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The EM-Q application was developed in collaboration with Fugro Airborne Surveys (now the Airborne Surveys Division of CGG). It provides a simple and quick way of using moments of the impulse response to interpret a transient electromagnetic (TEM) anomaly. Two types of model are available:

  • A simple sphere.
  • A "dipping" sphere, where the induced current is constrained to flow in a plane.

The results can be used either directly to give estimates of position and physical properties, or as a starting point for more detailed interpretation.

Moments are integrals of the time-weighted impulse response. The moment M of order n is defined as:


The moment data that are used by EM-Q can be generated during processing of GEOTEM™ and MEGATEM™ surveys. CGG offers this option as a value-added service to their clients.

Using EM moments it is possible to apply modelling methods that are similar to those traditionally used for potential field data. For this reason, EM-Q is based on the interface used in PotentQ. Like PotentQ, EM-Q is run from Geosoft's Oasis montaj interface, using a set of customised "GX" applications. Moments allow very fast preliminary interpretation of anomalies in terms of spherical models.


Smith, R., 2001, On removing the primary field from fixed-wing time-domain airborne electromagnetic data: some consequences for quantitative modelling, estimating bird position and detecting perfect conductors. Geophys. Prosp. 49, 405-416.

Smith, R.S., and Lee, T.J., 2001, The impulse response moments of a conductive sphere in a uniform field, a versatile and efficient electromagnetic model: Expl. Geophys., 32, 113-118

Smith R.S. and Lee, T.J., 2002a, The moments of the impulse response: a new paradigm for the interpretation of transient electromagnetic data: Geophysics, 67, 1095-1103.

Smith, R.S., and Lee, T.J., 2002b, Using the moments of a thick layer to map the conductance and conductivity from airborne electromagnetic data: J App. Geophys. 49, 173-183.

Smith, R.S., Lee, T.J., Annan, A.P., and O'Connell, M.D., 2003, Using realizable moments of the impulse response to estimate the conductance and conductivity of the ground: submitted to Geophysics.


   (Click on an image to display an
enlarged version in a new window)

EM-Q Oasis interface (click to enlarge)
You can run EmQ from a marked section of an Oasis montaj database profile, as in this example. Here the user has highlighted an anomaly and displayed the appropriate Oasis montaj GX dialog. The dialog allows you to choose up to six moment channels to model simultaneously (allowing you to model all three components for up to two moments), and to specify various survey parameters.


EM-Q model (click to enlarge)
On clicking OK, the EmQ application runs with the moment data loaded, as shown. Here the first order X and Z moments have been interpreted as a "dipping" sphere with physical properties as stated in the dialog box. The interpretation process took only a few seconds.