The combined use of airborne magnetic and electromagnetic methods for cover characterisation in the exploration for buried mineral systems

Abstract:

Approximately 80% of Australia basement is mantled by a variably thick and complex regolith and sedimentary cover. This cover is also characterised by being conductive relative to the underlying fresh rocks. Consequently it lends itself to being mapped using airborne electromagnetic (AEM) methods. However, in some geological settings the cover is too conductive, limiting the depth of investigation and application of airborne EM systems. Parts of the Fowler Domain, located on the western flank of the Gawler Craton in South Australia, represents one of those areas, particularly where the basement is covered by sediments of the Eucla Basin. The region, prospective for Cu-Ni massive sulfide and orogenic gold deposits, is relatively under-explored primarily because of the extensive cover. Newly acquired airborne magnetics and electromagnetic data provided an opportunity to better understand the extent, thickness and variability of these materials. It also allowed us to investigate the complementarity of these airborne systems where the cover comprises a mix of weathered pre-Cenozoic basement, with overlying fluvial and marine sediments accumulated during the Eocene and Miocene.

We employed magnetic source-depth estimation using sweet-spot method in which a suitable anomaly is selected and a traverse extracted from the TMI grid through the peak of the anomaly and perpendicular to the local trend of the field. A background (regional) field variation along the profile is estimated and a tabular source model with horizontal top is generated and then inverted to match the residual anomaly resulting from subtraction of that regional. Selection of the most appropriate anomalies and intensive individual inversions of targeted data samples generates relatively confident depth solutions, although it cannot negate the inherent ambiguity of the inverse problem. Each inversion result provided an estimate of the local depth of the basement. These solutions are compared with estimates for cover thickness from 1D sharp inversions of the helicopter TDEM data. Regional depth-to- basement maps derived from the different airborne systems were compared. Where cover thickness estimates are compromised by limited investigation depths in the case of the AEM data, we have deferred to those derived from the magnetic inversions.