Fibre optic temperature sensing: application for subsurface and ground temperature measurements

Abstract

The Distributed Fibre Optic Temperature Sensing Technique represents a new physical approach for temperature measurements. It is based on optical time domain reflectometry (OTDR). A laser pulse is coupled into an optical fibre and a small part of the light is backscattered as the pulse propagates through the fibre. Intensity and spectral composition of the backscattered light are determined by the molecules in the optical fibre. The Raman backscattering component is caused by thermally influenced molecular vibrations. Thus, its intensity depends on temperature. The velocity of light propagation in an optical fibre is well known, therefore, the backscattering intensity can be allocated to distance using the travel time of the backscattered light. Thus, an optical fibre works as a distributed temperature sensor which gives temperature and distance simultaneously for the entire length of the optical fibre. To test the method studies have been made on the temperature resolution, the consistency with high-resolution borehole measurements using standard temperature probes, the capability of the method for measuring short-term temperature variations during fluid logging experiments, for long-term technically induced temperature variations in boreholes as well as for studying the two-dimensional distribution of the ground surface temperature for a given area. From the measurements it follows that the optical fibre temperature sensing technique can be used under field conditions. The optical sensing cable can be installed in any horizontal, vertical, inclined or areal configuration. The method should be used especially for surveying the temperature field and its variations with time rather than for standard borehole logging.