Electromagnetic methods in the detection of water hazards in coal mines: A review

Water hazards in coal mines are a major type of hidden danger in safe coal mining. The accurate detection of hidden factors to cause hazard is the prerequisite to solving the water hazards. Electromagnetic methods, which are sensitive to low-resistivity bodies and have multiple types and high adaptability, can be used to effectively detect water-rich hazard-causing factors. Traditional ground electromagnetic methods, including the controlled source audio-frequency magnetotellurics (CSAMT) method and the loop source transient electromagnetic method, play an important role in detecting hidden hazard-causing factors in coal mines. With the continuous development of new technologies, near-source electromagnetic methods have developed rapidly. These methods include the wide-field electromagnetic method (WFEM) and the short-offset transient electromagnetic method (SOTEM) and contribute to the increase in the exploration efficiency and signal-to-noise ratio and the expansion of the observation areas. The grounded clectrical source air-born transient electromagnetic method GREATEM is a new method with high power and rapid exploration. This method enjoys the advantage of fast acquisition of airborne electromagnetic methods and the advantage of high-power emission of ground electromagnetic methods and is especially suitable for detecting hidden hazard-causing factors in coal mines with complex terrain conditions. This study analyzed the formation mechanisms, temporal and spatial distribution, and physical characteristics of the typical hazard-causing factors in coal mines. Among these factors, concealed water-conducting structures are a major type, and therefore, much attention should be paid to them. Moreover, this study reviewed the primary characteristics and applicability of various electromagnetic methods, as well as the current status of their applications in the detection of water hazards in coal mines. Moreover, it summarized the technical parameters and observation modes of various electromagnetic methods, thus providing relevant technicians with a reference for the detection of hidden factors inducing hazards in coal mines. Finally, this study offered the developmental prospects of electromagnetic methods for coal mines, including (1) Near-source electromagnetic exploration methods such as the WFEM and the SOTEM will be further applied in coal exploration; (2) The three-dimensional inversion technology and deep learning-based neural network algorithms will be the major developmental directions of electromagnetic methods in the future; (3) The grounded clectrical source airborne transient electromagnetic method will undergo rapid development and tends to substitute traditional electromagnetic methods.