An inversion algorithm for time-domain airborne electromagnetic (AEM) data has been developed. A complete flight line of data are inverted in each inversion to simultaneously recover a 2D layered earth conductivity distribution and selected unmeasured elements of the AEM system geometry. The conductivity and thickness of each layer in the earth model and the system geometry, are all parameterised by separate along line 1D cubic B-splines. It is the coefficients of these splines that become the unknowns that we solve for in the inversion. This new method is motivated by the success of three separate concepts: the ?holistic approach? to the inversion and calibration of frequency-domain AEM data (Brodie and Sambridge, 2006); laterally constrained inversion (Auken et al., 2005; Vallée and Smith, 2008); and simultaneous estimation of subsurface conductivity and system geometry parameters (Lane et al., 2004; Sattel et al., 2004). The method could be described as a form of laterally constrained inversion with a spline model parameterisation and the additional capability of solving for system geometry.
A recent paradigm shift has occurred in our understanding of the importance of Neogene intra-plate tectonics for surface hydrology and groundwater systems in Australia. While active deformation zones are readily identified by seismicity monitoring and high-resolution satellite and airborne terrain mapping, advances in airborne electromagnetic (AEM) technology and data optimization have enabled rapid mapping of more extensive 'blind' Neogene intra-plate fault systems in the near-surface within seismically quiescent depositional landscapes. However, to date, the geometries, displacements, and properties of these Neogene faults have remained ambiguous due to a paucity of outcrop, AEM footprint resolution, and the non-uniqueness of the conductivity models and derived hydrostratigraphy and fault geometry solutions produced by AEM equivalent inversion models. In this study, a novel, inter-disciplinary approach has been developed to characterise the hydrogeology of intra-plate fault systems within unconsolidated, near-surface depositional landscapes and underlying sedimentary basins. The approach integrates morphotectonic analysis with sub-surface mapping of hydrostratigraphy and 'blind' intra-plate fault systems using airborne geophysics (AEM, airborne magnetics) and satellite remote sensing (e.g. Landsat). Fault zone geometry and displacement is examined using both 1D and 2.5D inversions of AEM data, validated at local scales by ground geophysics (e.g. seismic reflection, resistivity and ground nuclear magnetic resonance (GMR)) and drilling. Fault zone hydrogeology has been assessed through the integration of hydrogeophysics with hydrochemistry, hydrodynamics, and studies of vegetation response to water availability (using Landsat time-series analysis). The approach has also adapted deterministic and stochastic structural analysis approaches developed for use with seismic reflection data in the oil industry, to analyse AEM datasets. This inter-disciplinary approach has revealed complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and localised recharge and discharge. This approach has revealed that Neogene deformation is a 'keystone element' of the Lower Darling floodplain and other surface hydrological and groundwater systems in Australia.
The Broken Hill Managed Aquifer Recharge (BHMAR) project aimed to define key groundwater resources and aquifer storage options in the lower Darling River floodplain of western NSW. The project was multi-disciplinary and utilised airborne electromagnetics (AEM), borehole nuclear magnetic resonance (NMR) and LiDAR DEM data and lithological, hydrostratigraphic and hydrochemical information to develop a suite of hydrogeological and groundwater property maps and products.This abstract discusses the methods and results of estimating the transmissivity of the semi-confined target aquifer. Hydrostratigraphy and hydraulic texture classes were mapped by interpreting the AEM data in conjunction with borehole geophysics and lithological information. Aquifer transmissivity was statistically derived by combining borehole NMR hydraulic conductivity estimates with the mapped 3D distribution of texture classes and hydrostratigraphic units. Using a statistical and GIS approach, the derived aquifer thicknesses in the key areas ranged from 20 - 40 m and the lower and upper transmissivity bounds ranged from 1 to 10 m2/d, and 10 m2/d to 1000 m2/d, respectively.
Over the past decade, a relatively rich record of neotectonics has been revealed in continental Australia, however very few investigations into the hydrogeological implications have been undertaken. While the most active intra-plate deformation zones are readily identified by seismicity monitoring and satellite and airborne terrain mapping, advances in airborne electromagnetic (AEM) technology and data optimization have made it possible to map numerous, more subtle ‘blind’ intra-plate fault systems concealed in near-surface floodplain landscapes. To date, fault geometries, displacements, and fault zone properties remain ambiguous 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 stitched 1D AEM inversion models. The resultant uncertainty in fault zone characterisation inhibits investigations into the permeability heterogeneity and anisotropy introduced by these faults, making it difficult to resolve the significance of these structures for groundwater processes.In this study, a novel, inter-disciplinary approach has been developed that helps characterise the hydrogeology of one such intra-plate fault zone in unconsolidated, near-surface floodplain sediments. The approach integrates the mapping of tectonic geomorphology, with mapping of sub-surface hydrostratigraphy and ‘blind’ intra-plate fault systems using AEM. Validation of fault zone geometry and displacement at local scales is provided by ground geophysics (e.g. seismic reflection, resistivity and surface nuclear magnetic resonance (SNMR)) and drilling. Fault zone hydrogeology, including permeability variability, has been assessed through the integration of geophysics with hydrochemistry, hydrodynamics, and studies of vegetation response to water availability (using Landsat time-series analysis). A combination of deterministic and stochastic approaches is then used to unravel complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and recharge. This inter-disciplinary methodology has been used to parameterise numerical groundwater flow models and target potential groundwater resources.
In Australia, groundwater investigations in remote frontier areas face challenges including the cost and difficulty in obtaining drilling approvals due to often lengthy heritage and environmental approvals processes. Non-invasive geophysical techniques, including airborne electromagnetics (AEM) and new Ground Magnetic Resonance (GMR) techniques are particularly attractive in these circumstances, as key hydrogeological parameters including depth to watertable, bound and free water contents, and transmissivity can be obtained cost-effectively in short timeframes with limited clearance approvals required.
A RESOLVE frequency domain helicopter electromagnetic (HEM) survey has been flown in and around the Riverland irrigation districts of South Australia. The purpose was to map the distribution and thickness of near-surface clay-rich sediments which impact on irrigation, groundwater and salinity management strategies.The survey data were re-calibrated after their conventional processing by utilising independent ground geoelectric data.Data were inverted using a five-layer 1-D parameterisation of the Earth. Reduction of the ambiguity in the unknown aspects of the geological section was sought by constraining the inversion with as much local geological and hydrological information as was available. Groundwater depth information was incorporated as an extra datum to constrain the upper layer thicknesses. A combination of drillhole lithologic, groundwater and downhole conductivity data were used to construct a spatially variable reference model and impose constraints on the estimated parameters in the form of prior probability information.The resulting detailed map of the thickness of the Blanchetown Clay is more detailed than previous compilations based on drilling. The results also provide insight into other important hydrogeological features of the Riverland area.
The Australian Government has recently provided Aus$100.5M to Geoscience Australia over 4 years (2016-2020) to manage the Exploring for the Future (EFTF) programme designed to increase investment in minerals, energy and groundwater resources, primarily in Northern Australia. The programme includes Aus$30.8M for groundwater-specific investigations, recognizing that there are major gaps in our knowledge of Northern Australia’s groundwater systems and resources. The groundwater component of the EFTF programme is focused on addressing these knowledge gaps, with the aim of underpinning future opportunities for irrigated agriculture, mineral and energy development, and community water supply. The groundwater programme will include identification and assessment of potential groundwater resources and water banking options in priority regional areas, while also analyzing the salinity risk (including seawater intrusion).To rapidly map, characterise and assess regional groundwater systems and resources in the data-poor ‘frontier’ areas of Northern Australia, a multi-physics, inter-disciplinary approach has been developed. The programme involves the initial use of temporal remote sensing ‘data cube’ technologies for surface hydrology and landscape mapping, and acquisition of airborne electromagnetic (AEM) and Ground Magnetic Resonance (GMR) datasets. This provides a framework for targeted investigations including passive seismic, microgravity and GPR; borehole geophysics (Induction, gamma and Nuclear Magnetic Resonance (NMR)); drilling and pump testing; hydrochemistry and geochronology (water, landscapes and geology); as well as soils, regolith and basin/bedrock geological, hydrogeological and structural mapping and modelling.This methodology has enabled rapid identification and assessment of potential groundwater resources, salinity and seawater intrusion hazards, and groundwater dependent ecosystems in several priority regions.
Summary Geoscience Australia has undertaken a number of regional drilling projects in collaboration with state and territory geological surveys. To reduce uncertainty associated with intersecting the targeted stratigraphy, we have actively used a range of geophysical methods to estimate cover thickness to identify prospective sites for drilling.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
