## 4 Leak Detection Technologies
### Hardware-Based Methods
#### 4.1.1 Non-Acoustic Methods  
Non-acoustic methods are integral within the hardware-based approaches for water leak detection. They encompass techniques such as visual surveys, gas injection, thermography, ground-penetrating radar (GPR), and negative-pressure waves. The visual survey method, a traditional mainstay, leans heavily on the observational judgment of the surveyor [28]. While this approach is accessible, its subjectivity and limited coverage present persistent challenges. The adjunct use of trained dogs, with their ability to detect leak-related substances through their acute sense of smell [15], offers promise. However, standardized training protocols and a comprehensive assessment of canine reliability across diverse field conditions are imperative to realize this potential. The gas injection technique has been used to detect water leaks. This method involves injecting a non-toxic gas that is both water-insoluble and lighter than air into a specific section of a water pipe that has been isolated [29]. This technique, especially the employment of sulfur hexafluoride SF6, stands under the ecological spotlight due to its high global warming potential. The gas injection method can detect water leakage in all pipe materials, with a diameter range of 75 mm to 1000 mm. However, the success of this method is heavily dependent on the specific site conditions, including soil type, wind speed, and temperature. Notably, its limitations in detecting smaller or pinhole leaks necessitate innovation to broaden its applicability [30]. Therography is a method rooted in detecting thermal anomalies in the soil adjacent to a buried pipe due to water leakage [31; 32]. Thermal anomalies above pipes can be detected using infrared cameras mounted on a vehicle or aircraft or held by hand [33]. This technique holds theoretical promise, and it is capable of covering large areas without requiring any excavation [34] but remains vulnerable to the influence of external factors, such as solar radiation, wind speed, and ground moisture. These environmental variables necessitate a meticulous consideration when employing this technique. Ground-penetrating radar (GPR) has been used as a non-destructive geophysical method for leak detection [16; 35; 36]. This technique operates by emitting electromagnetic waves into the ground, relying on anomalies in dielectric properties to reflect these waves back to the surface. The depth of these anomalies is determined by analyzing the time lag between the transmission and reception of waves, ultimately revealing the size and shape of buried objects. When it comes to detecting water leaks, GPR offers two methods: identifying soil voids formed due to leaks or pinpointing sections of pipes that appear deeper than they are, owing to changes in dielectric properties caused by saturated soil. This method applies to both metallic and plastic pipes, but it demands a substantial investment of both time and resources. Moreover, the challenge of selecting the appropriate radio frequency for diverse soil responses complicates its implementation. Additionally, the potential for false alarms stemming from reflected waves due to soil anomalies, such as metal objects, imparts uncertainty regarding its dependability [34]. The propagation of an incoming transient signal through a system generates reflections from any anomalies, encompassing leaks, blockages, or irregularities in surface conditions [37; 38]. Notably, the presence of a leak induces an increase in the dampingrate or gives rise to reflections within the pressure trace of the transient signal. Inducing hydraulic transient waves, frequently through a water hammer, has become a conventional method [39]. This hydraulic process triggers a negative-pressure wave upon abrupt pipe rupture, traversing in both directions and reflecting at the pipeline boundaries. The leak's location is ascertained by the temporal arrival of the known-speed negative wave at each pressure transducer, while the magnitude of the transient wave permits estimation of the breach size.