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Applied geophysics – Introduction

Techniques and targets

Introduction to geophysical methods:

Applied geophysics – Introduction

What do we mean by applied geophysics?

Applied geophysics – Introduction

Remote sensing

Constraining 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

Active

Transmit a signal into the subsurface and record what comes back

• Seismic arrival – explosions
• Electrical current
• Electromagnetic waves

Passive

Measure naturally occurring phenomena

• Gravity field
• Magnetic field
• Seismic arrivals - earthquakes

think scale

Applied geophysics – Introduction

Target identification

Applied geophysics – Introduction

Magnetics

Examples

Main use: locating steel

drums and pipes

Applied geophysics – Introduction

Examples

Seismic reflection

Applied geophysics – Introduction

Seismic reflection

Examples

Applied geophysics – Introduction

Techniques and targets

Magneto-telluric Resistivity s P P m m

Permitivity, conductivity m P P P s P P P

EM – Ground penetrating radar

EM - VLF Conductance, inductance m m P m s s s m m

Electromagnetic (EM) Conductance, inductance s P P P P P P P P m

Induced polarization Resistivity, capacitance m m P m s m m m m m

Spontaneous potential Potential differences P m P m m m

Resistivity Resistivity m m P P P P P s P m

Seismic reflection Elastic moduli, density P P m s s m

Seismic refraction Elastic moduli, density P P m P s s

Magnetic Susceptibility P P P s m P P

Gravity Density P P s s s s s

Dependent physical property

Geophysical Method Hydrocarbon exploration(coal, oil, gas)Regional geologic study(>100s km2)Exploration/development ofmineral depositsEngineering siteinvestigationsHydrogeologicalinvestigationsDetection of subsurfacecavitiesMapping leachate andcontamination plumesLocation of buried metallicobjectsArchaeogeophysicsForensic geophysics

P – primary method; s – secondary; m – maybe sometimes

Applied geophysics – Introduction

Profiling vs. mapping

What is the nature of the target?

• 2D or 3D?
• Preliminary site evaluation

necessary?

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

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, 2 nd^ 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 7

th

15% - Quiz 2 November 23 rd

25% - Final December 17

th

12:25pm

Announcements of any changes will be made at the lectures

and on the webpage.

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

Applied geophysics – Introduction

Electrical resistivity

Examples

Applied geophysics – Introduction

Electrical resistivity

Examples

polluted sand

clean sand

polluted sand

Applied geophysics – Introduction

Electromagnetic surveying

Examples

Applied geophysics – Introduction

Electromagnetic surveying

Examples