Abstract: The investigation of ultrasonic response characteristics of gas-bearing coal bodies can provide the basis for application of ultrasonic technologies for testing the physical properties of coal. An independently developed coal reservoir physical property experimental system was used. Coal samples were selected from the 16-17 Coal Seam of the Pingdingshan Eighth Mine to prepare three kinds of coal samples:the parallel bedding and surface cleat (x), parallel bedding and vertical surface cleat (y), and vertical bedding (z), in which the ultrasonic characteristics of nitrogen-containing coal bodies under different gas pressures and at different adsorption times were tested. The experimental results show that (1) when the axial pressure is constant, as the confining pressure increases, the elastic wave velocity of coal increases continuously, and the P-and S-wave velocity of the coal in a vacuum is slightly greater than that under normal pressure; (2) from the negative pressure state to the gas injection and adsorption state, the velocity of the coal column first decreases and then slowly increases with the extension of the gas injection duration; (3) when the axial pressure and confining pressure are constant, the P-and S-wave velocity increases with the change of gas pressure; (4) the anisotropy among the parallel bedding and surface cleat, parallel bedding and vertical surface cleat, and direction of the vertical bedding is significant, but the ultrasonic response patterns are basically the same; and (5) the relationship between the anisotropy degree of the vertical and parallel bedding and the adsorption time is significantly different between the P-and S-waves of the coal samples. The anisotropy degree of the P-wave is more sensitive to the adsorption time.

Key words: nitrogen-containing coal bodies, ultrasonic velocity, negative pressure state, anisotropy, cleat