## 3 Results
### Ultrasonic Characteristics  
#### 3.2.1 Wave Velocity and Amplitude  
The effect of various irradiation conditions on the ultrasonic-wave velocity and first-wave amplitude is investigated and presented in Figure 6. The ultrasonic-wave velocity was about 4.45 km/s for the original specimen, with the penetrating time of 22.5 \(\upmu\)s. However, the wave velocity decreased from 3.89 to 2.92 km/s for the irradiated specimens when the laser power ranged between 400 to 1000 W, with the penetrating time ranging from 24.9 to 32.5 \(\upmu\)s, as shown in Figure 6a. Compared with the value of the original specimen, the wave velocity of the irradiated rock decreased by 14.5-35.8% when the laser power increased in the range of 400-1000 W. In addition, the first-wave amplitude was also observed to be decreased under the same range of the laser-power output. For instance, the first-wave amplitude decreased from 134.25 (original specimen) to 102.1 dB (1000 W), with a reduction rate of 23.9%. The influence of the beam diameter on the wave velocity and the first-wave amplitude is illustrated in Figure 6b. It is seen from the figure that the wave velocity increases from 3.18 to 3.75 km/s when the beam diameter increases in the range of 6-12 mm, with an increasing rate of 18.0%. Similarly, an increase in the beam diameter with the same range also leads to an increased wave amplitude from 102.1 to 129.65 dB. The effect of moving speed on the wave velocity and the first-wave amplitude is presented in Figure 6c, which indicates that the wave velocity increases from 3.18 to 4.24 km/s when the moving speed is increased in the range of 0.5-4.0 mm/s. It is also found that the first-wave amplitude increases from 102.1 to 125.95 dB under the same range of the moving speed. In addition, the increasing rate of the wave velocity and first-wave amplitude is also observed to be significantly weakened when the laser beam moves faster. Figure 6 also indicates that all these influences of laser-power output, beam diameter and beam moving speed on the wave velocity and first-wave amplitude of the irradiated specimens are non-linear.  
It is believed that the ultrasonic-wave velocity is strongly related to the elastic characteristic and uniaxial compressive strength of the rock specimen [47]. When the ultrasonic wave encounters microcracks filled with air on the propagation path, it is time-consuming for the ultrasonic wave to penetrate the rock-air interface. The ultrasonic wave propagates along the edges of the cracks, which indicates that the time for the ultrasonic wave to penetrate the irradiated rock with microcracks is longer than that for the original specimen. Therefore, the cracks within the irradiated rock specimen caused by laser irradiation can lead to a longer penetrating time and a lower wave velocity. When the ultrasonic wave penetrates those rock specimens consisting of microcracks or sub-regions with a lower strength, an increase in the absorption and scattering attenuation could be observed, which results in a lower wave amplitude [48]. The cracks and defects within the irradiated specimens are equivalent to the obstacles that can interfere with the normal transmission of the ultrasonic wave. The variation of the first-wave amplitude is strongly dependent on the compactness of the rock specimens, which is more sensible than the penetrating time. Therefore, the more the cracks and defects, the larger the attenuation of the first-wave amplitude is [49]. Because the interference from other waves' superpositions may affect the validity of the results, the first-wave amplitude was used for the comparison in this study.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Mineral** & **Na2O** & **MgO** & **Al2O3** & **SiO2** & **K2O** & **CaO** & **Fe2O3** \\ \hline Original & 3.53 & 0.69 & 13.91 & 68.55 & 5.16 & 1.63 & 2.46 \\ Irradiated & 3.19 & 0.50 & 12.82 & 69.77 & 4.31 & 1.58 & 2.36 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Composition of original and irradiated granite (%).