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Rayleigh scattering based distributed acoustic sensing (DAS) systems use fibre optic cables to provide distributed strain sensing. In DAS, the optical fiber cable becomes the sensing element and measurements are made, and in part processed, using an attached optoelectronic device. Such a system allows acoustic frequency strain signals to be detected over large distances and in harsh environments.
In Rayleigh scatter based distributed fibre optic sensing, a coherent laser pulse is sent along an optic fibre, and scattering sites within the fibre cause the fibre to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fibre and back, the next laser pulse can be sent along the fibre. Changes in the reflected intensity of successive pulses from the same region of fibre are caused by changes in the optical path length of that section of fibre. This type of system is very sensitive to both strain and temperature variations of the fibre and measurements can be made simultaneously at all sections of the fibre.
The optical pulse is attenuated as it propagates it along the fibre. For a single mode fibre operating at 1550 nm, a typical attenuation is 0.2 dB/km. Since the light must make a double pass along each section of fibre, an extra 1 km of it induces an additional loss of 0.4dB. The maximum range of the system occurs when the amplitude of the reflected pulse becomes so low it is impossible to obtain a clear signal from it. It is not possible to counteract this effect by increasing the input power because above a certain level as this will induce nonlinear optical effects which will disrupt the operation of the system. Typically the maximum range that can be measured is around 40–50 km.
The minimum value of strain that can be measured depends on the carrier to noise ratio of the returning optical signal. The carrier level is largely determined by the amplitude of the optical signal while the noise is a combination of that from a variety of sources including, laser noise, electronic noise and detector noise.
The spatial resolution is mainly determined by the duration of the transmitted pulse, with a 100ns pulse giving 10m resolution being a typical value. The amount of reflected light is proportional to the pulse length so there is a trade-off between spatial resolution and maximum range. To improve the maximum range it would be desirable to use a longer pulse length to increase the reflected light level but this leads to a larger spatial resolution. In order for two signals to be independent they must be obtained from two points on the fibre that are separated by at least the spatial resolution. It is possible to obtain samples at separations less than the spatial reolution and although this produces signals that are not independent of each other such an approach does offer advantages in some applications. The separation between the sampling points is sometimes referred to as the spatial sampling period.
Before the next laser pulse can be transmitted the previous one must have had time to travel to the far end of the fibre and for the reflections from there to return, otherwise reflections would be returning from different sections of the fibre at the same time and the system would not operate properly. For a fibre 50 km long the maximum pulse rate is just over 2 kHz. Therefore strains can be measured which vary at frequencies up to the Nyquist frequency of 1 kHz. Shorter fibres clearly enable higher acquisition rates.
Although the system is sensitive to both temperature and strain variations these can often be separated as those due to temperature tend to occur at a lower frequency range than strain. Unlike other distributed fibre techniques such as those based on Brillouin or Raman scatter, distributed acoustic sensing is only able to detect changes in temperature rather than its absolute value.
Distributed Acoustic Sensing relies on light which is Raleigh backscattered from small variations in the refractive index of the fibre. The backscattered light has the same frequency as the transmitted light. There are a number of other distributed fibre sensing techniques that rely on different scattering mechanisms and can be used to measure other parameters. Brillouin scatter occurs due to the interaction between the light and acoustic phonons travelling in the fibre. As the light is scattered by a moving phonon its frequency is shifted by the Doppler effect by around 10 GHz. Light is generated at both at above (anti-Stokes shift) and below (Stokes shift) the original optical frequency. The intensity and frequency shifts of the two components are dependent on both temperature and strain and by measuring the shifts, absolute values of the two parameters can be calculated using a Distributed Temperature and Strain Sensing (DTSS) system. Brillouin scatter is much weaker than Rayleigh scatter and so the reflections from a number of pulses must be summed together to enable the measurements to be made. Therefore the maximum frequency at which changes can be measured using Brillouin scatter is typically a few 10’s of Hz. Raman scatter occurs when light is scattered in interaction with molecular vibrations in the fiber. As with Brillouin scattering both Stokes and anti-Stokes components are produced and these are shifted from the wavelength of the incident light by several tens of nanometers. By measuring the ratio in intensity between the Stokes and anti-Stokes components an absolute value of temperature can be measured by a Distributed Temperature Sensing (DTS) system. The larger wavelength shifts compared to Brillouin scatter mean that it is easier to separate the scattered Raman light from the un-shifted Rayleigh scattered component. However the intensity of the Raman scatter is even lower than the Brillouin scatter and so it is normally necessary to average for many seconds or even minutes in order to get reasonable results. Therefore Raman based systems are only suitable for measuring slowly varying temperatures.
The sensitivity and speed of Rayleigh based sensing allows distributed acoustic monitoring over distances of up to 100 km from each laser source. With suitable analysis software, continuous monitoring of pipelines for unwanted interference, as well as leaks or flow irregularities is possible. Roads, borders, perimeters etc. can be monitored for unusual activity with the position of the activity being determined to within approximately 10 metres. Due to the ability of the optic fibre to operate in harsh environments, the technology can also be used in oil well monitoring applications, allowing real-time information on the state of the well to be determined.
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