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Applied geophysics – Introduction
Techniques and targetsIntroduction to geophysical methods:
Reference:Sharma p1-10Reynolds p1-27
Applied geophysics – Introduction
What do we mean by applied geophysics?
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Applied geophysics – Introduction
Remote sensingConstraining the Earth’s sub-surface with observations at the surface
Geophysical techniques measure physical phenomena:• Gravity• Magnetism• Elastic waves• Electricity• Electromagnetic waves
Which are sensitive to sub-surface physical properties:• Density• Magnetic susceptibility• Seismic wave velocity and density• Resistivity• Conductance/inductance/permitivity
Oh, and it’s relatively cheap
Applied geophysics – Introduction
Active and passive
ActiveTransmit a signal into the subsurface and record what comes back
• Seismic arrival – explosions• Electrical current• Electromagnetic waves
PassiveMeasure naturally occurring phenomena
• Gravity field• Magnetic field• Seismic arrivals - earthquakes
think scale
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Applied geophysics – Introduction
Our scale…meters to kilometers
The techniques we’ll talk about are used in the fields of
• Environmental geophysics
• Engineering geophysics
• Exploration geophysics
Applied geophysics – Introduction
Planning a survey
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Applied geophysics – Introduction
Target identification
Applied geophysics – Introduction
MagneticsExamples
Main use: locating steel drums and pipes
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Applied geophysics – Introduction
ExamplesSeismic reflection
Applied geophysics – Introduction
Seismic reflectionExamples
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Applied geophysics – Introduction
Ground penetrating radarExamples
Applied geophysics – Introduction
Ground penetrating radarExamples
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Applied geophysics – Introduction
Techniques and targets
mmPPsResistivityMagneto-telluric
PPPsPPPmPermitivity, conductivityEM – Ground penetrating radar
mmsssmPmmConductance, inductanceEM - VLF
mPPPPPPPPsConductance, inductanceElectromagnetic (EM)
mmmmmsmPmmResistivity, capacitanceInduced polarization
mmmPmPPotential differencesSpontaneous potential
mPsPPPPPmmResistivityResistivity
mssmPPElastic moduli, densitySeismic reflection
ssPmPPElastic moduli, densitySeismic refraction
PPmsPPPSusceptibilityMagnetic
sssssPPDensityGravity
Dependent physical property
Geophysical Method
Hyd
roca
rbon
exp
lora
tion
(coa
l, oi
l, ga
s)
Reg
iona
l geo
logi
c st
udy
(>10
0s k
m2)
Expl
orat
ion/
deve
lopm
ent
of
min
eral
dep
osits
Engi
neer
ing
site
in
vest
igat
ions
Hyd
roge
olog
ical
inve
stig
atio
ns
Det
ectio
n of
sub
surf
ace
cavi
ties
Map
ping
leac
hate
and
cont
amin
atio
n pl
umes
Loca
tion
of b
urie
d m
etal
lic
obje
cts
Arch
aeog
eoph
ysic
s
Fore
nsic
geo
phys
ics
P – primary method; s – secondary; m – maybe sometimes
Applied geophysics – Introduction
Profiling vs. mappingWhat is the nature of the target?
• 2D or 3D?
• Preliminary site evaluation necessary?
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Applied geophysics – Introduction
Station spacing
Must ensure that spacing is sufficient to sample anticipated signal
Applied geophysics – Introduction
Limitations
• Methods require contrast in physical properties
• Nonuniqueness
Direct modeling: calculate the result of a specific structure
Inverse modeling: determine the causative structure from observations – usually ambiguous
e.g. small shallow body vs. large deep body
• Resolution is determined by the wavelength of the signal
sesimic: km, radar: cm
• Noise prevents recovery of low amplitude signal
e.g. wind, traffic, water pumps, power lines
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Applied geophysics – Introduction
Multiple methods…to improve uniqueness and cross-check interpretations
Applied geophysics – Introduction
Introductions
Instructors:
Richard AllenRm 106 Weeks Hall; Tel: 262-7513; rallen@geology.wisc.edu
Dante FrattaRm 2258 Engineering Hall; Tel: 265-5644; fratta@wisc.edu
Teaching Assistant:
Mei XueRm 111 Weeks Hall; 262 9784; meixue@geology.wisc.edu
Office hours: by appointment
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Applied geophysics – Introduction
Sources
Course webpage: http://www.geology.wisc.edu/courses/g594/
- lecture notes, homeworks etc
Required text
Environmental and Engineering Geophysics, 1st ed, Sharma, 1997
Other reference texts
An Introduction to Applied and Environmental Geophysics, Reynolds, 1997.
Applied Geophysics, 2nd ed, Telford, Geldart, Sheriff, 1990
Exploration geophysics of the shallow subsurface, Burger, 1992.
Whole Earth Geophysics, Lillie, 1999.
An introduction to geophysical exploration, Keary, Brooks and Hill, 2002
Applied geophysics – Introduction
Grading
50% - Homework: Bi-weekly problem sets
10% - Quiz 1 October 7th
15% - Quiz 2 November 23rd
25% - Final December 17th 12:25pm
Announcements of any changes will be made at the lectures and on the webpage.
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Applied geophysics – Introduction
Topic schedule
Weeks 1-2: Gravity Methods
Weeks 3-4: Magnetic Methods
Weeks 5-6: Resistivity Methods
Weeks 7-11: Seismic Reflection and Refraction Methods
Weeks 11-13: Electromagnetic Methods
Week 14: Ground Penetrating Radar Methods
Week 15: Borehole Methods
Applied geophysics – Introduction
Additional material
Summaries and examples of other methods
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Applied geophysics – Introduction
GravityExamples
Applied geophysics – Introduction
Seismic refractionExamples
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Applied geophysics – Introduction
Electrical resistivityExamples
Applied geophysics – Introduction
Electrical resistivityExamples
polluted sand
clean sand
polluted sand
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Applied geophysics – Introduction
Electromagnetic surveyingExamples
Applied geophysics – Introduction
Electromagnetic surveyingExamples
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