## 3 Experimental Results
### AE Activity  
The AE activity is accompanied by the rock damage process with different release intensities. Six sensors work independently to monitor the AE activity of the rock sample. When the energy released  
Figure 5: Ultrasonic wave characteristics of rock salt under a triaxial loading test: (**a**) the relationship of ultrasonic wave velocity and axial stress–stain curve; (**b**) the relationship of crack density and axial stress–stain curve; (**c**) the relationship of ultrasonic wave amplitude and axial stress-stain curve; and, (**d**) the relationship of ultrasonic wave attenuation coefficient and axial stress–stain curve. is monitored by four or more sensors, the location of this source can be determined by the difference of arrival time. The calculation method is shown in Equation (3) [36]. \[\left(x_{k}-x_{0}\right)^{2}+\left(y_{k}-y_{0}\right)^{2}+\left(z_{k}-z_{0} \right)^{2}={v_{p}}^{2}{\left(t_{k}-t_{0}\right)^{2}} \tag{3}\]  
where \(x_{k}\), \(y_{k}\), and \(z_{k}\) are the sensor's (No. K) coordinates. \(x_{0}\), \(y_{0}\), and \(z_{0}\) are the artificial testing coordinates. \(v_{p}\) is the P-wave velocity. \(t_{k}\) and \(t_{0}\) are the excitation and reception time of the signal, respectively. Figure 6 presents the AE event number of rock salt under the confining pressure of 5, 10, 15, and 20 MPa, respectively. The AE activity was weak and few AE events occurred at the beginning of the loading experiment. Subsequently, it started to be more active with increasing frequency. This was caused by salt crystal slippage in the sample and ensuing continuous energy release. Different from \"hard\" rocks, the main peak of AE activity of rock salt was difficult to find during the triaxial compression test for the energy was released by a series of continuous similar-strength earthquakes, and it was maintained at a high intensity until the end of the test. The AE event map, which is a display of rock damage status, was calculated by the arrival time of AE signals received by six sensors, as shown in Figure 7. The AE events are more concentrated at the middle part of the salt samples, since the most severe damage occurred there. This map is a conclusive verification of rock damage. The sample became \"thickened\" after the loading test, and the maximum lateral deformation occurred in the middle area, resembling the shape of a drum. All of these features were consistent with the AE event map. In summary, the AE event map can clearly express the distribution and severity for rock damage, and it lays a foundation for quantitative evaluation of the damage degree of rock salt. Figure 6: AE event number of rock salt under different confining pressure during the triaxial loading test: (**a**) confining pressure of 5 MPa; (**b**) confining pressure of 10 MPa; (**c**) confining pressure of 15 MPa; and, (**d**) confining pressure of 20 MPa. As for the experimental evidence of AE events under the confining stress of 5, 10, 15, or 20 MPa, the AE activity clearly varied under different confining pressures, as shown in Figures 6 and 7. According to the statistics results, 14763, 7673, 2674, and 703 AE events occurred from the test start to the axial strain of 8%, under confining pressures 5, 10, 15, and 20 MPa, respectively. The higher that the confining pressure was, the less the active the AE events were. Figure 7 presents the AE event maps of rock salt under different confining pressures, which clearly reflected that confining pressure had an inhibitory effect on AE activity.