An experimental study of the application of the spectral acoustic method to the advance detection of geologic structures in heading faces

Objective The advance detection of geologic structures is necessary for the safety of heading faces, with the advance detection results holding great significance for the safe production of coal mines. The spectral acoustic method has been applied to coal mine detection in Russia. However, there is a lack of systematic studies of this method based on field engineering data in China.

Methods This study developed a spectral acoustic method for coal mine detection based on the application experience in the coal mines of Russia. In this method, broadband artificial acoustic signals were excited in the rock masses during coal cutting using a tunnel boring machine (TBM), and these signals produced spectral resonance in roof strata. Then, spectral parameters were determined in real time by the monitoring and early warning system composed of geophones for signal receiving, monitoring substations for data transmission, and a monitoring host, allowing for continuous monitoring of the safety conditions of the heading face. Through cooperation between Chinese and Russian teams, an experimental study was conducted on the heading face of ventilation tunnel J₁₅-15080 in the Eighth Coal Mine of Pingdingshan Tianan Coal Mining Co., Ltd. During the experiment, the heading face advanced continuously for 756 m, with seven faults being exposed.

Results and Conclusions  The experimental results demonstrate the high consistency between the anomaly areas of the monitoring indicator (i.e., the geologic structure prediction coefficient) of geologic structures and distribution of geologic structures in the mining face, with all geologic structures in the mining face identified within the anomaly areas of the monitoring indicator. At a certain distance in front of the exposed geologic structures, the geologic structure prediction coefficient inevitably increased and exceeded its critical value, which can be set at 7 to ensure 100% reliability for the monitoring and early warning of geologic structures. The advance warning distances were calculated at 6.5‒27.3 m based on the normal distance from the mining face to the fault strike and determined at 13.5‒44 m based on the heading direction of the mining face. The geologic structure prediction coefficients exhibited two distribution morphologies on both sides of a fault, i.e., single- and double-wing morphologies. The former refers to the case where the coefficients exceed the critical value in either the hanging wall or the footwall of the fault, while the latter denotes that the coefficients exceed the critical value in both the hanging wall and footwall of the fault. Based on the distribution of relative stress coefficients on a fault plane and the fault's hanging wall and footwall, the stress-strain zones near the fault can be categorized into zones with unilateral high stress, bilateral high stress, and overall high stress. The distances from high-stress zones to fault planes typically ranged from 3.2 to 28 m. The findings of this study will provide a new monitoring indicator and method for continuous online early warning in the advance detection of geologic structures in heading faces.