Introduction to Environmental Geophysics Student ce of Emergency and Remedial Response Washington,...

Offi ce of Emergency andRemedial ResponseWashington, DC 20460

Introduction toEnvironmental Geophysics

Superfund

United StatesEnvironmental ProtectionAgency

Student Manual

July 2014www.epa.gov/superfund

jeffry.weidner

Not Approved

Overview of Environmental Geophysics 1

Overview of Geophysical Methods

OVERVIEW OF

GEOPHYSICAL METHODS

Geophysical Surveys

Characterize geology Characterize hydrogeology Locate metal targets and voids

Physical Properties Measured

Velocity Seismic Radar

Electrical Impedance Electromagnetics Resistivity

Magnetic Magnetics

Density Gravity



Magnetics

Measures natural magnetic field Map anomalies in magnetic field Detects iron and steel

Geometrics Cesium Magnetometer

Electromagnetics (EM)

Generates electrical and magnetic fields Measures the conductivity of target Locates metal targets



EM-31

Marion Landfill, Marion, IN

EM-61

Geonics EM-61 EM Metal Detector

Resistivity

Injects current into ground Measures resultant voltage Determines apparent resistivity of layers Maps geologic beds and water table



Sting Resistivity Unit

Seismic Methods

Uses acoustic energy Refraction - Determines velocity and

thickness of geologic beds Reflection - Maps geologic layers and bed

topography

Seistronix Seismograph



Gravity

Measures gravitational field Used to determine density of materials

under instrument Maps voids and intrusions

Scintrex Gravity Meter

Ground Penetrating Radar

Transmits and receives electromagnetic energy

Maps geology Locates cultural targets Has very high resolution



Noggin Ground Penetrating Radar Unit

Borehole Geophysical Methods

Variety of downhole tools available Optical and acoustic televiewers Parameters measured

Temperature Flow direction Conductivity and Resistivity Density Gamma

Borehole Geophysical Methods

Video

Caliper

Resistivity

Gamma / Temperature



Modeling for Interpretation

Two kinds of simple models Forward Inverse

Models depend on input conditions and data

Models can be very helpful in visualizing the site

Models are not reality

Geophysical MethodsAdvantages

Non-intrusive Rapid data collection Detects a variety of targets Screens large areas Fills in data gaps

Correct Interpretation



Methods require a specialist Interpretations are non-unique May be expensive Physical contrasts must exist Resolution varies by method and depth of

target

Geophysical MethodsLimitations

Problematic Interpretation

Geophysical Survey Design

1Overview of Environmental Geophysics

X

Y

A Good Survey Results In…

• A record of useful information– Background data to support survey

– Rationale for methods used

– Survey data - maps

– Conclusions in lay terms

• Efficient use time - money

• A document that maintains its value

Survey Design Rationale

• Establishes a plan

• Find potential pitfalls

• Maximize benefit

• Minimize surprises– Property line issues

– Archeological sites

– Utility lines

• Customize requests



Pre-survey Planning: Garbage IN – Garbage OUT

• Inadequate background information & planning dooms a survey before it starts:– Requires more time in the field

– Increases costs

– Missed targets

– Questionable data

Define Problem

• List issues of concern

• Can geophysics help?

• Data confirmable?

• How will results benefit your plan?

Background Paperwork Review

• Site history

• Previous studies

• Geology

• Geohydrology

• Geographic issues

• Health, safety & QAPP issues



Background Map Review

• Sanborn or other Public Maps– Historical site records & buildings

• Geologic Maps– Indirect conditions

• Topographic Maps– Terrain conditions

Sanborn Maps: Anacortes, Washington State

Feb. 1897

Nov. 1907

Nov. 1950

Oct. 1925

Sanborn UMI

Topographic & GeologicMaps



Background Photo Review

Recent Aerial Photo Historical Aerial Photo

Recent Site Photo Historical Site Photo

Photo Interpretation

April 5, 1988: Color

May 7, 1981: Color Infrared

Sept 25, 1936: B & W

Lammers BarrelBeavercreek, Ohio

U.S. EPAEnvironmentalPhotographicInterpretation

Center

Other Issues To Consider

• Property boundaries

• Consent for access

• Traffic & pedestrians

• Vegetation status

• “Noise” issues

• Utility location

• Archeological sites



Utility Locating

• Utility services require several days notice

• Service provides “dig” number for site area

• All utilities may not be members of service

• Have service remark area if necessary

• Know tolerances of service provider

Courtesy: Ohio State University

Dial 811on your phonefor local utility

location service

National Historic Preservation Act

• Why should we care?– It’s the law

– Regulations require it

– It’s EPA’s policy

– It’s a good idea

Public Law 89-665; 16 U.S.C 470 & Subsequent Amendments

EPA HQ Contact: [email protected] - 202.564.6646

State Contacts: www.ncshpo.org

Code of Federal Regulations (CFR)“Handling Drums & Containers”

• 1910.120 ( j ) (1) (x) “A ground-penetrating system or other type of detection system or device shall be used to estimate the location and depth of buried drums or containers”



Analyze Background Information to Determine..

• Area to be surveyed

• Size - number of suspect targets

• Potential problems

• Site reconnaissance needed?

Match Most Favorable Geophysical Techniques to Problem

• What method(s) contrast most from background?

• Note depth confines

• “Noise” issues

Ground Penetrating Radar

SeismicRefraction

Gradient Magnetometer

ElectromagneticGEM Unit

ElectromagneticEM-61 Unit



Optimize Data Collection

• Establish how data will be collected– Traverse pattern

– Grid spacing

– Axis labeling

– Data Location ID

Key Issues For Collecting Data

• Systematic collection (grid or lines)

• Spacing dependent on target size

• Accurate grid or line establishment

• Method to ensure location accuracy

• Label grids or lines reasonably

• Maintain good field notes

• Take plenty of photographs!

Data Collection(magnetics, electromagnetics, ground penetrating radar)



Consider Analogy Between Data Density & Photographic Pixels

As = Area of Site43,560

At = Area of Target4,356

Detection Probability(Using Individual Station Measurements)

(modified from Benson et al., 1988)

Probabilityof

DetectionAs/At= 10

10098907550

16131085

Number of data points required

As/At= 100

1601301008050


At = Area of Target435

As/At= 1000

160013001000800500


At = Area of Target43

Area of Target in ft 2

Area of Site in ft 2

= a in ft 2

Sampling PointsArea of Site in ft 2

= b

=b Grid Spacing in Feet

90% = 1.0

a x Probability Factor = Sampling Points (Approx.)

Probability Factors100% = 1.625

98% = 1.375% = 0.850% = 0.5

Determining Grid Spacing



Typical Acquisition Traverses

• Alternating mode– Most often used

• Random mode– Used for small or

large areas

• Parallel mode– Irregular shaped

sites

• Areas broken into rectangular shapes

• Irregular boundaries– Use multiple rectangles

• Positioning methods– Station

– Timed – collection

– Wheel encoder

– GPS

Random Survey Pattern(Small Area)

boulder

fence

startend

Random Survey Using GPS(Large Area)

• Maximize productivity

• Data linked to GPS

• Best in obstructed areas

• Areas must be free of:– Vegetative canopies

– Tall buildings

– Major power lines



Random Survey GPS Issues

• Data locations from Mag on ATV

• Dots show data points

• Note N-S dot spacing due to speed changes

• Note data gaps

(One dot per 5 data points)

Alternating TraverseNo GPS

Start

End

Alternating Traverse Grid Setup No GPS

• Layout grid markers at desired spacing– Flagging (plastic)

– Spray chalk or paint

– Ropes

– Alignment placards

– Wooden stakes

• Large sites require multiple marker lines

Tapes &markers

Siteboundary

Traversedirections

0,0



Alternating Traverse Parallel Swath GPS

Start

End

Parallel Swathing GPS

• Initialize start & end points of line

• GPS maintains parallel lines

• Operator follows cursor on lightbar

• Lat. - Long. output to sensor data

Photo: Geometrics

Lightbar Guidance

• Center: on line

• Left: move left

• Right: move right

• Outer edges yellow: nearing line end

• Outer edges red: at line end

• Advances to next spacing



Parallel Traverse – No GPS

Start

End

Linking Data to a Location

• Define X and Y

• X, line or longitude

• Y, position or latitude

• Several data collection options for tagging X, Y– Data recorder sets method

Data Recorder Methods

• Station position

• Time – distance

• Encoder wheel

• GPS



Correcting for Position (Y)

• Time-distance issue– Must correct for pace

• GPS– Correct for errors

– Use proper datum

• Wheel Encoder– Resolve distance errors

Continuous Data Acquisition Issuesfor Y Axis

• Operator inputs start & end points per line

• Unit auto “fits” data to input distance– Assumes same pace

• Obstacles usually slows pace

• Use data pause features as needed

Evenpace:real

Offpace:real

12pts

9pts

Posted

9pts

RealityVs

Processed

Line 1 Line 2 Line 2

Global Positioning Systems

• Accuracies vary by method & equip. used

• Example: some on a scale to locate an airport

• Example: others on a scale to find center of runway



Several GPS Methods

• Stand alone GPS receiver

• Differential correction (DGPS)– Real time using beacons, base stations

• Post processing GPS values

• 3 Grades of GPS accuracy– Recreational, mapping, survey

Drawing modified fromOmnistar corp. graphic

How a Differential GPS Service Works

Rover

Fixed Bases

Ground Based Local Positioning & Data Collection System



System Overview

• Laser beam tracking• Line-of-site system• Merges & stores

– Total station data+

– Geophysical dataor

– Radiological data

• Positioning options– guidance or tracking

• Real-time displays

Auto Tracking & Guidance

How It Works

• Laser tracks optical target

• Collects data– Position x, y, z data

– Sensor data

• Computes coordinates

• Merges data into one file

• Transmits to rover

• Displays data/position on HUD



• Blue Arrow shows direction related to X-axis baseline

• Coordinates

• Target path line

• Current path

• Distance L/R line

• Data readouts

• Recording indicator

2 Screen Views of HUD

Screen Color On Rover’s HUD Has Meaning

Red Zone

Yellow Zone

PlanViewGridArea

Yellow:Near End

White:Normal

Red:Passed End

Blue:Lost Tracking

Link

Pre-Planning for Seismic Survey

• Length of line required

• Number of lines & orientations

• Ambient “noise” issues

• Topography-elevation changes

• Good consistent ground coupling

• Line protection (traffic, etc.)



Which Method is Applied First?

• Dependent on site goals

• Generally........First– Methods having larger sensing areas

– Rapid data collection times

• Generally........Second– Methods with more definitive sensing

capabilities

Check List For Considering Geophysical Survey

• Define problem

• Research history

• Find area of concern

• Note site conditions

• Describe target(s)

• Estimate depth

• Will geophysics help?

• List methods that will show most contrast

• How will you use this information?

A Note About Contracting Geophysical Jobs

• Use source that is knowledgeable about all geophysical methods

• Write contract to assume several “what if” scenarios to deal with special issues

• Obtain copies of raw data & notebooks

• Be aware that interpretation & reports may be optional

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Magnetic Methods


Metal Detector ≠ Magnetic Method

METAL DETECTORS use internal power to create a electromagnetic field to locate metal

MAGNETOMETERS are passive instruments and only sense ambient magnetic fields

photo credit: Wikipedia

The Magnetic Method

• Senses presence of iron (ferrous metals)

• Measures magnetic fields

• Easy to apply and interpret

Magnetic Methods


iron cobalt

ironalloys

nickel

steel

Ferrous & Non Ferrous Metals

aluminum

brass stainlesssteel

titanium

copper

Why Is Magnetics Important?

• Non-invasive, passive detection method

• Quantitative results

• Large masses detectable at significant depths

• Complements other geophysical methods

Optimal Detectable FeaturesUnique to Magnetics

• Buried drums, tanks, pipes, valves

• Steel casing (abandoned wells)

• Mixed ferrous wastes (landfills)

• Steel reinforced foundations

• Natural occurring ferrous minerals

• Fired clays (bricks, clay pots)

Magnetic Methods


Why Are Baked Clays Magnetic?

• Magnetic force microscopy image showing magnetic domains

• Heated beyond curie point & when cooled domains realign

NDT Resource Center

Fire Pit Negev Desert

What Tools are Used to Measure Magnetic Fields?

• Instruments called

magnetometers

• Several types &

configurations

available

• Measures strength of

magnetic intensities

Magnetic Methods


Magnetic Survey Tools for Hazardous Waste Sites

• Generally 1 of 3 tool types used– Proton precession

– Overhauser precession

– Alkali vapor (cesium)

• All measure magnetic intensity

• Detects ferrous materials - minerals

• Tools are portable - independent systems

Selecting An Instrument

• Proton precession– Slow sampling cycle times: 3 – 6 seconds

– Rugged system can be linked to GPS

• Overhauser– Faster cycle times: 1s – GPS link possible

– Sensors sensitive to extreme heat (120°)

• Alkali Vapor– Fastest cycle times: 0.1s

– Sensors have high sensitivity but are fragile & $

– Most systems have direct hook-ups for GPS

Magnetometer Tool Options

• Several types available– Proton precession (2 types)

– Alkali vapor

• Each configurable– Total field mode

– Gradient mode

– Base station mode

• Sensor option alignments– Vertical

– Horizontal

Standard precessionOverhauser precessionAlkali Vapor

Total field (TF)TF + Base StationGradient*

* Vertical gradient* Horizontal gradient

Selection Options

Magnetic Methods


What Exactly Is Measured?

• An integration of magnetic properties– Earth’s magnetic field intensity

– Natural magnetic intensity rock/soil

– Cultural magnetic intensities

• Values either attractive or repulsive– Represented by + or - numbers

– (+) values same direction of inducing field

– (-) values oppose direction of inducing field

Earth’s Magnetic Field

• Always present

• Invisible to senses

• Viewed as background

• Sensitive to other ferrous influences

• Changes with latitude

A A

Strongest

Earth’s Magnetic Background

B

Moderate

BC

C

Weakest

Magnetic Methods


Ferrous Interactions• Ferrous metal has

its own magnetic field

• Capable of altering Earth’s field

• Limited influence

• Easily measured

• Provides accurate location method

Measurement Units

• Units measured in gammas or nano Teslas

• 1 gamma = 1 nano Tesla– 55 gallon drum lid about 40 or nT

– 250 gallon tank about 1000 or nT

Sensor Configurations

• Most systems can operate 1 or 2 sensors at same time

• 1 sensor– Obtains total field data

• 2 sensors– Collects total field &

gradient data

Magnetic Methods


Total Field Configuration: One Sensor

• Intensity measured from a single sensor

• Tool’s latitude defines background

• Anomalies: > or < than background

• Solar activity will influence data

Photo: Geometrics

Gradient Configuration: Two Sensors

• Intensity measured from two sensors

• Background is defined as “0”

• Anomalies: > or < than background

• Solar activity will not influence data

Gradient Configurations: Adjoining or Remote

Option A

Option B

GradientMode

BaseStationMode

Magnetic Methods


Gradient Readings

• Total field (bottom sensor) minus vertical gradient (top sensor) noted as or nT per unit of distance between sensors

• 55,900 - 55,200 = 700 /meter or nT/M

• Negative values are also possible

Why is Gradient Data Significant?

• Earth’s background fluctuates due to

solar disturbances

• Failure to neutralize a rapid background

change will result in misleading data

• Gradient data ignores solar changes

Solar Disturbances

Solar Forecasts: http://www.swpc.noaa.gov/today.html

Magnetic Methods


• Joint USAF/NOAA Solar Geophysical Activity Report and Forecast SDF Number 126 Issued at 2200Z on 06 May 2013

• IA. Analysis of Solar Active Regions and Activity from 05/2100Z to 06/2100Z: Solar activity has been at low levels for the past 24 hours. The largest solar event of the period was a C2 event observed at 06/0205Z from Region 1739 (N12E30). There are currently 8 numbered sunspot regions on the disk.

• IB. Solar Activity Forecast: Solar activity is expected to be low with a chance for M-class flares on days one, two, and three (07 May, 08 May, 09 May).

NOAA / Space Weather Prediction Center

3-day Report of Solar and Geophysical Activity

Last 75 Reports Today's Space Weather Space Weather Now

N

55,000

55,000

0 gamma

TypicalBackground

Gradient Measurements

55,200

55,900

700 gammas

TypicalAnomaly

N

700 gammas

omaly Withr Disturbance

AnSola

N

65,900

65,200

(Vertical Gradient)

Cesium Magnetometer

• Ionizing light “pumps” electrons to higher energy levels

• Magnetic fields affect rate energy gain/loss

• Constant “pumping” allows continuous data acquisition

• Accuracy of .1 gamma (detect several nails)

Magnetic Methods


Photo-Matic

(modified from Bloom, 1960)

IonizingLight

VaporChamber

Photocell

More Energy Emitted In Strong Ambient Field

Less Emitted In Weak Ambient Field

Cesium Mag Measurements

Cesium

Alkali Vapor Sensor

Orientation

0º Tilt

0 º Rotate

0º Tilt

45 º Rotate

Tilt = on horizontal plane parallel to

direction of travel

Rotate = on vertical plane perpendicular

to direction of travel

Good

Bad

Data Interpretation

• Data analyzed by computer program

• Typically by some contouring method– Lines connecting equal values at specific

intervals

• Displayed as 2D or pseudo 3D graphic

Magnetic Methods


DataValues

• Location over target affects data

• Strongest values closest to target

Data Signatures

NS

S

N

rS-S rN-N

rN-S rS-N

Pt. A Pt. B

rN-N rS-S=

rN-S rS-N=

= but opposite forces existsymmetrically @ Pt. A & Pt. B

rN

rS

rN rS<measured field

everywhere negative

Pt. C Pt. D Pt. E

rS-SrN-S rS-N

rN-N

rN-N rS-S>

rN-S rS-N>

strong positive field Pt. Dweak negative field Pt. E

measurement surfaces

Alan Witten

Data Display Options

Alen Witten – In Press

a) Contour plot: dashed = (-); Solid = (+)

Examples of synthetic magnetic data for a horizontally oriented dipole

b) Gray-scale plot: dark = (-); light = (+)

c) Surface plot (3D)

Magnetic Methods


+

-

Modified from The University of Melbourne

Estimating Target Depths

Credit: Alan Witten

½ Max. d

a)

b)

c)

2.4 m 3.12 m

1.0 m 1.30 m

1.6 m 2.08 m

Depth Estimate Calculation From Contour Map

• Solid & open circles are locations of max. value & ½ max. value: 3.6m (as measured from map scale)

• Contour interval 20 nT

• Target = horiz. metal bar– Depth: actual = 5m– Depth: est. = 4.68m

-

+

horizontal orientation = 45 degrees

vertical orientation = 0 degrees

Credit: Alan Witten

Magnetic Methods


Another Depth Estimate

• Solid & open circles are locations of max. value & ½ max. value: 1.8m (as measured from map scale)

• Contour interval 20 nT

• Target = vert. metal bar– Depth: actual = 3m– Depth: est. = 2.34m

+

horizontal orientation = 90 degrees

vertical orientation = 90 degrees

Credit: Alan Witten

Mag Data Example – 3DFrom Chicago Test Site

Pavement - Concrete & Rebar

Soil

Multiple Magnetic Sources

Drum Mass ≈ Rebar Mass: Difficult to Distinguish

Drum Mass > Rebar Mass: Easier to Distinguish

BuriedWaste

Magnetic Methods


Dealing With Noise Issues

• Accounting for un-wanted Interferences– Power lines, fences, cars

• Apply a “walk-away” test– Start at source

– Walk-away until readings normalize – note distance

Unwanted Magnetic Noise Examples

Data Interpretation Pitfalls

• Incorrect grid spacing

• Contour interval too large or small

• Cultural noise not properly addressed

• No data maps or reference points

• Use of color maps in reports that are photocopied in B&W

Magnetic Methods


Mag Anomaly Example 1

• 1 Crushed drum (lying vertical)

• Depth: -4.5’ to -8.5’• Values: +26 to -54• Contour interval: 10• Blues: pos. values• Reds: neg. values

10

10

10Feet

Contour Map

0

0

10

Example 1 Source


• 5 Crushed drums

• Depth: -5’ to -6’

• Values: +78 to -171

• Contour interval: 35

• Blue: pos. values

• Reds: neg. values

0

10

10

20

010 1020 20Feet

Contour Map

Magnetic Methods


Example 2 Source


• 1 Drum (horizontal)

• Depth: -3’ to -6’

• Values: +111 to -572


• Blues: pos. values


10

10

10Feet

Contour Map

0

0

10


• 2 Iron pipes: 10’ x 4”

• Depth: -1.7’ to -2’

• Values: +129 to -238




10

10

10

Feet

Contour Map

0

0

10

20

20 20

Magnetic Methods



• Two 500 gal. tanks

• Depth: -2’ to -7’

• Values: +1114, -120




10

10

Feet

Contour Map

0

10

20 2010

0

Tank Removal

In-Situ

500 Gallon Tanks

This Geophysical StuffActually Works!


Magnetic Methods


Example 6 Tank Removal


Mag Anomaly 7 Removal

Magnetic Methods


Confirmatory Methods for Magnetics

GPRDepth to Target

Top of Target Shape(dependent on soil conditions)

ElectromagneticsDetailed Lateral Dimensions

Generalized Depth Information(dependent on Tx & Rx range)

MagneticsRapid Data Collection

Establish Amount of MassGeneral Lateral Dimensions

Example of Landfill Mag Data

Magnetic Methods


More Landfill Mag Data

Vacant Field Mag Data

Marine Cesium Magnetometer

• Towed by boat

• X-Y location control by GPS

• Depth control by line & speed or floatation device

Geometrics

Magnetic Methods


Surface Mag in Aluminum Boat

Marine Applications

• Lake George Channel

• Indiana Harbor Canal

• Looking south Indianapolis blvd. bridge

Marine Cesium Magnetometer Data

Magnetic Methods


Search for CSS Hunley Using Magnetics

CSS H. L. Hunley – USS Housatonic

U.S. Naval historical Center PhotographAugust 29th 1862 - February 17th 1864

A, B, C Courtesy: Submerged Cultural Resources Unit - National Park Service – Santa Fe, NM

Battle Site Mag Anomalies

A B

C

Morris + Bailey

Magnetic Methods


What’s Wrong With This Picture?

Requesting A Survey(Questions Provider Should Ask You)

• How big is the site

• Composition of targets

• Orientation & size of targets

• Depth or burial method of targets

• Describe terrain & site conditions

• Explain special circumstances

Provider Submits Plan(Questions You Should Ask)

• Why are selected method(s) appropriate?

• What tool & configurations will be used?

• Method to ensure data location accuracy?

• What deliverables will be provided?

• Will data be presented for the layperson?

• How can I relocate area at a later date?

Magnetic Methods


Limitations

• Subject to cultural noise

• Detection of small objects

reduced with depth

• Depth estimates most

difficult for non-

homogenous masses

• Masses cannot be

uniquely characterized

Summary & Conclusion

• Magnetometers detects ferrous metal & fired clays

• Non-invasive, passive detection method

• Quantitative results relative to amount of mass

• Large masses detectable at significant depths

• Complements other geophysical methods

• Note: Magnetometers are different from metal detectors

– metal detectors emit energy to detect metal

– magnetometers passively measure ambient conditions


Electromagnetic Methods

ELECTROMAGNETIC (EM) METHODS

Module Goals

Describe electromagnetic methods in general

Explain the differences between these two types of electromagnetic instrumentation

Describe the application of the two types in the field of environmental geophysics

EM Methods

Often used with magnetics Fast and inexpensive Measures conductivity Frequency Domain Time Domain



Frequency Domain EM (FDEM)

Fixed Frequency - Fixed Depth

Multiple Frequency - Variable Depth

Reads Conductivity Directly

Metal Detection

T R

Frequency-Domain EM

Varying electric field

Varying magnetic field

Eddy currents

T R

Frequency-Domain EM

Varying electric field

Varying magnetic field

Eddy currents



FDEM Signal Components

The secondary magnetic field has two components Quadrature phase - used to measure ground

conductivity - 90° out of phase with primary field

In-phase - used to detect excellent conductors (metal) - 180° out of phase with primary field

EM-31

~ 4.5 meter maximum depth (3.66 m coil spacing)

Operating frequency 9.8 kHz

Soil conductivity (mS/m) - quadrature phase

Metal detection (ppt) - in-phase component

EM-31

Marion Landfill, Marion, IN



Depth of Penetration

~1.5 x coil spacing for vertical dipole

~.75 x coil spacing for horizontal dipole

EM-34

Three coil spacings – 10 m. (6.4 kHz) 20 m. (1.4 kHz) 40 m. (0.4 kHz)

Soil conductivity - quadrature phase Coil spacing - in-phase component

EM-34 transmitter

Air National Guard Base, Alpena, MI



EM-34 receiver

Gem-2 and 3

Multi-frequency signal Variable depth of investigation Output is secondary magnetic field (ppm)

to the primary magnetic field

Gem-2



Conditions Affecting Conductivity

Soil type

Moisture

Cultural debris

Pore fluid

Advantages/Limitations of FDEM Detectors

Advantages Fast, inexpensive

Reasonable lateral resolution

Limitations Limited depth of penetration

Sometimes difficult to interpret

Many noise sources

Frequency Domain EM

Examples



Vacant lot in Toronto



Lot after excavation

EM-31 In-phase survey

EPA Gem2 dataNorthridge, IL

21030 HzIn phase data



.GEM 2

Apparent Conductivity

Magnetic Susceptibility

GEM-2

Time Domain EM (TDEM)

Square Wave signal - Variable Depth

Conductivity at depth

Metal Detection



Time Domain EM (TDEM)

T R

PRIMARY

MAGNETIC FIELD DECAYING SECONDARY

MAGETIC FIELD

TDEM

SURFACE

TIME

AND

DEPTH

T R

TDEM Metal Detector



TDEM Metal Detector

EM-61 MK2

Site near Dayton, OH

Time Gates EM-61 MK2

Channel 1 – 216 μ seconds (bottom coil)




Channel T – 660 μ seconds (top coil)



35

EM-61 Data Display

TDEM Metal Detector

One transmitting coil

Two receiving coils

Ability to discriminate depth and screen surface metal

Depth of detection about 3.5 meters

Advantages and Limitations of TDEM Detectors

Advantages Fast and inexpensive

Easy to interpret

Excellent lateral resolution

Unaffected by conductive soil

Limitations Limited depth of penetration - 3.5 meters

No geologic data



Time Domain EM

Examples

Gravity Method


Gravity Method

GRAVITY

• Theory and measurement

• Targets

• Surveys

Gravity

Gravity Method


Gravitational Equationg=GM/R²

Where:• g = acceleration due to gravity• G = Universal Gravitational Constant

• 6.67191 x 10-11 m3 kg-1 S-2

• M = mass of Earth• R = radius of Earth

Unit of g

gal

1 µ gal = 1 x 10-6 cmsec2

Field Measurements

R elevation

g is measured

g is related to density

Gravity Method


Types of Measurements

high accuracyfixed instrumentactual acceleration

good precisionmobile instrumentrelated to density

Absolute -

Relative -

• Falling body

• Pendulum

• Metal zero-length spring

• L & R

• Quartz zero-length spring

• Scintrex

Gravimeters

Gravity Method


• Latitude

• Free air

• Bouger slab

• Terrain correction

• Earth tides

Corrections to Measurements

Gravity Corrections

BOUGER SLABFREE AIR

DATUM

• Density changes

• Voids

Gravity Targets

Gravity Method


Density Change

BURIED VALLEY

INTRUSION1.8 gm/cm3

2.8 gm/cm32.3 gm/cm3

Voids

CAVE

0 gm/cm3

1 gm/cm3

2.5 gm/cm3

0 gm/cm3

TANK

2.2 gm/cm3

Gravity Method


• Cultural sources

• Wind

• Microseisms

Noise Sources

• Unique property measured

• Void detection

• Low noise

Advantages

• Limited environmental targets

• Expensive

• Skilled operator needed

• Need a target

Limitations

Gravity Method


GRAVITYSURVEYS

• Transects

• Areal surveys

Types of Surveys

• Know the target

• Note linearities on maps, etc.

• Karst areas, note sinkholes, etc.

• Obtain elevations

• Establish base station if necessary

• Obtain Earth tide data

Survey Setup

Gravity Method


Instrument Setup

• Let instrument stabilize

• Place station markers

• Determine marker elevation

• Set up instrument

• Measure height

• Take measurement

Let Instrument Stabilize

Scintrex Gravimeter

Place Station Markers

Gravity Method


Set Up Instrument

Measure Height

Take Measurement

• Data processed

• Data plotted

• Compared with known features

• Modeled

Data Interpretation

Gravity Method


• Drilling

• Trenching

• Further geophysical surveys

Data Confirmation(Ground Truth)

Quick tides screen shot

Seismic Methods


Seismic Methods

ENERGY SOURCE

SEISMOGRAPH

GEOPHONES

BEDROCK

SOIL

DIRECT WAVE

REFRACTED WAVE

Module Goals

• Describe subsurface acoustic energy travel• Describe site geology effect on the seismic

method• Determine site subsurface information using

seismic data• Determine effectiveness of seismic method in

varying conditions• Operate seismic equipment

Seismic Methods


Seismic Method

Geophysical method that involves inducing

sound waves (energy) into the ground and

recording the time it takes for the energy to

travel through the subsurface

Seismic Method (cont.)

Because of different acoustic velocities, sound

waves reflect or refract as they cross

geologic boundaries

Reflection vs. Refraction

Reflection

Refraction

Seismic Methods


Subsurface Acoustic Wave Travel

Subsurface Wave Travel

Governed by principle of Index of Refraction

(Snell's Law)

Subsurface Wave TravelSnell's Law

Describes how acoustic waves react when they

encounter the boundary of two layers having

different velocities

Seismic Methods


Subsurface Wave Travel (cont.)

As an acoustic wave encounters a layer having

a higher velocity, part of the wave is

reflected and part is refracted

ENERGY SOURCE

Surface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone

Unconsolidated sand and gravel

DIRECT WAVE

REFLECTED WAVES

ENERGY SOURCE

Surface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone


DIRECT WAVE

Seismic Methods


ENERGY SOURCE

DIRECT WAVESurface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone


ENERGY SOURCE

DIRECT WAVESurface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone


ENERGY SOURCE

DIRECT WAVESurface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone


Seismic Methods


ENERGY SOURCE

DIRECT WAVESurface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone


REFRACTED WAVES

TRANSMITTED WAVES

ENERGY SOURCE

DIRECT WAVESurface

V = 10,000 ft/sec

V = 1500 ft/sec

Limestone

REFRACTED WAVES


TRANSMITTED WAVES

10 20 30 40 50 60 70 80 90 100

Direct Wave B First Arrival Geophones

SOURCE

V = 1500 ft/sec

V = 10,000 ft/sec

Seismic Methods


10 20 30 40 50 60 70 80 90 100

Direct Wave – First Arrival Geophones

SOURCE

V = 1500 ft/sec

V = 10,000 ft/sec

10 90807060504030200

10

20

30

40

50

60

70

80

90

100

110

120

Wave Signatures

Determine Subsurface Information

Seismic Methods


Time Travel

The time required for a wave to travel a given

distance is based on the velocity of the

geologic unit or units

Curve Plotting

By plotting arrival time vs. geophone distance

from source, velocities and depths to geologic

units can be determined

10 20 30 40 50 60 70 80 90 100 120110

SHOT POINT

V = 10,000 ft/secV = 10,000 ft/sec

Intercept time (ti)

Distance (in feet)

Crossover distance (XC)

1 2 7 8 9 10 116543

V = 10,000 ft/sec

V = 1500 ft/sec

12

Seismic Methods


Velocity Calculations B Curve Plotting

Geologic unit velocities and depths can be

derived from the time-distance plot using

mathematical or graphic methods

Mathematically-derived Velocities

Upper layer V =

Second layer V=

Third layer V =

xt

xt – t

x(t1 – t2)

Mathematically-derived Depths

•Intercept-time formula•Crossover distance formula

Seismic Methods


Intercept-time Formula

To determine depth to Layer 2, (Z1):

V2 – V1

V2 + V1(Z1) =

x2

Crossover Distance Formula

To determine depth to Layer 2, (Z1):

V1 V2

(V2)2 – (V1)2(Z1) =t i

2

( )

Intercept-time Formula

To determine depth to Layer 3, (Z ):

V3 V1

(Z2) = ½ t2 – 2Z1 (V3)2 – (V1)2 V3 V2

(V3)2 – (V2)2

Seismic Methods


Common Velocity Ranges

Sand and gravel (dry)Sand and gravel (saturated)ClayWaterSandstoneLimestoneMetamorphic rock

1,500 - 3,000 ft/sec2,000 - 6,000 ft/sec3,000 - 9,000 ft/sec

4,800 ft/sec6,000 - 13,000 ft/sec7,000 - 20,000 ft/sec

10,000 - 23,000 ft/sec

Reference: Bison Instruments, Inc.

Effectiveness of Seismic Refraction Method

Seismic Refraction

• Depth to saturated zone (groundwater)• Top of bedrock• Facies variation in an aquifer

Seismic refraction is used to determine:

Seismic Methods


Seismic Refraction (cont.)

• Mapping unconsolidated alluvial • Investigation of limestone or sandstone aquifer • Unsaturated consolidated deposits underlaid by

saturated consolidated deposits

Seismic refraction is generally used for:

Refraction Advantages

• Less expensive• Fast interpretation• Shallow investigations• Useful in a wide variety of geological

environments

Refraction Limitations

• Problems identifying• Drums• Thin beds• Velocity reversal

• Limited number of rock units identified• Complex data in steeply dipping formations• Poor lateral resolution of velocity variations

Seismic Methods


Seismic Reflection

• Acoustic energy encounters a boundary between two geologic layers

• If the acoustic impedance contrast is large enough some of the energy is reflected and the rest is transmitted

• Resolution of the thickness may be difficult for thin beds

Reflection Advantages

• No hidden bed problem• Less spread out arrays• Higher resolution

Reflection Disadvantages

• More involved to interpret• More expensive than refraction• Works only in special cases for shallow

investigations• Generally used for deeper investigations

(greater than 50 feet)

Seismic Methods


Seismic Equipment

Seismic EquipmentComponents

•Energy source

•Geophones

•Seismograph

Seismic EquipmentEnergy Sources

•Hammer•Weight drop•Shotgun slug (10 Gauge)•Explosives

Energy sources include:

Seismic Methods


Energy Sources Sledge Hammer

• Very Portable

Advantage:

• Strenuous operation• Limited Depth-20- 60 feet of unsaturated material-Maximum Depth ~ 100 feet below surface

Disadvantages:

Energy Sources Weight Drop and Shotguns

• Good depth range (100 - 300 feet)

Advantage:

• Not as field portable

Disadvantage:

Seisgun

Seismic Methods


Noise Sources

• Vehicular traffic• Construction zones• Wind blowing in trees• People walking

Seismic Equipment -Geophones

• Electromagnetic transducers that detect vibrations

• Placed into ground at appropriate spacing

• Spacing array determined by survey

Seismic line of geophones with cable

Seismic Methods


Geophone →

Cable connector

Seismograph

Stores and displays time when each

signal is received at each geophone

Geophone switch box

Laptop with seismic software

Cable to geophones

Seismic Methods


Seistronix control panel from laptop

Seismograph (cont.)

When energy source is repeated, the signals will be averaged or stacked.

This is done to improve the signal to noise ratio and to cancel out random noise.

Seismic refraction line interpretation

From MN DNR

http://www.dnr.state.mn.us/maps/geophysics.html

Seismic Methods


Tomographic Survey Lines

Tomographic Survey Diagram

Raw Seismic Data

Seismic Methods


Processed Data Plot

Target Revealed

Electrical Resistivity Method


ELECTRICALRESISTIVITYMETHOD

I

V

C1

(A)na

(M) (N)

P1 P2

a (B)

C2na

Module Goal

Determine the applicability of electrical resistivity in locating various targets

• Describe the method• Simple physics of resistivity

• Effects of the earth media

• Various arrays available – 1D, 2D, 3D and 4D

• Interpretation methods - models

• Describe advantages and limitations of the method

• Field and interpretation complications

• Operate the instrument

Module Objectives



Electric Flow in Geologic Materials

Flow controlled by the matrix mineralogy and the amount of moisture in interstitial pores

Terms

Coulomb – Fundamental unit of charge

Current – Charge/time (in amps) – I

Voltage – Work/charge (potential drop) – V

Ohm – Measurement of resistance ( friction) – R

Ohm's Law

V = IR

V = Potential difference between two surfaces of constant potential

Where:

I = Current in a conducting body

R = Resistance between surfaces

VR = I



V

T

C2P2P1C1 a a a

aA

Electrical Flow in Geologic Materials

Wenner Array

Apparent Resistivity ( app)

If earth were uniform, apparent resistivity would not vary.

Variations in resistivity can be interpreted as deviations from a uniform earth model.

Apparent Resistivity Calculation

Wenner: K = 2aSchlumberger: K = n(n+1)aDipole-Dipole: K = n(n + 1)(n+2)a

app = K VI



• Porosity

• Saturation

• Dissolved salts in groundwater

• Clay content

Resistivity Controlling Factors

• Most soil and rock minerals are electrical insulators with high resistivity (exceptions: clay, magnetite, graphite, pyrite)

Resistivity General Conditions

• Electrical current is transmitted through rock by ions in pore waters

• Low resistance contaminants may decrease apparent resistivity

Resistivity Method

Measurement of electric fields caused by current introduced into the ground as a means for studying the electrical resistivity structure of the earth



Resistivity Profiling

Fixed electrode spacing used to obtain horizontal changes in resistivity

Wenner

a a a

V

C1(A)

P1(M)

P2(N)

C2(B)

I

Resistivity Profiling

C1 P1 P2 C2

C1 P1 P2 C2

Measurement 1

Measurement 2



Profiling in Appropriate Terrain

Resistivity Sounding

Electrode spacing increased to obtain resistivity changes from successively greater depths (electric drilling)

Schlumberger

V

I

C1

(A)P1

(M)P2

(N)C2

(B)

na na



Resistivity Sounding

Measurement 1

Measurement 2

C1 P1 P2 C2

P1 P2C1 C2

Sounding in Appropriate Terrain

Dipole-dipole

I V

C2C1 P1 P2

(A) (B) (M) (N)

na aa



Dipole-dipole Data Collection

VI

1aa aC2C1 P2P1

VI

2aa aC2C1 P2P1

VI

3aa aC2C1 P2P1 P2

VI

4aa aC2C1 P1

Data Points fromDipole-dipole Array

Data Points fromDipole-dipole Array



Sting Resistivity Unit

Mini-Res Resistivity Unit

Summary of Surveys

1 D survey – Schlumberger, Wenner 2 D survey – Dipole-Dipole 3 D survey – multiple Dipole-Dipole or grid

of electrodes on surface 4 D survey – 3 D surveys carried out over

successive time periods



Modeling Examples

Forward Modeling 1 D inverse model 2 D inverse model 3 D inverse model

Data Chart 1 D survey

2.43% RMS

Electrode spacing (m)

Ap

par

ent

resi

stiv

ity

(oh

m-m

)

103

104

10

102

1

Ra

1 10 102 103 104L

1 D Model Chart

104

R103102

1

10

102

103

10–1

D

Resistivity (ohm-m)

Dep

th (

m)



2 D Inversion model

INVERSE MODEL RESISTIVITY SECTION

0.5

6.5

2.9

9.0

12.2

16.1

44.0 82.2 536154 287 1001 1869 3491Unit electrode spacing 3.0 m.Resistivity in ohm.m

LIMESTONECAVERN

SOIL3.06.0

9.0

2 D Forward and Inverse Models

• Quantitative modeling is possible

• Can estimate depths, thicknesses, and resistivities of subsurface layers

• May estimate resistivity of saturating fluid

• Finds void locations

Electrical Resistivity Advantages



• Cultural noise

• Large, clear area required

• Labor intensive (2- to 3-person crew)

• Lack of resolution in some cases

Electrical Resistivity Limitations

• Media

• Target

• Contrast

Can Target be Resolved?

• Buried stream channels

• Freshwater/saltwater interface

• Depth to groundwater/bedrock interface

• Cavities/sinkholes

Resistivity Targets

Borehole Geophysics


Borehole Geophysics Applied

to Bedrock Hydrogeologic

Evaluations

Don Bussey, CPG-08847USEPA/ERT – Las Vegas, Nevada

[email protected]

Borehole Geophysical Tools:

• Natural Gamma• Temperature• Caliper• Conductivity/Resistivity• Borehole Video• Heat-Pulse Flowmeter• Optical and Acoustic Televiewer• Borehole Deviation• Casing Collar Locator (Magnetic)• Cement Bond Log (Sonic)

Borehole Geophysics


Natural Gamma and Temperature Logging

Gamma logging is useful in evaluating stratigraphic sequences and for borehole to borehole correlation.

Temperature logging can aid in detection of groundwater flow in or out of a borehole.

Borehole Geophysics


Caliper Logging

Caliper logging measures borehole diameter, useful in detecting fractures or voids in open-hole bedrock boreholes.

Borehole Geophysics


Electrical Conductivity Logging

Resistivity/Conductivity Logging

12

34

Borehole Video Logging

Borehole video logging provides a visual picture of borehole conditions.

Useful in identifying fractures, voids, cascading water, well/boring blockage and other downhole trouble shooting.

Borehole Geophysics


Borehole Video Log

Borehole Geophysics


Heat-pulse Flowmeter logging is used to measure vertical flow within a well at discrete vertical intervals (> 0.1 gpm).

Useful in determining depths where water may be entering or leaving a borehole.

Heat-Pulse Flowmeter Logging

Optical and Acoustic Televiewer Logging

• Televiewer logging presents a 360-degree acoustic or optical digital borehole representation.

• Useful in evaluating fractures, bedding, and voids.

• Strike and dip of fractures can also be calculated.

Borehole Geophysics


Formation Gamma Caliper ATV OTV Resistiv Temp Flow

Borehole Geophysics


Virtual Core Using Optical Televiewer Data

Borehole Deviation Logging

• Useful to determine borehole deviation

• Useful to evaluate whether packer assemblies can be utilized downhole

Borehole Geophysics


Well Depth >700 feetWell Base Elev. Difference650 feetDeviation 143 feet0.215 ft/ft

Borehole Geophysics


Oil & Gas Well Abandonment ApplicationsCasing Collar Locator and Cement Bond Logging

Used in the oil patch during oil and gas well abandonment.

Casing Collar logs (magnetic) used to identify casing collarsfor targeting during casing shoot offs.

Cement Bond logs (sonic) identify presence of cement behindlogged casing – useful during casing perforating.

Cement Bond Logs important in Underground Injection wellcertification.

Borehole Geophysics


Borehole Geophysical Data - Uses

• Groundwater Sampling Strategy

• Packer Test Design

• Discrete-zone Multi-level Assembly Design (Westbay, Flute, Solinist , etc.)

Groundwater StraddlePacker Testing

Borehole Geophysics


Acoustic and Optical Televiewer Logs

Heat-Pulse Flowmeter Logs

Case Study Interpretations

Borehole Geophysics


Santana Community Production Well

Corozal, Puerto Rico

Cayuga County Groundwater Contamination Site

Cayuga County, New York

Borehole Geophysics


Geophysical, Stratigraphic, and Flow-Zone Logs EPA-1

LithologicL

Formation Gamma Caliper Acoustic Optic Resistiv Temp Flow

Ground water at the monitor wells in the Onondaga Limestone

flows NW and NE640

635

630630

Ground water at the EPA test wells in the Bertie Fm.

flows South then SW

560

555

550

545

Borehole Geophysics


Conclusion

Know Your Borehole !

Borehole Geophysics can help Understanding Geology, Hydrogeology, and Chemistry in Bedrock

Geologic Settings

Don Bussey, CPG-08847

USEPA-ERT/Las Vegas

[email protected]

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Ground‐Penetrating Radar


Ground Penetrating

Radar

What is GPR?

Technique to collect & record information about subsurface

Non-intrusive method for near surface zones

Provides most detail of all geophysical methods

Many parameters involved to apply method optimally Credit: GSSI & SERDP

GPR Basics Acronym for Ground

Penetrating Radar

“Ground” can be soil, ice, rock, concrete, water, wood - anything non-metallic

Emits a pulse into the ground

Records echoes

Builds an image from the echoes



GPR Similar to Fish Finder -Echo Sounder Processes

Sends pulsed focused signal

Signal scattered back from obstacles in path; fish, bottom

Result: single record has 2 blips at different timesCredit: Sensors & Software

Fish Finder & Echo Sounder Decoding Processes

Continuous data collection

Recordings displayed side by side

Result: a 2D cross-section of the subsurface

Credit: Sensors & Software

Where Similarities End Between GPR & Echo Sounder

Processes

Electromagnetic pulse

Electromagnetic wave propagation

Limited multi-matrix applications

Analyzes reflections

Acoustic pulse vs

Sound wave vspropagation

Only for aquatic vsconditions

Analyzes reflections

GPREcho Sounder



Typical GPR System

Digital video logger

Transmitter & Receiver antenna

Odometer controlled

GPS

GPR Theory

GPR: A Wave-Based Technique

• Wave energy travels at a characteristic wave speed

• Dependent on the material through which it travels

• This is the main difference between GPR and EMI.



Frequency refers to how many waves are made per time interval. This is usually described as how many waves are made per second, or as cycles per second.

The wavelength of a wave is the distance between any two adjacent corresponding locations on the wave train.

Wave Properties

Wavelength

Time

What Are Nanoseconds? GPR time is measured

in units of nanoseconds

1 nanosecond is 1 billionth of a second =1/1,000,000,000 second

GPR signals travel 1 ft (0.3m) in air in 1 nanosecond

ns is the abbreviation for nanosecond

Electromagnetic Spectrum

Top ScaleWavelength in Meters

Bottom ScaleFrequency in Hertz

GPR = 10 to 1000 MHz range

10-610-810-1010-12 10-4 10-2 1 100 104

X-Rays

106108101010121014101610181020

TV/RadioVisible

GPR

(GHz) (MHz)

104

106 108

100 1

EMI

(KHz)

Short wavelength

High frequency

Long wavelength

Low frequency



The Process Begins

Data collected along closely spaced transects within a grid

An active process of transmitting EM pulses from surface antennas into the ground

Measures time elapsed between when pulses are sent and received at the surface

Pulses transmitted through various matrices, target features, cause velocities to change

Changes in velocities plotted to create image

Transmitted Pulses & Why Their Velocities Change

Physical & electrical attributes of a medium control how fast electromagnetic waves travel

Physical attributes Moisture, density, porosity, chemical properties

Electrical attributes affecting energy propagation Relative Dielectric Permittivity (RDP)

Controls wave speed

Electrical conductivity Signal attenuation

Reflection Strength

Relative Dielectric Permittivity (RDP)

Also called: dielectric constant

Measure of ability of a material to store a charge from an applied EM field and then transmit that energy

The greater the RDP, the slower radar energy will move through it

Typical RDP Values (k)Air 1Ice 3-4Dry Sands 4Granite 5Limestone 6Saturated Sands 25Silts 5-30Clays 5-40Water 81



Electrical Conductivity

Ability of a material to conduct electric current

Conductivity increases with increase in water and/or clay content

Higher conductivities limit depth

Conversion of EM energy to heat

BoreholeConductivity

Data

Reflection Strength(2 Layer Model)

k 2 - k 1

k 2 + k 1

r =

k 1 = relative dielectric permittivity of first layer

k 2 = relative dielectric permittivity of second layer

Reflection Strength

r = 0 to 0.2 weak reflections

r = 0.2 to 0.3 moderate reflections

r = greater than 0.3 strong reflections

Metal reflects nearly 100% of a radar wave



EM Energy Transmission

• Dielectric mediums allow passage of significant electromagnetic energy without dissipating energy

• The more electrically conductive a material:– the less dielectric it is – energy will attenuate at a much shallower depth

• In highly conductive mediums – the electrical component of the propagating electromagnetic wave is

conducted away in the ground– when this happens the wave as a whole dissipates

• For propagation to occur the electrical and magnetic waves must constantly "feed" on each other during transmission.

Conjoined Electrical & Magnetic Waves

Credit: SERDP

Types of Factors Limiting Radar Penetration

Minerals in medium creating free ions & increase electrical conductivity

Sulfates, carbonate minerals, iron & salts or any charged elemental mineral = high conductivity

Iron rich soils have high magnetic permeability

Radar energy will not penetrate metal, it is reflected 100%

Pulse & Echo Traverses Through Soil Matrix & Target

AirDrySoil

Drum

WetSoil Clay

Multiple Echo Pulses From Target

Single Transmitted Pulse

Tx & RxAntenna

At Ground Surface

From Ground Surface to Increasing Depths



Transmitted Pulses Have Frequency Options

Operator has choices to select limited range of frequency options

Each frequency option has trade-offs

Higher freq. = better target resolution

Lower Freq. = greater depth of penetration

Antenna Characteristics

Frequency(MHz)

250

500

1000

Depth(feet)

5-45

1.5-12

0-1.5

Resolution(feet)

0.5

0.3

0.05

Antenna Frequency

(MHz)

Wavelength Frequency (meters)

Wavelength in medium

with RDP=5 (meters)


with RDP=15(meters)


with RDP=25(meters)

1000 0.3 0.13 0.08 0.06

900 0.33 0.15 0.09 0.07

500 0.6 0.27 0.15 0.12

300 1 0.45 0.26 0.2

120 2.5 1.12 0.65 0.5

100 3 1.34 0.77 0.6

80 3.75 1.68 0.97 0.75

40 7.5 3.35 1.94 1.5

32 9.38 4.19 2.42 1.88

20 15 6.71 3.87 3

10 30 13.42 7.75 6

Examples of Differing Frequency & RDP Options

Credit: SEDRP



Antenna Frequency

500

1000

250 MHz Credit: Sensors & Software

Viewing Data

What Creates Reflections? Changes in EM impedance associated with

material property variations Impedance reduces to resistance in circuits carrying

steady direct current

Changes in material property: density, porosity, texture

Changes in physical properties: dielectric permittivity, electrical conductivity and magnetic permeability

Greater changes in properties, the more signal reflected



Reflection pulse from ground surface

Attenuated reflections

Individual Waveform Reflection or Reflection Trace

Composed of 512 Digital Samples

Credit: SERDP

A composite of many wavelets recorded from many depths produces a series of reflections generated at one location, called a reflection trace

Many bed boundaries & other discontinuitiesreflect a wavelet of

energy to the surface & recorded

Two-Way Travel Time

Amount of time for the radio wave to make round-trip from the surface down to the reflector and back

Greater for deeper objects

Can be converted to depth if velocity is known

Measured in nanoseconds

Frequencies & Interface Reflections

1. Small λ at top (A) & bottom (B) produces a reflection, composite reflection trace of the two (C) can define both interfaces

2. Longer λ barely has enough definition from the top & bottom (D & E) to produce a composite reflection trace (F) that exhibits both interfaces

3. Low λ produces a wave reflecting off both interfaces (G & H), but composite reflection trace affected by constructive & destruction interference of two waves, only top interface is visible in composite reflection trace (I)

1. 2. 3.

λ = wavelength

Credit: SERDP



GPR Record Traces Widely Spaced

Distance along ground surface

Tw

o-W

ay T

rave

l Tim

e

GPR Display/RecordClosely SpacedDistance along surface

Two-

way

trav

el ti

me

Dep

th

GPR Energy Radiation Energy from GPR is NOT a pencil-like beam

Footprint size varies as a function of: Relative dielectric permittivity of material

Antenna Frequency

Credit: SERDP

Elliptical Cone



RDP Affecting Energy Radiation

Radar energy Tx cone becomes focused while traveling thru layers of increasing RDP, common in most soil conditions

Soil Profile Cross Section

Credit: SERDP

Example of RDP

Increasing with

Depth

Example of Energy Radiation

Cone of Tx cone becomes broader with depth when RDP is low

High RDP matrices - Tx cone is narrower

Credit: SERDP

Energy Focusing & Scattering

a) large area presented, most energy directed back

b) target presents small cross-section and scattered signal is not directed back to Rx

Credits: SERDPLarge Features

Small Features



Energy & Antenna Coupling

A Street inSouthernPortugal

• Cobbles had poor coupling properties, only propagated to 40-50 ns

• When antennas crossed onto asphalt (A) coupling improved

• Reflections visible at 60 & 90 ns at right are barely visible (B)

• What would happen moving from saturated soils to large paved parking lot?

Rounded Target Signatures

• Conical projection of energy allows waves to travel in oblique direction to buried point source (1) as seen in (A)

• 2-way time (t) recorded & plotted in depth below the antenna where it was recorded (2)

• As many reflections are recorded when antennas move to and from buried object, the result is a reflection hyperbola (3), when all traces are viewed in profile as seen in (B)

GPR Suitability Map



Examples & Applications

GPR Applications

Mapping near subsurface geology- Bedrock- Water Table- Faults and Fractures

Locating cultural objects- Drums and Tanks- Landfills and Pits- Contamination - plumes

GPR Unit Options

Credits: Sensors & Software



Underground Storage Tanks


1 Cross-Section Slice

Post Processed PlanView Time Slices


Geophysical Survey Systems, Inc.

Tim

e, n

s

0 -

40 -

Tank Tank

500 MHzMaine

- 0

-3

-9

-6 Dep

th, f

eet



Trench with Drums

Tim

e, n

s

Geophysical Survey Systems, Inc.

0 -

120 -120 MHz

Contaminated Groundwater

0 200Position, m

Original survey

Five years after remediation

0 -

600 -

0 -

600 -

Tim

e, n

sT

ime,

ns

Sensors & Software Inc.

0m

10m

20m

Water Table Mapping

0 75Distance, feet

0 -

400 -

Tim

e, n

s

Glacial sand and gravel deposits near Lake Superior, Ontario, Canada, 100 MHz, Sensors and Software, Inc.

Bedrock

Water tableDry sand & gravel

Wet sand & gravel

- 0

- 15

De

pth,

m



SinkholesT

ime,

ns

0 -

900 -

Distance, feet0 31

80 MHz GPR data showing developing sinkhole, Florida. GSSI

Sinkhole at Winter Park, Florida

Saltwater Intrusion

Freshwater constantly recharged by rainfall producing positive pressure head, forcing freshwater down & out to shoreline

A boundary between fresh & saline water occurs & is often abrupt

Credits: Sensors & Software

Water-borne GPR

Control Unit

Antenna



Post Processing GPR Data

Survey Grid

Y-Axis

X-Axis

Series of GPR Profiles

Grumman Exploration



3-D GPRGround Surface

Water Table

Sand and Gravel

Bedrock

Gravel lenses

Sand and Gravel

Grumman Exploration

Marquette, MI30 m by 6 m area6 - 8 m depth

3-D GPR

Time Slices

GPR dataset from Forum Novum site in the Tiber Valley, Italy.

Site is a Roman market place and church that were built in the 2nd century A.D.

GPR time slices revealed buried walls and foundations from the ancient Roman buildings.

Dean Goodman



Gas Station-Petroleum

Product

Elec.Lines Fuel

Piping

PumpIslandPad

Jeff Daniels and Dave Grumman, OSU

Time Slices

Color Assignment

Relative Amplitude3,000-3,000

Creosote Pit0

100

0

11.5

North

Creosote -filled pit

Estimating Target Depth



Depth Calibration:3 Methods to Measure Depth

Estimate matrix method

Depth to known target

Point target hyperbola matching

Depth Calibration:Method 1- Matrix Estimate

Measure travel time

Need material speed

Depth = velocity x time / 2

How …….?

Method 1 Matrix Estimate

Material Velocity (ft/ns)

Air 1.0

Ice 0.56

Dry Soil 0.43

Dry Rock 0.39

Moist Soil 0.33

Concrete 0.33

Wet Soil 0.22

Water 0.11Bet

ter D

epth

Cap

abili

ties



Method 2 Depth to Known Target

Known depth

Adjust velocity using inst. controls

Method 3 Point Target Hyperbola Matching

Benefit wide beam Localized features Hyperbolas (inverted

U’s) Shape determines

velocity Inst. can curve match


Dean Goodman




Adjust shape

Determines velocity

Example of shape fitting to a target response on a Sensors & Software PulseEKKO field monitor.


Optional Quick & Easy MethodEstimating Exploration Depth

Depth =35

meters

= conductivity in mS/m



Planning or Reviewinga GPR Survey

Two Methods to Collect Data

• Radom Method

• Grid or Systematic Method

Radom Data Collection Method

Systematic Grid Data Collection Method

Y-Axis

X-Axis



Survey Design

Proper design of GPR surveys is critical to success.

The most important step in a GPR survey is to clearly define the problem.

There are 5 fundamental questions to be asked before deciding if a radar survey is going to be effective.

EquipmentVariables1,

Constraints2

Composition2

Electrical properties

Water Content2

Signal Absorption2

Shape, profile2

Frequency1

Energy Radiation

Focusing & Scatter2

Energy Transmission2

Ground Coupling2

MediumConstraints2

1: What is the Estimated target depth?

The answer to this is usually the most important

If the target is beyond the range of ideal GPR conditions, GPR can be ruled out

?



2: What is the target geometry?

Most important target factor is size

If target is non-spherical, target orientation should be qualified

•height•length•width•strike•dip•etc.

3: What are the target electrical properties?

What is relative dielectric permittivity (K) and electrical conductivity ( ) of target?k

4: What is the host material?

Electrical properties of the host needs to be defined

Need contrast in electrical properties with host environment

Variations of electrical properties in the host material can create noise

k



5: What is the survey environment like?

GPR sensitive to surroundings

Extensive metal structures

Radio frequency EM sources & transmitters

Site accessibility

GPR Summary Reflection technique which uses radio

waves to detect changes in subsurface electrical properties

Limited exploration depth in conductive soils

GPR provides the highest resolution* of any surface geophysical method

The most important step in a GPR survey is to clearly define the problem

Disclaimers

Disclosure of product names in this report is not an implied or direct recommendation of the equipment used for this survey. It is only provided for its scientific value related to a specific method or tool used.

SERDP citations or credits refer to Strategic Environmental Research & Development Program, a Department of Defense Environmental Research Program.

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