Wireless underground sensor networks (WUSNs) can enable many important applications such as intelligent agriculture, pipeline fault diagnosis, mine disaster rescue, concealed border patrol, crude oil exploration, among others. The key challenge to realize WUSNs is the wireless communication in underground environments. Most existing wireless communication systems utilize the dipole antenna to transmit and receive propagating electromagnetic (EM) waves, which do not work well in underground environments due to the very high material absorption loss. The Magnetic Induction (MI) technique provides a promising alternative solution that could address the current problem in underground. Although the MI-based underground communication has been intensively investigated theoretically, to date, seldom effort has been made in developing a testbed for the MI-based underground communication that can validate the theoretical results. In this paper, a testbed of MI-based communication system is designed and implemented in an in-lab underground environment. The testbed realizes and tests not only the original MI mechanism that utilizes single coil but also recent developed techniques that use the MI waveguide and the 3-directional (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 paper provides the principles and guidelines for developing the MI underground communications testbed, which is very complicated and time-consuming due to the new communication mechanism and the new wireless transmission medium.
The concept of WUSNs is first introduced in @cite_2 , after which many novel applications are presented based on the WUSNs. In @cite_7 , a WUSN is implemented to monitor the underground tunnels to ensure safe working conditions in coal mines. In @cite_3 , the WUSNs are deployed during the hydraulic fracturing process in the crude oil extraction, which can provide real-time physical and chemical measurements deep inside oil reservoirs. In [6,7], the WUSNs are utilized for pipeline leakage detection where MI communications are used to connect the sensors along pipelines.
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The real-time and in-situ monitoring capability in oil reservoirs is highly desired to increase the current recovery factor of crude oil and natural gas. To this end, the wireless sensor networks (WSNs) are envisioned to be deployed deep inside oil reservoirs to collect and report the physical and chemical information in real time. However, none of the existing wireless communication and networking technologies can support WSNs in oil reservoirs due to the very challenging environment and the extremely small device size. To address the problem, this paper proposes a new self-contained micro wireless sensor network framework based on the Magnetic Induction (MI) technique, which can enable the real-time and in-situ monitoring in oil reservoirs. Rigorous analytical models are developed to characterize the oil reservoir channel for both MI communication and energy transfer, which confirm the feasibility of the proposed self-contained sensor network framework. To enhance the system efficiency and reliability, high-permeability proppants are injected in the hydraulic fracture to increase mutual induction; while the tri-directional MI coil antenna is designed to achieve omnidirectional coverage. The theoretical models and numerical results are validated by the widely used finite element simulation software COMSOL Multiphysics. Environment monitoring in coal mines is an important application of wireless sensor networks (WSNs) that has commercial potential. We discuss the design of a structure-aware self-adaptive WSN system, SASA. By regulating the mesh sensor network deployment and formulating a collaborative mechanism based on a regular beacon strategy, SASA is able to rapidly detect structure variations caused by underground collapses. A prototype is deployed with 27 Mica2 motes. We present our implementation experiences as well as the experimental results. To better evaluate the scalability and reliability of SASA, we also conduct a large-scale trace-driven simulation based on real data collected from the experiments. This work introduces the concept of a Wireless Underground Sensor Network (WUSN). WUSNs can be used to monitor a variety of conditions, such as soil properties for agricultural applications and toxic substances for environmental monitoring. Unlike existing methods of monitoring underground conditions, which rely on buried sensors connected via wire to the surface, WUSN devices are deployed completely belowground and do not require any wired connections. Each device contains all necessary sensors, memory, a processor, a radio, an antenna, and a power source. This makes their deployment much simpler than existing underground sensing solutions. Wireless communication within a dense substance such as soil or rock is, however, significantly more challenging than through air. This factor, combined with the necessity to conserve energy due to the difficulty of unearthing and recharging WUSN devices, requires that communication protocols be redesigned to be as efficient as possible. This work provides an extensive overview of applications and design challenges for WUSNs, challenges for the underground communication channel including methods for predicting path losses in an underground link, and challenges at each layer of the communication protocol stack. � 2006 Elsevier B.V. All rights reserved.