A Guide to Resistivity Investigation and Monitoring of Embankment Dams

Abstract

The resistivity method is an established geophysical method with a broad range of engineering and environmental applications. It has been tried numerous times on embankment dams, mainly for seepage investigations, dam status control and investigations of known defects. In previous use of the method on embankment dams some success has been reported, but only occasionally. The method is still not completely adapted to customary industrial use, although there is rapid progress.

The purpose of this guide is to help the user to optimize use of the resistivity method for dam status control and dam seepage investigations. For this purpose the guide covers resistivity survey design, equipment, data acquisition and other practical issues. The guide includes a few recent examples, where the method has been used both for investigation and for long-term monitoring. To get the full picture, the guide also briefly covers theoretical discussions on the method. It is assumed that the reader of this guide is acquainted with basic theory of geophysics and knowledge about design and function of embankment dams.

The method can be used in two ways. Firstly, resistivity investigations as a one time survey may detect spatially anomalous zones along the dam, and can be used to investigate suspected structural weaknesses. Secondly, long-term resistivity monitoring make use of the seepage-induced seasonal variation inside the embankment to detect anomalies not only in space, but more importantly in time, by studying deviations from the time-variation pattern. The second approach is more powerful as repetition of measurements provides additional evaluation possibilities for seepage analysis.

The use of the resistivity method on embankment dams can be challenging and the anomalies are often small. Complicating factors for interpretation, such as for instance complex dam geometry, plentiful noise sources, rather small signals and reservoir level fluctuations are discussed. Advantageous factors for the method are also mentioned. These include it being non-destructive, the possibility to cover large volumes, the possibility to install on existing dams and the sensitivity of the method to changes in material properties and seepage flow among others.

The monitoring approach is based on the principle that the resistivity in an embankment dam varies seasonally, mainly due to variations in temperature and ion content of the seepage water. Both these parameters vary seasonally, and their variation in the dam depends on the seepage flow. This implies that areas in the dam with larger seepage may stand out as areas with larger seasonal resistivity variation, and increasing seepage may be noticed as increasing variations. Moreover, material change due to washout of fines, may be detectable through resistivity measurements implying that trends of changing resistivity over time may relate to internal erosion.

This guide also discusses practical aspects of performing resistivity measurements on dams. Most common is still to perform 2D-measurements using an array of electrodes placed along a line. The complex geometry of the dam leaves two options, i.e. measurements where the survey line is placed along the dam, usually along the dam crest, or measurements where the survey line crosses the dam axis. The latter is often difficult to conduct in practise but whenever possible, it is a good complement providing detailed information in a specific part of the dam. Using a survey line along the dam is the most straightforward option and provides information on a larger part of the dam, although less detailed. The final choice of survey design should always depend on site-specific conditions.

Standard resistivity surveying equipment is used for dam investigations. For repeated measurements it is advisable to leave the electrodes in the ground between measurements. It is essential to make sure that good electrode contact is provided, especially in the case of permanent installations where the contact can often not be improved after installation works are completed. Processing of resistivity data includes data quality assessment, inverse numerical modelling and presentation and analysis of the results. Data quality is preferably checked in the field, typically by examining the pseudosection. Standard inversion packages may be used for data processing of 2D-measurements. Interpretation should be made with as much reference data as possible.

A few case studies from Scandinavia are briefly presented, and some other examples in literature are referred to. Based on experiences from the case studies it is confirmed that long-term monitoring is more powerful than one time surveys, which however still may be useful in many cases. Repeated measurements confirm that resistivity variations inside the dams are obvious. A zone with an increasing resistivity trend has been detected at one of the dams. This may be explained by ongoing internal erosion, which is supported by other observations but has not been confirmed by direct investigations in the zone. Under good circumstances quantitative seepage evaluation from resistivity monitoring data can be performed.

• author: Dahlin, Torleif ; Sjödahl, Pontus and Johansson, Sam
• organization: Engineering Geology
• publishing date: 2008
• type: Book/Report
• publication status: published
• subject: Geotechnical Engineering
• publisher: CEA Technologies Inc. (CEATI), Montreal
• report number: T992100-0205B/4
• language: English
• LU publication?: yes
• id: c6ea6f2a-70e7-4af1-8688-4b6fa8e74243 (old id 2369819)
• date added to LUP: 2016-04-04 10:54:48
• date last changed: 2018-11-21 21:01:32