Artículos

 

Advanced optical technologies for monitoring estuaries and coastal environments

 

Tecnologías ópticas avanzadas para monitorear estuarios y ambientes costeros

 

D. Pereira¹*, O. Frazão¹, J. Ferreira¹, I. Dias¹, J.M. Dias² M. Teixeira⁴, N. Vaz², A. Quintela⁴, J.F. Lopes² and J.L. Santos^1,3

 

¹ INESC Porto UOSE, Rua do Campo Alegre 687 4169-007 Porto, Portugal. * E-mail: dap@goe.fc.up.pt

² CESAM Departamento de Física Universidade de Aveiro 3810-193 Aveiro, Portugal.

³ Faculdade de Ciências da Universidade do Porto Rua do Campo Alegre 687 4169-007 Porto, Portugal.

⁴ CABELTE S.A. Apartado 3 4412 Praia da Granja Valadares Codex, Portugal.

 

Recibido en junio de 2003;
aceptado en abril de 2004.

 

Abstract

In recent years the need to monitor different parameters has led to the development of several architectures able to make real time, un-attended measurements. Particularly in regard to environmental issues, it is very important that the equipment and measuring systems be small and light to avoid maximum interference of the ecosystems studied. In this context, optical fibre sensors have become extremely attractive for use in natural environments to monitor different parameters of biological interest, due to their intrinsic small weight and size and low reactivity to chemical and biological parameters. In this paper we present an innovative and technologically advanced system for the simultaneous measurement of temperature and salinity based on optical fibre Bragg grating sensors and on an optical sensing cable especially designed for large-scale and distributed or quasi-distributed measurements. A prototype of this optical cable was installed in the Mira Channel at Ria de Aveiro, Portugal. Temperature variation during three weeks from 4 to 23 June 2002 is investigated and presented here.

Key words: Fiber Bragg graing, wavelength, temperature, salinity, monitoring.

 

Resumen

En años recientes la necesidad de monitorear diferentes parámetros condujo al desarrollo de diversas arquitecturas capaces de hacer mediciones en tiempo real, de manera automatizada. Particularmente cuando se trata de parámetros ambientales es muy importante que equipos y sistemas de medición sean pequeños y ligeros para evitar al máximo la interferencia con los ecosistemas estudiados. En este contexto, los sensores de fibra óptica han resultado muy atractivos para monitorear diferentes parámetros de interés biológico en ambientes naturales gracias a sus reducidos peso y tamaño, y a su baja reactividad hacia parámetros químicos y biológicos. En este artículo se presenta un sistema innovador y tecnológicamente avanzado para la medición simultánea de temperatura y salinidad, basado en sensores de fibra óptica con rejillas de Bragg y un cable de fibra óptica especialmente diseñado para mediciones a gran escala y distribuidas o quasi-distribuidas. En este trabajo se presentan los resultados obtenidos de investigar la variabilidad térmica mediante un prototipo de este cable óptico que fue instalado en el Canal de Mira de la Ría de Aveiro (Portugal) durante tres semanas entre el 4 y el 23 de junio de 2002.

Palabras clave: Rejillas de Bragg, longitud de onda, temperatura, salinidad, seguimiento.

 

Introduction

It is nowadays recognized that coastal and estuarine environments are privileged zones for bio-diversity evolution. By their own nature they are extremely sensitive to changes in the boundary conditions, particularly in regards to water properties. In this sense, emphasis should be placed on the diagnosis and surveillance of the most fragile and mutable environments that are often subjected to considerable stress due to an excess of human activities.

Coastal and estuarine ecosystems have special characteristics, since they are areas where the salt water carried by the ocean tides converges with fresh water input by rivers. The temporal variation of these water flows, together with the geometric characteristics of the area and different meteorological conditions, determine the equilibrium mixture between salt and fresh water. The temperature and salinity gradients established, as well as the salt- and fresh-water flows, have a decisive influence on the water circulation and on the evolution of different vegetal and animal species in these coastal habitats, providing some of the most varied and unstable environments on Earth.

Ría de Aveiro (Portugal) is an example of this kind of zone. It is a shallow lagoon with a very complex geometry, connected to the Atlantic Ocean through a single artificial channel. Most of its water exchange with the ocean takes place by tidal input/output across this narrow entrance. The input of fresh water occurs through several small rivers by different channels discharging on the lagoon. The most important river, the Vouga River, is responsible for approximately 2/3 of the fresh-water input to the lagoon. These characteristics make Ría de Aveiro an ideal place to implement and test innovative sensing systems for monitoring estuaries, lagoons and coastal zones.

The use of optical fibre sensors in the marine environment has, so far, been limited; however, they offer many potential advantages, such as low attenuation, allowing remote location of the electro-optical power supply and control units from the measuring area; capability to perform distributed measurements over many kilometres; small physical size and non-intrusive nature, even when in armoured form; high speed of response; and excellent salt-water corrosion resistance of optical fibres.

In this paper we present two new technologies for monitoring estuaries, lagoons and coastal zones. One is an optical head sensor for the simultaneous measurement of temperature and salinity, and the other is an optical sensing cable to monitor temperature. The main advantage of these two technologies is the possibility of integrating both in a system for real-time monitoring these environmentally relevant measurements. Other advantages arise from the low costs involved in implementing and maintaining this technology. Conventional technology, which requires electrical transducers especially designed to measure variations in the electrical conductivity of water, is hard to install, has a short durability and, most importantly, is not immune to electromagnetic interference, which in some cases is responsible for troubleshooting in measurements.

In this context it should be mentioned that in several cases some sensitive measurements performed by electrical transducers can be affected by electromagnetic interference or noise. This happens most often in gauge systems as well as in LVDTs (linear variable differential transformers) when the external electrical supply has slight fluctuations in tension. Furthermore, when placed in field environments under severe weather conditions like thunders, erroneous measurements due to electrical discharges and humidity can occur. In this particular case, electromagnetic interference can be due to the own nature of the installation which demands an individual cable for each immersed sensor. These are then collected in the proximity to the observation station and, as a result, cross electromagnetic signal effects could occur.

It should also be mentioned that the electrical conductivity of water is an indicator not only of the degree of salinity (ionic presence) but, in addition, it may be used to determine the water flow in a dynamic system. In fact, the conductivity is a measurement of the amount of electrical charges, that if being adverted by a water current field, and subjected to the Earth magnetic field, will induce an electrical potential difference proportional to the water flow. Measurement techniques based on motion-induced electric fields (MIV) have increased the interest of oceanographers in studies of ocean flow to provide detailed information about the velocity structure of ocean currents.

 

Fibre Bragg grating theory

Optical fibre is a glass element that comprises two concentrical cylinders with a slightly different refractive index covered by a polymeric material that ensures mechanical protection and resistance. The inner cylinder is the core and the outer the cladding. The effective refractive index is a characteristic parameter of the glass material and is associated with the propagation speed of light inside it; the difference between the refractive indexes of the materials in the optical fibre is responsible for its waveguide capacity.

In addition to the high potential for developing optical fibre communications, investigations of the photosensitivity mechanisms in optical fibres allow the construction of a new type of sensors: optical fibre sensors.

A fibre Bragg grating (FBG) is a periodic modulation of the refractive index of the core of a single mode optical fibre, written by exposure to UV light in the region around 248 nm. This fabrication process is based on the photosensitive mechanism, which is observed in Ge-doped optical fibres. If broadband light is travelling through an optical fibre containing such a periodic structure, its diffractive properties promote that a very narrow wavelength band is reflected back. The centre wavelength of this band can be represented by the well known Bragg condition: λ_B = 2n_eff Λ, where λ_B is the centre wavelength, n_eff is the effective index of the guided mode and A is the period of the refractive-index modulation. The FBG resonance wavelength will vary according to the changes in temperature or refractive index experienced by the fibre. For a silica fibre the wavelength-temperature sensitivity is ~13 pm°C^-1 for a Bragg wavelength centred at 1555 nm.

Temperature and index variations applied to the gratings cause shift in the Bragg wavelengths as

The thermal sensitivity, KT, depends on the thermal expansion of the fibre and, essentially, on the thermo-optic coefficient. On the other hand, the index sensitivity, Kn, depends on the relative diameters between the core and the cladding. In this way, equation (1) can be used to discriminate between the effects of the changes in temperature and refractive index applied to the sensor head:

where Δ = K_T1 K_n2 - K_n1 K_T2.

 

Temperature and salinity sensor

In this work, refractive index measurements are achieved by etching the fibre cladding in the region of the grating to a diameter such that the evanescent field of the mode interacts with the immediate surrounding environment. With this configuration, the value of the effective refractive index of the waveguide mode is directly affected by the refractive index of the medium where the fibre is immersed.

To different concentrations of salt correspond different surrounding refractive indexes, which can be measured. The reduction of the cladding diameter is made by chemical attack with an aqueous solution of fluoridic acid (HF 40%).

Considering the Bragg relation mentioned above, this means that associated with the etching process there is a variation of the Bragg wavelength of the FBG given by

Therefore, in such a stage the variation of the Bragg wavelength of the FBG is only related to the variation of the refractive index of the surrounding medium (Asseh et al., 1998). This constitutes the principle of the salinity sensor proposed in this work.

The relation between the refractive index and the corresponding salt concentration is shown in figure 1 (Quan et al., 1995). The values were measured with an Abbe refractometer. Figure 2 shows the evolution of the refractive index with temperature. From these two graphs it is easy to see that the influence of temperature on the refractive index, and consequently on the sensor response, is one order of magnitude less than that produced by a direct index change at room temperature. Hence, in a first approximation, it is possible to ignore the influence of temperature on the refraction index. The peak power and wavelength variations during the chemical attack are presented in figures 3 and 4, respectively. The abrupt zone observed indicated that the core boundary of the fibre is almost exposed to air, which alters the guided wave conditions and consequently the wavelength FBG response. The abrupt zone in the peak power graph indicates a faster degradation of the sensor in the neighbourhood of the core.

The temperature calibrations of both sensors are presented in figure 5. As expected, both sensors have the same behaviour regarding temperature. The response of both sensors to refractive index changes in a relatively large range and the response to refraction index changes of the salinity sensor in the range corresponding to the salt concentrations in nature are shown in figures 6 and 7, respectively. The main aspect that arises from these results is the linearity of the results, making them attractive for environmental monitoring (Esteban et al., 1999).

Figure 8 shows the setup used in the laboratory to test the salinity sensor. The light emitted from a broadband source (BBS) is directed toward the sensors. The light reflected from each sensor corresponding to the Bragg wavelength is then analyzed by an optical spectrum analyzer (OSA) controlled by an acquisition program done in LabViewTM for this particular purpose. Once some temperature or salinity fluctuation occurs, the shift induced in the Bragg wavelengths is observed following the principles outlined above.

 

Optical cable sensing

Here we present an environmental monitoring system that was designed and implemented for the PROTEU project (PROTEU: Advanced technologies for monitoring estuaries and coastal environments, FCT-PDCTM/P/MAR/15275/1999) and that is currently under field tests. At present, only water temperature in the Mira Channel of Ría de Aveiro (see fig. 9) has been measured, but in the near future the salinity sensor (previously mentioned) will also record measurements in the extremity of the optical cable. Monitoring is made by a specially designed optical fibre cable (TON GERE), with Bragg grating sensors, manufactured by CABELTE S.A.

The optical cable comprises three optical fibres (SMF 28®) with nine FBGs each. Groups of three sensors were formed in the same section of the cable in such a way that all nine groups are distributed along the cable extension. This distribution allows not only a distributed monitoring of temperature but also a statistical treatment for each section since it provides three individual readings.

The cable was installed near the Vagueira Bridge, at Mira Channel towards Costa Nova, in an extension of approximately 850 m. To guarantee that the sensors were measuring the bottom temperature, a concrete support was used (see fig. 10). The measuring equipment consists of a super luminescent erbium-doped fiber source (FIBERWHITE-SP) to illuminate a FBG sensor, an optical spectrum analyzer (ANDO-Q6330) and an optical fibre switch (JDS Fitel), inter-connected all through a GPIB bus and controlled by a personal computer running LabView software (see fig. 11).

In figure 12 we can see the FBG sensor, the position inside the cable and the attenuation of optical cable sensing using an optical time domain reflectometer (OTDR). A floating raft was used for installing of the optical cable (see fig. 13).

The values of temperature variation for the period between 4 and 23 June, 2002, are presented herein. The water temperature variations in shallow tidal environments, such as this one, are essentially dependent on the tidal dynamics and on surface heating. However, temperature variations also occur due to fresh-water inflow. The analysis of figures 14 and 15 reveals that the values measured have periodic evolution, similar to that observed by Dias et al. (1999) using STD observations in nearby locations. The period observed in this variation is about 12 h 25 min, which corresponds to the period of the lunar principal (M2) constituent of the tide (M2 is the main tidal constituent in Ría de Aveiro). The maximum values of the curves are observed at the local low tide, while the minimum values occur at the local high tide. In this season the oceanic water, which propagates through the Mira Channel during the flooding, is cooler than both the fresh water input and the water inside the lagoon. Then the observed temperature pattern is coherent with the tidal dominance of the channel dynamics.

The analysis of figure 14 reveals that the first sensor has a pattern different from the other sensors because, due to the local dynamics, bottom sediments covered that sensor and the measurements thus correspond to the sediment temperature variation. The temperature variations for the other sensors show different results between the first week and the remaining sampling period. The first week corresponds to a period of neap tides and, therefore, tidal modulation is less effective. During this period there was also occurrence of precipitation and, therefore, the fresh-water inputs at the far end of the channel were more significant. These reasons explain the more irregular temperature pattern observed during the first week. The tidal mean values of the water temperature also increased after the first week, in response to the 30°C increase in air temperature and cloudy conditions, and hence, the surface heating.

Figure 15 shows the temperature difference between three points located at different distances from the mouth of the lagoon. During the local low tide the temperature is the same in all the points, but during the local high tide minimum values decrease toward the mouth of the lagoon (there is a difference of about 1°C between the 50-m point and the 850-m point). This pattern is explained considering that the 850-m point is closer to the mouth of the lagoon and, therefore, is under a stronger tidal influence. During its propagation along the Mira Channel, the tidal wave amplitude decreases toward the channel head (Dias et al., 2000).

In summary, we have presented two novel technologies based on FBG sensors. The sensing head for the simultaneous measurement of salinity and temperature showed good results and is an interesting alternative to conventional monitoring technology. The sensing cable performed very well and has a good potential for widespread application due to its low size and weight, almost non-intrusive character and its possibile expansion to perform other relevant measurements, including those made with the salinity sensor mentioned above.

 

Acknowledgements

This research would not have been possible without the support from Fundagao para Ciencia e Tecnologia to the PROTEU project. The authors would also like to thank Irmaos Cavaco, S.A.

 

References

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Dias, J.M., Lopes, J.F. and Dekeyser, I. (1999). Hydrological characterisation of Ría de Aveiro, Portugal, in early summer. Oceanol. Acta: 22(5): 473-485.         [ Links ]

Dias, J.M., Lopes, J.F. and Dekeyser, I. (2000). Tidal propagation in Ría de Aveiro Lagoon, Portugal. Phys. Chem. Earth, 25(4): 369-374.         [ Links ]

Esteban, Ó., Navarrete, M.C., Cano, A.G. and Bernabeu, E. (1999). Measurement of the degree of salinity of water with a fiber-optic sensor. Appl.Opt, 38: 5267-5271.         [ Links ]

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