The Study Of Transient Electromagnetic Response Modeling In Dispersive Medium

In recent years, TEM was wildly used in electrical prospecting at home and abroad.,its application is also expanding.The wave used in TEM exploration is transmitting in lossy medium underground, so it follows dispersion phenomena. There are certain relationships between media electrical parameters and frequency. It indicates that physical characters change with the frequency. Generally speaking, the phenomenon of the dispersion is caused in the induced polarization by the low-frequency and in the electric polarization by the very high-frequency. When we use transient electromagnetic method to do some measurement, we can often observed that the decay curves cause a phenomenon which we can called"sign reversal". This is mainly due to induced polarization effect near–surface. This phenomenon would have an impact on the transient electromagnetic response. This article introduced the development of the transient electromagnetic method in recent years,outlined the basic principles of transient electromagnetic method, and deduced the formula of 1-D transient electromagnetic response, we combined Debye model, Cole-Cole model and multi-Cole-Cole model to simulate 1-D transient electromagnetic response of the layered media, the we analyzed the impact of the negative response, and contrast the differences of transient electromagnetic response between dispersive state and nondispersive state, we also had discussed the effect of dispersion phenomenon on transient electromagnetic detection, and based on the simulation calculation, we combined damped least-squares method to inverse the forward data obtain from Debye model, and analyzed the result to prove that the forward method is effective and accurate.When we calculate the transient electromagnetic response of layered media, we use linear filtering method to transform frequency domain into time domain, as the linear filtering has the advantage of efficiency and accuracy in low frequency range, it can calculate the Hankel transform fast and need not to calculate the complex Bessel function. In our paper, the linear filtering algorithm using high density sampling techniques is applied to calculate cosine transform, and simulate the transient electromagnetic response in layered medium with dispersive and nondispersive.In the simulation, first we choose different parameter of high resistance half-space model and low resistance half-space model to investigate the effect of response in different model parameters, and we find when we consider the dispersion phenomenon, the greater the polarization, the negative response occurs sooner, and the greater the response amplitude, the smaller the time constant, the negative response occurs sooner the greater the response amplitude, the smaller the frequency correlation coefficient, the negative response occurs sooner the response amplitude is also smaller. We also find that the time which negative response appears in low resistance half space is later than in high resistance half space, that is because the eddy current decay fast in high resistance half space, it makes the electromagnetic decay faster in high resistance half space, so the IP effect appears earlier and also the negative response appears earlier. Contrast the response amplitude of high resistance half space and low resistance half space, we can see, the response of low resistance half space is 2-3 orders of magnitude lager than high resistance half space. So transient electromagnetic method is more applicable to low resistivity anomaly detection than to high resistivity.Combine the simulation diagram of high resistance half space and low resistance half space, we can give explanation from the model as follow :As the polarization and the frequency correlation coefficient is greater and the time constant is smaller, the difference betweenρ(ω) andρ0 is lager, it makes the resistance decrease greater. So when we considers dispersive phenomenon the resistance of the medium is lower, so the transient electromagnetic response is greater, and the curve will decay faster, the negative response will also appears earlier.Then we contrast the curves between dispersive and nondispersive in different geoelectrical models, such as G, D, H, K, Q, and A. We can clearly see that the dispersive phenomenon makes the transient electromagnetic response appear negative response, it also makes the curves decay faster, and the performance is different in different geoelectrical models. Overall transient electromagnetic exploration is more effective in low resistance, especially when the surface layer is low resistance. That is in low resistance half space, G, K and A model the effect of response amplitude and occurred time of negative response is more obvious, in high resistance half space, D, H and Q model the effect is not so clear, however it still exists transient electromagnetic negative transient and the effects could not be ignored.In the inversion, we used damped least square method to inverse the resistance, the polarizability, the time constant and the depth of the subsurface. From the result of the inversion we can see, the simulation of this paper is correct and the inversion is feasible. The inversion is not only the verification to the simulation method, but also these inversion results will provide the basic for high accuracy engineering and environmental geophysical exploration. In the inversion, because of the lacking information, some inversed parameters were large and this can be solved by improving the damping or increasing the transceiver distance or increasing the information by extending the range of the frequency.This paper provide 1-D simulation and inversion to transient electromagnetic response in the dispersive condition. The results show that it is very necessary to consider the dispersion phenomenon in practical work if we want to improve the detection accuracy. We hope this paper can provide some reference to high precision detection of transient electromagnetic.