Silixa has developed a technique called Distributed Thermal Acoustics (DTA) to monitor simultaneously the thermal and acoustic profile along a hybrid optical fibre cable in submarine environments. Using its world leading distributed fibre optic technology (ULTIMATM DTS and iDASTM) Silixa deployed an optical fibre cable along the seabed and across the water column in an active gas seepage field. Measurements were carried out to investigate the effects of gas seeping from the seafloor.
Submarine gas seepages are closely related to petroleum reservoirs and geohazards. The seepage mechanism has not yet been fully explained. This is partly due to the difficulties encountered when attempting to detect and subsequently monitor seepage zones. Up to now, seepage detection has relied on expensive geophysical surveys, the quality of which is highly dependent on the ability to integrate different types of data (eg acoustic, temperature, etc.). Once seepage zones have been detected, the subsequent monitoring is generally performed using expensive and cumbersome point sensors. As a result, researchers attempting to advance the body of knowledge in this area are often limited by equipment costs and deployment challenges.
Distributed fibre optic sensing overcomes the inherent limitations of traditional technologies and offers unparalleled advantages in monitoring critical assets, especially in inaccessible or inhospitable environments. A single optical fibre can replace thousands of traditional single point sensors. In addition, optical fibres are passive, immune to electromagnetic interference, are light and pliable, require no maintenance, and the installation cost are much lower too. These characteristics make distributed fibre optic sensing a flexible and cost effective technology.
Silixa’s fibre optic–based distributed measurement systems (iDAS and ULTIMA) make it possible to measure the temperature and acoustic signal continuously along a single optical fibre cable. This enables seepage observation with a resolution and ease not previously seen in geophysical research.
The intelligent Distributed Acoustic Sensor (iDAS) measures the full acoustic field along unmodified optical fibre up to tens of kilometers in length with a spatial resolution down to one meter, capturing the full amplitude and phase of the incident wave on the sensing optical fibre at frequencies up to several kHz with a wide dynamic range (›120dB). The system enables high-quality synchronized coherent digital recording of acoustic waves, with no cross-talk, along the fibre. The distributed temperature sensor ULTIMA-DTS offers a spatial resolution from 25cm and a temperature resolution of 0.01⁰C over a wide operating temperature range. The range of the sensor can be extended to several km.
Cold gas seeps have been related to sudden temperature drops in the order of 0.1-1°C between the seep location and the ambient sea temperature; however, lower gradients have also been reported. Silixa offers advanced detection due to the high temperature and sampling resolution (0.01 ⁰C and from 12.5 cm respectively). High spatial resolution measurement enables detailed vertical thermal & acoustic profiling of the water column and thermal imaging of the seafloor. The combined ULTIMA and iDAS approach facilitates discharge detection and its quantification, as well as the monitoring of other oceanographic processes and human activities in the marine environment. Current applications for the Distributed Thermal-Acoustic (DTA) system are based on thermal trace and acoustic signature detection associated with hydrocarbon release. In addition, using hybrid cables with multiple fibres for both acoustic and temperature measurements, the Distributed Thermal Acoustic Data Integration is envisaged to be used in monitoring of environmental behaviour, such as diurnal and seasonal changes.
Silixa’s DTA is a new integrated approach to geophysical observation that for the first time simultaneously provides spatially distributed thermal and accurate acoustic monitoring of the subsea environment. Gas seepages are precisely detected and located, as well as interpreted in space and time. By integrating iDAS and DTS into a single cable, thermal-acoustic data fusion becomes a simple exercise, even over massive observation arrays.
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