Application of integrated geophysical surveys in the identification of hidden fire sources in spontaneous combustion zones in coal seams

Spontaneous combustion in coal seams, a factor influencing the development of many mining areas in China, can cause severe coal resource loss and environmental pollution. The positioning and distribution identification of hidden fire sources can provide an important basis for coal mine enterprises to prepare prevention and control measures. They are also a hot research topic and challenge in the field of coal fire disasters. Suitable channels for ventilation and oxygen supply are necessary for spontaneous combustion in coal seams. When the oxidative heat release rate of coals exceeds the heat dissipation rate, coal temperature will rise until the occurrence of spontaneous combustion in coal seams. Therefore, the primary task for coal fire surveys is to pinpoint the combustion centers of coal fire zones and identify the burning-out areas and caving zones. This study investigated the Gongwusu open-pit coal mine in Wuhai City, Inner Mongolia. Based on the correlation analysis of coal seam resistivity and temperature, this study conducted surveys of the coal fire locations and their distribution range using the ground penetrating radar (GPR) and conical source-based transient electromagnetic (CSTEM) methods. Then, by combining the surface smoking positions, this study acquired the geophysical characteristics of the underground channels of the fire zones and further ascertained the distribution range of the coal fire channels. The results indicate that: (1) Both the protolith and sintering zones exhibited relatively consistent physical stability, with their top boundaries present as distinct reflection events in radar images and as transversely continuous resistivity distribution in the CSTEM outcomes. (2) The underground hidden fire channels, caused by factors like rock fragmentation and the presence of pores and fissures, exhibited weak, discontinuous in-phase echoes in the GPR profiles. In contrast, they show transverse concaves or dislocations of resistivity contours in the CSTEM pseudosections. (3) Along the survey lines, the apparent resistivity range varied with the elevation drop. Therefore, to determine the distribution patterns of geophysical anomalies in the protoliths, it is necessary to delve into the localized changes in physical property along survey lines based on the analysis of overall resistivity across the study area. As indicated by these results, potential spontaneous combustion in coal seams and its distribution range can be pinpointed by integrating the CSTEM and GPR methods, thus providing a basis for delineating coal fire zones and analyzing coal fire patterns.