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Geophysical Survey Design
October 2007
Global Positioning Systems
Accuracies vary by
method & equip, used
Some on a scale to
locate an airport
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
3 Grades of GPS accuracy
- Recreational, mapping, survey
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Geophysical Survey Design
October 2007
How a Differential GPS Service Works
Fixed Bases
Ground Based Local Positioning &
Data Collection System
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Geophysical Survey Design
October 2007
System Overview
Laser beam tracking
Line-of-site system
Merges & stores
- Total station data
+
- Geophysical data
or
- Radiological data
Positioning options
guidance or tracking
Real-time displays
Auto Tracking & Guidance
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Geophysical Survey Design
October 2007
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
2 Screen Views of HUD
Blue Arrow shows
direction related to X-
axis baseline
Coordinates
Target path line
Current path
Distance L/R line
Data readouts
Recording indicator
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Geophysical Survey Design
October 2007
Screen Color On Rover's HUD Has
Meaning
4.0
0.7
4.0
24.5
21.7
24.5
White:
Normal
Yellow:
Near End
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.)
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Geophysical Survey Design
October 2007
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?
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Geophysical Survey Design
October 2007
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
October 2007
Metal Detector ^ Magnetic
Method
pholo credit Wikipedia
METAL DETECTORS use internal power to create a electromagnetic field to locate metal
MAGNETOMETERS are passive instruments and only sense ambient magnetic fields
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
The Magnetic Method
Iron
Senses presence of iron
Measures magnetic fields
Easy to apply and interpret
Ferrous & Non Ferrous Metals
ium
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Ferrous & Non Ferrous Metals
Why Is Magnetics Important?
Non-invasive, passive detection method
Quantitative results
Large masses detectable at significant depths
Complements other geophysical methods
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Optimal Detectable Features
Unique to Magnetics
Buried drums, tanks, pipes, valves
Steel casing (abandoned wells)
Mixed ferrous wastes (landfills)
Steel reinforced foundations
Fired clays (bricks, clay pots)
Natural occurring ferrous minerals
What Tools are Used to
Measure Magnetic Fields?
Instruments called
magnetometers
Several types &
configurations
available
Measures strength of
magnetic intensities
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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
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Magnetic Methods
October 2007
Earth's Magnetic Background
Strongest
B
Moderate
Weakest
Ferrous Interactions
Ferrous metal has
its own magnetic
field
Capable of altering
Earth's field
Limited influence
Easily measured
Provides accurate
location method
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Measurement Units
Units measured in gammas or
nano Teslas
1 gamma = 1 nano Tesla
- 55 gallon drum lid about 40 y or nT
- 250 gallon tank about 1000 y 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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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: Geometries
Gradient Configuration: Two
Sensors
Intensity measured
from two sensors
Background is
defined as "0"
Anomalies: > or <
than background
Solar activity will not
influence data
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Magnetic Methods
October 2007
Gradient Configurations:
Adjoining or Remote
Option A
Gradient
Mode
Option B
Base
Station
Mode
Gradient Readings
Total field (bottom sensor) minus
vertical gradient (top sensor) noted as y
or nT per unit of distance between
sensors
55,900 - 55,200 = 700 y /meter or nT/M
Negative values are also possible
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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.sel.noaa.gov/today.html
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Gradient Measurements
(Vertical Gradient)
55,000
0 gamma
Typical
Background
700 gammas
Typical
Anomaly
700 gammas
Anomaly With
Solar Disturbance
Cesium Magnetometer
Ionizing light "pumps"
elections to higher
energy levels
Magnetic fields affect
rate energy gain/loss
Constant "pumping"
allows continuous
data acquisition
Accuracy of .1 gamma
(detect several nails)
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Cesium Mag Measurements
More Energy Emitted In Strong Ambient Field
Ionizing
Light
Cesium
Vapor
Chamber
Photocell
Less Emitted In Weak Ambient Field
(modified from Bloom, 1960)
Alkali Vapor
Sensor
Orientation
Tilt = parallel plane to direction of travel
Rotate = perpendicular plane to direction
of travel
Signal values [green] lets than 15* wteale non-ooeiabte condition
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Detection Probability
100
98
90
75
50
(modified from Benson et al., 1988)
. ป.
At'= Area ofjTarget? -~..^
'' ~ ''
';, As.=' Area of-Site" , :>-
:' -'"143,560' ' ":-. '''',,
Probability
of As/At As/At As/At
Detection =10 =100 =1000
'13'";
10 *
's '
,;, -160
'M30
100
-so
50,
1600-
1300-
1000
800
500
Number of data points required
Determining Grid Spacing
Area of Site in ft:
= ainft:
Area of Target in ft2
a x Probability Factor = Approx. Sampling Points
Area of Site in ft2
Sampling Points
- Grid Spacing in Feet
Probability Factors
100% =1.625 75% = 0.8
98% = 1.3 50% = 0.5
90% = 1.0
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Magnetic Methods
October 2007
Continuous Data Acquisition Issues
Operator inputs start
& end points per line
Unit auto "fits" data
to input distance
- Assumes same pace
Obstacles usually
slow pace
12
pts
Even
pace:
real
Linel
9
pts
9
pts
Reality
Vs
Processed
Off
pace:
real
Line 2
Posted
Line 2
Establishing A Grid
Layout grid markers
at desired spacing
- Flagging (plastic)
- Spray chalk or paint
- Ropes
- Alignment placards
- Wooden stakes
Large sites require
multiple marker lines
Site
boundary
Tapes &
markers
\\
Traverse
directions
0,0
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Using GPS For Grid
Establishment
Initialize start & end
points of line
GPS maintains
parallel lines
Operator follows
cursor on lightbar
Lat. - Long, output
to mag data
' - I M
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Random Survey Using GPS
Used for large areas
Maximize productivity
Data linked to GPS
Best in obstructed areas
Areas must be free of:
- Vegetative canopies
- Tall buildings
- 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)'.' .
Timber Pile
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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
Data
Values
Location over
target effects
data
Strongest
values closest
to target
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Data Signatures
Pt. A
Pt. B
Pt. C
measurement surfaces
Pt. D
N-N
rN-N- rS-S
rN-s = rs.N
= but opposite forces exist
symmetrically ฎ Pt. A & Pt. B
Pt. E
N-N
measured field
everywhere negative
strong positive field Pt. D
weak negative field Pt. E
Alan WWen
Contoured Data Signatures
Anomaly #
1
2
3
4
2
1
1
Signatures - 0ฐ
Depth Dipole Moment
Magnitude Horizontal A Vertical Z
1 150 -135 0
1.5 500 0 0
2.5 1500 90 -15
2 400 0 -90
4 ,.-. 4
(ii) ฎ
v^sSE^y
dash = neg.
solid = pos.
f \ "ป
';-ฉ) v(O)';'- )
'v^^ -^s *-*'
Credit: Alan Winen
Dip Angle Signatures - 75ฐ Dip Angle
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Magnetic Methods
October 2007
Estimating Target Depths
d = Depth
_ i . Axy2 = Half maximum value
/i~ h = Distance above ground surface at
which measurements are made
% Max. d
a) 2.4 m 3.12 m
b) 1.0m 1.30m
c) 1 .6 m 2.08 m
ฃ ^N. / **
.ซ,..__,!_^i>Jr,ir.,!r^r4 .ซ,
fa)
_A^L/V
gQtfl J _^ST
ft) <<=)
Credit: Man Witten
Depth Estimate Calculation
From Contour Map
Solid & open circles are
locations of max. value &
1/2 max. value: 3.6m
Contour interval 20 nT
Target = horiz. metal bar
- Depth: actual = 5m
- Depth: est. = 4.68m
1G.OO 18.CO 200C 22.ffi) 2<.HJ 2S.3& 28 E> 33.0D 3ZOC 3
DisBncetKl(jn)
Credit: Alan WWen
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Magnetic Methods
October 2007
Another Depth Estimate
Solid & open circles are
locations of max. value
& 1/a max. value: 1.8m
Contour interval 20 nT
Target = vert, metal bar
- Depth: actual = 3m
- Depth: est. = 2.34m
horizontal orientation = 90 degrees
vertical orientation = 90 degrees
~ :
39.00
.
Wti
35 00 35 00 37 CO 3B.OU J&.OQ ซ.OC <1.0C ซ OC 43 DO -U PC 45 X>
Credit: Alan Witten
Mass & Depth Examples
X-Section - Buried Targets
Data Plots of Buried Targets
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Multiple Magnetic Sources
Pavement - Concrete & Rebar
Soil
Buried
Waste
Drum Mass = Rebar Mass: Difficult to Distinguish
Drum Mass > Rebar Mass: Easier to Distinguish
Dealing With Noise Issues
Accounting for un-
wanted Interferences
- Power lines, fences, cars
Apply a "walk-away"
- Start at source
- Walk-away until readings
normalize - note distance
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
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
\
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
Contour Map
10
0
Feet
10
10
10
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Example 1 Source
Mag Anomaly Example 2
5 Crushed drums
Depth: -5' to -6'
Values:+78 to-171
Contour interval: 35
Blue: pos. values
Reds: neg. values
Contou
Map
20
10
10
20 10
0 10
Feet
20
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Example 2 Source
Mag Anomaly Example 3
1 Drum (horizontal)
Depth: -3' to -6'
Values: +111 to -572
Contour interval: 35
Blues: pos. values
Reds: neg. values
Contour Map
10
10
10 0 10
Feet
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Mag Anomaly Example 4
2 Iron pipes: 10'x4"
Depth:-1.7'to-2'
Values: +129 to-238
Contour interval: 35
Blues: pos. values
Reds: neg. values
Contour Map
20
10
10
20 10 0 10 20
Feet
Mag Anomaly Example 5
Two 500 gal. tanks
Depth: -2' to -7'
Values:+1114,-120
Contour interval: 35
Blues: pos. values
Reds: neg. values
20
Contour Map
10 0
Feet
10
10
10
20
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Tank Removal
In-Situ
500 Gallon Tanks
Mag Anomaly Example 6
0 5 10 15 20 25 30 35 4[ 45 ฃ0 55 60 65 7] 75 BO 95 90 95 1301C5I10
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Magnetic Methods
October 2007
Example 6 Tank Removal
Mag Anomaly Example 7
o
Approximate Anomalous Area
Marked For Excavation
Grid 1 Point D
Grid3 Point J
Grid Area 5
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Mag Anomaly 7 Removal
Environmental Anomaly
Comparisons
1) 1 crushed drum
-4.5' to -8.5' depth
2) 5 crushed drums
-5' to -6' depth
3) 1 whole drum horiz.
-3' to -6' depth
4) 2pipes10'x4"
-1.7' to -2' depth
5) 2 tanks 500 gal. ea.
-2' to -7' depth
Value Range in Gammas
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Confirmatory Methods
for Magnetics
Magnetics
Rapid Data Collection
Establish Amount of Mass
General Lateral Dimensions
GPR
Depth to Target
Top of Target Shape
(dependent on soil conditions)
Electromagnetics
Detailed Lateral Dimensions
Generalized Depth Information
(dependent on Tx & Rx range)
Marine Cesium Magnetometer
Towed by boat
X-Y location
control by GPS
Depth control by
line & speed or
floatation device
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Marine Applications
Lake George
Channel
Indiana Harbor
Canal
Looking south
Indianapolis
blvd. bridge
Mar
ine Cesium Magnetometer Data
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Magnetic Methods
October 2007
Search for CSS Hunley Using
Magnetics
Towed Marine
Magnetometer
Cross-Section 5
Outer Charleston Harbor
Sea Floor JBKSSJW
6'
CSS H. L. Hunley- USS Housatonic
Inside the Hunley
2. 3. 4. 5. 6. 7. I. 9.
I. Rear Balla.i Tank
2. -3. III Officer's Positions (Cronk.Bollo si Tank)
t.-1. Ctew PoiitiorulCranksJinf!)
.
10. Bellows Equipm*nt
II. Captain's Position (Holm, Ballast tank)
12- Fotwoid Ballon Tank
in+b
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Battle Site Mag Anomalies
Planned Lines and Data Points
A, B, C Courtesy: Submerged Cultural Resources Unit - National Park Service - Santa Fe, NM
Gradient Magnetic Data
ConlOiif inwrrval 2
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Magnetic Methods
October 2007
Advantages
Magnetics ideal for ferrous targets
Method fast, simple and easy to interpret
Targets identified by area and mass
Use of other geophysical methods can
increase value of magnetic data
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
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Electromagnetic (EM) Methods
October 2007
ELECTROMAGNETIC
(EM) METHODS
Module Goals
Describe electromagnetic methods in
general
Explain the differences between the two
major types of electromagnetic
instrumentation
Describe the application of the two
types in the field of Environmental
Geophysics
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
Time Domain EM (TDEM)
Square Wave signal - Variable Depth
Conductivity at depth
"Metal Detection
Frequency-Domain EM
T
R
Varying electric field
1
Varying magnetic field
Eddy currents
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
Frequency-Domain EM
R
varying magnetic field
Depth of Penetration
~1.5 x coil spacing for vertical dipole
-.75 x coil spacing for horizontal dipole
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
FDEM Signal Components
The secondary magnetic field has two
components
Quadrature phase - used to measure ground
conductivity - 90ฐ out of phase with primary
field
In-phase - used to detect excellent conductors
(metal) -180ฐ out of phase with primary field
EM-31
~ 4.5 meter maximum depth
Soil conductivity - quadrature phase
Metal detection - in-phase component
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS-
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Electromagnetic (EM) Methods
October 2007
EM-34
Three coil spacings -10m., 20 m., 40 m.
Soil conductivity - quadrature phase
Coil spacing - in-phase component
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
Gem-2 and 3
Multi-frequency signal
Variable depth of investigation
Output is secondary magnetic field (ppm)
to the primary magnetic field
Gem-2
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
Frequency Domain EM
Case Studies
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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-------
Electromagnetic (EM) Methods
October 2007
Apparent
Conductivity
Data
Inphase
'Shaded
, Relief
Map
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
P5*~~
Livestock
, Feeder
Overhead
Powerltne
X
US IปA FJiYlKONMENI AL RESPONSE Tt AM CENTER
RESPONSE ENGINCERING AND ANALYTICAL COOTRACT
M-CW-223
WORK ASSIGNMENT NO. R1AOOI76
ROUW4
Expanded View o( Northern Grid Aral
EM31 Terrain Condtictn u> Dau, I -inc
And OOTphvsioil Anonulv Numbers
BOY1XS a
-------
Electromagnetic (EM) Methods
October 2007
EPA Gem2 data
Northridge, IL
21030 Hz
In phase data
Time Domain EM (IDEM)
R
PRIMARY
MAGNETIC FIELD
DECAYING SECONDARY
MAGETIC FIEL
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
IDEM
T
R
SURFACE
TIME
AND
DEPTH
IDEM Metal Detector
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
DEM Metal Detector
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
IDEM 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 IDEM 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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
Telephone pole
Telephone pole
Underground storage tanks
Abandoned Gas Station, East St. Louis, Illinois
0 30
Scale
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Electromagnetic (EM) Methods
October 2007
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
SEISMIC METHODS
Seismic Refraction
Seismic Reflection
Seismic Methods
Acoustic energy induced in the ground
Refraction relies on increasing acoustic
velocities to refract energy
Reflection relies on velocity contrasts to
reflect the energy
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
\
October 2007
Environmental Seismic
Methods
Shallow targets
Simple geometry/geology
Generally only P waves (compressional
wave) used
Seismic Refraction
Acoustic energy (wave) encounters a
boundary between two geologic layers
If the velocity is higher in the lower layer,
some energy is reflected and some is
refracted upward
If the velocity is lower in the lower layer
the layer is "hidden" from the refraction
method
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
First Arrival
SOURCE
Geophones
20 30 40 50 60 70 80 ^,90 100
Refraction Equipment
Seismometer - instrumentation
Geophones - acoustic sensors
Source - acoustic energy source
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
Wave Signatures
10 20 30 40 50 60 70 80 90
Common Velocity Ranges
Sand and gravel (dry)
Sand and gravel (saturated)
Clay
Water
Sandstone
Limestone
Metamorphic rock
1,500-3,000 ft/sec
2,000-6,000 ft/sec
3,000-9,000 ft/sec
4,800 ft/sec
6,000-13,000 ft/sec
7,000-20,000 ft/sec
10,000-23,000 ft/sec
Reference: Bison Instruments, Inc.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
Seismic Refraction Uses
Depth to groundwater
Top of bedrock
Mapping unconsolidated alluvial deposits
Rippability
Determination of rock types from seismic
velocities
Refraction Advantages
Rapid data collection
Simple field procedure
Fast preliminary interpretation
Useful in a wide variety of geologic
settings
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS-
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Seismic Methods
October 2007
Refraction Limitations
Velocities of layers must increase
Poor resolution for simple surveys
Complex interpretation in dipping
formations
Lateral velocity variations complicate
interpretations
Weathered layer absorbs acoustic energy
and is hidden
Seismic Reflection
Acoustic energy encounters a boundary
between two geologic layers
If the contrast is high enough some of
the energy is transmitted and some is
reflected
Thickness of the layer determines if it is
detected or "hidden"
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
ENERGY
SOURCE
Seismic Reflection Equipment
In most cases identical to refraction
equipment
Geophone arrangement may be different
Data is taken from later in the seismic
record
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
Seismic Reflection Uses
Subsurface geometry/geology
Finding faults and intrusions
High resolution mapping of beds
Seismic Reflection Advantages
No problem with low velocity layers
Better resolution of thin beds
Higher resolution overall
Deeper imaging with same source
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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Seismic Methods
October 2007
Seismic Reflection Limitations
More complex to interpret
May be more expensive than refraction
Works only in some environments
Generally for deeper investigations
High resolution requires high frequency
signal
Acoustic Velocity Logging
Downhole seismic technique
Used for fracture studies and stratigraphic
determinations
Very high resolution
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
10
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Seismic Methods
October 2007
Crosshole Seismic
Three dimensional imaging
Velocity and stress determinations
Very high resolution
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
11
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What is GPR?
October 2007
What is GPR?
acronym for Ground
Penetrating Radar
ground can be soil, rock,
concrete, wood
- anything non-metallic
emits a pulse into the
ground
records echoes
builds an image from
the echoes
GPR is Just Like a Fish Finder
& Echo Sounder
sends out a ping
signal scattered
back from fish
signal scattered
back from bottom
in this example a
single record has
2 blips at different
times
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
Ground Penetrating Radar
Electromagnetic
technique
Same principles and
theory as radar used
to detect aircraft
Sensitive to changes
in electrical properties
A Little History!!
First GPR survey
was performed in
Austria, 1929
Sound depth of a
glacier
Technology then
largely forgotten
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
1950'sUSAF Greenland
_ Runway
Apollo 17 Surface Electrical
Properties Experiment
NASA
December 1972
Transmitting antenna
(1-32.1 MHz) near
Lunar Module
Receiver on Lunar
Rover
Results: Upper 2 km
lunar surface extremely
dry
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
Commercial Systems
GSSI
1960's had to build
your own system
Changed in 1972
Geophysical Survey
Systems Inc.
Sell first commercial
GPR system
Several companies
now make systems
GPR: A True Wave-Based Technique
Wave energy travels at a characteristic wave speed that
depends on the material through which it travels. This is
the main difference between GPR and EMI.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
4
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What is GPR?
October 2007
Wave Properties
The wavelength of a wave is the distance between any
two adjacent corresponding locations on the wave train.
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.
Electromagnetic Spectrum
Wavelength in meters
Short wavelength
Long wavelength
10-12 10-10 10-8 10-6 10-4 10-2 1 100 104 106 108
i i i i i i i
TV/Radio
--GPR-
EMI
i I
1020 1018 1016 1014 1012 1010 108 106 104 100 1
(GHz) (MHz) (KHz)
High frequency
Low frequency
Frequency in hertz
GPR = 10 to 1000 MHz range
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
5
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What is GPR?
October 2007
Ground Penetrating Radar
(GPR) Data Collection
Land
Surface
Ground Penetrating Radar
(GPR) Data Collection
TR
Land
Surface
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
6
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What is GPR?
October 2007
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
What Are Nanoseconds?
1 foot
= 1 ns
Position
I
20ns
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
GPR Trace
o
E
i-
1
CO
GPR Record
Distance along ground surface
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
8
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What is GPR?
October 2007
0)
I 8.249
9.3'
GPR Display/Record
Distance along surface -*
!:i>9 i:i?9
16.8' 32.B'
i:!>
bi
Q.
0>
Q
Typical GPR System
Digital video logger
Transmitter & Receiver
antenna
Odometer controlled
GPS
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
What Creates GPR
Reflections?
Caused by an
abrupt change in
electrical properties
of the subsurface
Primarily the relative
dielectric permittivity
Relative Dielectric
Permittivity
aka: Dielectric Constant
Measure of the capacity of a material to
store charge when an electric field is
applied
Controls wave velocity
Reflections occur when radio waves
encounter a change in velocity
Values range from 1 to 81
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
10
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What is GPR?
October 2007
Typical RDP Values (K)
Air 1
Water 81
Dry Sands 4
Saturated Sands 25
Silts 5-30
Clays 5-40
Limestone 6
Granite 5
Ice 3-4
Reflection Strength
K
r =
K2+
K,
= relative dielectric permittivity
of first layer
= relative dielectric permittivity
of second layer
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
11
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What is GPR?
October 2007
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
How Deep Can GPR See?
Radio waves do not
normally penetrate far
through most materials
Loss of radio reception
or cell phone connection
in a tunnel attests to this
GPR works because of
very sensitive
measuring systems and
specialized
circumstances
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
12
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What is GPR?
October 2007
How Deep Can GPR See?
Conductivity prime
factor
Higher conductivities
limit depth
Conductivity controlled
by material type
Frequency
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
13
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What is GPR?
October 2007
Estimating Exploration Depth
Depth =
35
o
meters
a = conductivity in mS/m
Material Type
Sensors & Software Inc.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
14
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What is GPR?
October 2007
GPR Suitability Map
Ground-Penetrating Radar Soil Suitability Map
of the Conterminous United States
fe: I
'&A
Antenna Frequency
1000
250 MHz
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
15
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What is GPR?
October 2007
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
Depth Calibration
How Do I Measure Depth?
Measure travel time
Need material speed
depth =velocity x time / 2
How ?
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
16
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What is GPR?
October 2007
Method 1
Estimate
Material
Air
Ice
Dry Soil
Dry Rock
Moist Soil
Concrete
Wet Soil
Water
Velocity (ft/ns)
1.0
0.56
0.43
0.39
0.33
0.33
0.22
0.11
.
.387
.8'
Method 2
Depth to Known Target
16.8'
32.B1
Know
depth
Adjust
velocity
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
17
-------
What is GPR?
October 2007
Method 3
Point Target Hyperbola
Position
o
P
Wide beam
Localized features
Hyperbolas (inverted
U's)
Shape determine
velocity
Method 3
Point Target Hyperbola
Hypwbota v* Velocity
3. 30 4.2 48 54
Dean Goodman
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
18
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What is GPR?
October 2007
Method 3
Point Target Hyperbola
.3B7
16.11
8.387
9.71
32.B1
Adjust
shape
Determines
velocity
Profile and Mark
&&ฃ3ง@PiS? >flSซ5Sซ:
* ?txl y.* '*A'ป'jy"^VwSh* ,-(*..,' ^'i^fปr\. 5**^v
u^ f/f.jif ' v>...jซ*..V./^^Cป TCV" '..ซ''***'*' _ j>i'ซK..'^V..V*^*, j.1
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
19
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What is GPR?
October 2007
Survey Grid
Series of GPR Profiles
Grumman Exploration
** "f^NfcJ&'itf iiJJ li*. ** ' ** ' '^** *^T
;aS5ป
- :?f xป
ซr - 'ปปซ.?/- 'f*s*jsrz
-------
What is GPR?
October 2007
3-D GPR
Marquette, Ml
30 m by 6 m area
6 - 8 m depth
Ground Surface
Sand and Gravel
Water Table
Sand and Gravel
Gravel lenses
Bedrock
Grumman Exploration
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
21
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What is GPR?
October 2007
GPR and GPS
12-
10-
2-
Legend
DGPS trace positioning
s Actual GPR survey grid
4 6 8
Local Easting (m)
10 12
10 m by 10 m grid
0.5 m line spacing
Differential GPS
Area wide open
Ideal conditions
Not accurate
GPR Applications
Mapping subsurface geology
- Bedrock
- Water Table
- Faults and Fractures
Locating cultural objects
- Drums and Tanks
- Landfills and pits
- Contamination
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
22
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What is GPR?
October 2007
GSSI
Bedrock
0-
100-
GPR data
Delta
Sediments
25MHz
270
1200-
Stratigraphy
Peyto Lake Delta
Banff National Park
Distance, m
-0
-35
Derald Smith, University ot Calgary
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
23
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What is GPR?
October 2007
Water Table Mapping
Dry sand & graved
Water table
400 411 Hill lilllilllllll fill! Ill llllll Illlllllilllllliniiillllilllll -15
Glacial sand and gravel deposits near Lake Superior, Ontario, Canada, 100 MHz, Sensors and Software, Inc.
Faults and Fractures
,- Annotated data. Profile 1350 Ml Use 4
0.00. 10.00. 20.00. 30.00m 40.00. 50.00.*
Ventersdorp Contact Reef - South Africa
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS-
24
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What is GPR?
October 2007
Sinkholes
Sinkhole at Winter Park, Florida
0-
of
900-
80 MHz GPR data showing developing sinkhole, Florida. GSSI
Underground Storage Tanks
Geophysical Survey Systems, Inc.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
25
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What is GPR?
October 2007
Trench with Drums
O-s
m
. c "i "!- i..*^"88?^^^! ;.. ''; ; ;" >. j. -\ , VSB|5KJ-;3
.,"A, \:>v' ' 'v "'- ". ''' ~ > '' -""-?1
ฃ&ปฃ.ฃป Wrt^lv/v/A^ป^V,ซ^\vW/-U''/^;f5-v''*fe}?sV,''l*3I
;ป,S:>w-*v... K^V.Vjj*^. ^y,ซซ\v.ซ^vซ.**S?Vf '-^ %ฃjV,r5
120
ซ*.-.. .^y-*ป>- *>^/^'ป.*rt-ซ^,S*^^w;'Si?>V'*:'
MmM^Mซp4M| -ii** ^"*'*(i'^' ซM"MMปU ป,**> ซซi*i^*fcvซS*-r -^W^*J*S'*-
120MHz
Geophysical Survey Systems, Inc.
Drainage Pipe Detection
I IIU' l>lltnn
-------
What is GPR?
October 2007
Laboratory Waste Pits
Pit Pit
Pit
200
OSU - Brookhaven National Laboratory
Water
Table
80MHz
Gas Station-
Petroleum
Product
=. 3^. ^SaBBESfe. -'g-^H:
Time Slices
Jeff Daniels and Dave Grumman, OSU
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
27
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What is GPR?
October 2007
Contaminated Groundwater
CO
c
0>
E
CO
c
0)
ฃ
600 J
Position, m
Five years after remediation
200
Sensors & Software Inc.
Creosote Pit
Distance (ft) along survey line
*#>i
Color Assignment
Relative Amplitude
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
28
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What is GPR?
October 2007
Underground Mine Workings
Cresson Open-Pit Mine
Cripple Creek, Colorado
Pikes Peak Mining Co./ GSSI
Ice / Glaciology
Ice very transparent
to GPR signals
RDP = 3-4
Penetration depth as
much as 500 meters
Used to study
thickness and
structure of glaciers
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
29
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What is GPR?
October 2007
The Lost Squadron
Snow 24 m
Ice 61 m
GPR locates aircraft at depth of 279
feet below surface in Greenland
Distance (ft)
1200
400
i".; :'
350
Water-borne GPR
Antenna
Control Unit
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
30
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What is GPR?
October 2007
Bathymetry & Sub-bottom Profiling
Connecticut River, Hartford, CT, 100 MHz
Riverbed scour near
bridge piers is a
widespread problem
throughout the United
States
Iceland
Archaeology
Burials
0-
9 i A\ *, v i ซ A>
-------
What is GPR?
October 2007
Objective to image
permafrost layer similar
to that presumed on
Mars
Haughton Meteorite
Impact Structure
Devon Island, Canada
Mars Analog
2002
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 five fundamental questions to
be asked before deciding if a radar
survey is going to be effective.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
32
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What is GPR?
October 2007
What is the 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.
What is the target geometry?
height
length
width
strike-
dip
Most important
target factor is size
If target is non-
spherical, target
orientation should
be qualified.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
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What is GPR?
October 2007
What are the target
electrical properties?
What is relative
dielectric permittivity
and electrical
conductivity of
target?
What is the host material?
Electrical properties
of the host need to
be defined.
Need contrast in
electrical properties
with host
environment.
Variations of
electrical properties
in the host material
can create noise.
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
34
-------
What is GPR?
October 2007
What is the survey
environment like?
GPR method is
sensitive to
surroundings
Extensive metal
structures
Radio frequency EM
sources and
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
INTRODUCTION TO ENVIRONMENTAL GEOPHYSICS
35
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