeability.In addition,no layered heat reservoirs with high water richness were observed in the northern part of the study area.However,the application of enhanced geothermal system(EGS) technology in the northern region would be essential to improving the power generation capacity of the Yangbajain Geothermal Field.In addition,the study found no deep high-temperature heat storage areas in the southern region of the study area.