Response of triaxial velocity and acceleration geophones to channel waves in a 1-m thick coal seam

Abstract

Intelligent coal mining techniques are developing rapidly in China and require geological transparency of the working face. Coal seam channel waves are particularly promising because they only propagate in coal seams and are sensitive to changes in seam thickness and local geological structure. Previous channel wave studies focused mostly on medium-thick coal seams, while channel wave signal characteristics in thin coal seams remain poorly understood. We have conducted experiments on the transmission of channel waves in a 1-m thick coal seam using triaxial velocity geophones and acceleration geophones embedded in the coal seam and its roof and floor rock mass to better understand guided wave characteristics in thin coal seams. We find that a wide-frequency triaxial acceleration geophone with stable amplitude-frequency characteristics enables collection of all of the 1-m thick coal seam guided wave information. However, the velocity geophones did not effectively respond to different coal seam guided waves. Signal analysis reveals that the thin coal seam channel wave features with an Airy seismic phase frequency of 570 Hz and velocity of 950 m/s. There was a high-energy and high-frequency guided wave between the P and S direct waves of the coal seam roof and floor. Combined with time-frequency analysis, polarization analysis, and numerical simulation, the guided wave is inferred to be a P-P interference wave with potential exploration value propagating in the same direction along the roof and floor of the coal seam. Acceleration geophones with a wide frequency band and stable sensitivity are therefore the key point to acquiring coal seam channel wave exploration data.