Distortion effects caused by target abnormal bodies in CSAMT exploration
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RESEARCH ARTICLE - APPLIED GEOPHYSICS
Distortion effects caused by target abnormal bodies in CSAMT exploration Xian‑Xiang Wang1 · Ju‑Zhi Deng1 · Jing‑Li Ren1 Received: 11 September 2019 / Accepted: 26 September 2020 © Institute of Geophysics, Polish Academy of Sciences & Polish Academy of Sciences 2020
Abstract In CSAMT exploration, the using of the artificial sources not only improves the signal-to-noise ratio of the data, but also brings a series of distortion effects, such as shadow and source overprint effects. This paper attempts to introduce a distortion effect caused by the target in the survey area. Although it is often ignored, it always plagues the data interpretation. In CSAMT method, the primary current has determined direction due to the source. When the primary current encounters electrical interfaces, the induced charge will accumulate on it and generate local current, causing local distortion. The anomaly body stretches in the direction of the vertical primary current, and a false anomaly with opposite polarity appears on both sides of the target. If the direction of the primary current is different, the accumulation position of the induced charge is also different, which will result in different shapes of the anomalies in observed data. This paper confirms the existence of the distortion by taking four simple models as examples and explains it from the physical mechanism. On this basis, the paper summarizes the relationship between inversion and distortion. If our code can simulate the distortion effect in the forward, we do not need to remove it before the inversion. Otherwise, it must be removed. Keywords Controlled source audio-frequency magnetotellurics · Distortion effects · Galvanic effects · Target abnormal body
Introduction Controlled source audio-frequency magnetotellurics (CSAMT) is an artificial source electromagnetic method derived from magnetotellurics (MT). The method was first proposed by Professor Strangway and Goldstein of the University of Toronto (1975). The frequency range of the CSAMT is usually 0.1–105 Hz, and the exploration depth is about 1–2 km. Due to artificial sources, this method has stronger anti-interference ability. In recent years, it has been widely applied in the exploration of metal ore, geothermal, groundwater, and hydrocarbons and has been developed into an effective method in geophysical exploration (Di et al 2002, 2004, 2018; An et al. 2013a, b, 2016; Fu et al. 2013; Wang et al. 2015; Lei et al. 2017a, b; Wynn et al. 2016).
* Xian‑Xiang Wang [email protected] 1
Jiangxi Engineering Laboratory on Radioactive Geoscience and Big Data Technology, East China University of Technology, Nanchang 330013, Jiangxi, China
In electromagnetic exploration, distortions due to local anomalies at the surface are ubiquitous. Berdichevsky and Dmitriev (1976) conducted a study on it and divided it into inductive and galvanic distortion. Many techniques have been proposed to remove galvanic effects, such as impedance tensor decomposition (Swift 1967; Groom and Bai
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