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1 Applied geophysics – Introduction Techniques and targets Introduction to geophysical methods: Reference: Sharma p1-10 Reynolds p1-27 Applied geophysics – Introduction What do we mean by applied geophysics?

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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; [email protected]

Dante FrattaRm 2258 Engineering Hall; Tel: 265-5644; [email protected]

Teaching Assistant:

Mei XueRm 111 Weeks Hall; 262 9784; [email protected]

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