Accurate delineation of fault systems and characterisation of fault zone hydrogeology in sedimentary basins and paleovalleys is challenging and expensive. Over the past few years, considerable progress has been made in the development of airborne electromagnetic (AEM) technologies for rapid and accurate mapping of hydrostratigraphy, structural elements and groundwater systems and resources. Compared to seismic operations, AEM surveys are orders-of-magnitude cheaper and being airborne, have advantages in terms of land access and impacts.
New calibrated AEM systems have the proven ability to map conductivity contrasts in the near-surface at high-resolution. AEM inversions map the subsurface distribution of bulk electrical conductivity, hence provide a different perspective to seismic datasets, and support the mapping of multi-layered hydrostratigraphy (lithology + porosity), hydrology (groundwater salinity + water saturation), as well as evidence of geological displacement (faults). However, until recently, fault geometries, displacements, and fault zone properties remain ambiguous. This is due to the combination of AEM footprint resolution, the non-uniqueness of the conductivity models and derived hydrostratigraphy and fault geometry solutions produced by AEM equivalent inversion models, and the inherent uncertainty of ‘standard’ 1D AEM inversion models. The resultant uncertainty in fault zone characterisation inhibits investigations into the permeability heterogeneity and anisotropy introduced by faults, making it difficult to resolve the significance of these structures for groundwater processes.
In recent projects new inter-disciplinary workflows have been developed to optimize AEM data. This commences with AEM system suitability assessment, while acquisition strategies take into consideration all relevant prior data and knowledge. Careful consideration of AEM data processing workflows is also vitally important, particularly in removing noise (e.g. from VLF sources and cultural coupling), while the use of spatial filters must be used judiciously, if at all, particularly for 3D inversions. The development of new 2.5D and 3D AEM inversion procedures is particularly important in removing topographic effects and imaging the hydrostratigraphy and geometry of faults with high fidelity.
While AEM mapping technologies and inversion procedures have evolved to permit accurate 3D mapping of fault geometries, supporting borehole, hydrochemical and hydrodynamic data are required to assess the influence of faults on dynamic groundwater processes. A combination of deterministic and stochastic modelling approaches are required to understand complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and recharge. These methodologies can foster more robust, realistic and sophisticated parameterisation of numerical groundwater flow models, including the incorporation of structural elements, like faults.