Estimation Of Terrestrial Heat Flow In Hydrothermally Active Areas Based On Long-Term Bedrock Temperature Observations

High ground temperature is an important engineering geological problem that must be faced in the planning and construction of underground projects such as tunnels.At present,many underground projects have been adversely affected by hot disaster of high temperature to the project progress or project participants.The terrestrial heat flow is a physical parameter that characterizes the underground thermal state and thermal process,and is also an important research content of geodynamics.The measurement of terrestrial heat flow usually utilizes deep boreholes.In hydrothermally active areas,boreholes need to reach a depth of 500-1000meters to exclude the influence of fluid migrates.Limited by the difficulties,costs and observations range of drilling work,underground temperature observation data is very limited.In recent years,the borehole multi-channel bedrock ground temperature observation technology has made a major breakthrough.Based on high-precision and multi-depth bedrock temperature observations,fluid migration information in shallow crust can be analyzed.Because the subsurface fluid migrates causes thermal convection to change the temperature field,the heat flux in the hydrothermally active area is easily disturbed,it is necessary to remove the influence of thermal convection in the observation of terrestrial heat flow.Based on previous studies,this paper presents a new method of calculating geothermal heat flow suitable for removing the influence of underground fluid migration in hydrothermal active areas:Based on the multiyear observation results of bedrock geothermal temperature at different depths of shallow boreholes,the underground fluid migration informations such as velocity and flow direction is solved by temperature and depth profiles,and then the heat flow is fitted by one-dimensional heat conduction equation.The value of geothermal heat flow that removes the influence of fluid migration is calculated.Through the theoretical model verification,the data are in good agreement,which verifies the reliability of the method.Using this method,based on the observation data of the bedrock geothermal observation network with six boreholes in the northwest Yunnan area formed by the Geological Institute of China Earthquake Administration at the end of 2017 for more than four years,the paper calculates the underground fluid migration information in this area,and on this basis,calculates the corrected heat flux value of the observation point.The main insights and progress gained in the paper are as follows:（1）Based on the results of shallow long-term bedrock temperature-time series observations,this paper exploratively proposes a new method for calculating geothermal flow in hydrothermally active area:firstly,the underground fluid migration information such as flow velocity and flow direction are solved by temperature-depth profile,and then the heat flow is fitted with a one-dimensional heat transfer equation to remove the influence of fluid transport,and then the geothermal flow value is calculated.Through the theoretical model verification,the data match is very good,which verifies the reliability of the method.（2）Based on the four-year bedrock temperature observation data of six measuring points in northwestern Yunnan,the flow direction and velocity of each measuring point are calculated:The results show that the flow rates of underground fluid at the measuring points of GYG,DL and EY are 3.170×10^-8,9.468×10^-9and 3.420×10^-8m/s,respectively,and the flow direction is downward.The flow rates of underground fluid at the measuring points of ZNC,LJ and MD are-6.275×10^-9,-3.345×10^-9,-2.085×10^-8m/s,respectively,and the flow direction is upward.（3）Using the calculated flow velocity,the heat flow is corrected,and the regional deep heat flow background value is obtained.The corrected heat fluxes at GYG,ZNC,Dali,EY,LJ and MD are 130.3,115.7,97.1,103.1,54.5 and 90.4 m W/m².（4）Comparing and analyzing the corrected heat flux and uncorrected heat flow values,the results show that the relationship between the corrected heat flux and the uncorrected heat flow is related to the groundwater flow rate.After the groundwater migrates downward,the corrected heat flux is greater than the uncorrected heat flow.Therefore,the groundwater migration mode has an effect on the correction of heat flow.