8 Intro EM - University of British Columbia · GEM 3 Frequency range: 30 Hz to 24 kHz Single or multiple, frequencies. Airborne Surveys: RESOLVE Horizontal coplanar coils: 400Hz,

Slide 1

Electromagnetic Induction

Let’s first recap the motivation for using geophysics

We have problems to solve

SEG Distinguished Lectureslide 2

Finding resourcesMinerals

Ground Water

Hydrocarbons

Geothermal Energy

Natural Hazards

Tsunami Earthquake

Volcano Landslide


Geotechnical engineeringTunnels and highways In-mine safety

Subsurface voidsSlope stability


Environmental

?UXO

Water contamination

UXO detection

Saline water intrusion


Surface or Underground StorageCO2 sequestration Aquifer storage and recovery

Industrial and radioactive waste


What are basic challenges?

• Need to “see” inside the earth without drilling (noninvasive technology)

• Geophysics is the only technique that is devoted to this

• How do we do this?



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…


Electrical conductivity can be diagnostic


Electrical conductivity: units and range

Slide 11

Electromagnetic Induction

Everybody at an airport goes through a security scan.

How does it work?


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:


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


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


EM induction example: larger scale

EOSC 350 ‘06 Slide 16

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 fieldB: magnetic field

Think about electric field asvoltage in a circuit.Units of E are Volts/meter

Electromagnetics

σ ρJ=E

• Ohm’s Law: Tx Rx


J : current density (Amp/m^2)electrical conductivity

Think about V=IR for a circuitV: voltage (Volts)I: current (Amperes).R: Resistance (Ohms) resistivity


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 fieldJ: 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).


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

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

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)

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

Transmitter

Magnetic field of a loop of current is like a magnetic dipole

Dipole moment m = I A (current x area)

Orientation of loop shows direction of primary field

Tx

Couple with the target

Max flux Zero flux

Induced Currents in the Target

Think of target as an electrical circuit

Resistance R (small R means large current)

Inductance L (accounts for interaction of currents in the target)

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.

Frequency domain EM data

Transmitter ( )tωcos

Receiver

I(t)

V(t)

A

-A

ψ

amplitude

( )ψω +tAcos

Measure amplitude and phase (A, )ψ

In-phaseReal

Out-of-phaseImaginary

Or

Data Signal in receiver is harmonic (sinusoid)

but not in phase with the primary.

Decompose into portion In-Phase (Real) Out-of-phase (Quadrature, Imaginary)


Effect of buried objects

See GPG Ch3.h.

Source field moves with receiver.

Graph measurements vs line position

Inst

rum

ent,

fie

lds,

an

d t

arg

et

FIELD

PHYSICS

DATA

Data We have the shape of the response.

What about amplitude?

Relative amplitudes depend upon the conductivity of the body

Relative amplitudes of In-Phase and

Out-of-phase components.

See Electromagnetics 1.0 Fundamentals

EM Induction: Summary

Time varying magnetic magnetic field generates an electric field E

J=σE (induced currents) (Coupling is important) Induced currents generate secondary magnetic fields Secondary magnetic fields are recorded at the receiver.

Coupling is important. Receiver outputs In-Phase and Out-of-phase data

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

ρδ 506= meter

where is resistivity in and f is frequency in Hz.

ρ mΩ

deptham

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

Multilayered Earth

Note: Integration begins at the plane of the instruments! Sensitivity is carried with the instrument height.

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)

σaApparent conductivity.


Implications over “real” earth

Reading are “true” values of the ground’s physical property ONLY over uniform ground.

Therefore, result is “apparent” conductivity.

Result over NON-uniform ground is a complicated weighted average of all materials.

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

EM31

EM in phase and quad phase?

Other Frequency Domain Systems

EM34 Different frequencies

10m at 6.4kHz20m at 1.6kHz40m at 0.4kHz

Operate at different orientations

Mapping deeper groundwater contaminant plumes,and groundwater exploration.

Very sensitive to vertical geological anomalies. spacing.

EM34

GEM 3

Frequency range: 30 Hz to 24 kHz Single or multiple, frequencies

Airborne Surveys: RESOLVE

Horizontal coplanar coils: 400Hz, 1600Hz, 6400Hz, 25kHz, 100kHz

Coaxial coils at 1600Hz

DIGHEM: RESOLVE

Solar Storm and Earth’s field


Auroral electrojet expansion: Currents


Auroral Electrojets and Aurora


Storm 1989


Storm 1989


Quebec power grid

Closed loops of grid 500km x 300 km


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


Next

EM quiz

TBL EM-31Geophysical Applications to Solid Waste AnalysisHutchinson

Documents

8 Intro EM - University of British Columbia · GEM 3 Frequency range: 30 Hz to 24 kHz Single or multiple, frequencies. Airborne Surveys: RESOLVE Horizontal coplanar coils: 400Hz,