Near-source electromagnetic technology has been developed and applied in the exploration of petroleum, metallic ore, coal, and engineering geology due to its high efficiency, high detection accuracy, and deep depth of investigation. In this paper, research and applications of the frequency-domain electromagnetic sounding method (FDEM), wide-field electromagnetic method (WFEM), modified central-loop transient electromagnetic method (TEM), and short-offset grounded-wire TEM (SOTEM) with obvious near-source characteristics, were reviewed and analyzed. From the 1960s to 1990s, the FDEM method and equipment were extensively developed in China. These methods have played important roles in the exploration of coal resources. Based on controlled source audio-frequency magnetotelluric (CSAMT) and FDEM methods, a new method has been developed by deriving a new expression to calculate apparent resistivity. This method, which is referred to as WFEM, has been studied, applied, and received great attention in China. To increase work efficiency and reduce the influence of local transverse anisotropy on the detection processes, a modified central-loop TEM detection technology based on the central loop transient electromagnetic method was developed in China. The advantages of SOTEM in near-source surveys with high resolution and increased depth detection stimulated academic research interest to further develop grounded-wire TEM techniques.
[Figure: see text]
Relative multiscale deep depth from focus
