Wireless Underground Sensor Networks: MI-based communication systems for underground applications.

Wireless underground sensor networks (WUSNs) can enable many important applications such as intelligent agriculture, pipeline fault diagnosis, mine disaster rescue, concealed border patrol, and crude oil exploration. The key challenge to realize WUSNs is the wireless communication in underground environments. Most existing wireless communication systems utilize a dipole antenna to transmit and receive propagating electromagnetic (EM) waves, a method that does not work well in underground environments due to the high material absorption loss. The magnetic induction (MI) technique provides a promising alternative solution that could address the current problem in underground environments. Although MI-based underground communication has been intensively investigated theoretically, little effort has been made so far to develop a testbed for MI-based underground communication that can validate the theoretical results. In this article, a testbed of several MI-based communication systems is designed and implemented in an in-lab underground environment. The testbed realizes and tests not only the original MI mechanism utilizing a single coil but also recently developed techniques that use the MI waveguide and threedirectional (3D) MI coils. The experiments are conducted in an in-lab underground environment with reconfigurable environmental parameters, such as soil composition and water content. This article provides the principles and guidelines for developing the MI underground communication testbed, which is very complicated and time consuming due to the new communication mechanism and the new wireless transmission medium.