| Transient electromagnetic method,a new method for electrical prospecting,has been used widely in hydrogeological investigation,because of its sensitivity to low resistivity.The magnetic source TEM ihas been a common construction method due to its high construction efficiency,large depth of detection and low cost.In this paper,based on the principle of transient electromagnetic exploration,the transient electromagnetic response characteristics of the magnetic source are studied:(1)Based on the theory of large fixed source transient electromagnetic theory and the Maxwell's fundamental equation,the formula for calculating the response of large source is derived.(2)On the plane rectangular coordinate system,the large frame of the magnetic source is divided into a plurality of small electric dipole,the responset to a point in the coordinate system is calculated,and then accumulate the integral to calculate the total response value.(3)Use the mathematical dichotomy,and program to calculate the apparent resistivity.When there is a double solution,the discrimination method is given:the magnetic field integral is used in the early stage,and the latter is calculated by the method of the central loop.(4)In this paper,the spatial distribution of the TEM response of a large magnetic source is studied.By changing the size of the frame and the resistivity value of the model,the characteristics of the boundary effect and the response value are analyzed.(5)Write the calculation program.The stability and validity of the program for calculating the apparent resistivity of the large fixed source TEM are shown by the simulation of different geoelectric models and examples.The research show that the method of calculating the apparent resistivity by using the large fixed source can better reflect the electrical characteristics of the shallow stratum,and it is better for the stratification of strata. |
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