# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 1 Introduction  
In recent years, with the progress of coal mining technology and the improvement of mechanical equipment performance, highly efficient intensive coal mining has become the dominant trend of major coal mining countries in the world, such as China, the United States, and Australia [1; 2; 3]. Inevitably, the high-intensity coal mining activities would incur the enlargement of roadway cross-section and the drastic strata pressure behaviors. Under the complex disturbance of multiple stress, severe roof fall and floor have are highly likely to occur in the roadway. Meanwhile, coal wall collapse is also highly probable in the roadway, which poses an extreme threat to the safety of production and miners [4; 5]. When large-scale coal wall collapse occurs in the roadway with hard roof, it can trigger active movements in the top covers, which in turn are inductive to the overall rock burst [6; 7]. However, coal wall instability does not happen overnight; instead, it is the consequence of gradual evolutionunder progressive load. Therefore, it is of significance to consider the role of progressive load in analyzing the characteristics of pore and fracture evolution from a micro perspective. Scholars, both at home and abroad, have conducted a large number of studies using a variety of testing methods. Coal and rock under the load would deform, and the crack would grow, so as to release energy rapidly and generate transient elastic waves. It is due to this property of rock that acoustic emission technique has been used to study the evolution law of pore and fracture. [PERSON] et al. [8] studied the characteristics of acoustic emission distribution under different loads. [PERSON] et al. [9] analyzed the reasons for the difference in uniaxial compressive strength and acoustic emission energy caused by the anisotropy of coal. [PERSON] et al. [10] conducted uniaxial loading tests at different loading rates to explore the relationship between the cracks in coal sample, stress and energy, and acoustic emission. With the rapid development of ultrasonic technology, it has been widely used to study the mechanical property and structural characteristics of rock. [PERSON] et al. [11] conducted a contrastive study of the anisotropy of ultrasonic velocity and fracture distribution in the coal. [PERSON] et al. [12] took the effect of axial compression on the velocity and attenuation of s-wave and longitudinal wave into consideration and analyzed the ultrasonic response characteristics of coal under uniaxial loading. However, acoustic emission and ultrasonic testing cannot directly illustrate the evolution law of pore and fracture on the microscopic level. The main characterization methods for pore and fracture structures on micro level are as follows: electron microscope method [13; 14; 15], mercury intrusion method [16; 17; 18], gas adsorption method [17; 19], CT scanning method [20; 21; 22], and low-field nuclear magnetic resonance (NMR) method [23; 24; 25]. By electron microscope method, the distribution characteristics of pore and fracture can be observed directly. However, this method is applicable only in a relatively confined area, thus being unable to provide a panoramic view of the whole evolution process when the coal sample is under loading. Mercury intrusion method can be used to characterize the macro pores in the coal sample, but the relatively high pressure for mercury intrusion would damage the pore structure, thus being not applicable to measure the micro-pores in the coal sample. Gas adsorption method is commonly used to measure the specific surface area and pore structure of coal samples, but mainly for the micro-pore measurement. CT scanning method can be used for non-destructive pore structure testing within the coal sample, but the pores tested are mainly mesopores and macropores.