CSAMT Static Correction Based On Wavelet Method

Electromagnetic measurement technique generally refers to any methods that use electromagnetic theory to achieve the physical parameters measurement of measured object. As one of the electromagnetic measurement techniques, CSAMT(Controlled Source Audio-frequency Magnetotelluric) has been widely used to survey the electric inhomogeneous body in the earth. This method is based on the principle of electromagnetic induction with a grounded wire as field source and transmitting AC primary field of different frequency. In CSAMT method, the x-component electrical field and y-component magnetic field are measured to achieve the identification of the electric anomalous body in the earth.However, in the process of practical application of CSAMT, there is a very serious electromagnetic interference named static effect which is caused by the local conductive inhomogeneous body. When the electric current flowing through the inhomogeneous body, charge accumulation effect occurs on its surface. The accumulated charge will produce additional electric field proportional to the external current field, leading to the distortion of collected electric field component. This makes the apparent resistivity data significantly different from the actual situation, misleading the recognition of underground anomalies. As a result, this paper presents a CSAMT static correction method based on wavelet analysis. Exactly, it is based on the wavelet transform modulus maxima and threshold method to achieve CSAMT static correction. The concrete research content is as follows:1) The Cagniard resistivity data is firstly processed by wavelet decomposition, then the Lipschitz exponent is calculated based on the modulus maxima on every scale. The static effect is identified according to positive or negative sign of the Lipschitz exponent in combination with the difference of Lipschitz exponent of the underground anomalies and the static effect. This lays a foundation for the suppression of static effect.2) Based on wavelet multi-scale analysis theory, the apparent resistivity data is decomposed into two parts: the approximate part and detail part. So the anomalies and static effect are separated with each other to some extent.3) Different from traditional wavelet static correction method that set all the detail coefficients to zero, this paper sets different thresholds for modulus maxima on every scale. Then we reconstruct the signal by Mallat alternating projection method to obtain the static corrected Cagniard resistivity data.4) Based on Integral Equations numerical simulation algorithm, we check the strong feasibility of the CSAMT static correction method based on the wavelet transform modulus maxima and threshold by designing simulation model. We further verify the practicality and accuracy of the method proposed in this paper by applying it to the actual experimental data.5) Based on.NET component and Type-Safe Interfaces, this paper designs CSAMT static correction software with C# and MATLAB mixed programming. The software not only contains the method proposed in this paper, but also supplies five-points, median, EMAP static correction methods, facilitating the user to select and compare different static correction methods.