**1. Introduction**

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While RFID technology is nowadays very common in many commercial and industrial sectors, from items tracking to personal identification, few studies have dealt with the chance to use RFID systems in marine or fluvial environments for underwater monitoring operations. While the technical limitations for these scenarios can be in some cases insur‐ mountable, ad-hoc studies have proven that in some cases RFID technology can work even under water.

RFID, like all radio technologies, in unsuitable to work in presence of water. Still water is not a natural conductor, but the presence of dissolved salts or other materials turns it into a partial conductor. Electromagnetic waves cannot travel through electrical conductors: this means that in most cases radio waves cannot be used to communicate under water. Any‐ way, studies have proven that the chance to transmit radio signals under water mainly de‐ pends on two factors: the conductivity of water and the frequency of the radio wave. While the conductivity of water is a factor that cannot be modified to increase the possibility to use radio waves under water, the only factor that can be modified to increase the performances is obviously the radio frequency.

This factor has already been employed when using the electromagnetic fields for the com‐ mon radio transmissions: Very Low Frequency radio waves (VLF – 3-30kHz) have proven to be able to penetrate sea water to a depth up to 20 meters, while Extremely Low Fre‐ quency radio waves (ELF - 3-300Hz) can travel in sea water up to hundreds of meters. Anyway, these frequency bands present severe technical limitations. First of all, their ex‐ tremely long wavelengths require antennas of very big dimensions: frequencies lower than 100Hz have wavelengths of thousands of kms, forcing to use antennas covering wide areas. Secondly, due to their narrow bandwidth, these frequencies can be used to transmit only text signals al slow data rates.

© 2013 Benelli and Pozzebon; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Benelli and Pozzebon; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Some of these considerations can be applied also to RFID systems. First of all the use of ac‐ tive technologies is discouraged by many factors: at lower frequencies only passive systems can be found; moreover, the use of active systems is also impeded by the required dimen‐ sions of the antennas. Due to these limitations, only two RFID technologies can be employed for underwater applications: the High Frequency systems, operating at 13.56MHz and the Low Frequency systems, operating in the 125-134kHz band. The first solution (13.56MHz) still presents some severe limitations due to the reduction of the reading range: with com‐ mon desktop antennas the reduction in the range is up to 80%, forcing to bring the trans‐ ponder practically in contact with reader antenna. For the second solution (125-134kHz) the reduction is lower (around 30%) and the reading at a distance is still achievable. Laboratory tests proved that, with long-range antennas, a 50cm reading range is still achievable.

charge, while the two ends have a partial positive charge. A molecule with such a charge equilibrium is called electric dipole, and is characterized by its dipolar momentum µ, de‐ fined as the product between the absolute value of one of the two charges and the dis‐ tance between them. This value indicates the tendency of a dipole to orientate under the

RFID Under Water: Technical Issues and Applications

http://dx.doi.org/10.5772/53934

381

While still water has a very low electrical conductivity, this value increases in presence of ionized molecules, in proportion to their concentration. When a salt is melt in still water, the single molecules are equally perfused in the whole liquid so that each single volume portion of the solution dissociates, creating many positive and negative ions that remain in the solution together with all the other molecules that aren't dissociated. This phenom‐ enon is called electrolytic dissociation, and the so created solutions are called electrolytic solutions. These solutions can be crossed by an electrical current, in contraposition with

The chemical composition of marine water is influenced by several biological, chemical and physical factors: one simple example is the presence of rivers that add every day new chemi‐ cal materials to the water. On the other side, other materials are removed by the action of organisms and due to erosion. Anyway, the most part of the salts dissolved in marine water remains almost constant due this continuous interchange phenomenon. The most important factors that influence the chemical composition of the marine water are the following:

The elements that can be found is marine water are around 70, but only 6 of them represent

The symbol (wt%) stands for the mass fraction, and represents the concentration of a solu‐ tion or the entity of the presence of an element in a solution. The quantity of these ions is proportional to the salinity of water, a parameter describing the concentration of dissolved salts in water. Due to the evaporation, this value is lower at the poles (around 3.1%) and

effect of a uniform electric field.

still water that acts as a pure insulator.

**•** The draining of materials deriving from human activities; **•** The interaction between the sea surface and the atmosphere;

**•** The processes between the ions in solution;

the 99% of the total. These predominant salts are:

**•** The biochemical processes.

**•** Chloride (Cl): 55.04 wt% **•** Sodium (Na): 30.61 wt% **•** Sulphate (SO42-): 7.68 wt% **•** Magnesium (Mg): 3.69 wt%

**•** Calcium (Ca): 1.16 wt% **•** Potassium (K): 1.10 wt%

**2.2. Marine water**

Both these two solutions can be anyway employed to set up RFID systems working in under water environments. Some solutions can already be found in some parts of the world [1]. USS Navy is testing the use of RFID technology for their applications based on the use of Unmanned Underwater Vehicles. Other applications foresee the use of RFID for the moni‐ toring of underwater pipelines, with RFID transponders employed as markers to guarantee the integrity of the pipes. RFID has also been employed in aquariums to identify fishes, in the same way as Low Frequency RFID capsules are employed in cattle breeding. Finally RFID has been employed as a way to track the movement of pebbles on beaches, in order to analyse the impact of coastal erosion during sea storms.

The chapter will be subdivided in four main sections.

In the first section, the transmission of radio signals in water will be analysed. Details will be given on how the presence of water affects the electromagnetic fields, and examples of ap‐ plications working in the VLF and ELF bands will be provided.

The second section will focus only on RFID. Technical data will be provided concerning the signal attenuation due to the presence of water. Some results will be given to prove the agreement of experimental data with the theoretical analysis.

In the third section the state of the art concerning under water RFID applications already existing all around the world will be provided. The few already tested applications will be described in detail.

Finally, in the fourth section some future applications based on this technology will be proposed.
