## 3 Experimental Setup and Instrumentation
### Sensor Deployment and Measurement Techniques  
This section details the deployment of sensors and the measurement techniques used to capture both low- and high-frequency wave phenomena, forming part of a comprehensive instrumentation strategy to monitor the bridge's dynamic behavior and internal condition. During the monitoring campaign, several measurement sessions were conducted using a combination of four vibration sensors, four navigation sensors for six-degree-of-freedom (6-DOF) measurements, and twelve geophones. To ensure consistent sensor positioning and adequate spacing throughout the monitoring period, a non-symmetrical configuration across the two spans was adopted. However, since the focus of this paper is on extracting shear wave propagation from the vertical vibrations of the bridge, the configurations of the other sensors are not discussed further. Free vibration measurements were conducted using a Trillium Compact broadband seismometer to capture low-frequency wave propagation (Figure 4). This highly sensitive instrument measured the vertical translational responses of the bridge at a recording rate of 200 Hz. Figure 3: The post-tensioning system, composed of hydraulic cylinders at the bridge ends and two unbonded tendons beneath the structure. Figure 2: The inverse U-shaped cross-section along the bridge length, highlighting unbonded tendon ducts (diameter 70) beneath the structure. The numbers 1–6, 12, and 13 represent different types of rebar based on their sizes. For high-frequency wave monitoring, embedded piezoelectric ultrasonic transducers (Figure 5), specifically the Acsys-SO807 model designed by Acoustic Control Systems, Ltd., Moscow, Russia for BAM, were employed. These transducers were embedded in the bridge model from an early age, specifically during concreting, to enable continuous structural monitoring throughout its service life. The hollow piezoceramic design allows each transducer to function as both a transmitter and a receiver, operating at a sampling rate of 2 MHz. Their stable signal quality and low sensitivity to environmental disturbances make them particularly well-suited for in situ wave velocity estimation and crack detection in concrete structures [36; 37]. Figure 6 illustrates the deployment of four Trillium Compact vibration sensors. Each is spaced 6 m apart to ensure consistent low-frequency wave propagation paths between adjacent sensors. The bridge features a monolithic cross-section and symmetric spans, with two sensors placed on each span--Stations 1 and 2 on the south and Stations 3 and 4 on the north. Equal spacing between Stations 1-2, 2-3, and 3-4 ensures uniform measurement intervals. An arrow marked with an \"N\" in Figure 6 indicates the direction of the north. Additionally, ultrasonic transducers were embedded at cross-sections A, B, C, D, and E. The experimental design ensured that the distances between ultrasonic sensor pairs were shorter than those between vibration recording stations. This arrangement facilitates the validation of low-frequency wave propagation using high-frequency ultrasonic measurements within the same measurement region. Figure 4: Trillium Compact broadband seismometer used for vibration measurement. Figure 5: Embedded ultrasonic transducers, designed by Acoustic Control Systems, Ltd. for BAM [16], and their embedment. This configuration was selected for two main reasons. First, it maintains consistent sensor spacing throughout the monitoring period, ensuring reliable data collection despite the asymmetry relative to the beam centerline. While direct comparison of mid-span displacements between spans is not possible, the setup still provides sufficient spatial resolution to capture structural mode shapes. Second, the layout supports directional comparison between seismic interferometry and ultrasonic wave propagation.