Lecture series potential methods

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Lecture series potential methods

• Mark van der Meijde
• ITC
• vandermeijde_at_itc.nl

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Time-Domain EM
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TDEM

• The continuous wave field (FDEM) methods
  discussed previously essentially involve
  measurement of a small secondary field in the
  presence of a much larger primary field which
  results in a decrease of accuracy.
• This problem is overcome in time-domain
  electromagnetic systems (TDEM) by use of
  transient pulses instead of continuous waves.

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TDEM

• A large direct current is passed through a large
  ungrounded loop transmitter.
• After a discrete period of time the current is
  turned off and the measurements begin.
• Opposite to Freq EM
• Direct current instead of alternating
• Measurement during time off

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• Because transmitting current is off during
  measurements the secondary current which is
  generated can be measured relatively easy.

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TDEM

• Question
• How can we apply Faraday and Amperes law in the
  case of TDEM????
• What is causing electromagnetic field in this
  case?

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TDEM

• After a discrete time (tens of milliseconds),
  during which transient signals due to turning
  current on dissipated, the applied current is
  interrupted abruptly.
• If a conductor is present in the vicinity, the
  sharp change in the primary field will induce
  secondary currents within the conductor,
  initially at the surface only

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TDEM

• This is known as early-time stage of the
  transient process. These surface currents start
  dissipating through Ohmic losses.
• The zone immediately within the conductor then
  experiences a decreasing magnetic field with a
  consequential flow of secondary currents through
  it (Eddy currents).
• This is the start of inward diffusion of the
  current pattern, of the secondary field, towards
  the interior of the conductor
• ? Intermediate-time transient process

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TDEM

• The final or late-time stage of this process is
  reached when the induced current distribution is
  invariant with time
• The only change observed is a decrease in the
  overall amplitude with time
• ? No variation, no contribution from secondary
  field due to conductor anymore!!

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TDEM

• The initial distribution of surface current is
  dependent only on the size and shape of the
  conductor (as geometrical factor) and NOT due to
  the conductivity of the body!!!
• If the conductor is very large compared to the
  used dipole source the secondary field might not
  only diffuse inside the body but also spread
  outward. This is dependent on the size, shape AND
  conductivity of the body.

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TDEM

• The whole process of the step-wise excitation of
  the current loop is repeated many times and are
  stacked for each location
• Within highly conductive bodies the decay of the
  eddy (secondary) currents and therefore the
  secondary magnetic field is significantly slower
  than that in poor conductors.
• Measurement of the rate of decay of the secondary
  field thus provides a means of detecting
  conductive bodies and estimating their
  conductivity.

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TDEM configurations

• Two possible TDEM configurations
• Loop-loop slingram
• Central loop sounding mode

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Principle of TDEM signal transmission
Loopsize 25x25 to 500x500 m
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Principle of TDEM receiver
Receiver output wave form
Transient current flow into the ground
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Receiver signal
Receiver signal is measured in time-slices
Logplot of receiver output voltage vs time
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Apparent resistivity vs time
Apparent resistivity for a homogeneous halfspace
Apparent resistivity for a two-layered earth
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Receiver output two layered earth
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Two-layered earth models
Forward calculated curves for 2-layered earth
models
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Inversion example
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Inversion example
blocking Layer
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• Estimating amount of layers visually (like in
  resistivity) is not straightforward in TDEM
• Inversion is needed for interpretation, almost
  never done manually, always with computer
• No standard interpretation curves available
• Conductive layers limit depth resolution and can
  function as barrier for deeper investigations
• Depth of penetration is strongly depending on
  this factor! Any reason for less depth
  penetration can most often be explained by high
  conductivity bodies/layers

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Case study groundwater
HoekstraBlohm, 1990
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Case study groundwater
HoekstraBlohm, 1990
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Case study groundwater
HoekstraBlohm, 1990
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Case study contamination
Hoekstra et al, 1992
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Case study contamination
Model ?
Hoekstra et al, 1992
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Case study contamination
Hoekstra et al, 1992
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Different TDEM

• Last note on different TDEM equipments. Some
  measure in microseconds others in nanoseconds.
• The latter are called nanotem and have far more
  accuracy and get more and more popular.
• First one was called (surprise) NanoTem, nowadays
  also FastTem