Near subsurface moisture sensing

Subsurface sensing often implies the intention to look deep into the hidden structures. A special task is the analysis of the "skin" itself, which covers the deep structures and builds the surface layer which can not or may not be penetrated. There are also many cases where thin material layers are embedded in another materials and can be measured only from one side, from the outside. The object of the investigations are often the quality, the moisture content or hydraulic conductivity of this "skin". The measurement of these layers is influenced by their neighborhood materials. A new method has been developed to measure the water content of these thin layers independent of the surroundings. Its patent application is submitted. The sensor itself is a partially shielded flat band cable attached to the outer layer side. The electromagnetic field of the cable penetrates into the layer, which changes its dielectric properties depending on the water content. The cable parameters, like capacitance and conductance per length will be measured. The extension of the measuring electromagnetic field is adapted to the thickness of the layer by spacing and arrangement of the cable conductors. Design rules and field calculations will be given on how to confine the field into the layer besides preserving the impedance matching between sensor and measuring device. A civil engineering application will be presented. Tunnels, dams and waste disposals are often sealed with geosynthetic clay liners (GCL) against water percolation. These mineral (bentonite) liners have a thickness between 5 and 15 mm. For a reliable long-term performance GCLs have to maintain a certain water content. In order to control this water content the new sensor measures the water content inside the GCL independent of the varying water content of their soil bed. Measurement results from field experiments will be presented.