## 2 Experimental Apparatus and Procedure  
### Experimental Apparatus  
(1) Integrated Testing Apparatus for Ultrasonic Wave and Acoustic Emissions  
The apparatus for ultrasonic wave measurement and AE monitoring was composed of an ultrasonic wave testing system (UWS), an AE testing system (AES), and a data recording system (DRS). A schematic of this system was shown in Figure 1; it has been patented in China (ZL2014101711015). It can be used to measure ultrasonic velocity as well as record AE information and location of a rock sample under uniaxial or triaxial loading. In this way, resources and costs can be saved, and it helped to reveal the coupling laws among rock cracks, ultrasonic wave and acoustic emission for future research. _Part 1, Ultrasonic Wave System (UWS):_ the UWS includes two steel indenters, UW-1 and UW-2, made of steel with the maximum allowable stress of 140 MPa. One P-wave sensor and two S-wave sensors are sealed in each (Figure 1). Sensors in UW-1 linking with the ultrasonic wave transmission channel can transmit the high-voltage signals positively. Sensors in UW-2 linking with the ultrasonic wave reception channel can receive the signal from UW-1. During the loading test, each indenter was placed in the same direction to make the P-wave sensor in UW-1 face the P-wave sensor in UW-2 and the S-wave sensor in UW-1 face the S-wave sensor in UW-2. The ultrasonic velocity spreading in the rock was calculated by the elapsed time and distance of the pulsing signals. Errors in the ultrasonic testing system were determined before the test was conducted. Firstly, the coupling agent was smeared on both sides of the indenters to ensure a good contact; secondly, the two indenters were set face-to-face with the same direction; then, we turned on the data recording system, and the acoustic signals from UW-1 were received by sensors in UW-2, the spread time was calculated as the error of the acoustic wave testing system. S-waves had an error of 12 us and P-waves had an error of 9 us. _Part 2, Acoustic Emission System (AES):_ AE sensors were located at different positions in the rock sample and connected with the AE received channel to record AE activities, which were applied to determine the locations of AE events. We used NANO-30 sensors (produced by the Physical Acoustics Company, Princeton Junction, NJ, USA) as the AE receivers; they were sealed in protective shells with sufficient stiffness, as shown in Section 2.3  
The sensitivities of the AE sensors were tested before use. We placed a crushed pencil lead of 0.5 mm thickness on the face of the AE receivers, and the data recording system recorded a signal. If the signal amplitude was higher than 90 dB, it indicated that the receiver was sensitive to the AE source; if this was not the case then we rechecked the link conditions and coupling quality of the sensors and repeated the tests until the sensitivity met the requirements. _Part 3, Data Recording System (DRS):_ the DRS included a personal computer (PC), a high-voltage signal transmitting card (Model ARB-1410, produced by the Physical Acoustics Company), and signal receiving cards (Model PCI-2, produced by the Physical Acoustics Company). ARB-1410, which was connected with UW-1, can emit different types of ultrasonic signals with enough power to overcome the energy attenuation in the spread process; the emission time interval, cycle time, and signal energy can be set by experimenters. PCI-2 was connected with UW-2 and the AE sensors, and they were used to record the ultrasonic wave and AE signals. Figure 1: Schematic of integrated testing of ultrasonic waves and acoustic emissions for rock under uniaxial stress. AE, acoustic emissions; PCI, peripheral component interconnect; P-sensor, pressure wave sensor; S-sensor, shear wave sensor; UW-1, ultrasonic wave system part #1; UW-2, ultrasonic wave system part #2. (2) Uniaxial Loading System  
A uniaxial loading system was used to apply a uniaxial compression on salt rock samples. The maximum output axial loading was 4600 kN.