  Underground in-situ sensing plays a vital role in precision agriculture and
infrastructure monitoring. While existing sensing systems utilize wires to
connect an array of sensors at various depths for spatial-temporal data
collection, wireless underground sensor networks offer a cable-free
alternative. However, these wireless sensors are typically battery-powered,
necessitating periodic recharging or replacement. This paper proposes a
battery-free sensor array which can be used for wireless multi-depth in-situ
sensing. Utilizing Near Field Communication (NFC)-which can penetrate soil with
negligible signal power loss-this sensor array can form a virtual magnetic
waveguide, achieving long communication ranges. An analytical model has been
developed to offer insights and determine optimal design parameters. Moreover,
a prototype, constructed using off-the-shelf NFC sensors, was tested to
validate the proposed concept. While this system is primarily designed for
underground applications, it holds potential for other multi-depth in-situ
sensing scenarios, including underwater environments.
