Finite-difference Approach For 3D Transient Electromagnetic Modeling Based On Non-uniform Grid

Transient electromagnetic method has made great progress, but there are still deficiencies. In the forward modeling, it should not be confined to the theoretical physical model such as magnetic dipole and electric dipole. It should focus on the galvanic sources and the loop sources. The excitation waveform should not stay in the pulse wave and step wave, the instrument should be used in the study of the waveform.Time domain finite difference method three-dimensional forward also need to solve the problem of low computational efficiency. In the data processing, the current is still using the center line of the late point of the apparent resistivity approximation approach. It is important to improve data efficiency and data processing accuracy of large loop source devices. In view of the above problems, this article has done the following work.Based on the time domain finite difference, this thesis completes three-dimensional Transient electromagnetic forward algorithm of the non-uniform grid.Here, the underground boundary of the non-uniform finite difference algorithm adopts the nature boundary condition, and the ground-null boundary adopts the extension method. In terms of the stability of the algorithm, the maximum time iteration step is given on the basis of the Courant condition.The accuracy of the non-uniform algorithm is verified by the uniform half-space,the layered model and the three-dimensional low resistance model. The plate-like and three-dimensional anomalies are forwarded and their response characteristics are analyzed. The loop current is introduced into Maxwell equation by means of current density according to Ampere circuital theorem. The method can realize arbitrary waveform launch the calculation of electromagnetic response, and the calculation of the source is no longer dependent on analytical solutions of uniform half space. Here, we use the source current density as the initial condition and use non-uniform grid scheme steps Maxwell's equations. I study the difference between the trapezoidal wave response and step wave response. The results show that smaller the trapezoidal pulse width is,the earlier results deviate from the theoretical value of step wave response.The theoretical response of the magnetic field strength of the non - central point in the loop is calculated by the electric dipole integral method. A method for rapid calculation of global apparent resistivity is proposed. According to the theory of smoke circle to complete the resistivity and depth. The theoretical model proves that this method is effective.