## 5 Discussion  
To summarize, it is more comprehensive and precise to describe resistivity variation regularities adopting the joint test method of resistivity and AE technique during the process of damage evolution of loading coal mass. Thus, it is conducive to understand the evolution process of coal-rock dynamic disasters and provide an important theoretical basis for the application of coal and rock dynamic disasters prediction. Moreover, there is a good correspondence relationship between the trend of resistivity and damage variable in coal mass, and the curve of damage variable has a stepladder upward trend, while the resistivity curve shows a stepladder downward trend with time. And AE characteristic parameters can mirror the damage and failure process of coal mass, which can be used to further describe resistivity variation regularities in each loading stage with establishing the relationship between the AE damage variable and electrical resistivity. And then improving the theoretical basis of electrical prospecting technology for predicting coal-rock dynamic disasters. In previous studies, it could be retrieving the damage evolution process of coal-rock mass by utilizing electrical resistivity. [PERSON] et al. conducted studies on resistivity variation regularities and  
Figure 8: The electrical resistivity curve under damage conditions. accurately retrieved the formation, expansion and failure of microcracks in concrete specimens [42]. Nevertheless, it was usually limited to a fixed sampling frequency in resistivity testing of loading coal, which could not fully reflect the change of the coal mass at any time due to polarization effect [43]. Thus, collected resistivity data could not correspond to the damage evolution process of coal mass completely, so that it was difficult to determine the point-in-time of damage and failure in coal mass by resistivity variation regularities and accurately predict the occurrence of coal-rock dynamic disasters. And it is necessary for accurately calculating electrical resistivity by establishing the relationship between electrical resistivity and damage variable. [PERSON] et al. calculated the damage evolution expression among resistance, stress and strain, and the rationality of the evolution equation is verified by fitting the data [44]. [PERSON] et al. calculated and fitted the elastic modulus and collected resistivity data under cyclic loading and unloading, but it was not comprehensive to describe the relationship between resistivity and damage variable [45]. As shown in Figure 5, the cumulative AE ringing counts of coal mass have a long quiet period at the initial stage of loading, and they are gradually increasing with increase of stress and loading time. But there are few AE signals at a constant loading stage, and it indicates that the damage degree of loading coal sample does not increase. After loading for a while, the cumulative AE ringing counts increases at a speed of 2-3 times, which indicates that the main fracture of loading coal will occur and coal samples will be completely destroyed. In recent years, the electrical prospecting technology plays a significant role in predicting and forecasting the occurrence time of coal-rock dynamic disasters, and we all know electrical resistivity is an important parameter of coal mass [46; 47]. As shown in Figure 7, when damage variable and electrical resistivity increases suddenly, this shows that electrical resistivity and cumulative AE ringing count has the same change tendency, so it is feasible that calculating the electrical resistivity by using cumulative AE ringing count. As can be seen from the foregoing description, the coal mass is a kind of solid conductor material and there are mainly two kinds of conductive forms, ionic conduction and electronic conduction, and the electrical resistivity is sensitive to temperature, frequency, applied electric field and its properties, it will gradually decrease with the increase of temperature and frequency [48; 49; 50].