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Saturday, 16 Dec 2017

Resistivity

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Resistivity surveying is a geophysical method widely used for a variety of engineering and environmental applications. In characterizing the subsurface the method makes use of the fact that earth materials have different ability to conduct electrical current. Being non-destructive the method may be attractive for installations on existing dams and it has been found useful for status control in dam safety investigations. Another more ambitious and more powerful approach is using the method for long-term monitoring of dams. In that case additional information is received from an analysis of the seasonal resistivity variations in the dam. Moreover, changes in resistivity can be analysed by comparing measurements on different occasions, e.g. different reservoir levels.

Fundamentals

The resistivity in the dam depends mainly on the temperature and the ion content of the seepage water. Both these parameters vary seasonally, and their variation in the dam depends on the seepage flow. Seepage areas in the dam will then be shown as areas with larger seasonal variation. This approach may allow for both detection and quantification of seepage flows. Moreover, changes in the average resistivity in the soil are also relevant for dam monitoring. A systematic change in resistivity is a sign of changing material properties, such as changes in fine contents, which could be related to internal erosion.

The seasonal resistivity variation in the reservoir water is separated into two parts when the seepage water passes through the dam. The solutes penetrate into the dam with the pore velocity vn while the temperature travels with the thermal velocity vT. The resistivity variation in the dam is therefore a combined result of these two transport processes. Seepage flow can be analysed applying these assumptions. The status of the method today is mainly qualitative, i.e. areas with increased seepage can be located. The method is sensitive for seepage flow changes, in the order of 10-5m3/s and m dam.

The method is dependent on good installations to minimize effects from noise and increase the monitoring accuracy. On existing dams it is preferable to install electrodes in the dam core, which is the most conductive part and the main target of investigation. Installation in cold climates should be performed below the freezing depths. The spacing needed between the electrodes is typically around 5m, but may vary widely with dam size. Electrodes can be placed anywhere on the dam or in the upstream water. Measurements can be performed in 2D or 3D.