applied geophysics electromagnetics...applied geophysics – electromagnetics em induction methods...
TRANSCRIPT
![Page 1: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/1.jpg)
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
![Page 2: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/2.jpg)
SEG Distinguished Lecture
slide 2
Electrical conductivity: units and range
![Page 3: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/3.jpg)
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.
![Page 4: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/4.jpg)
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)
![Page 5: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/5.jpg)
EOSC 350 ‘06 Slide 5
EM 31 Data from Expo Site
![Page 6: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/6.jpg)
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
![Page 7: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/7.jpg)
Electromagnetics
JE
• 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
![Page 8: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/8.jpg)
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
![Page 9: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/9.jpg)
Electromagnetics
JH
• Amperes Law:
A current generates a magnetic field
Tx Rx
Electromagnetic induction
H: magnetic field
J: current source density
![Page 10: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/10.jpg)
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).
![Page 11: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/11.jpg)
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
![Page 12: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/12.jpg)
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).
![Page 13: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/13.jpg)
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
![Page 14: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/14.jpg)
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
![Page 15: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/15.jpg)
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
![Page 16: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/16.jpg)
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
![Page 17: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/17.jpg)
Airborne (Inductive source)
![Page 18: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/18.jpg)
Elements of EM Induction
Transmitter and primary magnetic field
Magnetic flux and coupling
Target and induced currents
Secondary magnetic fields
Receiver
Data
![Page 19: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/19.jpg)
Generic EM system
Tx: transmitter Rx: receiver
Tx, Rx, target body are represented as circuits
![Page 20: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/20.jpg)
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
![Page 21: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/21.jpg)
Couple with the target
Max flux Zero flux
![Page 22: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/22.jpg)
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
![Page 23: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/23.jpg)
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.
![Page 24: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/24.jpg)
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.
![Page 25: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/25.jpg)
Frequency domain EM data
Transmitter tcos
Receiver
I(t)
V(t)
A
-A
amplitude
tAcos
Measure amplitude and phase (A, )
tAtAtA sinsincoscoscos
In-phase Real
Out-of-phase Imaginary
Or
![Page 26: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/26.jpg)
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)
![Page 27: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/27.jpg)
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
![Page 28: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/28.jpg)
EOSC 350 ‘07 Slide 28
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
![Page 29: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/29.jpg)
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
![Page 30: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/30.jpg)
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.....
![Page 31: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/31.jpg)
Earth is also a conductor
Depth of investigation depends upon
skin depth
source receiver geometry
![Page 32: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/32.jpg)
EOSC 350 ‘06 Slide 32
Meaning of readings over earth
Earth is a conductor so currents are induced
everywhere.
Reading is two numbers.
Rcv
![Page 33: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/33.jpg)
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)
![Page 34: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/34.jpg)
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.
![Page 35: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/35.jpg)
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
![Page 36: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/36.jpg)
![Page 37: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/37.jpg)
![Page 38: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/38.jpg)
EM in phase and quad phase?
![Page 39: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/39.jpg)
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?
![Page 40: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/40.jpg)
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
![Page 41: Applied Geophysics Electromagnetics...Applied Geophysics – Electromagnetics EM induction methods Amperes Law Faradays Law Basics of EM induction Use EM31 as a specific learning example](https://reader030.vdocuments.mx/reader030/viewer/2022040100/5edb14aa09ac2c67fa68c6ca/html5/thumbnails/41.jpg)
Readings for Electromagnetics
Electromagnetics 1.0 Fundamentals
GPG.h