Surpassing the detection limits of current distributed acoustic sensing systems (Conference Presentation)

Abstract

The detection range of Distributed Acoustic Sensor (DAS) systems is limited by signal attenuation to approximately 75 km. The ability to increase the detection range is of great commercial interest to the offshore wind farm operators interested in structural health monitoring of their subsea cables. In most cases, the operators are interested in monitoring 200km~400km subsea cable where the fibres can be accessed at two ends of the cable. In this paper, we present a new, commercially viable, ultra low-loss sensing element, comprising of discrete broadband reflectors. Optical time domain reflectometry measurements were performed on a 100 m sample of the fibre. The sample contained reflectors placed at 3 m intervals. At reflector sites, the recorded trace revealed increases in the backscatter signal two hundred times that of the unmodified regions of the fibre. Theoretically, the spatial resolution of a system utilising this new element is only restricted by the ability to resolve two reflector points. Therefore, the ultra low loss fibre also offers the potential for high spatial resolution measurements over large distances, as long as sufficient data acquisition and processing techniques are employed. The significant enhancement means no amplification of the reflected signal is required, further reducing the cost of the system. To verify the long distance capability of the fibre, the sample was subjected to optical side scattering radiometry measurements. The largest side-scattered loss from a reflector point was 10^-4 dB per reflector. If a reflector was placed every meter, the total fibre attenuation is predicted to be 0.3 dB per km.