## 4 Parameter Sensitivity Analysis
### Effect of Excitation Position on Wave Behaviour  
One of the most challenging issues in this work is caused by the practical restriction on where guided waves can be generated. In the laboratory testing, impact on the top of a 5 m long wood pole generates the pure longitudinal wave, making the analysis of the wave behaviour straightforward. To investigate the wave behaviour caused by the top excitation, both isotropic and orthotropic numerical models are created. The cylindrical pole is modelled as a tapered shape with 1 m soil embedment. The diameter of the top is 270 mm and the bottom is 300 mm. Figure 20 portrays the wave propagation in the pole with top excitation for both isotropic and orthotropic models. It is noticeable that the wave behaviours for both models are quite similar, even though their material characteristics are definitely different. However, most in-situ wood poles are 12 m long with 1.2-1.5 m embedment length underground, making the top excitation unfeasible because of practical limitations. Hence, in practice the excitation is usually imparted at a reachable location from the ground level with a certain of angle through a waveguide, which may result in two types of waves: longitudinal wave and bending wave as well as two directional wave propagations: upward and downward. To study this complicated wave behaviour, a full-sized wood pole model with 12 m length is built up. The pole is of tapered cylindrical shape and the top and bottom diameters are 228 mm and 300 mm, respectively. The impact excitation is generated at the location of 1.5 m above the ground with the angle of 45 degrees. Similarly, both isotropic and orthotropic models are generated, and the wave traces are shown in Figure 21. It is clearly observed that the up-going and down-going waves are generated simultaneously in the isotropic material model. Figure 22 shows a close-view of wave traces in the isotropic material model based on a fixed sensor (SN6), which is deployed at 400 mm above the soil level. It is obviously seen that the travel-down wave will be first captured by the sensor, which is recorded as first peak denoted  
Figure 20: Wave trace of a 5 m pole for top excitation with (**a**) isotropic and (**b**) orthotropic material models. by 1a. Then, the wave will be reflected from the bottom of the pole and captured by the same sensor as second peak denoted by 2a. At the same time, the travel-up wave reflected from the top of the wood pole will then arrive at the sensor as its first peak denoted by 1b, followed by its reflections from the bottom of the pole as its second peak denoted by 2b. Also, the figure indicates wave propagation in the soil which is below the pole. From the interface between the pole and soil (dashed line), it can be clearly observed that some of the incident waves are reflected by the bottom boundary of the pole while some of them are refracted into the soil. According to the wave trace, it can be seen that the wave travelling in the pole captured by the sensor will not be affected by the wave reflected from the boundary of the soil, because the wave velocity in the soil is extremely low and the reflected energy is also very small. Therefore, they can difficultly be detected by the sensor within the recoding time. On the other hand, the wave behaviour in the 12 m orthotropic material pole model with side excitation is pretty much complicated. It is noticeable that from the impact point, two types of waves are generated simultaneously: longitudinal and bending waves. Additionally, different modes of waves are generated because of the orthotropic characteristic of the wood, which means that reflection and refraction happen in the meantime inside the orthotropic material. Due to different types of waves with various modes, reflected wave identification becomes challenging. Accordingly, further investigation in signal processing is necessary to effectively separate longitudinal and bending waves as well as different modes.