Lecture 7c - CSAMT and TEM methods

Controlled Source AMT

  • An extension of the Magnetotelluric (AMT) method which uses naturally occurring electrical magnetic field variations from thunderstorm activity.
  • The 'control source' variant augments natural signals with an artificial source to fill gaps in the frequency spectrum where natural signals are weak.
  • Operates at relatively low frequencies, allowing for deeper exploration (up to multiple kilometers).
  • Uses separate measurements of electric and magnetic fields with electrodes and orthogonal magnetometers (oriented at 90 degrees), respectively.
  • Requires direct contact with the ground via electrodes, limiting its use in rapid, mobile systems like aircraft.
  • Apparent resistivities are derived from the ratio between the magnitudes of the electric and magnetic fields.
  • Example application: Mapping saline groundwaters in permafrost regions in Svalbard, where saline fluids (remnants from when the area was below sea level) exist at depth and provide a low resistivity contrast to the resistive permafrost. This is critical for understanding the impacts of melting permafrost due to global warming.

Time Domain Method (TEM)

  • Evolved more recently than frequency domain methods, driven by the need to measure below thick conducting upper layers (developed in Australia in the 1980s).
  • Equipment consists of a large transmitter loop and a smaller receiver loop, making it more complex and expensive than frequency domain systems but providing richer information on vertical conductivity variations.
  • Two loop arrangements:
    • Central loop: Receiver coil located in the middle of the transmitter loop; suitable for depths around 50 meters (e.g., 40-meter transmitter loop with a 1-meter receiver loop).
    • Offset configuration: Receiver moved outside the transmitter loop to avoid interference, used for shallower investigations.
  • Combines electromagnetic principles with a time-domain approach:
    • A current is sent through the transmitter coil, generating a primary magnetic field.
    • The current is suddenly switched off, causing the primary magnetic field to collapse.
    • According to Faraday's law, the changing magnetic field induces potential differences, generating currents within the subsurface.
    • These subsurface currents create a secondary magnetic field, which is measured at the surface.
  • Transient signals are recorded during the 'off' period between transmitter pulses.
  • Signals recorded at longer delay times provide information from deeper depths, making TEM primarily a sounding method.
  • Apparent resistivity is plotted against time (in milliseconds), resembling a sounding curve.
  • Measurements can be taken rapidly, allowing for multiple adjacent vertical soundings to be compiled into a 2D image.
  • Applications:
    • Mapping salinity of groundwater underneath the Sea Of Galilee, where TEM systems towed by boats are used to create resistivity depth profiles. The lake body has a resistivity of around 5Ω⋅m5 \Omega \cdot m, but it decreases to about 1Ω⋅m1 \Omega \cdot m or less within 10-15 meters of the lake bed surface.
    • Groundwater resource protection from agricultural pollution in Belgium, using helicopter-mounted TEM systems. Clay layers, identified as low resistivity regions (pinks and reds), protect underlying aquifers, and mapping the depth to the top of the clay layer informs resource management decisions.

Summary

  • Control source AMT and TEM provide deeper insights than higher frequency domain methods.
  • Control source AMT works in the frequency domain, using natural and artificial sources simultaneously.
  • TEM methods are more complex and expensive but offer better insight into vertical conductivity changes.
  • The choice of EM method depends on the environment, depth, and lateral extent of the investigation required.