Link Quality Analysis of Wireless Sensor Networks for Underground Infrastructure Monitoring: A Non-Backfilled Scenario

Wireless underground sensor networks (WUSNs) have been preliminarily applied in underground infrastructure monitoring (UIM) to alert entities about any anomaly in a timely manner. Some key repeaters are deployed along a typical borehole to ensure reliable connectivity with the aboveground gateways. However, the link quality characteristics of the bi-directional communications between the underground repeaters and aboveground devices have not been experimentally analyzed for UIM-WUSNs. To this end, a semi-empirical evaluation of underground-to-aboveground (UG2AG) and aboveground-to-underground (AG2UG) communications are presented in non-backfilled scenarios. In this paper, the effects of burial depth, internode distance, propagation direction, and backfilled condition on link quality are quantitatively investigated by real-life experiments. First, the non-backfilled UG2AG and AG2UG channel model is proposed and verified by the measured data. Then, the symmetry between the UG2AG and AG2UG links is demonstrated in the non-backfilled communication. Finally, the backfilled UG2AG/AG2UG communications show asymmetry behavior, which is largely due to the channel sensitivity to the soil properties. These findings can provide guidelines for the design and development of new communication protocols in UIM-WUSNs.