Step 1: Back to signal processing
Figure shows transmitter pulse shape, repeated periodically, as a modified squared signal. Secondary fields occur when transmitter current varies.
Step 2: Field equipment
Transient EM is acquired with a magnetic dipole as a transmitter and magnetic field is usually acquired vertically (sometimes horizontally depending on technologies and objectives at stake). With electric current about 300 A in the 26-meter transmitter loop, depth of investigation could reach 800 meters.
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Step 3: Transmitter current shapes library (from Allard, 2007)
Step 4: Acquisition mode example
Primary and secondary magnetic fields with a transmitter current shape from HELIGEOTEM system (from Allard, 2007).
Key information from the figure is that decay of the secondary field is faster for poor conductors during off-times. However, that is not surprising since current loops vanish rapidly in poor conductors.
That could be easily demonstrated by taking the analogy of an electric circuit crossed by a magnetic field and containing a resistance and a self-inductance.
