Onshore and offshore fibre optic sensing – examples and lessons learned

Distributed acoustic sensing (DAS) has been a research topic for several decades, and in conjunction with the steadily increasing number of various practical applications the interest for this relatively new technology has increased significantly in the past years. There has been a number of significant developments on the hardware side making it possible to interrogate large distance using a fibre cable. Today it is feasible to interrogate between 100 to 200 km, and recent experiments indicate that this can be extended to several thousands km.

During the past decade, we have tested various geophysical applications of fibre optic sensing. In this paper we will show offshore examples including whale tracking, detection of ships and distant storms and other oceanographic examples such as ocean gravity waves, as well as earthquakes. Onshore examples include road traffic, eigenfrequencies of bridges and development of warning systems for rockfall and avalanches. In Norway most railways are equipped with fibre optic cables close to the railway and often deployed in specific pipes 1-2 m away from the line. This causes variation in coupling along the line and we will discuss how these variations can be determined and accounted for in processing. The major challenge for warning systems is to reduce the number of false positive events, and this will be addressed in the talk.

Recently we have demonstrated that it is possible to use fibre optic cables trenched at the seabed to detect silent whales. Using DAS-data containing ultra-low frequencies we show that it is possible to detect the water movements of a whale if it swims closer than approximately 40 m from the fibre. For ships the corresponding distances are typically 400-500 m. Since a fibre optic cable has a long antenna, the fibre can detect low frequency signals that are very hard to measure by a single hydrophone. The huge advantage of the seabed fibre cable is of course the capacity of monitoring large distances (100 km or more), which makes the DAS-technology very attractive for a multifold of applications. 

In the future we expect that both the range and signal to noise ratio of DAS-data will increase significantly. In parallel with these huge technical achievements the need for secure handling of data will increase, leaving challenges to the research community that it is important to address and handle.