Study on the electromagnetic spectrum characteristics of underground coal fire hazardous and the detection criteria of high temperature anomaly area

Electromagnetic radiation (EMR) is an optimal non-contact and directional geophysical detection method, which has been applied in geotechnical engineering, monitoring and giving early warning about the coal and rock dynamic disaster. In this paper, the different frequencies EMR signals generated in the process of coal heating and spontaneous combustion are tested and analyzed, and the detection criterion of locating anomalous heating areas by EMR is illustrated. During the spontaneously-heating process of coal, the EMR signals are detected by antennas with frequencies of 30 kHz, 100 kHz, 300 kHz and 1 MHz, covering both low and high frequencies. In the process of coal spontaneous combustion, the spectrum of EMR signal changes with the increase of coal temperature. The amplitude of EMR also fluctuates, showing the characteristics of high frequency and high amplitude, and the corresponding thermal damage of coal occurs in different degrees (intensities). Further, in order to use EMR as an effective detection method of coal fire, the relationship between the intensity of EMR and coal size, test distance and temperature is discussed. With the dominant spectrum of EMR determined by field tests, researchers can employ different EMR frequency to detect the fire risk based on the concrete conditions of coal field. This study provides a meaningful method for improving the level of safety and environmental protection during mining process.