## 3 Results
### Probe Rod Axial Stress and Acceleration Analysis  
Figure 3 shows the axial stress change with time at the top of the probe rod and at the water-soil contact surface under different water depths. In this representation, negative stress indicates the probe rod being subjected to compression, while positive stress indicates tension on the probe rod. It can be clearly observed that there are multiple stress peak points, which are caused by secondary hammering due to rebound after the anvil is struck by the falling hammer. In this paper, only the axial stresses and acceleration changes of the rod produced by the first hammering are analyzed. As can be seen from Figure 3, the stress wave formed by the impact of the falling hammer on the probe rod gradually decays in the process of downward propagation, resulting in a significant reduction in the peak axial stress from the top to the bottom of the rod. With the increase in the test water depth, there \begin{table}
\begin{tabular}{c c c} \hline \hline
**Distances between Two Test** & **Time Intervals between Two** & \\
**Point/m** & **Test Point/s** & **Stress Wave Velocities /m** \\ \hline
18.6 & 0.004090 & 4547.7 \\
18.6 & 0.004052 & 4590.3 \\
18.6 & 0.004058 & 4583.5 \\
28.6 & 0.006056 & 4722.6 \\
28.6 & 0.006121 & 4672.4 \\
28.6 & 0.006135 & 4661.8 \\
38.6 & 0.008124 & 4751.4 \\
38.6 & 0.008198 & 4708.5 \\
38.6 & 0.008173 & 4722.9 \\
48.6 & 0.010226 & 4752.6 \\
48.6 & 0.010273 & 4730.8 \\
48.6 & 0.010366 & 4688.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Times of peak axial stress for test points. is no significant difference in the peak axial stress at the top test point, but the peak stress at the bottom test point decreases with the increase in water depth. Compared with the test group with a test water depth of 7.1 m, the peak stress at the top test point in the test group with a test water depth of 17.1 m decreased by 2.1%, while the peak stress at the bottom test point decreased by 28.2%. Figure 3: _Cont._  
Figure 4 shows the effect of survey depth on the axial stress at the top and bottom test points of the probe rod under a water depth of 15 m. As can be seen from Figure 4, there is a significant difference in the magnitude of peak stress between the top and bottom test points of the probe rod. The axial stress at the bottom test point is notably smaller than that at the top test point. For example, at a survey depth of 10 m, the peak stress at the top test point is 308.4 MPa, while the peak stress at the bottom test point is 124.6 MPa, a reduction of 59.6%. With the increase in the length of the probe rod, there is no significant difference in the peak stress at the top test point, and the peak stress at the bottom test point decreases with the increase in the rod length. Compared with the test group with a survey depth of 10 m, the stress at the top test point was reduced by 4.8% and the stress at the bottom test point was reduced by 11.5% for the test group with a survey depth of 20 m.  
Figure 4: _Cont._  
Figure 3: Relationship between axial stress of probe rod and time under different water depths. (**a**) Water depth of 7.1 m; (**b**) water depth of 8.9 m; (**c**) water depth of 11.1 m; (**d**) water depth of 11.6 m; (**e**) water depth of 15.0 m; (**f**) water depth of 17.1 m; (**g**) water depth of 19.4 m; (**h**) water depth of 20.7 m.  
Figure 4: _Cont._  
Figure 5 shows the variation curve of the probe acceleration with time for different rod lengths. With the increase in rod length, the peak acceleration of the rod decreases obviously. For the test group with survey depth of 5-50 m, the peak accelerations are 4975.8, 4923.2, 4672.1, 4501.7, 4399.1, 4267.5, 4192.3, 4087.2, 3957.8, and 3952.7 m/s\({}^{2}\), respectively. Figure 4: Relationship between the axial stress of the probe rod and time.