Geobandwidth: comparing time domain electromagnetic waveforms with a wire loop model

Abstract

We compare time domain systems of different waveform shape, power and receiver sampling times using a wire loop conductor model to define a comprehensive ‘geobandwidth’ that shows the strength of the response over a range of time constants, analogous to a range of conductance. Frequency domain EM responses can also be calculated as a function of time constant for a wire loop model, giving a consistent comparison method for all time domain waveforms and frequency domain. Arbitrary waveforms can be modelled as a sum of simple short ramps, and the geobandwidth determined numerically. Peak time constant (time constant of peak response) or equivalent frequency can be determined analytically or numerically. The frequency content of a time-domain EM system can be characterised by the peak time constant or the equivalent frequency. The results of these calculations are used to compare response amplitude across a wide range of geological target conductance. Systems can be compared on the basis of signal or signal/noise ratio.