applied geophysics – electromagnetics · slide 1 applied geophysics – electromagnetics em...
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Slide 1
Applied Geophysics – Electromagnetics
EM induction methods
Amperes Law
Faradays Law
Basics of EM induction
Use EM31 as a specific learning example
Tx Rx
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SEG Distinguished Lecture slide 2
Electrical conductivity: units and range
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EOSC 350 ‘06 Slide 3
HO
M
Previous work: Magnetic induction
Instrument is a sensor only Source is Earth’s field – always present. Relatively uniform at survey scales Source field does not change with time.
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EOSC 350 ‘06 Slide 4
Tx Rx
Electromagnetic induction
Survey involves a transmitter and receiver Source field changes – it is NOT a static field
Frequency ~ 101 – 104 Hz (GPR is ~ 106 – 109 Hz)
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EOSC 350 ‘06 Slide 5
EM 31 Data from Expo Site
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Electromagnetics
dtdB /−=×∇ E
• Faraday’s Law: A time varying magnetic field generates an electric field
Tx Rx
Electromagnetic induction
E: electric field B: magnetic field
Think about electric field as voltage in a circuit. Units of E are Volts/meter
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Electromagnetics
ρJ=E
• Ohm’s Law: Tx Rx
Electromagnetic induction
J : current density (Amp/m^2) σ : electrical conductivity
Think about V=IR for a circuit V: voltage (Volts) I: current (Amperes). R: Resistance (Ohms)
J=σE
E = Jρ ρ = 1/σ
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EOSC 350 ‘06 Slide 8
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
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Electromagnetics
JH =×∇
• Amperes Law: A current generates a magnetic field
Tx Rx
Electromagnetic induction
H: magnetic field J: current source density
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EOSC 350 ‘06 Slide 10
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).
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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
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EOSC 350 ‘06 Slide 12
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).
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SEG Distinguished Lecture slide 13
Basic principles of EM induction
secondary
primary
primary transmitter
loop
primary
secondary
receiver loop
• 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
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SEG Distinguished Lecture slide 14
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
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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
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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
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Airborne (Inductive source)
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Elements of EM Induction
Transmitter and primary magnetic field Magnetic flux and coupling Target and induced currents Secondary magnetic fields Receiver Data
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Generic EM system
Tx: transmitter Rx: receiver
Tx, Rx, target body are represented as circuits
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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
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Couple with the target
Max flux Zero flux
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Induced Currents in the Target
Max flux Zero flux
Think of target as an electrical circuit Resistance R (small R means large current) Inductance L (accounts for interaction of currents in the target
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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.
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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.
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Frequency domain EM data
Transmitter ( )tωcos
Receiver
I(t)
V(t)
A
-A
ψ
amplitude ( )ψω +tAcos
Measure amplitude and phase (A, ) ψ
( ) ( ) ( ) tAtAtA ωψωψφψω sinsincoscoscos +=+In-phase
Real Out-of-phase
Imaginary
Or
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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)
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Understanding the Data
Read the electromagnetic notes
Learn how to understand the response of a system like the EM31 as it goes over a conductive plate.
The first task is to work with the basic principles of EM induction and sketch out the signal caused by geometry
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EOSC 350 ‘07 Slide 28
Effect of buried objects
See GPG Ch3.h. Source field moves with receiver.
Graph measurements vs line position
Inst
rum
ent,
field
s, a
nd ta
rget
FIELD
PHYSICS
DATA
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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
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Understanding the Data
We can now sketch the effects of geometry and coupling on an EM31 as it passes over a plate. This is essentially controlled by geometry and coupling.
Next we’ll consider the information in the in-phase and quadrature phase
Read the EM notes Matlab routine to estimate the responses. demos.....
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Earth is also a conductor
Depth of investigation depends upon
skin depth
source receiver geometry
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EOSC 350 ‘06 Slide 32
Meaning of readings over earth
Earth is a conductor so currents are induced everywhere.
Reading is two numbers.
Rcv
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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)
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σa Apparent conductivity.
EOSC 350 ‘06 Slide 34
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.
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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
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EM in phase and quad phase?
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EOSC 350 ‘06 Slide 39
Effect of buried objects
Contour plotted area data:
Where are peak & trough patterns?
Where are large responses?
Where are negative responses?
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EM Summary so far Basics of EM induction
Sketch approximate anomalies for a simple system
(EM31) that traverses a confined body
Responses for EM31 and application
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Readings for Electromagnetics
Electromagnetics 1.0 Fundamentals
GPG.h