Application Of CSAMT 3D Inversion To Geothermal Exploration In Manjiang Region

As a kind of clean energy,geothermal resources have great development potential and play an important role in my country's economic development.Changbai Mountain is an area with relatively strong volcanic activity in modern times,and there are abundant geothermal resources underground.At present,many hot springs and geothermal wells have been discovered in Songjianghe,Erdaobaihe and other places.Maniang is covered by volcanic activities in Changbai Mountain,which has potential conditions for geothermal resources.In order to find out the geological characteristics under the basalt in Manjiang and the distribution of strata and structures in the study area,a lot of geophysical exploration work has been carried out in this area by predecessors,among which the controllable source audio frequency magnetotelluric method is the most widely used.However,due to the limitation of data inversion theory and technology,the data collected in the past mainly used one-dimensional and two-dimensional inversion.Because of the complex geological structure in Manjiang area,the two inversion methods can not reflect the complex underground electrical structure.In this paper,the finite memory quasi Newton three-dimensional inversion method is used to process and inverse the field measured data of a work area in ManJiang town,and the geothermal occurrence conditions in the study area are evaluated objectively.In order to find geothermal resources in Manjiang town area and lay a good foundation for the establishment of new energy sources.Firstly,a theoretical model is established to verify the effectiveness of the inversion method.Then,the geophysical and drilling data of this area are collected,and the controllable source audio frequency magnetotelluric data in this area are checked point by point.The data are smoothed and the jumping point data are eliminated.Combined with the geological data of the work area,10000?·m is set as the background resistivity of the three-dimensional inversion model.The inversion grid adopts 1/2 point spacing,and the inversion unit is divided into 462875(115×115×35)grids,the Ex component of complex electric field is selected for 3D inversion,and the influence of different frequency on inversion effect is discussed.Finally,the inversion results of appropriate frequency are selected for comprehensive geological interpretation.Combined with drilling results and inversion results,the strata are divided into three structural layers.The first structural layer is quaternary+Miocene basaltic rocks;the second structure is andesite,rhyolite and clastic rock of Guolin formation(J2-3g)of middle and Upper Jurassic;The third structural layer is andesite and granite layer of Triassic Changbai formation(T3c).four NW trending faults F1?F4 are delineated in the ne resistivity profile perpendicular to the overtopping valley;and two NE trending faults F5?F6 are also delineated in the NW resistivity profile parallel to the Manjiang valley.Finally,three thermal reservoir structures are inferred based on the above results.The results and conclusions are as follows1.In this paper,the finite memory quasi Newton inversion method is used to study the method and process of three-dimensional inversion of controllable source audio-frequency magnetotelluric data,the measured data of Manjiang work area are inversed,and the three-dimensional electrical structure diagram is obtained;2.In this paper,the inversion results of different frequency bands are compared and studied.It is found that the frequency of partial inversion plays a decisive role in the inversion results,and the appropriate frequency should be selected for inversion of measured data;3.This paper studies the stratigraphic structure of Manjiang and its surrounding areas,divides the geological structural layers,analyzes the lithology and water rich property of each structural layer,divides the concealed fault structure,and discusses the nature and water bearing property of the fault.It provides background information for accurate interpretation of deep geothermal resources exploration and electromagnetic detection results;4.This paper infers three thermal reservoir structures,which provide important information for the further development of geothermal resources.