| Recent years, the demand of mine of resources is increasing with the rapid development of economy, the problem of resources supply has became an important obstacle for the Chinese economy development. Therefore, in our country, an efficient resource detection method is a key factor for the solution of resource shortage. The grounded electrical source airborne transient electromagnetic(GREATEM) system combines the advantages of both ground and airborne electromagnetic system such as: considerable prospecting depth, lateral resolution and detection efficiency. Therefore, it is suitable for rapid exploration of deep mineral and groundwater resource under complex geological conditions. Numerical solution is an effective method for the research of transient electromagnetic response. In the field of EM modeling, four methods can be adopted: the integral equation(IE), finite-element(FEM), finite-difference time-domain(FDTD) and finite volume. Because FDTD method has the characteristics of simplicity and flexibility, therefore, it has been widely used to solve the problems of 3D transient-electromagnetic modeling. At present, the researches of FDTD are mainly concentrated on the 1D and 2D models. But the distributions of mineral resources are irregular shape, it is difficult to accurately simulate the electromagnetic responses of mineral resources only using the 1D or 2D FDTD method. It is necessary to d evelop the 3D FDTD for the GREATEM system. In addition, the resolution of GREATEM system is related to the SNR(Signal to Noise Ratio), and the amplitude of GREATEM response has a large dynamic range, with a span of three to four orders of magnitude from the early time to the latter time. The latter time response is only approximately a few nanovolts, therefore, it is extremely easy to mix with noise and difficult to distinguish.Focus on the above problems, this paper is supported by the Key Technology an d Development Program of Jilin province and 21 th Talent project from Education Ministry, we have studied the 3D FDTD and the electromagnetic noise reduction of GREATEM system. The main research contents are as:(1) Based on the research of 3D transient electromagnetic modeling proposed by Wang and Hohmann, we focus on the operating mode of GREATEM system(using a long grounded electric wire as transmitter and a receive sensor carried by aircraft), the formula of electric field generated by long grounded electric wire is decuced for the calculation of initial field.(2) The modeling of 3D transient electromagnetic is time-consuming, therefore, the rapid numerical simulation is researched in this paper. The parallel calculation is achieved by using the computing fabric of CPU+GPU. According to the attenuation characteristic of transient electromagnetic signal, the accelerated time-stepping of FDTD is studied. The accelerated approach can greatly reduce the time steps with a high calculation accuracy. The FDTD method of this paper is verified by the homogeneous half-space and layered half-space, and is applied to the calculation of 3D complex models caused by anomalous body and linear conductivity, the electromagnetic response is analyzed.(3) The characters of GREATEM response and measured noise are analyzed, and a noise reduction method is proposed based on the theories of wavelet threshold and exponential fitting. The background noise and the random electromagnetic spike are removed by using the proposed method. The de-noising method is also applied to the noise reduction of measured electromagnetic data collected at Inner Mogolia.(4) The shortages of wavelet threshold-exponential fitting are improved, and proposed a combined wavelet noise reduction method for the GREATEM signal. And the new de-noising method is applied to the measured electromagnetic data collected at Jiangsu province and Hengsha island near shanghai and obtain a great de-noising effect. |
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