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. Fugro offer 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.