Most people are familiar with the use of fibre optic cables in the communications industry. Light is shone into an optical fibre and, using the property of total internal reflection, can be transmitted across long distances with little attenuation. The fibres allow the transmission of high data rates or bandwidth and are in increasing use in Internet connections rather than carriers such as conventional telephone wires. Fibres have also been used in the oil and gas industry to relay downhole measurements from boreholes and in some cabled seismic acquisition systems.
But there is another property of fibre optic cables that can be exploited to make measurements. Minute pressure changes along the length of a cable create strain on the fibre. When light is passed down the cable, some of it is reflected back as a result of the strain differences. Knowledge of the speed of light tells us that if the backscattered energy from a ten nanosecond light pulse is detected and analysed, every ten nanoseconds of these returns will represent a one metre section of the cable. If a light pulse is generated for every 100 microsecond interval, measurements can be made for every metre of the cable for a total length of 10 km at a 10 kHz sample rate. As the fibre cable responds directly to pressure, there is no need for external devices; the cable acts as a continuous array of sensors.
This property of fibre optic cables is being used by specialist fibre optic sensing company Silixa in a wide variety of industrial applications, including an acoustic detection security system that detects breaches of boundaries. (See GEO ExPro iPad App 4, ‘New Technologies’.) A development of this system is called iDAS (intelligent Distributed Acoustic Sensor).
iDAS for VSPs
Using proprietary processing technology, the company can measure the full acoustic pressure field over a wide frequency range and with a 120dB dynamic range at every point along the fibre. Directivity, equivalent to three-component recording with geophones, can be measured by configuring the fibre to shape its directionality. Using the measurements of many adjacent recording points, large scale directivity can also be obtained by array processing and beam-forming techniques.
Silixa realised that the continuous acoustic measurement possible along a fibre cable could be of great benefit when recording a Vertical Seismic Profile (VSP). Rather than recording the acoustic signal on wireline at discrete intervals – a process that can take several hours as the tool with the detectors is pulled up the hole and clamped in place at each location – the entire acoustic signal could be recorded at one time on an optical cable placed down the borehole. Silixa’s proprietary cables are acoustically sensitive to enable these measurements to be taken. However, even where the technology is retrofitted to pre-existing fibre cables that are less sensitive, many records can be taken quickly and summed together, still saving substantial time and cost.
In March 2011, the company embarked on a test VSP at a site in Texas that had also been used for conventional VSP testing. The results of the onshore VSP test demonstrated the potential of the method and compared well with a previous conventional VSP test. In this test, the cable was coupled frictionally to the borehole – it is expected that with a permanent emplacement where the cable is cemented in place that the results would be even better. In 2011, Silixa also conducted a successful offshore VSP test in a near horizonal well in the Norwegian North Sea. Such systems would be ideal for permanent borehole monitoring or seismic applications.
Silixa has a companion DTS (Distributed Temperature Sensor) system to iDAS that also uses fibre optic cables – the Ultima. This can measure downhole temperatures with a spatial resolution of 25cm. By using the Ultima and iDAS in tandem, the temperature and acoustic profile can be monitored together. Silixa has developed a method to do this using the fibre cables that is called Distributed Thermal Acoustics (DTA). This technique can be used to identify discharges such as gas seepages and monitor the temperature and acoustic profile of the seabed over a large area.
Leakage and movement of liquid and gaseous hydrocarbons from offshore vents and fractures can provide valuable information for many studies, including petroleum geology, exploration tectonics, geo-hazards and global warming. Observations of marine seeps with bubbles dissolved in the water column are difficult to interpret and most techniques merely identify the seep locations. Quantitative measurements can provide much more information and a permanent emplacement could offer real-time seabed monitoring.
In 2012, Silixa conducted a subsea gas survey feasibility study together with the University of Patras in Greece in the harbour of Katakolo, a small town in the western Peloponnese, Greece. Katakolo is visited by large cruise ships as it is the closest port to the site of Ancient Olympia. The harbour also contains a prolific thermogenic gas seepage zone where faults reach the seabed. Because of its accessibility and scientific interest, the seepage zone has been studied extensively and many surveys have been conducted over it, including sub-bottom profiling, side-scan sonar, long term gas flux and isotopic composition measurements, etc.
When the gas bubbles are released into the water column, they ‘ring’ with characteristic resonance frequencies. These can be detected and recorded by the iDAS acoustic system. The methane gas is also expected to be at a lower temperature than the surrounding ambient temperature, so thermal measurements using the Ultima system can show seep locations. As the seep locations are well known here, Silixa used a system where the cable carrying four fibre optics was wrapped around a pyramid-shaped metal frame placed at 7.5m water depth in the harbour. This allowed three-dimensional spatial distribution of the measured parameters and also the sensors to be tested, together with tidal effects.
The initial results demonstrated that the DTA system can be used to both detect seeps and make measurements of their acoustic signatures. Further experimental work is planned to refine the method and add to the observations by, for example, trying to detect seismic activity. Analysis of the results is in progress in order to try to correlate the measurements with the known properties of the seeps.
Silixa has already developed systems for continuous borehole monitoring with iDAS and a concept for profiling the flow of fluids in a producing well. The VSP test in a horizontal borehole demonstrated the potential of the system offshore. As well as permanent borehole instrumentation, it could also possibly be used in future permanent seismic sea bed installations, removing the need for costly sea bottom receivers. Silixa has also conducted tests for onshore seismic where the fibre cable was buried just below the surface.
The many other possible uses that can be envisaged are shown by the Katakolo example that uses the combined DTA system. Permanent emplacements could monitor offshore seeps remotely, or mobile systems be used to characterise them for a variety of studies.
Silixa is working with academia, oil companies and service companies to develop and apply iDAS and DTS capabilities. Several borehole systems are already in place. Wherever we need to measure acoustic signals or thermal profiles, it should be possible to do it without additional sensors, in a cost effective way, yet with much finer spatial sampling than is possible with conventional equipment.