This document discusses electrical resistivity tomography (ERT) and its applications for dam safety assessment. ERT is a non-destructive testing technique that uses electrical currents and measurements of potential to image subsurface structures. It can detect anomalies associated with cracks, voids, water intrusion and other issues. The document provides an overview of ERT methodology, gives examples of its use in interpreting dam structures and monitoring for signs of issues like seepage. It also presents a case study showing how ERT detected seepage in a reservoir embankment several days before it was visually observed.

1. ELECTRICAL RESISTIVITY TOMOGRAPHY
ON DAM SAFETY ASSESSMENT

2. Overview
0 Introduction
0 Advantages of NDT
0 NDT for concrete structures
0 Tomography, Tomogram & Imaging softwares
0 ERT and its Methodology
0 Interpretation of ERT
0 Resistivity Monitoring
0 Importance of Dam Safety
0 Case Study
0 Scope of ERT in India
0 Conclusion
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3. Introduction
0 Reliability of structures can be ensured only through testing
0 Non-destructive testing (NDT) ensures the structural integrity,
quality and reliability of components without damage
0 Versatility of NDT techniques allows it to be used for a wide
variety of analysis.
0 Today NDT finds extensive applications for condition
monitoring, residual life assessment, energy audit, etc.
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4. Advantages of NDT
0 Preserves the tested material
0 Save both money and time in product evaluation,
troubleshooting, and research.
0 Highly sensitive methods, capable of finding extreme
flaws.
0 The tests can be conducted on a wide variety of
materials.
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5. NDT for concrete structures
0 Vibration analysis
0 Infrared Thermography
0 Acoustic Emission Analysis
0 Ultrasonic Imaging
0 X-ray Computer Tomography
0 Ground Penetrating Radar
0 Digital Radiography
0 Electrical Resistivity Tomography
0 Magneto-Inductive Cable Testing
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6. Tomography
0 Invented by Sir Godfrey Hounsfield (as CT scanner)
0 “tomos” – slice/ section & “graphy” – to write
0 Refers to imaging by sections, through the use of a
penetrating wave
0 Involves gathering projection data from multiple
directions & feeding it to a tomographic reconstruction
software algorithm to create a tomogram
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7. Tomograms
0 Derived using physical phenomena like
0 X-rays, Gamma rays, Radio-frequency waves, Electrical
Resistance, Magnetic particles, Ions etc
0 Thus resulted in different types of tomography
0 X-ray tomography
0 Single photon emission computed tomography
0 Ocean acoustic tomography
0 Electrical Resistivity tomography
0 Cryo-electron tomography
0 Ultrasonic Pulse Velocity Tomography
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8. Tomographic Imaging Softwares
0 Popular softwares:
0TOMO
0Earth Imager
0InGEOHT - 2D+
0 Used for detecting of cracks, honeycomb, voids, uncured
or weak concrete, water intrusions, soil velocity profiles
0 In concrete drilled shafts, Mat Foundations, Dams,
Bridge Substructure, Rock, Wood, Masonry
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9. Electrical Resistivity Tomography
0 Electrical current is introduced into the medium through
electrodes and the resulting potential distribution is
measured
0 Typically, one pair of steel electrodes is used to inject
current and another pair is used to measure potentials
0 Resistivity ρa =K *V/I
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10. Resistivity Chart
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11. Methodology of ERT
0 Altering electrode distances, resistivities at different
depths are obtained.
0 Relationship between electrode spacing and resistivity
is used to back calculate the resistivity profile
0 Results in an extended 2D collection of measurements
at different depths
0 The resulting data represents a 2D section of the
subsurface
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12. Methodology of ERT (cont…)
0 The inversion of the 2D survey data gives 2D ERT
0 Combination of separately measured 2D ERT in
parallel gives us 3D image
0 Field conditions often obstruct 3D ERT.
0 Hence, 2D ERT remains to be the state of the practice
due to its simplicity in field works and less space
requirement.
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13. 2D Tomogram
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C:UsersSREELEKSHMIDesktopResistivity imaging (ERT) with ZondRes2D.mp4

14. 3D Tomogram
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15. Interpretation of ERT
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16. Resistivity monitoring in dams
0 Based on the fact that a change in the porosity leads to
the changes in water content and fine particles
0 Resistivity is known to be very sensitive to the
changes in porosity
0 Hence internal erosion can easily be detected
0 It can also detect leakage zones
0 Seasonal fluctuations in the dam due to variations of
temperature and water level can be elucidated
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17. Importance of Dam Safety
0 The dams are national property large investments and other
resources have been deployed
0 Important aspect for safeguarding the national investment &
the benefits derived by the nation from the project
0 Unsafe dam constitutes a hazard to human life & property
0 To be examined for ensuring public confidence
0 To protect the d/s area from any potential hazard
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18. Causes of Failure of Dam
0 Overtopping, caused by water spilling over the top of
a dam
0 Structural failure of materials used in dam
construction
0 Stability failure of the foundation or other features
that hold the dam in place
0 Cracking caused by movements like the natural
settling of a dam
0 Inadequate maintenance and upkeep
0 Piping
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19. Case Study
0 Area: Embankments of East Ayrshire Reservoir of USA
0 ERT interpretations for seasonal variations in the dam
Oct 2010 to Nov 2010
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20. March 2011 to May 2011
Nov 2011 to Dec 2011
CaseStudy(cont…)
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21. Case Study (cont…)
0 3 days after the last survey, visible evidence of
seepage was recorded.
0 It was decided to drain the reservoir in order to
protect the dam from premature failure.
0 ERT was able to detect signs of seepage several days
before it was visually observed on the d/s
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22. Scope of ERT in India
0 Test was conducted on Mullaperiyar Dam on to find
the amount of Surki present in it
0 Water loss test was also conducted
0 Tests were carried out through boreholes of 910 feet
0 Conducted by Parson Overseas Ltd. as per the
direction of the honourable Supreme Court
0 Committee reported the dam as seismically safe
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23. Conclusion
0 ERT is an elegant measure to analyse the dam of cracks and
seepage
0 Though expensive, it’s very useful & interpretation of results is
more accurate
0 It enables 2D as well as 3D imaging of dam & the embankment.
0 Monitoring of dams through ERT helps to avoid catastrophic
events due to dam failure
0 Assessment of seasonal fluctuations within dam is possible
through ERT
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24. References
1. Jared S. Case ( May 2012) Inspection of earthen embankment
dams using time lapse electrical resistivity tomography , pp. 71-
87, Ole miss project 90034.
2. Karlis Dreimanis, Dr. Philip Sentenac (2012) Electrical
Resistivity Tomography and Electromagnetic Assessment of East
Ayrshire Reservoir Embankments , University of Strathclyde.
3. Christophe Michaux & Michel Grill (2011) NDT 3D tomographic
testing cases on concrete and national heritage buildings , pp. 7-
13.
4. Yannick Fargier & Cyrille Fauchard (2011) Methodology
Applied to the Diagnosis and Monitoring of Dikes and Dams, Vol.
14 pp. 274 – 278.
5. The Hindu , March 26, 2012 Tomography test at Mullaperiyar
dam, by correspondent
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TKMCE
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