On the Time Decay constant of AEM systems: A Semi- Heuristic Algorithm to validate calculations

Abstract

The time decay constant or “tau” of airborne EM systems (AEM) is commonly used to indicate the presence and the relative conductivity or conductance of conductors in the survey area. In fact, it is not a constant since 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 simply the time at which the transient voltage decays to 37%, or V37, of some initial value. The model utilises a semi-heuristic algorithm that estimates V37 for each transient in the database and then it calculates the delay time that voltage is measures, which is the estimates tau value. No calculation is involved with the data, instead tau is given by a weighted average of the delay times associated with the windows either side of the V37 value. We illustrate how this algorithm works using data collected by MEGATEM II in the Reid-Mahaffy test site. Results shown a good agreement between tau-grids reported in previous studies and those calculated with our V37 – method. To account for all effects coming from the acquisition and processing of EM data, the algorithm allows the emphasis to be shifted from early to late time parts of the transient. It is envisage that since this method does not apply any mathematical operation to the data it may serve as a robust means of for validating other methods.