Analysis Of Electrical Structure And Heat Source In The Crust Of Songyuan City

Fossil fuels such as coal and Oil play a leading role in China's energy structure.At the same time,in order to solve the problem of environmental degradation and the depletion of non renewable energy,as a new type of renewable geothermal energy has become the goal of gradual exploration.Songliao basin is the largest Mesozoic Cenozoic sedimentary basin in Northeast China,and it is also a potential oil and heat storage basin.Songyuan City,located in the central depression of Songliao Basin,has extremely high heat flow and geothermal gradient.Therefore,in order to explore the geothermal system of Songyuan City,it is necessary to study the underground heat source in this area.We have set up 25 broadband magnetotelluric observation points in Songyuan City and its surrounding areas.The distance between the points is about 10 km,and the observation time of each point is not less than 20 hours.After the original time series are processed into EDI file of standard format by the software of the instrument,we analyze the phase tensor of all the measuring points.The results show that the measuring points below and in the north of Songyuan City show strong threedimensional,so it is reasonable to choose three-dimensional inversion.In order to obtain a stable conductivity model,we use different grid sizes and initial models to determine the stable inversion results.At the same time,in order to verify the reliability of the inversion results,we also carried out the forward and inverse verification of the resistivity of the replacement model for the low resistivity anomaly in the middle crust,which proved the existence of the low resistivity anomaly.The conductivity model shows that in the upper crust of Songyuan City,there is a low resistivity anomaly with a trend of approximately north-south,and most of the anomalies are distributed near large faults or concentrated epicenters;in the middle crust,there is a low resistivity anomaly with a trend of approximately east-west,and compared with the upper crust,the location of the low resistivity anomaly is concentrated in the lower part of Songyuan City and the northwest side,which shows a trend Banded low resistivity anomaly.Based on the previous geophysical exploration results in this area and the three-dimensional inversion results in this paper,it is shown that there is a partial melting layer in the crust below Songyuan City.According to the corresponding relationship between the attenuation amplitude of seismic S-wave velocity and the melting ratio,it is inferred that the melting ratio is about 4%-8%.The higher heat in the partial melting layer will continue to transfer upward along the large faults,and will be hosted in the high resistivity rock or fluid in the middle and upper crust after being blocked by the surface mudstone caprock.Therefore,the partial melting layer may form a heat source in the geothermal system.