Magnetotelluric Imaging Of The Zhangzhou Basin Geothermal Zone,southeastern China

With the continuous growth of human demand for energy and the continuous consumption of fossil energy,the world is increasingly faced with the problem of exhaustion of fossil energy and environmental pollution.For a long time,scholars around the world have been exploring and trying to find alternative energy sources.Among them,geothermal energy,as a renewable energy,has been gradually recognized and utilized.As a supplement to traditional energy,it plays a pivotal role in energy structure restructuring,energy conservation and emission reduction,coping with global climate change and improving people's living standards.Zhangzhou Basin,located in the geothermal region along the southeast coast,has abundant natural geothermal resources.In order to understand the formation mechanism of the geothermal resources in the study area,this paper mainly use four magnetotelluric profiles to carry out a detailed study on the electrical structure of the study area.The thermal structure,some new knowledge about the formation mechanism of the heat source and geothermal resources in the study area is obtained.A total of 157 high-quality magnetotelluric sites are selected through fourier transform,robust estimation,remote reference processing and power spectral selection.The Swift,Bahr and phase tensor decomposition algorithm are adopted to extract the dimensional information.The study area is two-dimensional structure while there is three-dimensional feature in the local and deep regions.In addition,The GB multi-frequency multi-site decomposition method is used to analyze the electrical strike of the study area.It is determined that the tectonic strike direction of the study area is NE,which is consistent with the regional tetonic strike direction.To obtain the reliable underground electrical structure,this paper also used the Niblett-Bostick method to calculate the penetration depth,and finally determined the inversion depth of 50 km.Because the study area is close to the coast of the east China sea and the resistivity of sea water is greatly different from that of the mainland,the "coastal effect" is bound to distort the electrical data.In this paper,a model is established for the actual situation of the study area to conduct forward and inversion research,analyze the influence of the coastal effect on the measured data,and find the suitable inversion method.In this paper,2D and 3D inversion of electromagnetic method data are carried out respectively,and finally the electrical structure model along the section of the study area was obtained.Based on these electrical structure models and combined with the existing geological and geophysical data of the study area,we have the following understanding of the electrical structure and the formation mechanism of geothermal resources in the study area:(1)The structure of Zhangzhou Basin is roughly divided into three electrical layers from shallow to deep.The first layer is a low resistivity layer,resistivity of dozens to hundreds ohms,vertical distribution is not uniform;The second layer is a high-resistivity layer with resistivity ranging from several thousand to tens of thousands ohms,which is widely distributed in the upper and middle crust of the study area.The third layer is a relatively low resistivity layer with a resistivity of several hundred ohms.It is also found in this study that there are non-uniform high conductors in the middle high resistivity layer.(2)The inversion results of four profiles show that there are 200-400 m low resistivity layers on the surface of the study area,mainly Quaternary sediments and Jurassic volcanic rocks.The low resistivity layer is thicker in the southwest and east of the study area at the mouth of Jiulongjiang river.(3)The results show that high resistivity bodies are widely distributed in the upper and middle crust at a depth of 20 km.As the study area is located at the contact position between the Philippine plate and the Eurasian plate,it has undergone many magmatic tectonic activities,especially the influence of Yanshanian activity.The extensive distribution of granitic bodies and the electrical structure indicate that the intrusive rocks have a wide distribution range and a large depth.Due to the existence of Th,U and K,radioactive heat is one of the main heat sources in the study area.(4)The middle and upper crust is distributed with uneven high conductors,which are mainly distributed along deep and large faults.The longitudinal extension of fault zone controls the transverse distribution of high conductors.According to preliminary judgment,the existence of high conductors is related to the upwelling of asthenosphere along the fault zone in the extensional environment.Surface thermal resources show that hot springs' temperature is relatively high in the places where there are high conductors in the upper crust,indicating that the existence of high conductors can promote the formation of high temperature hot springs.(5)The cold atmospheric precipitation,which permeated and converged in the faults,became warmed by the bedrock.The heat of the bedrock mainly comes from heat generated by the radioactive decay of granites and upper mantle,forming medium and low-temperature hot springs in the favorable parts due to the effect of pressure difference.In this process,the faults supplied the space and channels for groundwater transportation.In some places,due to the presence of anomalous low-resistivity zones in the middle crust,the temperature of the hot springs will be higher.