Forward And Inverse Calculation And Data Interpretation Of Transient Electromagnetic Method In Urban Underground Spac

With the rapid development of the domestic economy,accurate detection of underground space is the key to ensuring its safe operation.As a commonly used underground detection method,the transient electromagnetic method has the advantages of non-destructive testing,high detection resolution,large detection depth,convenience,and low cost.It is widely used in mineral resource exploration,hydrological environmental survey,geological survey,and archaeological excavation and other fields.The electromagnetic data processing method is the key link to realize electromagnetic detection and high-resolution imaging.This paper mainly focuses on two aspects of the forward and inverse calculation of the time domain electromagnetic method in the typical structure of urban underground space,and the preprocessing and interpretation of the measured data.Starting from the working principle of the transient electromagnetic detection instrument,the thesis introduces the transient electromagnetic response theory of the layered medium model of the center loop source and uses the numerical filtering method of fast Hankel transform and cosine transform to simplify the calculation process.Combined with the simulation,it is determined that the imaginary component of the frequency domain response is substituted into the solution of the transient electromagnetic response to improve the calculation accuracy.Aiming at the special application environment—urban underground space detection,a simulation model based on the section structure of the subway station was designed,and reasonable dielectric layer parameters were selected.Perform forward simulation analysis on the model,and qualitatively analyze the influence of underground abnormal structures on the response based on the simulation results.This can provide references for the inversion calculation results and accumulate observation experience to provide guidance for the interpretation of measured data.The basic principle of the damped least squares inversion algorithm and the calculation method of the Jacobian matrix and the damping factor are systematically introduced.Aiming at the problems of large difference in magnitude of inversion model parameters and low calculation efficiency in the application of transient electromagnetic method in urban underground space detection,an improved scheme of damped least squares method is introduced.In order to improve the stability of the inversion iteration convergence and the calculation speed,under the premise of ensuring the accuracy requirements,the inversion calculation method for fixing all the permeability parameters is determined according to the simulation calculation.Perform inversion simulation analysis on the designed typical singlepoint model to solve the conductivity parameters and thickness parameters of each layer of media.In the calculation process,the inversion results under the three selected effective electromagnetic parameters of reinforced concrete were compared.According to the inversion imaging effect,the approximate distribution of the underground space structure can be distinguished.The typical underground space environment-tunnels and subway stations are explored in the field,and the self-developed transient electromagnetic method instrument system is used to measure point by point.The test results are preprocessed by transient superposition,FIR filtering,and channel superposition to improve the signal-to-noise ratio.According to the test results and the observations of the field environment,the horizontal position distribution of the subway station and tunnel construction structure is qualitatively analyzed.According to the specific conditions of the survey area and the inversion simulation experience,a reasonable geoelectric model is designed to improve the calculation stability and calculation speed.Take the inversion calculation results of the underground space structure part to draw a twodimensional cross-sectional image of the underground space structure.The imaging results show that the inversion effects of the two survey areas are basically consistent with the actual observations.The algorithm can more accurately calculate the approximate contour distribution of the underground structure.