Fibre optic measuring methods

Distributed fibre-optic temperature measurements

The modern fibre-optic temperature measurement methods measure temperatures along a conventional fibre optic cable from telecommunications technology with lengths up to 60 km, providing linear profiles. A fibre optic cable can be integrated into a structure during the construction or during remediation measures. Then, the temperature within the structure can be measured along the low-cost fibre optic cable to detect and precisely locate possible leaks. For cable lengths of up to 10 km, a temperature measurement accuracy of better than 0.1°C and a spatial resolution of approx. 0.7 m can be achieved.

Measuring principle

The measuring principle of fibre optic temperature measurement is based on the backscattering of a short laser pulse (< 10 ns) coupled into the glass fibre. The temperature is determined by Raman spectroscopy on the backscattered light. The temperature is calculated from the ratio of the intensities of Stokes and anti-Stokes lines. The spatial assignment of the measured value is done by means of a very precise time measurement, taking into account the propagation speed of the light in the glass fibre. The temperature profile along the entire fibre optic cable is complete after time interval of a couple of minutes..

Measurement methods

Gradient method

Basically, two different types of application can be distinguished. The first type is applied when the temperature in the environment of the fibre optic cable and the seepage temperature are sufficiently different. A leakage can be recognised by a significant decrease of the temperature gradient between the two initial temperatures, i.e. the temperature of the structure or the soil equalises to the temperature of the leachate.This method is therefore called the gradient method.

Heat pulse method

The second type is applied if the temperature difference is not suffcient. e.g. due to a small distance between the water body and the cable for construction reasons, or due to constant water temperatures for a long time interval. n this case the heat-up or Heat Pulse Method (HPM) is used.

Distributed fibre optic strain measurements

Fibre optic strain measurements are increasingly used to detect deformations or movements in the subsurface. For distributed fibre optic strain measurements, a resolution of up to 0.2 m and a magnitude of 10 μm/m is achieved, i.e. movements of 0.01 mm/m can already be detected and localised with a location accuracy of 0.2 m. An optical fibre (single mode) can be stretched up to approx. 10 000 μm/m (1%). Therefore, the cladding must transfer the change in strain of the measurement object to the fibre as optimally as possible.

In concrete, (micro-)cracks in the structure caused by stresses can thus be detected. With standard single-mode fibres, all possible deformation states in concrete can be measured.