## 1 Introduction  
Salt formations are almost perfectly impermeable and they have good creep properties; therefore, many countries have used them to store energy and nuclear waste [1; 2; 3; 4]. Underground salt caverns are the safest way to protect energy storage containers from fire and explosion. However, several accidents have occurred, resulting in substantial damage to property and causing environmental disaster in the past forty years [5; 6; 7; 8]. According to statistical analyses, 38% of accidents were caused by salt over-creep or cavern structural instability. Therefore, research on the damage evolution laws of rock salt can provide scientific guidance for the design, construction, and operation of rock salt gas storage. Up to now, many methods have been applied to study rock damage, such as scanning electron microscopy,computed tomography (CT), and density or infrared radiation detection. However, they are not widely used, owing to the strict requirements on testing conditions or equipment. In recent years, an active method, as represented by ultrasonic wave detection, and a passive method, as represented by acoustic emission (AE) monitoring, have been accepted by many scholars, and they have become essential tools in analyzing and predicting rock damage. Ultrasonic waves are a good index for evaluating the change of internal structure and mechanical properties of rocks. When the elastic wave encounters certain obstacles during propagation, such as inclusions, holes, and cracks, ultrasonic attenuation will occur. This process is presented by the decline of ultrasonic wave velocity or amplitude [9]. In the 1970s, [PERSON] and [PERSON] [10; 11] studied the differences of the elastic modulus in crack-bearing rocks and crack-free rocks, and obtained a relationship between ultrasonic wave velocity and the elastic modulus of rock mass. An equation depicting ultrasonic wave velocity and fracture density was established. However, it was not widely used, owing to the difficulty of P-wave (Primary wave) and S-wave (Shear wave) velocity measurement at that time. In the 1990s, [PERSON] et al. [12] evaluated the damage process of limestone during the freeze and thaw cycles using ultrasonic methods and [PERSON] and [PERSON] [13] determined the excavation-disturbed zone of a granite tunnel using AE and ultrasonic techniques. In the 21 st century, research on rock damage using ultrasonic methods became popular. Researchers [14; 15; 16] applied ultrasonic wave attenuation to detect and study damage development in granite and then demonstrated that the ultrasonic wave velocity was a good indicator for the physical and mechanical capability of rocks, as well as the gestation environment. [PERSON] and [PERSON] [17] and [PERSON] et al. [18] showed the developing velocities of S-waves and P-waves through shale during uniaxial and triaxial loading, separately. [PERSON] et al. [3] determined the dilatancy boundary of rock salt through the change in ultrasonic wave velocity and permeability. [PERSON] et al. [19; 20] carried out multiple sets of experiments on rock salt to obtain the relationship between the ultrasonic wave velocity variations and the process of salt crystal self-healing. The AE activity is highly contrasted with the damage evolution of rock materials, which has become an important indicator in predicting rock rupture. [PERSON] [21] was the first to study the differences in AE sequences between rock samples and synthetic samples and revealed the AE pattern of rocks during the damage process. [PERSON] and [PERSON] [22], [PERSON] et al. [23], and [PERSON] et al. [24] investigated the AE activities of marble, salt rock, and granite under cyclic loading, respectively; and, the memory effect of AE of rock had been confirmed. [PERSON] et al. [25] and [PERSON] and [PERSON] [26] analyzed rock damage during different freezing-thawing cycles using AE activity. [PERSON] et al. [27; 28] studied the effect of uniaxial loading rate on the AE frequency of rock salt and the number of AE events. [PERSON] et al.