Electromagnetic methods

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Electromagnetic methods

• Mark van der Meijde
• Assistant Professor
• Earth Systems Analysis

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Contents

• Objectives
• Introduction
• Theory EM
• Applications
• Field experiment
• Assignment / Self-study
• Presentations (groups)
• Assignment Serrata

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Introduction

• If ground is very resistive we use inductive
  methods to create currents in the ground
• Frozen ground
• Very resistive (or insulating) surface, e.g. sand

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Resistivity of Earth material
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Theory EM

• A time-varying electromagnetic field on the
  surface will create currents in the ground
• These currents will create potential differences
  in conductive bodies
• These potential will create new electromagnetic
  fields

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Theory - equations

• Electromagnetic theory is based on Maxwell
  equations (1864)
• Maxwell equations are mathematical expressions of
  two important laws
• Faradays Law (1832)
• Amperes Law (?)

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Amperes Law

• Amperes Law states that
• A magnetic field is produced by an electric
  current
• The magnetic field is proportional to the
  electric current

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Amperes Law

• H magnetic field intensity (A/m)
• J current density (A/m2)
• D electric displacement (C/m2)

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Amperes Law

• A magnetic field is generated in space by current
  flow and the field is proportional to the total
  current

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Faradays Law

• Faradays Law states that
• An electric voltage is produced wherever there is
  a time varying magnetic field.
• The voltage is proportianal to the time rate of
  change of the magnetic field

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Faradays Law

• E electric field intensity (V/m)
• B magnetic flux density (Teslas (T))

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Faradays Law

• An electric field exists in the region of a
  time-varying magnetic field

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EM methods

• EM methods are based on these two laws
• For illustration of the technique we will look at
  a horizontal two loop EM system
• One loop is the transmitter, the other is the
  receiver

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EM theory creation of mag field
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EM theory - induction
surface
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EM theory secondary field
-
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Theory - EM fields

• General phase diagram

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Theory - EM fields

• Sinusoidally varying field A
• Can be represented in plane diagram by
• rotating vector P

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Theory - EM fields

• A secondary field B results in second vector S
• Difference in phase between vectors

Resulting vector R combination PS
P
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Theory - EM fields
Primary field A in time can be represented
by Second field B has phase difference with
respect to P
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Theory - EM fields
Resulting field in time can be represented by
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Theory - EM fields

• Similar for the
• resultant-field phase
• Secondary field amplitude
• Secondary field phase

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Theory - EM fields

• It is important to realize what we measure.
• We measure resultant field R and its phase.
• Amplitude P of primary field A can be calculated
  from source-receiver geometry
• If P and R are known, we can calculate S and its
  phase

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Theory - EM fields
The field R can be split up in a
real and an imaginary component
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Theory - EM fields

• Different names for same parameters
• real component
• in-phase component
• imaginary component
• out-of-phase component
• quadrature component

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Theory - EM fields

• A very common representation for electromagnetic
  fields it to treat the axis as parts of a complex
  plane

imagnary
Real
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Single frequency EM EM31
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Single frequency EM EM31
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Single frequency EM EM31
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Single frequency EM - minerals
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Single frequency EM - environment

• Relation pollution - conductivity

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Multi-frequency EM34 - transmitter
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Receiver
34
Connected by cable
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EM34
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EM34
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Depth of investigation

• Investigation depth is a function of
• Coil separation
• Resistivity of medium of propagation
• Depth of investigation (skin depth) is defined
  as the depth of which 1/3 of energy is
  attenuated

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Depth of investigation

• Skindepth
• This is valid for frequency below 100 kHz
• at higher frequencies displacement curves are
  generated
• And permittivity becomes important (GPR)

operating frequency
conductivity
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Depth of investigation

• Depth of exploration for EM
• Also called skin-depth

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Field exercise EM31

• See behaviour of EM signals on burried objects
  and objects on surface (like cars)
• Detect buried objects in ITC backyard
• Analyze signal
• Bring paper, pen

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Assignment

• 2 groups
• Study of specific topic
• Max 5-10 min presentation tomorrow
• Different material supplied, check library,
  internet, articles, lecture books

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Assignment 1

• Study the different behaviour of EM signals over
  specific targets
• Make an overview of characteristic behaviour
  (location of troughs, shoulders depending on
  anomaly)
• Present to other students

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Assignment 2

• Investigate the different EM methods/techniques
  (e.g. EM31, EM34, VLF, GPR, etc)
• Summarize characteristics for each method
  (frequency, depth of exploration, most used
  for..)
• Present to other students

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Literature

• Parasnis
• Principles of applied geophysics
• Telford, Geldart, Sheriff
• Applied geophysics
• Ch 6, Unesco manual
• TN6 by Geonics