关键词: 可控源音频大地电磁法, 各向异性, 激发极化, 有限元, 2.5维, 正演

Abstract:

The Controlled Source Audio Magnetotelluric(CSAMT)method is a frequency-domain electromagnetic sounding technique that utilizes an artificial field source. It has been widely used in the exploration of various resources. Due to tectonic stress, temperature variations, material migration, and geological deposition within the Earth, the electrical parameters of underground media may vary with the direction of current, exhibiting electrical anisotropy. Additionally, rocks and ores exhibit complex electrochemical phenomena under the influence of an artificial field source, resulting in resistivity being a frequency-dependent complex number, known as the induced polarization(IP)effect. Electrical anisotropy and the IP effect are common phenomena in Earth's internal media, significantly impacting CSAMT observation data. Traditional CSAMT methods typically assume that the underground medium is electrically isotropic and lacks IP effects when performing forward and inversion numerical simulations. However, the actual underground medium often exhibits electrical anisotropy, the IP effect, or both. Adopting an isotropic model assumption during data processing and interpretation can therefore lead to serious errors in the results. Current research on CSAMT mainly considers the influence of either anisotropy or the IP effect individually. Since the anisotropy and polarization effect model involves multiple parameters, this complexity increases the difficulty of research. Consequently, there are relatively few studies on the combined influence of anisotropy and the IP effect on CSAMT responses. With increasing demands for exploration accuracy, it is imperative to study the response characteristics of the CSAMT electromagnetic field under the combined effects of anisotropy and the IP effect. In this paper, we employ a 2.5-dimensional model to derive the electromagnetic field partial differential equation for a 2.5-dimensional anisotropic medium based on Maxwell's equations with a source. The 2.5-dimensional CSAMT numerical simulation of anisotropy is achieved using the Galerkin finite element method. The real resistivity is replaced by complex resistivity using the Cole-Cole model. We discuss the effects of the Euler angle and principal axis resistivity in electrical anisotropy parameters, as well as three polarization parameters in the Cole-Cole model, on the CSAMT electromagnetic field response. Finally, we study the controllable source electromagnetic field response considering both anisotropy and IP effects.
The results show that electrical anisotropy significantly impacts the response of 2.5-D CSAMT, primarily depending on the Euler angle and principal axis resistivity values. Depending on these values, electrical anisotropy can either increase or decrease the CSAMT response value. Among the three polarization parameters, polarizability has the greatest influence on the calculation results, with the apparent resistivity value gradually decreasing as polarizability increases. The time constant and frequency correlation coefficient have relatively smaller influences on the calculation results, and their effects are consistent. The combined effect of anisotropy and the IP effect on the 2.5-D CSAMT response is more complex. Electrical anisotropy, influenced by the Euler angle and principal axis resistivity, can either increase or decrease the CSAMT response result, while the polarization effect generally decreases the CSAMT response result. Thus, when both factors act simultaneously, their effects can overlap or offset each other depending on the values of the Euler angle and principal axis resistivity. The findings of this paper have significant theoretical implications for improving the accuracy of CSAMT data processing and interpretation.

Key words: CSAMT, anisotropy, induced polarization, finite element, 2.5-D, forward