Hybrid Wireless Underground Sensor Networks: Quantification of Signal Attenuation in Soil

Wireless sensor network technology allows real-time soil water content monitoring with a high spatial and temporal resolution for observing hydrological processes in small watersheds. The novel wireless soil water content network SoilNet uses the low-cost ZigBee radio network for communication and a hybrid topology with a mixture of underground end devices each wired to several soil sensors and aboveground router devices. Data communication between the end and router devices occurs partially through the soil, and this causes concerns with respect to the feasibility of data communication due to signal attenuation by the soil. In this study, we determined the impact of soil depth, soil water content, and soil electrical conductivity on the signal transmission strength of SoilNet. In a first step, we developed a laboratory experimental setup to measure the impact of soil water content and bulk electrical conductivity on signal transmission strength. The laboratory data were then used to validate a semi-empirical model that simulates signal attenuation due to soil adsorption and reflection and transmission at the soil boundaries. With the validated model, it was possible to show that in the case of a soil layer of 5 cm, sufficient power will remain to ensure data communication over longer distances for most soil conditions. These calculations are fairly simplified and should be considered as a first approximation of the impact of attenuation. In actual field situations, signal transmission may be more complex. Therefore, a field evaluation of signal attenuation is a crucial next step.