## 2 System
### UAV-TDEM System  
The UAV-TDEM system is meticulously designed for efficient and high-resolution subsurface exploration, particularly for detecting shallow targets. The architecture incorporates a sophisticated UAV platform, a host control system, and electromagnetic transceiver sensors, working in concert to accurately capture electromagnetic field data. Real-time georeferencing through RTK GPS technology ensures spatial synchronization. Figure 1 presents the structural layout of the UAV-TDEM system.  
The UAV platform forms the foundation of the system, offering stability and precision in navigation, both of which are essential for achieving high-quality data in extensive survey missions. Following comprehensive performance evaluations, a six-rotor UAV from Sunward Technology Co., Ltd. (Zhuzhou, China) was selected, distinguished by its payload capacity, stability, and reliability under survey conditions. This UAV can support sustained flight with a considerable payload for up to 40 min, enabling efficient, large-scale surveys in a single mission.  
The host system within the UAV-TDEM setup consists of a control module, transmitter, and data acquisition module. The control module oversees signal generation and data collection timing, transmitting commands to adjust frequency and timing settings. This coordination ensures synchronized operations between the UAV's flight and the data acquisition process, maintaining precise control over the electromagnetic data capture.  
The transmitter employs an H-bridge circuit to manage current direction within the transmitting coil, creating a bipolar waveform to produce a consistent primary magnetic field with a maximum current output of 8 A. Once transmission ceases, the receiver captures the secondary field generated by eddy currents within subsurface anomalies. The data acquisition module then amplifies and digitizes the signals, with onboard storage for post  
Figure 1: UAV-TDEM structure diagram.  
processing. Real-time data visualization is available via Wi-Fi connection to a base station, allowing immediate quality control and data validation during field operations.  
The transceiver sensor array includes a 1 m \(\times\) 1 m transmitting coil for primary magnetic field generation and a 0.5 m \(\times\) 0.5 m receiving coil to capture the induced secondary field. The receiving coil operates with a high bandwidth of 140 kHz, ensuring precise transient response measurements essential for characterizing subsurface anomalies. Advanced signal processing features, such as low-pass filtering and analog-to-digital conversion, enhance data accuracy by reducing noise. The RTK GPS integration further ensures precise geospatial alignment, providing centimeter-level accuracy critical for spatially synchronized measurements.  
The transceiver sensor array is suspended from the UAV via non-magnetic nylon ropes to mitigate interference from the UAV's frame and electronic components. This suspension mechanism significantly reduces platform-induced noise, isolating the electromagnetic response of shallow subsurface targets. The UAV's flight path is programmed to maintain a 0.5 m line spacing at a controlled speed of 1 m/s, ensuring data consistency and high lateral resolution across the survey area.