Three-dimensional Finite Difference Numerical Simulation And Transient Electromagnetic Response Characteristics Study

Transient electromagnetic method(TEM)is favored by geophysical workers because of its many advantages and is widely used.The TEM method achieves the geological exploration objective by observing the change of the secondary field induced in the underground medium after the primary field power off.Therefore,it is necessary to further study the response characteristics of the TEM secondary field,which is meaningful for the further research and development of the TEM method.The research work in this paper includes: 1.Study the theoretical basis of the TEM method,the frequency domain electromagnetic field expression are derived;study the frequency-time conversion algorithm of the vertical magnetic dipole source and the rectangular loop source.2.Systematic study of TEM 3D finite difference numerical simulation theory and technical methods.The discrete difference formula is derived to determine the initial conditions and boundary condition processing methods.On this basis,the 3D finite-difference numerical simulation algorithm of TEM method is realized by programming,and the correctness of the algorithm is verified by the comparison with the predecessors.3.Carry out a 3D FDTD numerical simulation study on the TEM response behavior and its characteristics.Considered with the actual work in the field,a variety of common typical 3D geoelectric models are designed,and the 3D combined meshing is performed on the calculation domain of the model,and then the numerical simulation and analysis of the TEM response characteristics are carried out.The response characteristics are summarized as follows:(1)The TEM response characteristics of uniform earth.The high-resistance earth TEM response decays fast and spreads faster in the horizontal direction earlier.The low-resistance earth TEM response decays slowly and spreads slowly in the horizontal direction.The low-resistance earth TEM response helps to increase the depth of exploration and provides information on the deeper parts of the coil where it is located.The high-resistance earth helps to provide information on the lateral side of the coil where it is located.(2)The TEM response characteristic of the low resistance overlay.The low-resistance cap layer enhances the amplitude of the early TEM response(the size is related to the thickness of the overlay and the resistivity),which slows down the attenuation rate of the mid-late TEM response and expands the horizontal distribution range.The expansion of the range may blur the boundaries between the geological body and the surrounding earth,making it difficult to accurately determine the actual limits of the two.(3)The TEM response characteristics of a high resistance base.In the early times,the high-resistance basement effect was small.In the middle and late stages,the influence is obvious.Not only the abnormal response contour is observed to be “squeezed”,but also the contour inflection point corresponding to the high-resistance base and the overlying earth interface is observed.In addition,the high-resistance base reduces the distribution of late anomalous response and reduces the amplitude significantly,thus reducing the resolving power of deep geological bodies.(4)The TEM response characteristics of different scale transmitting loop.Under the current conditions,the larger the loop scale is,the higher the transient response amplitude is excited in the earth,the plane/section contour shape of the abnormal response is clearer,and the resolution of the buried geological body is stronger;in addition,a large-scale transmit loop can detect the geological body earlier at the side position than the small ones.(5)The TEM response characteristics of different geological bodies buried in depth.The geological body with small buried depth has the best coupling with the transmitting loop.The TEM response of the geological body can be observed during the observation period,and the plane/section contour of the abnormal response display is clear.With the passage of time,the anomaly display becomes more and more obvious.The deep buried geological body and the transmitting loop are poorly coupled.During the observation period,it is difficult to see the anomaly caused by the deep buried geological body in the plane view,and only the weaker anomaly is observed from the sectional view.Therefore,it is necessary to design a reasonable device size according to the possible burial depth of the target body in actual work.(6)The TEM response characteristics of geological bodies of different scales.The TEM response of different geological bodies in the early times is not much different,and the difference(amplitude,range,decay rate,etc.)in middle and late appears.Therefore,it is more advantageous to distinguish the size of the geological body from the middle and late stages of the TEM response.