EOSC 350 slide 1

The University of British Columbia Geophysical Inversion Facility

http://gpg.geosci.xyz

EOSC 350 : Environmental, Geotechnical and Exploration

Geophysics I

September 2016

EOSC 350 slide 2

Outline:

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

• Geophysical surveys and data images

• Inversion

• Mineral exploration example

• Summary/discussion

EOSC 350 slide 3

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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?

Mineral exploration

• What do we want? – Subsurface structures and materials

Map of surface geology

Subsurface: Physical Properties and Contrasts

Source Data

Solutions … Geophysics

EOSC 350 slide 11

Geophysics: Sources

• On the ground

• In the air

• Inside the earth

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Geophysics: Physical Properties

–Density

–Magnetic susceptibility

–Electrical conductivity

–Chargeability

–Electrical permittivity

–Elastic moduli

Important to have

contrasts in physical

Properties.

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Geophysics: Surveys and Data

• On the ground

• In the air

• Inside the earth

SEG Distinguished Lecture slide 14

How do we distinguish bodies?

• Characterize materials by physical properties:

– Density

– Magnetic susceptibility

– Electrical conductivity

– Chargeability

– Electrical permittivity

– Elastic moduli

• If we know the physical properties then we might be able to answer our question…

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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 50 0

10

20

30

40

50

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mortar Popcan Frag 76mm

Operational Task: Dig

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100 metres Approximate Scale

3.5 KM of Electrical Resistance Imaging along tunnels

Geotechnical survey data (potash mine)

NTS

UNDERGROUND 2D ERI GOCAD VISUALIZATION VIEW FROM NE

FIGURE 7

PROJECT No. DESIGN CADD CHECK

REVIEW

FILE No. ---- REV. SCALE

TITLE

PROJECT

06OCT03 28SEP04

Max CB/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

Inversion processing

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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 = Data Pre-processing Inversion

Physical property distributions = MODELS

Prior 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 aeromagnetic data

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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 survey Data: ”pseudosection”

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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: Elevation Meters

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)

Vη Measurement is: How easily does material retain charge.

Non-zero area occurs because charges 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: Elevation Meters

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

• Read the paper: “A Geophysical Journey around Ireland” by James Hodgson • Download the TBL and complete it by evaluating one of the case

histories in the paper. This is handed in at the beginning of the next class.

Upcoming events: • Quiz involving “Foundations”

• Team exercise: Physical properties and case histories from

Ireland. EOSC 350

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Homework