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In 1970s, there was a rapid progress in fibre optic technology, as indicated by Giallorenzi et al. There were many isolated approaches in developing new optical sensing techniques. Over last 30 years, a number of experimental studies have been conducted to investigate the application of fibre optic technology in various fields (Hurtig et al., 1994, Boiarski et al., 1995, Jackson 1995, Ishii et al., 1995, Hurtig et al., 1998, Betta et al., 2000, Vogel et al., 2001). Some of the researches were focused on application of this technology in the mines (Dubaniewicz et al., 1991 and 1996, Sen et al., 1992, Zhang et al., 2000, Kher et al., 2002, Wolinski et al., 2002, Li et al., 2006, Soto et al., 2007).
This chapter reviews the theoretical background of fibre optics sensing and different experiments which were carried out to determine the different applications of this technology. Because DTS is a type of optical sensing technology, it is worthwhile to start with a brief review of the different theories and their applications. The theoretical background of DTS system used in the present study will be discussed in next chapter.
2.2 Applications of optical fibre
During the early development of fibre optic technology, optical fibres were used with conventional electrical, hydraulic, pneumatic components to facilitate in sensing of the physical quantity. Giallorenzi et al. (1982) described different fibre optic sensors. Wojtek (1992) described stress sensor working on the fibre optic principle. Using the same principle Werthen et al. (1996) measured current in electrical equipments. Riza et al. (2005) also conducted successful experiment for measuring extreme temperature (10000C to 25000C) using fibre optic sensor.
On the other hand, Kher et al. (1991) indicated that optical fibres can be used for transmitting purpose because of itââ‚¬â„¢s immunity to electromagnetic waves, radio wave, secure data transmission and higher speed of response. Sen et al. (1991) were successful in connecting 16 sensors using optical fibre, which was used as a transmitting medium. Murtaza et al. (1996) used optical fibre to transmit the spectral emission of LED which was used as sensing element. Tomaz et al. (2002) successfully utilised optical fibre to transfer data collected by Liquid Crystal (LC) cell for measuring temperature in a mine.
Due to more research in the field of fibre optics, many new potential technologies evolved and optical fibres were used as sensors and transmitting medium as indicated by Dubaniewicz et al. (1991), Bioarski et al. (1995), Kovalchik et al. (1997), Hurtig et al. (1998), Zhang et al. (2000).
2.3 Theories of optical sensing
Many measuring systems were developed using different optical theories, which can be broadly classified into two as discussed below.
2.3.1 Fibre optic based theories
Raman Optical Time Domain Reflectometry (OTDR)
Dubaniewica et al. (1991) first presented experimental results of measuring distributed temperature along the length of fibre using Raman Effect based on OTDR in a laboratory which was a converted lime stone mine. Later, this technology was applied in different fields and various experiments were conducted by Hurtig et al. (1994), Ishii et al. (1997), Hurtig et al. (1998), Zhang et al. (2000). On the other hand, Vogel et al. (2001), Zhang et al. (2001), Kher et al. (2002), Soto et al. (2007) have reported different techniques for improving the performance, reliability, sensitivity and spatial resolution of DTS system. Present DTS system which works on OTDR principle will be discussed in next chapter.
Kersey et al. (1992) utilised optical fibre Bragg-Grating elements in Distributed Temperature sensors. The temperature differences due to high sensitivity of thermally induced Bragg wavelength shifts were recorded using an interferometric detection. Continuing research in this area, Nellen et al. (2000) conducted experiments in laboratory and Tunnel in Switzerland during 1999, to show the potential of this technology in measuring strain and temperature. Further, fibre Bragg-Grating technique was used successfully to measure distributed temperature in harsh environment at 32 points by Yu et al. (2007), but this system had limitations with the wave length drift and further research was recommended.
Wait et al. (1997) presented detail theoretical comparison of the Spontaneous Raman and Brillouin base fibre optic DTS system as shown in Figure 2.1 and reported that Brillouin base DTS has more range limit for given spatial and temperature resolution.
Figure 2.1: Effect of temperature on Raman & Brillouin wave (Wait et al., 1997)
Yang et al. (2002) conducted many experiments on Distributed Temperature Sensor (DTS) they were able to measure temperature by measuring Brillouin backscattered wave after being separated form Rayleigh scattering wave using Mach-Zehnder interferometer as shown in Figure 2.2.
Figure 2.2: Schematic diagram of double pass all-fibre
Mach-Zehnder interferometer (Yang et al., 2002)
Boiarski et al. (1995) carried out experiment using DTS, based on novel technique for ultra-high spatial resolution distributed temperature measurement using Rayleigh OTDR to measure the temperature of electrical equipment in the power plant. Rayleigh OTDR using standard single-mode fibre was presented by Gifford et al. (2006). They used Swept Wavelength Interferometry (SWI) technique to measure the Rayleigh backscatter as a function of length in optical fibre, set up is shown in the Figure 2.3.
Figure 2.3: Optical network for Rayleigh DTS system (Gifford et al., 2006).
2.3.2 Other optical theories
There were different isolated researches for using optical technology other than fibre optics technique, for measuring the parameters like temperature, velocity, strain, methane gas. Murtaza et al. (1996) successfully used LED as an optoelectronic temperature sensor and obtained a ratiometric response of LED spectral emission as a function of temperature.
Passia et al. (2002) demonstrated a laser anemometer working on Doppler Effect, for measuring velocity (1 to 15 m/s) in underground mine. They indicated the potential usage of this technology for measuring displacement, vibration and other biomedical applications.
Wolinski et al. (2002) have presented preliminary results using liquid crystal cell. The measurements were based on the principle of nematic-isotropic phase transition due to temperature variation in the liquid crystal (LC) cell. Although the measurements were not distributed for larger length but this system was envisaged for harsh environment conditions of the underground coal mine with potential application in fire detection in coal-mines.
The usage of optical sensor based on single crystal Silicon Carbide (SiC) in high temperature (10000C) environment was reported by Riza et al. (2005). He demonstrated working of sensor using free-space beam targeted single crystal SiC chip frontends having a resolution of 10 cm and indicated the potential of using this technique for monitoring temperature up to 25000C.
Li et al. (2005 and 2006) have proposed and demonstrated a fibre optical gas sensor for measuring CH4 quantity in a coal mine. They utilised the optical fibre to measure and transmit the mid-infrared and near-infrared absorption spectra of CH4 because optical fibre can transmit the near infrared signals safely with less loss.
2.4 Applications of Distribute Temperature Sensing (DTS)
Fibre optic sensing technology is very versatile and potential applications are enormous. In this study the literature on applications of DTS technology is categorized into three sections
Fire detection applications and
Heat detection system in mines.
2.4.1 General applications
Boiarski et al. (1995) carried experiments using DTS base on Rayleigh OTDR to measure the temperature of equipment in the power plant. They compared the Raman and Rayleigh OTDR techniques and indicated that spatial resolution of 10 cm is achievable by Rayleigh method, their research was focused on the application of DTS in measuring the temperature of electrical equipment. Also, Betta et al. (2000) carried out number of tests on 25 kVA oil-cooled transformer to measure the hotspot temperature of the fluid, with the help of DTS system. They obtained average temperature resolution of 50C and confirms the absence of electromagnetic susceptibility in DTS system. Werthen et al. (1996) designed and tested two optical powered current measuring systems in high voltage electrical equipments, for the improvement in the protection of high voltage electrical equipments.
Jackson (1995) first presented two optical fibre based structural monitoring systems. It was not until 1998, when Hurtig et al. (1998) were able to successfully apply DTS technique for monitoring building facilities (power supply lines, supply pipes) through temperature measurement and leakage detection. They also indicated one, two and three dimensional fibre optics temperature measurement application in facility management of building or structure. Later, Vogel et al. (2001) carried further research emphasizing more on leakage detection using DTS technology, were leakage was detected due to temperature variation in surroundings because of Joule-Thomson effect.
Fibre optic technology was also applied in mining industry. Dubaniewicz et al. (1991), have first reported the applications of DTS technology in mine. Later they presented further research with review of potentials and problems of this technology in 1993. In the same year, Hurtig et al. (1993) carried out experiment to measure temperature in shallow bore holes for fluid logging, using the concept of temperature variation in leakage area due to Joule-Thomson effect. The results were used to characterize hydraulic properties of fractured rocks, they also stated potential of using this method in geothermal applications.
Dubaniewicz et al. (1996 and 1998) and Kovalchick et al. (1997) focused on the measurement of conductor temperature of mine trailing cables (especially shuttle cars) carrying high current and voltage. This research was able to solve the problem of electrocution of miners due to deterioration of outer insulation of cables by overheating and harsh working conditions. This research was carried out by U.S Bureau of mine and National Institute for Occupational Health and Safety of U.S. The Test setup is shown in the Figure 2.4.
Figure 2.4: Test setup (Kovalchick et al., 1997)
Monitoring of gases in underground mine was later researched by Li et al. (2005 and 2006), they proposed and demonstrated a fibre optical gas sensor for measuring quantity of CH4 and potential of being utilised for measuring other explosive gases like C2H2, C3H8, C2H4.
Figure 2.5: Setup of optical fibre gas sensing system (Li et al., 2005)
Figure 2.5 shows the schematic representation of sensor, in this study optical fibre was used to transmit and measure the mid-infrared and near-infrared absorption spectra of CH4. Because optical fibre can transmit these near infrared signals safely with less loss, this technology had a potential to replace the conventional pellistor sensors.
Naruse et al. (2007) have conducted a series of experiments in EI Teniente mine located in foothills of Andes Mountains, to detect changes in the state of mine resulting from mining activities using Brillouin Optical Time Domain Reflectometry (BOTDR). They installed optical fibre held by rockbolts in the haulage tunnels of the mine. From the results obtained they conformed the applicability and practicality of this system in an underground mine.
2.4.2 Fire detection applications
Distributed Temperature Sensing (DTS) technology has been available commercially for twenty five years and in past 10 years this technology increasingly became best fibre optic technology solution in number of applications (Hartog, 2001). One of the important applications is in fire detection and control.
Ishii et al. (1997) first successfully examined a fire detection system for utility tunnels of 2 km in length. They also discussed the method of locating the fire source and calculating the thermal energy from the thermal distribution data obtained. In results they recommended DTS system for fire detection and also state a relation between the temperature near the sealing and heat released by the heat source. Below Figure 2.6, 2.7 show the results obtained during this study.
Figure 2.6: Temperature distribution in the tunnel (Ishii et al., 1997 )
Figure 2.7: Thermal energy and temperature relation (Ishii et al., 1997 )
The use of DTS technology in fire detection is exemplified by Arthur, 2001. He stated that about 600 to 700 systems were installed till 2001 and about 50 % of them were installed during 1997 to 2001.
SENSA (2008) has installed DTS systems for fire detection in:
Dartford Road Tunnel, London, England. About 1.4 km long.
Tempi Tunnel, between Athens and Thessaloniki, Greece. A high speed (250-300 km/h) railway tunnel.
Britomart Station, Auckland, New Zealand. A vital bus, ferry transport centre.
Madrid Utility Tunnel, Madrid, Spain. Consisting of valuable telephone, electrical, water and cable networks.
SENSORNET (2008) has installed ire detection system using linear heat detection technique in the Xuanwu Lake tunnel in Nanjing, China.
All these fire detection DTS systems work on Raman scattering base on OTDR principle. Fire detection systems have common system architecture as shown in Figure 2.8.
Figure 2.8: DTS system architecture (Hampson, 2006)
On the other hand, this technology was also applied in underground mines, which is discussed in next section.
2.4.3 Heat detection system in mines
The first approach for heat detection in the mine was done by Dubaniewicz et al. (1991), they conducted experiments to monitor methane, carbon dioxide and distributed temperature in Lake Lynn laboratory in south-western Pennsylvania.
The test setup consist of a network of optical fibre cables connecting a reel which was ignited and an electrically heated roller, which was held by six poles, as shown in Figure 2.9.
Figure 2.9: Test set up at Lake Lynn mine ( Dubaniewicz et al., 1991)
In this study, they successfully tested the DTS system and obtained distributed temperature profile (Figure 2.10) and also they were able to record higher temperature (5100C) for first time and recommended further research in using this technique for safe operation of the electrical equipments.
Figure 2.10: Distributed Temperature profile ( Dubaniewicz et al., 1991)
Later their research mainly concentrated on measuring the temperature of the mine trialling cables carrying heavy current and voltage for the safe operation in mines reported in his works in 1996 and 1998.
Zhang et al. (2000) conducted series of test in an underground explosion test gallery, 896 m in length and obtained the test result of temperature variation in a belt conveyor, which was heated roller shaped heater. The temperature recorded by the DTS system was compared to the temperature of thermocouple. This system was equipped with alarm system to alarm if the temperature of conveyor belt reaches more than 400C. Further in 2001, they have successfully tested this system for 10 km length optical fibre. A new demodulated configuration of DTS measurement was presented, as shown in Figure 2.11.
Figure 2.11: DTS configuration for tests (Zhang et al., 2001)
Although most of the research and experimental study relating to application of DTS technology for temperature measurement, monitoring and heat detection was applied in mine, but none of the study was carried in an actual underground mine. Most of them were carried out in and laboratory (converted mine) or underground tunnel stimulating the underground mine environment.
Present study was conducted in an actual underground metalliferous mine using the latest Distributed Temperature Sensing technology form sensonet. The details of the facility at University of Queensland Experimental Mine and the results obtain are discussed in next chapters.