Fixed wing vs. helicopter airborne EM for mapping fine scale conductivity variations in transported sedimentary cover associated with sedimentary uranium mineralisation

Abstract

The constant development and improvement of Airborne Electromagnetic (AEM) systems is not questionable. Contractors are constantly pushing their systems to see deeper, fly faster, output higher moment and to achieve a better resolution. Recording and archiving system parameters is becoming more standard practice than it used to be â?? making it easier to derive accurate conductivity-depth products from the acquired data. Appropriate processing and inversion of the data to achieve reliable conductivity-depth products becomes particularly important when trying to define finer scale conductivity variations. This variability may be relevant when determining aquifer bounds, groundwater quality differences, regolith discontinuities to assist geochemical sampling strategies and/or geological unit boundaries that have exploration significance. In this paper, we examine the importance of understanding how a variable vertical constraint (or roughness) can influence the detail obtained from an inversion of AEM data within an alternating sequence of lacustrine and fluvial sediments in the southern Frome Embayment in South Australia. Specifically we compare results from the inversion of coincident fixed-wing (TEMPEST), helicopter time domain EM (SkyTEM508) data and ground EM data. We demonstrate how the manipulation of inversion parameters can significantly influence the detail obtained from an inversion of TEMPEST data when exploring through these transported sedimentary sequences. At local scales, vertical changes in conductivity can be resolved that better match those defined in the higher resolution SkyTEM data, borehole conductivity logs and ground TEM soundings We also show how the use of both smooth and blocky model inversions can assist the understanding of the geometry and variability of these sediment packages that have associated uranium mineralisation. Our results emphasise the importance of carefully considering the output product being used and the processing employed, particularly when exploring through cover.