recap the motivation for using geophysics we have problems ... · seg distinguished lecture slide...

Electromagnetic Induction

Recap the motivation for using geophysics

We have problems to solve

SEG Distinguished Lecture

slide 2

Finding resourcesMinerals

Ground Water

Hydrocarbons

Geothermal Energy

Natural Hazards

Tsunami Earthquake

Volcano Landslide


slide 3

Geotechnical engineeringTunnels and highways In-mine safety

Subsurface voidsSlope stability


slide 4

Environmental

?

UXO

Water contamination

UXO detection

Saline water intrusion


slide 5

Surface or Underground StorageCO2 sequestration Aquifer storage and recovery

Industrial and radioactive waste


slide 6


slide 8

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…


slide 9

Electrical conductivity can be diagnostic


slide 10

Electrical conductivity: units and range

Electromagnetic Induction

Everybody at an airport goes through a security scan.

How does it work?


slide 13

Electrical conductivity: basic equations

• Faraday’s law:

• Ampere’s law: are sources

• Other important relationships:

• Solution depends upon the sources and boundary conditions

Frequency: Time-domain:


slide 14

Basic principles of EM induction

secondary

primary

primarytransmitter

loop

primary

secondary

receiverloop

• Time-varying transmitter current generates a time-varying magnetic field

• Time-varying magnetic field generates an EMF (i.e. electric field) in the earth

• Currents are generated ( )

• Currents in the conductor generate magnetic fields (secondary)

• Measure the secondary fields and the primary fields of the transmitter


slide 15

EM induction example: small scale

• Transmit alternating primary magnetic field– Induces eddy currents in conducting object

• Eddy currents produce secondary magnetic field– Induces current in receiver coil


slide 16

EM induction example: larger scale

EOSC 350 ‘06 Slide 17

Tx Rx

Electromagnetic induction

Survey involves a transmitter and receiver

Frequency ~ 101 – 104 Hz

(GPR is ~ 106 – 109 Hz)

EM in phase and quad phase?


EM 31 Data from Expo Site

Electromagnetics

• Faraday’s Law: A time varying

magnetic field generates an electric

field

Tx Rx


E: electric field

B: magnetic field

Think about electric field as

voltage in a circuit.

Units of E are Volts/meter

Electromagnetics

JE

• Ohm’s Law: Tx Rx


J : current density (Amp/m^2)

electrical conductivity

V: voltage (Volts)

I: current (Amperes).

R: Resistance (Ohms)

E: electric field (Volts/meter)

J: current density (Amperes/meter^2).

: Resistivity (Ohm-meters)


EM induction

Faraday’s law

Time varying magnetic fields cause electric fields

Electric fields produce currents in a conductor

Hence current flows in conductors that are near an

oscillating magnetic field

Electromagnetics

• Amperes Law:

A current generates a magnetic field

Tx Rx


H: magnetic field

J: current source density


EM induction

Ampere’s law -

Currents generate magnetic fields

Oscillating current will cause an oscillating magnetic field

Current in wire causes a

magnetic field to sur-

round it (iron filings).

Direction of the Field of a Long Straight Wire

Right Hand Rule

Grasp the wire in your right hand

Point your thumb in the direction of the current

Your fingers will curl in the direction of the field


EM induction

Lens’ law -

The direction of the induced currents will be in such a

direction as to oppose any change in magnetic flux.

Current in wire causes a

magnetic field to sur-

round it (iron filings).


slide 28


secondary

primary

primarytransmitter

loop

primary

secondary

receiverloop

Time-varying transmitter current generates a time-varying magnetic field

Time-varying magnetic field generates an EMF (i.e. electric field) in the earth

Currents are generated

Currents in the conductor generate magnetic fields (secondary)

Measure the secondary fields and the primary fields of the transmitter

An alternating “primary” magnetic field induces eddy currents in a

conducting object moved through the detector.

The eddy currents in turn produce an alternating “secondary” magnetic

field and this field induces a current in the detector’s receiver coil.

EM induction example: metal detector

Important elements

Primary field must couple with the target

Strength of the induced currents must be big

enough to generate signal

Need to choose which fields to measure

Airborne (Inductive source)


slide 33


secondary

primary

primarytransmitter

loop

primary

secondary

receiverloop

Time-varying transmitter current generates a time-varying magnetic field

Time-varying magnetic field generates an EMF (i.e. electric field) in the earth

Currents are generated

Currents in the conductor generate magnetic fields (secondary)

Measure the secondary fields and the primary fields of the transmitter

Elements of EM Induction

Transmitter and primary magnetic field

Magnetic flux and coupling

Target and induced currents

Secondary magnetic fields

Receiver

Data

Generic EM system

Tx: transmitter Rx: receiver

Tx, Rx, target body are represented as circuits

Magnetic fields from a circular current

produce a magnetic dipole

Slide 36

Dipole moment: m = IA I : Current (amperes)

A: area of loop (meter^2)

Transmitter: Time varying current

Magnetic field of a loop of current is

like a magnetic dipole

Orientation of loop shows direction of primary field

Time varying current generates time varying magnetic field

Tx

Couple with the target

Max flux Zero flux

Secondary Magnetic Fields

Currents in the target generate magnetic fields

If target is modelled by a current circuit then

secondary magnetic fields are like those of a

magnetic dipole.

Receiver

Receiver is a coil. A time varying flux generates a

voltage.

For some instruments Hp is known and subtracted.

Then receiver measures only Hs.

Understanding the data

Simple circuit

Sketch induced currents

Secondary magnetic fields (Hs) at receiver

EOSC 350 ‘06Slide 44


Effect of buried objects

See GPG Ch3.h.

Source field moves with receiver.

Graph measurements vs line position

Instr

um

en

t, f

ield

s,

an

d t

arg

et

FIELD

PHYSICS

DATA

Understanding the data

Simple circuit

Plotted

But data are complex.

EOSC 350 ‘06Slide 46

Frequency domain EM data

Transmitter tcos

Receiver

I(t)

V(t)

A

-A

amplitude tAcos

Measure amplitude and phase (A, )

In-phaseReal

Out-of-phaseImaginary

Data

Primary field:

Secondary field:

Decompose secondary field

In-phase Out –of-phase

Relative amplitudes of In-Phase and

Out-of-phase components.

See GPG

In-Phase:

Out-of-phase


See GPG Ch3.h.



Instr

um

en

t, f

ield

s,

an

d t

arg

et

FIELD

PHYSICS

DATA

Relative sizes for a good conductor

Slide 53

EM Loop Demo

Slide 55


See GPG Ch3.h.



Instr

um

en

t, f

ield

s,

an

d t

arg

et

FIELD

PHYSICS

DATA

Relative sizes for a good conductor

Slide 72

Earth is also a conductor

Depth of investigation depends upon

skin depth

source receiver geometry

EM waves inside the earth

Vertical dimension is not to scale

Frequency Domain

EM waves decay when propagating in a

conducting earth

Skin depth

f

506 meter

where is resistivity in and f is frequency in Hz.

m

depth

am

plitu

de

7.2

11

e

Sensitivity: horizontal coplanar system (HCP)

Sometimes called vertical magnetic dipole system

φ is the sensitivity

Horizontal axis scaled by s (separation between source and receiver

Note: System responds most strongly to conductivity at 0.4s

Obtaining apparent conductivity

For s << δ

Apparent conductivity is

General Formula

Note: Apparent conductivity depends upon height and

orientation of the instrument

Horizontal Dipole

Vertical Dipole

Multilayered Earth

Note: Integration begins at the plane of the instruments!

Sensitivity is carried with the instrument height.

Horizontal Dipole

Vertical Dipole

Inphase/Quadrature

The EM-31 gives two measurements called the In-phase and Quadrature

In-phase: (also called “real”)

Particularly useful for find good conductors (metal pipes, drums)

Quadrature: (also called “imaginary” or (out of phase)

Yields apparent conductivity (if s>δ)

CH2: Sand and Gravel Quarries Setup: Find sand and gravel quarries. Area has granitic mountains, rolling hills and lakes. Glacial

deposits are responsible for potential sand and gravel resources. Some of the area is bog and

agricultural land. (Picture)

Properties: Bog material is wet and conductive. Gravel deposits are resistive (low conductivity).

Gravels are unconsolidated and have a low seismic velocity.

Survey: Preliminary EM survey (EM31) Logistically easy and gives an estimate of ground

conductivity in the top few meters. Good reconnaissance tool. More detailed follow-up using DC

resistivity to get 2D conductivity structure and seismic to find the base of the gravel.

Data: EM31data. Also DC and seismic are acquired along selected line profiles.

Apparent Conductivities from EM31

CH2: Sand and Gravel Quarries Processing: EM31 data is converted to ground conductivity. (Picture). DC resistivity

data is inverted to get a 2D cross section. Seismic data are inverted to provide

location of refracting interfaces.

Interpretation: Areas of low conductivity are identified from the EM survey. The

inversion of DC and seismic data outline a gravel lens along one of the transects.

Gravel lens is 5-8 meters in thickness and 40-50 meters in length.

Synthesis: Seems successful. Have found gravel lenses and results have helped

assess the potential tonnage across the site.

Case History Project: Expo Site

Integrated site investigation of contaminated waste

site in Vancouver

Combines all the geophysical methods covered in

EOSC 350: Magnetics, GPR, Seismic refraction and

EM induction

EM in phase and quad phase?

Other Frequency Domain Systems

EM34 Different frequencies

10m at 6.4kHz

20m at 1.6kHz

40m at 0.4kHz

Operate at different orientations

Mapping deeper groundwater contaminant plumes,

and groundwater exploration.

Very sensitive to vertical geological anomalies.

spacing.

GEM 3

Frequency range: 30 Hz to 24 kHz

Single or multiple, frequencies

Airborne Surveys: RESOLVEHorizontal coplanar coils:

400Hz, 1600Hz, 6400Hz,

25kHz, 100kHz

Coaxial coils at 1600Hz

DIGHEM: RESOLVE

Solar Storm and Earth’s field

EOSC 350 ‘06

Slide 94

Auroral electrojet expansion: Currents

EOSC 350 ‘06

Slide 95

Auroral Electrojets and Aurora

EOSC 350 ‘06

Slide 96

Storm 1989

EOSC 350 ‘06

Slide 97

Storm 1989

EOSC 350 ‘06

Slide 98

Quebec power grid

Closed loops of grid 500km x 300 km

EOSC 350 ‘06

Slide 99

Some calculations B changes 400 nT in 2 minutes

One grid loop in Quebec is 300 x 500 km

Voltage induced: 500 Volts

Equivalent circuit: V=IR

Area of wire 4 cm^2

Length of loop: 1600km

Conductivity of copper: ~10^7 S/m

Induced current = 1 Amp

EOSC 350 ‘06

Slide 100

Next

EM quiz

TBL EM-31

Geophysical Applications to Solid Waste Analysis

Hutchinson

recap the motivation for using geophysics we have problems ... · seg distinguished lecture slide...

Documents