Wireless underground sensor networks: Research challenges

Abstract

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.

Introduction

Sensor networks are currently a very active area of research. The richness of existing and potential applications from commercial agriculture and geology to security and navigation has stimulated significant attention to their capabilities for monitoring various underground conditions. In particular, agriculture uses underground sensors to monitor soil conditions such as water and mineral content [1]. Sensors are also successfully used to monitor the integrity of belowground infrastructures such as plumbing [32], and landslide and earthquake monitoring are accomplished using buried seismometers [13].

The current technology for underground sensing consists of deploying a buried sensor, such as that shown in Fig. 1, and wiring it to a data-logger on the surface which stores sensor readings for later retrieval. Dataloggers (see Fig. 2), may be equipped with a device for wired or single-hop wireless back-haul to a centralized sink, but often data is manually retrieved by physically visiting the datalogger [4]. All of these existing solutions require sensor devices to be deployed at the surface and wired to a buried sensor [20]. While the usefulness of these applications of sensor network technology is clear, there remain shortcomings that can impede new and more varied uses. These shortcomings include: visibility (versus concealment), ease of deployment, timeliness of the data, reliability, and potential for coverage density.

This paper departs from current technology and introduces the concept of Wireless Underground Sensor Networks (WUSNs), where the majority of sensor devices, including their means of transmitting and receiving, are deployed completely below the ground. WUSNs can address the cited shortcomings of current existing underground sensor networks in the following ways:

• Concealment – Current underground sensing systems require dataloggers or motes deployed at the surface with wiring leading to underground sensors [5], [20] in order to avoid the challenge of wireless communication in the underground. The aboveground components of the sensing system are vulnerable to agricultural and landscaping equipment such as lawnmowers and tractors, which can cause damage to devices. Visible devices may also be unacceptable for performance or aesthetic reasons when monitoring sports fields or gardens. WUSNs, on the other hand, place all equipment required for sensing and transmitting underground, where it is out of sight, protected from damage by surface equipment and secure from theft or vandalism.

• Ease of deployment – Expansion of the coverage area of existing underground sensing systems requires deployment of additional dataloggers and underground wiring. Even if terrestrial WSN technology is used for underground monitoring as in [20], underground wiring must still be deployed to connect a sensor to a surface device. Additional sensors in a WUSN can be deployed simply by placing them in the desired location and ensuring that they are within communication range of another device.

• Timeliness of data – Dataloggers often store sensor readings for later retrieval. WUSNs are able to wirelessly forward sensor readings to a central sink in real time.

• Reliability – A datalogger may have tens of sensors connected to it and represents a single point of failure for all of them. Since the sensors of a single datalogger may be spread over a large physical area, a failure of a datalogger could be catastrophic to a sensing application. WUSNs, however, give each sensor the ability to independently forward readings, eliminating the need for a datalogger as well as the wire that must be buried between a datalogger and a sensor. Additionally, WUSNs are self-healing. Device failures can be automatically routed around, and the network operator can be alerted to device failure in real time.

• Coverage density – Sensors in existing underground applications are typically deployed close to their controlling datalogger to minimize the distance between them. Coverage density can therefore be uneven – high in the vicinity of the datalogger, but low elsewhere in the environment. WUSNs allow sensors to be deployed independent of the location of a datalogger.

While the benefits of WUSNs should be clear from the above, there are a number of research challenges that must be addressed to make them feasible. WUSNs may appear to be similar to their terrestrial counterparts, but the underground environment is a hostile place for wireless communication and requires that existing architectures for terrestrial WSNs, including hardware and communication protocols, be reexamined.

The remainder of the paper is organized as follows. In Section 2 we provide an overview of potential applications for WUSNs. Section 3 describes several factors that are important to consider in the design of WUSNs and proposes possible network topologies. In Section 4, we present an overview of the underground channel and the associated challenges. Section 5 examines the communication architecture of WUSNs and explains the challenges existing at each layer of the protocol stack. We conclude the paper in Section 6.