David John Hill

Fiber laser hydrophone array

In recent years growing interest has surrounded the development of fiber laser sensors (FLS). This is due to their ultra high sensitivity to temperature and strain as well as their ability to be multiplexed along a single fiber using WDM techniques. It is their extreme sensitivity that has led to them being considered as acoustic pressure sensors rather than standard fiber Bragg gratings. The work presented here describes the development of an array of FLS configured as hydrophones. We discuss the design of the single mode fiber laser used throughout our system; comparing examples based upon distributed Bragg reflectors (DBR) and distributed feedback (DFB). In addition we discuss both the theoretical and experimental acoustic sensitivity enhancements obtained by the application of an elasto-plastic coating to the FLS. The array configuration is described, as is the heterodyne interrogation scheme using an unbalanced Mach-Zehnder interferometer with WDM channel selection. Results from the measurement of the minimal detectable acoustic signal of a bare fiber laser are shown to be -69 dB re.Pa/(root)Hz at 1 kHz when using a 200 m path imbalanced readout interferometer. Further gains in the sensitivity due to the application of various coatings are reported, as is a full characterization of an array of fiber laser hydrophones. Finally we discuss the future research of the FLS, and the areas in which the technology is particularly applicable.

Large-scale multiplexed fiber optic arrays for geophysical applications

Fiber optic sensors are becoming a well-established technology for a range of geophysical applications, and static pressure and temperature sensors in particular are now comparatively well developed. However, rather less attention has been paid to systems for measuring dynamic quantities such as acoustic and seismic signals. Furthermore, the very large multiplexing potential of fiber optic sensing systems has yet to be fully explored for geophysical applications. However, development of fiber optic sonar systems for military applications has proven the viability of large multiplexed arrays, and demonstrated advantages which include electrically passive arrays, long term reliability and the potential for operation in very deep (

Progress toward an ultrathin optical hydrophone array

GTR3000m) water. This paper describes the applications for large scale fiber optic sensing arrays in geophysical metrology. The main applications considered here are ocean bottom cables and streamers for marine seismic, and downwell seismic systems. Systems can require up to several thousand channels and the use of multi- component sensors, which include 3-axis geophones and hydrophones. The paper discusses the specific requirements for each application, and shows how these requirements can be met using a system approach based on time and wavelength multiplexing of interferometric sensors. Experimental and theoretical studies at DERA into the performance of highly multiplexed systems are also described, together with initial development work on fiber optic hydrophones and geophones.

Pipeline Leak Detection Using Four Mode Fibre-Optic Based Distributed Sensing

Evaluation and Research Agency (DERA) has initiated a project with the Universitiesof Aston and Kent to investigate the use of FBG sensors in an acoustic array. Tocompete with existing technology the specification required is very demanding witheach sensor having to be capable of detecting an applied radial pressure of 1O Pa.In bare fibre, this corresponds to an axial strain of O.5x1015 (or 0.5 femto-strain). Inaddition, the system must handle signals with a wide acoustic bandwidth and must

Improvements in distributed fibre optic sensing

Fibre-optic based Distributed Acoustic Sensing (DAS), is now commonly used around the world for providing pipeline operators with real-time and early warning of intrusions on their pipeline Right of Ways. Also open to analysis within the DAS signal return are multiple signatures derived from the egress of product leaking from pipelines. Over the last three years, focussing attention on these alternative signals, OptaSense have developed a novel 4-mode External Leak Detection capability, fusing outputs from both DAS, Negative Pressure Pulse (NPP) and Distributed Temperature Gradient Sensing (DTGS) signals (the latter not to be confused with Distributed Temperature Sensing or DTS). The first commercially deployed 4-mode Leak Detection products are now being seen on the market, for both gas and liquids pipelines.In this paper, we report how DAS can be used to provide these four modes of leak detection — including (listed in order of typical detection latency, fastest to slowest)i) negative pressure waves created in the pipeline product from the leak eventii) acoustic noise from turbulent flow through the leak orificeiii) temperature gradients in the soil due to the presence of the leaked product (positive and negative), andiv) local strain/ground heave due to soil displacement by the leaked product.These acoustic, temperature and strain measurements using a fibre-optic cable buried next to a pipeline can be fused together to provide highly sensitive and reliable alerts for pipeline leaks. The pipeline industry has always sought to detect smaller leaks faster, with better locational accuracy. This paper, which draws upon industry sponsored test results and commercial deployment data, provides an update to the industry on leak detection possibilities using DAS.Copyright