Applicability of an imaging method for ambient noise in coal mines based on triangular and linear arrays

Ambient noise surface wave imaging has been widely applied in the engineering exploration of large-scale regional structures and shallow parts. However, there are limited studies on the exploration of mineral resources at depths ranging from several hundreds of meters to one kilometer. The noise source utilized for exploration at this depth range is human environmental noise with frequencies from a few Hz to over ten Hz, varying greatly in time and space. To examine the applicability of ambient noise imaging in the exploration of coal mines, this study systematically analyzed noise source distribution and the adaptability of various array dispersion imaging schemes using experimental data from the Libi Coal Mine. As revealed by the results, the noise in the coal mine is dominated by that with frequencies below 2Hz at night and by that above 2Hz during the day. The noise frequency band (2~10 Hz) utilized for the No. 3 coal seam at a depth of 700m is primarily distributed in the southeast. In the case of simple frequencies and azimuths of the noise sources, a linear array in the noise source direction can obtain dispersion data with higher quality than a triangular array. Finally, by extracting dispersion data from a linear array in the NW direction, the 1D velocity structure below the linear array was obtained. By comparison with the lithology column of borehole ZK101 near the linear array, the 1D velocity structure, obtained through ambient noise imaging, corresponded well with the underground lithology. This result indicates that when fully considering noise distribution, the ambient noise imaging based on a linear array can yield reliable velocity structures for layers at depths less than 1 km in a coal mine.