Advances in research on borehole geophysical exploration and associated equipment for advance detection of underground water hazards in coal mines

Background Deep coal mining is facing increasingly complex hidden disaster-causing factors, including faults, collapse columns, water accumulation in goaves, confined water in coal seam floors, and hydraulically conductive fracture zones. In this context, conventional modes based on ground exploration, roadway geophysical exploration, and drilling verification suffer from limitations in terms of detection distance, positioning accuracy, dynamic perception, and real-time feedback. In contrast, borehole geophysical exploration extends the detection space into coal-rock masses using underground boreholes, enabling the acquisition of information about continuous physical properties around, between, and ahead of boreholes. Therefore, this technology provides important support for the advance detection of water hazards, fine-scale geological guarantee, and transparent geological modeling for coal mines. Advances Based on the databases of China National Knowledge Infrastructure (CNKI) and Web of Science (WoS) Core Collection, this study presents a summary of the current status of research on borehole geophysical exploration using methods including literature metrology, keyword co-occurrence analysis, and hot research topic analysis. In accordance with the theoretical basis of physical fields and the methodology for forward modeling and inversion, this study classifies borehole geophysical exploration technology for the advance detection of underground water hazards in coal mines into three categories: wavefield propagation, electromagnetic induction, and potential field methods. Existing studies indicate that techniques including borehole radar, borehole elastic wave detection, cross-hole electromagnetic wave computed tomography (CT), borehole transient electromagnetic (TEM) detection, the borehole induced polarization (IP) method, and the borehole direct current resistivity method have found widespread applications in the identification of water-bearing anomalies, the detection of hydraulically conductive structures, the delineation of water accumulation in goaves, the determination of coal-rock interfaces, and the evaluation of grouting effects. Different methods place varying emphases on detection distance, resolution, anomaly sensitivity, and applicable conditions. The collaborative detection and comprehensive interpretations using multiple methods help reduce the solution multiplicity of inversion results, thereby improving the reliability of water hazard identification. In terms of equipment, instruments for borehole geophysical exploration are shifting from general-purpose and single-function devices toward specialized, miniaturized, integrated, and intelligent systems. Nevertheless, further breakthroughs are yet to be achieved in the adaptability, attitude control, real-time data transmission, synchronous multi-parameter acquisition, and stable detection under strong interference of long-distance directional boreholes. Prospects To meet the requirements of deep mining, intelligent tunneling, and long-distance tunneling and detection, it is necessary to shift borehole geophysical exploration from static anomaly identification to dynamic process perception. Future research should focus on the identification of the coupling relationships among the stress, fracture, seepage, and multi-physical fields under coal mining, aiming to establish multi-parameter response spectra of typical water hazards. It is advisable to make breakthroughs in both equipment for drilling - geophysical exploration integrated real-time perception and technologies for detection while drilling. This effort will help form the collaborative closed loop of drilling, detection, interpretation, and geosteering. Furthermore, intelligent interpretation methods, including multi-source information fusion, physics-constrained inversion, machine learning-based identification, and digital twin, should be developed to enhance the capabilities for the identification of complex water hazards, the assessment of water yield properties, and the risk grading and control effect evaluation of water inrushes.