Wireless Underground Sensor Network
Sensor nodes can be deployed under the ground surface or above the ground surface depending on the application. For example, for the purpose of predicting land slide in an area, sensors like pore pressure sensor, moisture sensor etc. need to be deployed at certain specified depths beneath the earth surface. There can be one or more similar type of sensor nodes depending on the application. If there is more than one sensor node deployed beneath the surface, then there may arise the need for single hop or multi hop ad-hoc communication depending on the communication protocol based on which that sensor network is implemented. Hence if there are multiple nodes deployed under the earth surface, from the communication aspects, there are mainly two different kinds of communication.
One is the communication between two underground sensor nodes and other one is the communication between two sensor nodes placed above and below the ground surface. So if two sensor nodes want to communicate each other, then they can use either a wired connection or a wireless connection. Since wireless connection has some advantages like ease of deployment, increased node density, proper concealment of nodes, timeliness of data etc, it is preferred to a wired connection. So a wireless sensor network can be set up in the underground as shown in the figure.1. This set up consists of two sensor nodes N1 and N2 separated by a distance D. d1 and d2 shows the depth of the nodes from the surface. A base station is also shown placed above the ground surface to which each node will send the sensed data.
Figure.1 System Architecture
The main factor that will affect the communication is the path loss during the communication through the soil. The transmission losses are mainly due to the constituents of the soil. This can be reduced to a great extent by carefully choosing parameters like frequency of operation, transmission power, inter node distance. In order to choose those parameters, a proper understanding of the channel is necessary. That is, all the constituents of the soil should be identified by testing the soil. After identifying the soil components, a path loss model should be developed for identifying the loss inducing factors. After the identification of loss inducing factors, the above mentioned parameters can be chosen so as to enable an efficient communication in a sensor network. So for enabling a communication with minimal loss between sensor nodes, a path loss model is essential. By developing a path loss model, we can efficiently choose the above mentioned parameters and can be used for developing an efficient communication protocol which can be used for proficient communication between underground sensor nodes.