Thermal leak detection - structural monitoring

Principle of thermal leak detection

Fluid movements in the subsurface often lead to temperature anomalies, as the seepage water usually has a different initial temperature than the soil. The advective heat transport coupled with the fluid movement leads to an approximation of the ground temperature to the temperature of the seepage water in the flowed-through areas and in their immediate vicinity. This makes it suitable as a tracer, i.e. for the detection and containment of fluid movement.

Applications

With the temperature sounding method developed and patented by GTC Kappelmeyer® in 1989, already

• more than 500 km of dams and numerous lock structures were investigated.
• Many damages to sealing elements could be precisely localised
• and dam areas with increased throughflow or underflow could be clearly delimited in time.
• The measurement method was also used to check dammed flood protection dikes.
• The preservation of evidence during new construction or rehabilitation measures and the monitoring of hydraulic engineering works, such as undercutting and seal work, are further applications of the method.
• Long-term measurements can be used to detect changes in and on structures.
• Determination of further parameters, such as bedding density, permeability and in-situ flow velocity, is possible.

Temperature soundings can be carried out in sediments and bulk solids - depending on the bedding density and grain distribution - down to depths of approx. 40 m. Pre-drilling is required for greater exploration depths, very high bedding densities or a high proportion of coarse grains.

Permanent monitoring

In addition to individual measurements, permanent monitoring with automatic evaluation is also possible. The data is usually recorded with battery-operated loggers, uploaded via the mobile phone network and graphically processed and can be retrieved on the internet via online monitoring. With longer measurement series, analyses of the phase shift are possible and seepage velocities can be determined. The monitoring system calculates the parameters. When threshold values are exceeded, warning messages are sent by SMS or email.

Fibre optic temperature measurements

Optical fibres can be used to measure temperatures over a distance of more than 30 km with a spatial resolution of 0.5 m. The installation of the optical fibres is ideally carried out during the construction of the structure to be investigated, but can also be carried out subsequently, depending on the case (retrofit method).

If there is a sufficiently large difference between the temperature in the vicinity of the fibre optic cable and the water temperature, a leakage can be recognised by the fact that the temperature gradient between the undisturbed soil temperature and that of the seepage water decreases significantly (gradient method).

If there is no sufficient temperature difference, the heat-up or heat-pulse method (HPM) is used. Hybrid fibre optic cables with electrical conductors are installed. An electrical voltage is applied to these conductors so that the cable heats up. The temperature rise is measured with the glass fibres. When a fluid (e.g. water) flows around the cable, additional advective heat transport occurs. This leads to a significantly lower heating of the cable and makes it possible to detect flow processes and locate leaks. Almost all sealing systems can be checked regardless of seasonal temperature fluctuations.

• Thermal leak detection in pipelines
• Thermal leak detection in excavations
• Thermal leak detection in dams

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