Presentation 17:
Geophysics for Spillway and Seepage
Evaluation

FEMA Workshop - Issues, Remedies and Research Needs Relating 
to Service and/or Emergency Spillways

Geophysics for Spillway & 
Seepage Evaluation 

So what is Geophysics

The study of the earth (and other materials)
using non-intrusive measurements of
physical properties.

Some typical examples of parameters we measure are:

* Resistivity
* Seismic velocity
* Localized magnetic field!
* Gravity field
* Radar wave velocity & reflectance

FEMA Workshop - Issues, Remedies and Research Needs Relating 
to Service and/or Emergency Spillways

How is Geophysics Helpful?

* Non-lnvasive "Screening " Tool
* Does not Generate Waste (Environmental)
* Supplements Subsurface Data Between Borings.
* Help Subsurface Characterization by Seeing the Big Picture
* Quickly Search for Specific Targets
* Trace What is Not Easily Seen (Water Seepage)
* In-Situ Estimation of Engineering Properties of Subsurface Earth Materials

Some of Our Common Tools

* Resistivity Sounding and Imaging (2-D & 3-D)
* Electromagnetics (EM)
* Ground Penetrating Radar (G P R)
* Magnetics
* Induced Potential (IP)
* Spontaneous Potential (SP)
* Seismic Techniques
* Microgravity

FEMA Workshop - Issues, Remedies and Research Needs Relating 
to Service and/or Emergency Spillways

What Can you Do with Geophysics?

* Trace Seepage through 
Embankment Dams/Ponds
* Define Limits of Voids underneath Spillway Slabs
* Determine Shear Wave Velocities for Seismic Design
* Karst Investigations 
* Detect Abandoned Mines
* Map Voids and Sinkhole Potential
* Subsurface Stratigraphy
* Characterize Geologic Structure
* Determine Depth to Bedrock
* Determine Depth to Non-rippable Bedrock
* Map Contaminant Plumes
* Locate steel Reinforcing in Concrete Slabs
* Locate Underground Storage tanks (USTs)
* Define Limits of Abandoned Landfills
* Confirm Fractures in Bedrock for Groundwater Well Siting
* Assess Concrete Quality
* Monitor Vibrations from Blasting/Construction/Demolition
* Define GW Well Capture Zones in Fractured Bedrock
* Locate Buried Metallic Debris

Performing Geophysical Investigations

Investigation Design > Data Collection > Data Reduction > Analysis > Interpretation

FEMA Workshop - Issues, Remedies and Research Needs Relating 
to Service and/or Emergency Spillways

Object of This Presentation

NOT to teach the theory of geophysics;

RATHER, to provide examples where geophysics is used to provide valuable information

Example 1, Moores Creek Dam, Lexington, VA

Outlet Structure

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to Service and/or Emergency Spillways

Sinkhole in downstream face above spillway conduit

Boil at Toe of Dam

Geophysical Investigation to Define Seepage Pathways

Complimentary
Geophysical
Techniques

* Self Potential (SP)
(measures voltages from water
moving through porous
medium)

* Two-Dimensional
Resistivity
(measures low resistivity
zones caused by increased
water saturation)

FEMA Workshop - Issues, Remedies and Research Needs Relating 
to Service and/or Emergency Spillways

Spontaneous Potential

Causes of SP Anomaly

Ground Surface
Fixed Electrode
Roaming Electrode
Electrical Potentials
Non-Polarizable Electrode

* Mineralization
* Geothermal gradients
* Bioelectric activity
* Varying electrolytic concentrations in ground water
* Geochemical variations
* Corrosion
* Changes in topography
* Telluric currents
* Streaming

Plan View of the Dam

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to Service and/or Emergency Spillways

Plan View of SP Results

Self Potential (milli-volts)

Resistivity Data Layout

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to Service and/or Emergency Spillways

Electrical Flow Through Earth Materials

Resistivity Results

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to Service and/or Emergency Spillways

Example 2: SP on Water, Chagrin Falls, OH

Seepage Through or Around Spillway?

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to Service and/or Emergency Spillways

Through Embankment Adjacent to Intake

Example 3, Lake Logan, NC 
Sonic Analysis of a
Concrete Arch Spillway 

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to Service and/or Emergency Spillways

Tomographic Analysis of the Sonic Data to Locate Poor Concrete Sections

Example 4, Concrete Spillway Slab

Finding Voids Under Spillways

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to Service and/or Emergency Spillways

Ground Penetrating Radar (GPR)

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to Service and/or Emergency Spillways

GPR Results  Typical Voids

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to Service and/or Emergency Spillways



Approximately 12 feet
Footing and Piles
Rebar
Potential Voids
Bottom of Concrete and Stone Subbase
Spillway with Shaded Areas Showing Voids and Areas with no Rebar in the Slab

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways

GEOPHYSICS
* Will NOT solve every problem.
* Each method has strong and weak points, therefore often best when several complimentary methods used.
* Can be extremely useful and cost effective if used Properly to see the big picture, or to search for targets.
* Necessary to understand geophysical principles, geology, construction methods and design, and what the client wants in order to provide USEFUL interpretation and subsurface characterization.

FEMA Workshop - Issues, Remedies and Research Needs Relating to Service and/or Emergency Spillways