Prototype Design of a Distributed Fibre Temperature Sensor for Borehole Monitoring

Abstract

This Thesis details the development of a distributed fibre temperature sensor, that will be installed at the DFDP-2 borehole in Whataroa. The sensor works by observing the Raman backscatter generated from an input pulse of monochromatic light into an optical fibre, and then filtering this backscatter into Stokes and anti-Stokes components. By taking the ratio of the intensity of these components, the temperature can be calculated at every point along the fibre. We present our prototype design, by first making an Optical Time Domain Reflectometery (OTDR) measurement. This is to quantify the ability of our system, by measuring the length and optical loss of a spool of test fibre. By testing the sensor with different fibre spools, varying the pulse width of the input light, and exploring the effects of amplification, we were able to catalogue the performance of the system across a variety of setup configurations. After iterative development of the sensor design through these stages, our system was compared against a commercial OTDR. This sensor design was then further developed to make a distributed temperature measurement. An insulating Styrofoam box filled with ice was used to change the temperature of the fibre spool under test, and the sensor used to resolve this change. A successful demonstration of recovering the changing temperature profile is presented, observed in singlemode and multimode fibre over a period of 12 hours. A discussion of all the results and sensor developments follows, where we detail our assumptions, unexpected challenges and an analysis of their effects on the performance of the system. A list of recommendations is put forward, based on the successes and pitfalls of this sensor design. This ensures that the next iteration of our distributed fibre sensor will have an improved performance, thus taking a step closer towards the final aim of building a distributed fibre temperature sensor, capable of continuously acquiring data to show the evolving temperature profile of the DFDP-2 borehole.