We demonstrate Distributed Acoustic Sensing (DAS) with extended distance range utilising repeaterless all-optical amplification and single-side interrogation schemes. This work addresses the need for over 200 km range of distributed fibre-optic sensing in applications where very long assets need to be monitored, e.g., subsea power cable monito ring. A commercially available DAS interrogator and an advanced prototype interrogator setup with increased system performance are used. Both are based on coherent OTDR to detect Rayleigh backscattering. Range extension is achieved by compensating optical losses with amplifying the pulse traveling down the fibre as well as the Rayleigh backscattered signal coming back to the interrogator. We do this by launching CW pump light into the sensing fibre to create a combination of distributed Raman amplification and a remote optically pumped amplifier in an erbium doped fibre. We analyse the DAS interrogator’s ability to detect acoustic events at distances between 170 km and 200 km of ultra-low loss telecom fibres in a quantitative and linear way. To this end, events are simulated by periodically modulating the length of a short fibre at those distances with a piezo fibre stretcher. Results show that the thereby created optical phase shift is correctly measured and that amplitude and frequency of the applied signal are successfully reconstructed. It is thereby proven that singleended DAS is possible with the presented interrogators and all optical amplification schemes to achieve at least 200 km range.
Rayleigh Backscatter has been used for many years as the source signal for fibre optic distributed acoustic sensing (DAS) in many industrial and civilian activities such as situational information monitoring and down-hole hydrocarbon exploitation. The signal from a DRS system is affected by temperature, strain and acoustics/vibrations. The very low frequency content representing temperature and strain contributions has historically been overlooked in favour of the higher frequency acoustic content. In this paper we describe the potential of a quantitative DRS system to deliver low frequency strain monitoring together with an understanding of the degree of thermal coupling to the system which allows useful results. A long term (50 day) measurement using a quantitative DRS system was carried out to investigate how much drift there was in the system and demonstrate a compensation approach. An example of strain monitoring in a novel pipeline system is illustrated to show the degree of measurement resolution deliverable as compared to conventional strain gauges.
Fibre-optic seismic sensor systems for permanent monitoring of oil and gas reservoirs have been under development for
several years. Such systems, which may comprise over 30 000 sensor channels, require an efficient optical multiplexing
architecture. We report test results from a time and wavelength based multiplexing architecture combining up to 256
sensor channels onto a single optical fibre pair, while still allowing low system noise floor and dynamic range over 180
An array of fibre optic seismic intruder detection sensors has recently been tested by QinetiQ. The array consisted of a set of distributed cable sensors and accelerometers, each being interrogated by an interferometric effect. Both types of sensor were able to detect a person crossing over the array, and frequency analysis of the signals suggests ways in which automatic intruder detection could be achieved.
QinetiQ have been developing security systems for land and sea applications using interferometric based fiber optic sensors. We have constructed and tested a multi-channel fiber-optic hydrophone seabed array, which is designed for maritime surveillance and harbor security applications. During a recent trial it was deployed in a coastal location for an 8 day period during which it successfully detected and tracked a wide variety of traffic. The array can be interfaced with an open architecture processing system that carries out automatic detection and tracking of targets. For land based applications we have developed a system that uses high sensitivity fiber optic accelerometers and buried fiber optic cable as sensor elements. This uses the same opto-electronic interrogator as the seabed array, so a combined land and sea security system for coastal assets could be monitored using a single interrogator.