eosc 350 : an introduction to applied geophysics€¦ · summary for applied geophysics •...

slide 1© UBC-GIF 2005

The University of British ColumbiaThe University of British ColumbiaGeophysical Inversion FacilityGeophysical Inversion Facility

http://www.eos.ubc.ca/research/ubcgif

EOSC 350 : An Introduction to Applied Geophysics

September 2009

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Outline:

• Introduction: Example problems– Environmental, geotechnical, resource exploration

• Geophysical surveys and data images

• Inversion

• Mineral exploration example

• Summary/discussion

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Environmental: UXO

• Military proving grounds

• Regions of conflict

• Avalanche control

http://www.nohowinc.com/http://www.dma.state.mn.us/

http://www.centennialofflight.gov

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Various types of UXO

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Environmental: How do we find UXO?

?

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Geotechnical: A Canadian potash mining

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Geotechnical problem

• Slide: water gushing into the mine

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Mineral exploration

• What do we have?

Map of surface geology

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?

• What do we want?– Subsurface structures and materials

Mineral exploration

Map of surface geology

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Solutions … Geophysics

Energy from sourceEnergy from source

Physical propertiesand contrasts

Physical propertiesand contrasts

Measurements = DataMeasurements = Data

- Density- Magnetic susceptibility- Electrical resistivity- Chargeability- others …

- Gravity- Magnetic field anomalies- DC or electromagnetics- Induced polarization- etc. …

Data are related to physical properties

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Physical properties

• UXO:– Electrical conductivity and magnetic susceptibility

• Water (at potash mine): – Electrical conductivity: high if it has dissolved salt

• Minerals: – magnetic susceptibility (at Raglan)

– electrical conductivity,

– chargeability,

– density

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Environmental : Magnetic Survey

TM4

Ferrex

nT

-200

-100

0

100

200

Easting (m)

Nor

thin

g (m

)

0 10 20 30 40 500

10

20

30

40

50

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mortarPopcanFrag76mm

Operational Task: Dig

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100 metresApproximateScale

3.5 KM of Electrical Resistance Imaging along tunnels

Geotechnical survey data (potash mine)

NTS

UNDERGROUND 2D ERIGOCAD VISUALIZATIONVIEW FROM NE

FIGURE 7

PROJECT No.DESIGNCADDCHECKREVIEW

FILE No. ----REV.SCALE

TITLE

PROJECT

06OCT0328SEP04

MaxCB/Max

04-1419-007

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WEST EAST

143 m 148 m 155 m 159 m 163 m

Ground Penetrating Radar: Drill-holes with no water

Water found

Direct Current Resistivity image: Water found here

Two geophysical surveys along tunnels

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Our mineral exploration example

• Data: magnetic measurements.

• Image: map of anomalies in earth’s field.

Map of surface geology

Image provides some information …but not enough.

Total field aeromagnetics

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Getting more from data …

• Image provides some information but not enough.

Total field aeromagnetics

We want a “model” of subsurface materials.

Invert the data

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What is Inversion?

Goal: Estimate the Earth model that generated the data

Model

??

Data

Airborne, surface or borehole

measurements are data

Inversionprocessing

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Inversion procedure:

• Divide Earth into cells (each with fixed size and unknown value).

• Inversion: find values for cells such that data are explained.

• Use mathematical optimization theory.

• Difficulties:

– Solution is non-unique.

– Computationally demanding.

Measurements = DataMeasurements = Data Pre-processingPre-processing InversionInversion

Physical property distributions = MODELS

Physical property distributions = MODELS

Prior informationPrior information

3D, and ~ 105 cells

1κ

2κ

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Geophysical inversion is analogous to medical imaging

• We need a 3D image for pinpointing a target or making a final assessment.

Image of a brain based on MRI measurements.

Image of an ore body based on magnetic field measurements.

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Viewing an inversion result

• 3D volume can be viewed many ways. Here:– Data on top

– Pixels showing material property value:visible along the slice.

– Isosurface:

hide all cells with values less than a chosen value.

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Exploration at Raglan: Inversion image

? ?

Invert data => 3D images of property distribution.

Total field aeromagneticdata

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Framework for Applied Geophysics: 7 Steps

• Setup: What is the question to be answered?

• What are the diagnostic physical properties?

• Choose survey and design data acquistion.

• Data collection

• Processing of field data

• Interpretation

• Synthesis

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Mineral Exploration: The “Cluny” copper/lead/zinc deposit

Physical Properties of the Rocks

Rock unit Conductivity Chargeability

Adjacent shale high low

Host volcanic low-moderate low rocks

Mineral zone:- Siltstone containing (Cu, Pb, Zn)

- Galena, pyrrhotite, pyrite, sphalerite

moderate high

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Electrical survey: concept

DC surveyData: ”pseudosection”

• Basic equations:

dvr1or ⎟⎠⎞

⎜⎝⎛⋅⎟

⎠⎞

⎜⎝⎛ ∇⋅∇

= ∫ σφσφS=∇⋅∇ φσ )(

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Data set #1:

Apparent resistivity,dipole - pole.

Electrical survey: data• Eight survey lines

• Two survey configurations.

Easting (m) Easting (m)

mS/m

Easting (m) Easting (m)

mS/m

Data set #2:

Apparent conductivity,pole - dipole.

10500 11500 12500

13000

14000

15000

16000

400

450

500

Easting (m)

Nor

thin

g (m

)

Surface topography:ElevationMeters

10 lines surveyed

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3D conductivity model from 3D inversion:

Click image to see the AVI movie

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IP data: what is being measured?

Source Current (Amps)

Measured Voltage (Volts)

Vσ Vs(t)

VηMeasurement is:How easily doesmaterial retaincharge.

Non-zero area occurs becausecharges took time to equilibrate.

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Apparent chargeability,dipole - pole.

3D induced polarization

10500 11500 12500

13000

14000

15000

16000

400

450

500

Easting (m)

Nor

thin

g (m

)

Surface topography:ElevationMeters

10 lines surveyed

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3D conductivity and chargeability: Cluny

Volume rendered resistivity model Volume rendered chargeability model

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Summary For Applied Geophysics

• Multi-disciplinary:– brings together mathematics, physics, computer science,

geology, engineering into a single context.

• Requires team work and ability to communicate among different disciplines

• Two communication tools are:– Expressing problems in terms of physical properties

– Images

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Framework for Applied Geophysics: 7 Steps

• Setup: What is the question to be answered?

• What are the diagnostic physical properties?

• Choose survey and design data acquistion.

• Data collection

• Processing of field data

• Interpretation

• Synthesis

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Slide 33

Homework• Read the course syllabushttp://www.eos.ubc.ca/courses/eosc350/

• Read the paper:

“A Geophysical Journey around Ireland” by James Hodgson

Upcoming events:

• Quiz involving “Seeing underground”

• Team exercise: Physical properties and case histories from Ireland.