The time decay constant or “tau” of airborne electromagnetic (AEM) systems is commonly used to indicate the presence and relative conductivity or conductance of conductors in the survey area. In fact, it is not a constant because it depends on the system, the survey design and the method of calculation. The system dependence is a consequence of parameters relating to the acquisition and pre- and post-processing of the signal. Here, we propose a method for calculating tau, which is the time at which the transient voltage decays to 37%, or V37, of some initial value. The model utilises a semi-heuristic algorithm to estimate V37 for each transient in the database and then calculates the delay time at which that voltage is measured, which estimates tau. No calculation is involved with the data, instead, tau is given by a weighted average of the delay times associated with windows either side of the V37 value. We illustrate how this algorithm works using data collected using MEGATEM II at the Reid Mahaffy test site. The results show good agreement between tau-grids reported previously and those calculated using our V37 method. To account for all effects due to the acquisition and processing of EM data, the algorithm allows emphasis to be shifted away from early-time to late-time parts of the transient. It is envisaged that because this method does not apply any mathematical operation to the data it may serve as a robust means of validating other methods.
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