Transient Electromagnetic Responses of Water-Rich Zones in Coal Seam Roofs

Abstract The presence of water-rich zones in coal seams greatly increases the risk of water inrush during mining operations. Currently, there is a lack of quantitative research on the detection capability of the mine transient electromagnetic method for single- and double-layer water-rich zones in coal seam roofs. This study integrates numerical simulations and physical modelling based on whole-space theory to systematically investigate transient electromagnetic (TEM) responses of single- and double-layer water-rich zones in coal seam roofs, providing more specific understanding of the propagation and attenuation patterns of electromagnetic waves in multilayer media, and offers technical support for enhancing coal mine water hazard early warning capabilities and accurately detecting roof aquifers. Numerical results show that large single-layer zones (>30 m × 30 m) produce enhanced anomalies characterised by distinct convex multi-measurement profiles and clearer apparent resistivity contrasts. Adjacent zones separated by > 10 m horizontally yield bimodal measurement curves, enabling identification through apparent resistivity contours. For double-layer configurations, early-stage responses are mainly controlled by the lower layer, while low-resistivity anomalies attenuate as the distance from the coil or the interlayer spacing increases. When the lower layer lies deeper than 55 m, responses approximate those of a single layer, whereas upper-layer anomalies become discernible at interlayer spacings < 35 m. Physical modelling corroborated these patterns: TEM detection capability varied with zone parameters, and the characteristic curve morphologies observed under specific conditions closely matched numerical predictions, validating the simulation model. When detecting water-rich areas in practice, the detection capabilities can be reasonably predicted based on the scale of the numerical model discussed in the text and in combination with specific geological conditions.