Physical properties of coal from different depths

With the reduction of shallow resources gradually,a deep mining will be a major way to acquire mineral and energy resources. Therefore,it is of great importance to explore the coal from different covered depths,especially from more than 1 000 meters. Previous studies have commonly simulated the external environment of the rock mass at differ- ent depths by changing conditions such as the confining pressure and temperature. However,these studies did not test the rock samples from different depths and overlooked the initial structure,material composition and properties of in-si- tu rock mass. In this study,the coal samples were collected from one coal seam at five different depths (300,600,700, 850 and 1 050 meters) in a mining area. A series of tests were conducted to assess the effect of depth on the composi- tion,meso-structure,gas absorption and acoustic properties of coal. The results showed that as the depth increased,the compositions and meso-structure changed significantly:the metamorphic grade of the coal increased,and its structure became more compact. With the depth increase,the gas absorption capacity of the coal raised,the probability of ultra- sonic scattering and refraction during wave propagation decreased,thus,the energy dissipation decreased and the ultrasonic longitudinal wave velocity increased. These results lay a foundation for the subsequent studies of the effect of depth on the macro-mechanical properties of coal.