Appendix Ii: Theory Of Operation - Geometrics MetalMapper 2x2 Operation Manual

Appendix II: Theory of Operation

The figure above shows how the instrument performs background cancellation. The example is
for 60 Hz, but of course the same diagram can be used to determine what to do for 50Hz or
180Hz, etc. The X scale is microseconds. The Y scale is amperes or volts depending on which
signal is plotted.
The red cosine wave represents background 60 Hz. One 2π cycle is 16667 microseconds. Note
that the phase is completely arbitrary with respect to the acquisition events. The magnitude of
this signal is also arbitrary.
The green square represents the positive transmitter current. In the current system this is 10A.
During this time, eddy currents are induced in a conductor within range. When the transmitter
is turned off, the eddy currents start to 'decay' and thus produce significant magnetic fields,
which is measured with the receiver coils. The initial behavior is quite complex, but after 100
microseconds or so it's very well characterized by an exponential decay: f(t) = ae^(-bt). (This can
represent the voltage in the receiver coils or the rate of change of the magnetic field.)
The magenta colored exponential tail curve is the positive decay. The blue square is the
negative transmitter current. The grey exponential tail is the negative decay.
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