AEM base frequency and depth of investigation

Recent advances in time domain airborne electromagnetic (AEM) data acquisition include lower noise levels and subsequently the use of 12.5 Hz and even 6.25 Hz base frequencies instead of the dominant 25 Hz. The main advantages associated with lower base frequencies are increased depth of investigation (DOI) and improved conductor discrimination and detection in the high conductance range. A study was undertaken to quantify these improvements for the Xcite system that resulted from implementing a new suspension system. A synthetic model study was done, evaluating the effect of base frequency and noise levels (utilising noise samples collected at high altitude) on a variety of conductance models. The results were displayed in different formats to illustrate the combined effects of conductance, base frequency and noise levels on DOI. There is no simple formula or correlation to quantify DOI as a function of all relevant parameters and visual analysis of model and inversion results for relevant conductance models is recommended to determine the best acquisition parameters for a survey. The results highlighted the importance of noise levels compared to base frequency to achieve larger depths of investigation and the value of lower base frequencies when exploring for conductive targets in a relatively conductive host environment.