**Year of publication**

**Figure 3.** Distribution of number of relevant studies published between 2000 and 2018. **Figure 3.** Distribution of number of relevant studies published between 2000 and 2018.

#### *4.2. Overview of Previous Review Articles Related to Underground Water and Sewer Pipeline Monitoring 4.2. Overview of Previous Review Articles Related to Underground Water and Sewer Pipeline Monitoring*

The WSN system has been tested by monitoring various types of faults in pipelines carrying different substances such as gas, oil, and water [54,55]. Various researchers have reviewed previous methods used for monitoring underground water and sewer pipelines, and they have suggested prospects for improvement. A hybrid method (combining any two different pipeline leakage monitoring methods) to form a new and more reliable solution was suggested in articles [56] and [57]. According to [58,59], there are two critical issues that need improvement in terms of development of a preferable underground WSN system—(1) efficiency of network-communication range and (2) energy efficiency of WSN. The WSN system has been tested by monitoring various types of faults in pipelines carrying different substances such as gas, oil, and water [54,55]. Various researchers have reviewed previous methods used for monitoring underground water and sewer pipelines, and they have suggested prospects for improvement. A hybrid method (combining any two different pipeline leakage monitoring methods) to form a new and more reliable solution was suggested in articles [56] and [57]. According to [58,59], there are two critical issues that need improvement in terms of development of a preferable underground WSN system—(1) efficiency of network-communication range and (2) energy efficiency of WSN.

encountered during each review.

articles.

Abdelhafidh et al. [56]

Adedeji et al. [60]

Datta et al. [61]

Sheltami et al. [57]

Obeid et al. [62]

is applicable to various linear pipeline layouts (i.e., straight, short, long, and zigzag) while being one of the most energy efficient and reliable methods. Table 4 lists extant review articles published on monitoring underground water and sewer pipeline leakage using WSN-based systems in the reverse chronological order. The listed articles also discuss suggestions for future research and challenges

**Table 4.** Summary of future research suggestions pertaining to underground water and sewer pipeline leakage and sinkhole monitoring using WSN-based systems, as discussed in extant review

**Article Future Research Suggestions Research Field Publication** 

one hybrid system for better results. Computer Sciences <sup>2018</sup>

considered. Electrical Engineering <sup>2017</sup>

Mechanical

Electronics and

Engineering <sup>2016</sup>

Computer Sciences 2016

Communication <sup>2016</sup>

Combination of several leak detection methods to form

Sensors deployment strategies in WSN nodes need to be

Acoustic reflectometry is most suitable for leakage and blockage in underground pipelines.

Hybrids of different WSN-based pipeline monitoring techniques to enhance the detection and localization of leakage.

Integrated energy-aware system on chip solution for noninvasive pipeline monitoring.

**Year**

Adedeji et al. [60], in their review article, discussed and suggested the importance of sensordeployment strategy for reliable and scalable data propagation in the soil channel. Similarly, different

Adedeji et al. [60], in their review article, discussed and suggested the importance of sensor-deployment strategy for reliable and scalable data propagation in the soil channel. Similarly, different methods for underground pipeline-leakage monitoring were compared in [61]. Acoustic reflectometry was found to be the most reliable method among all the reviewed methods, because it is applicable to various linear pipeline layouts (i.e., straight, short, long, and zigzag) while being one of the most energy efficient and reliable methods. Table 4 lists extant review articles published on monitoring underground water and sewer pipeline leakage using WSN-based systems in the reverse chronological order. The listed articles also discuss suggestions for future research and challenges encountered during each review.


**Table 4.** Summary of future research suggestions pertaining to underground water and sewer pipeline leakage and sinkhole monitoring using WSN-based systems, as discussed in extant review articles.

From Table 4, it can be concluded that all suggestions for future research are related to the fields of computer science, electrical, electronics, and telecommunication sectors. Therefore, it can be considered that all researchers, in their respective review articles, aimed to present the challenges encountered in the field of WSN, sensor-deployment strategies, and communication radius of the system. However, in the fields of soil mechanics, civil infrastructure, and geology, the after effects of leakage on the surrounding soil must be considered. As mentioned earlier, leakage can lead to soil erosion and ultimately results in sinkholes.
