FIBER OPTIC TEMPERATURE SENSING - APPLICATION FOR SUBSURFACE AND GROUND TEMPERATURE-MEASUREMENTS

The Distributed Fibre Optic Temperature Sensing Technique represents a new physical approach for temperature measurements. It is based on op tical 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 compositio n of the backscattered light are determined by the molecules in the op tical fibre. The Raman backscattering component is caused by thermally influenced molecular vibrations. Thus, its intensity depends on tempe rature. The velocity of light propagation in an optical fibre is well known, therefore, the backscattering intensity can be allocated to dis tance using the travel time of the backscattered light. Thus, an optic al fibre works as a distributed temperature sensor which gives tempera ture and distance simultaneously for the entire length of the optical fibre. To test the method studies have been made on the temperature re solution, the consistency with high-resolution borehole measurements u sing standard temperature probes, the capability of the method for mea suring short-term temperature variations during fluid logging experime nts, for long-term technically induced temperature variations in boreh oles as well as for studying the two-dimensional distribution of the g round surface temperature for a given area. From the measurements it f ollows that the optical fibre temperature sensing technique can be use d under field conditions. The optical sensing cable can be installed i n any horizontal, vertical, inclined or areal configuration. The metho d should be used especially for surveying the temperature field and it s variations with time rather than for standard borehole logging.