SummaryAirborne electromagnetic data generated by the AusAEM Survey are shown to map mineral deposit host rocks and regional geological features within the AusAEM Survey area. We have developed new functionality in Geoscience Australia’s sample-by-sample layered earth inversion algorithm, allowing inversion of the magnitude of the combined vector sum of the X- and Z-components of TEMPEST AEM data. This functionality improves the clarity of inverted interpretation products by reducing the degree of along-line incoherency inherent to stitched 1D inversions. The new inversion approach improves the interpretability of sub-horizontal conductors, allowing better mapping of geological features under cover.Examples of geological mapping by the AusAEM survey highlight the utility of wide line spacing, regional AEM surveying to improve geological, mineral systems and groundwater resource understanding in the regions flanking outcropping mineral deposit host rocks in northern Australia.
SummaryLarge geophysical data has traditionally been difficult to manage in a consistent, open, and efficient manner. The demands of modern, large-scale computing techniques, coupled with the need for sound data and metadata management, mean that established data formats and access methods are no longer adequate.Geoscience Australia (GA) has been working with its partners to leverage and extend existing data standards to represent various geophysical data in modern scientific container formats including netCDF & HDF. The new data encodings support rapid and efficient data subsetting, either directly from a file or remotely via web services. These will underpin GA’s future data delivery pipelines for Australian government-funded geophysical data.NetCDF efficiently handles multi-variate raster, line, and point data, as well as n-dimensional data structures supporting more demanding applications such as AEM and airborne gravity data. Structural and metadata standards deliver interoperability, and existing and emerging data types are supported without loss of precision or other information.This extended abstract will cover: The rationale for Modernising GA’s geophysical data holdings into modern open standard container formatsAn outline of the netCDF4 file format and associated tools, and some of the benefits they provideThe open-source tools and methodology used to translate grid, line, point and other data into netCDF4, and to perform metadata synchronisationA brief description of a live use case exploiting web services
In the Tintinara area, located south-east South Australia, airborne geophysics was recognized as<br>having potential to provide valuable biophysical data relevant to the management of irrigation<br>development and groundwater recharge reduction in the area. The groundwater of the area sustains<br>irrigation and other dryland agriculture. However the lifetime of this resource is limited by the leaching<br>of salt that has accumulated in the soil prior to land clearing and agricultural development. For some<br>areas, the groundwater may be saline and unusable for irrigation within ten to twenty years. The<br>presence or absence of a near surface clay unit can have an important influence on the rate and timing of<br>this deterioration by slowing recharge. Forward modelling suggested that a frequency domain helicopter<br>electromagnetic (HEM) system could map spatial variability associated with this unit. A survey was<br>conducted using the RESOLVE® HEM system and demonstrated that, through the use of a constrained<br>inversion approach, this unit could be mapped. This was confirmed with shallow drilling. We have<br>generated a product, namely clay thickness, which is now being used as an input into a hydrogeological<br>model to help predict recharge rates and influence management decisions in the area.
SummaryData from a VTEM™ airborne electromagnetic survey over resistive terrain is examined. Forward modelling and analysis of high-altitude lines shows that the amplitudes of random noise, bucking error, processing corrections and geological signals can be large compared to the geological signal in the resistive terrain. The negative impacts of the low geological signal to noise ratio on conductivity estimates generated by layered-earth inversion and conductivity transformations are demonstrated. The reader is alerted to the degree of uncertainty and non-uniqueness that is inherent in conductivity estimates generated from similar datasets.
High-resolution hydrogeophysical data are increasingly acquired as part of investigations to underpin groundwater mapping. However, optimization of AEM data requires careful consideration of AEM system suitability, calibration, validation and inversion methods.In modern laterally-correlated inversions of AEM data, the usefulness of the resulting inversion models depends critically on an optimal choice of the vertical and horizontal regularization of the inversion. Set the constraints too tight, and the resulting models will become overly smooth and potential resolution is lost. Set the constraints too loose, and spurious model details will appear that have no bearing on the hydrogeology. There are several approaches to an automatic choice of the regularization level in AEM inversion based predominantly on obtaining a certain pre-defined data misfit with the smoothest possible model.However, we advocate a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. In this approach, in a process of both confirming and negating established interpretations and underlying assumptions, the inversion results are judged by their ability to support a coherent conceptual model based on all available information. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project.
Geophysical methods are used in Australia to provide detailed spatial information to help predict the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. RESOLVE frequency domain helicopter electromagnetic data were acquired over the Chowilla Floodplains, in the Lower Murray region of southern Australia, to provide detailed baseline data on the spatial distribution of near‐surface salt stores and materials in the floodplain and their relationship with in‐river salinity. Degradation across the floodplain and wetlands has resulted primarily from a significant reduction in flood events, and overgrazing. Restoration of the floodplain will involve the reduction of salinity flow from groundwater into the river and increasing environmental flows across the floodplain. Conductivity models predicted from HEM data help identify local recharge and discharge areas, and links with river salinity. The baseline data provided by the airborne data are used with high resolution ground EM surveys including EM31 and time‐domain EM, over targeted areas. Ground methods can be repeated, to monitor affects of artificial flooding designed to restore vegetation health. Similarly the combination of airborne and ground data, allows piezometers to be effectively targeted with the resulting information interpreted within the context of the baseline conductivity structure defined form airborne data.
The integration of high resolution, image-processed aeromagnetic data with regional geological, magnetic, gravity and seismic data-sets has provided new insights into the structural architecture, rifting history, and petroleum potential of the western onshore and offshore Otway Basin, south-eastern Australia.Three principal structural directions are evident from the magnetic data: NS, NE-ENE and NW-WNW. The structural fabric and regional geological data suggest that the rifting history of the basin may have taken place in two distinct stages, rather than within a simple rift-to-drift framework. The initial stage, from 150 to ~120 Ma, took place within a stress regime dominated by NW-SE extensional transport, similar to that of the basins within the Great Australian Bight to the west. ENE-striking extensional rift segments, such as the Crayfish Platform-Robe Trough and the Torquay Sub-Basin, developed during this period, contemporaneous with the deposition of thick sediments of the Early Cretaceous (Tithonian-Hauterivian) Crayfish Subgroup. In other parts of the basin, NW-striking rift segments, such as the Penola, and perhaps Ardonachie, Troughs onshore, developed within a strongly trans-tensional (left-lateral strike-slip) environment. At ~120 Ma, the regional stress field changed, and the Crayfish Subgroup-aged rift segments were reactivated, with uplift and block faulting extending through to perhaps 117 Ma. Rifting then recommenced at about 117 Ma (contemporaneous with the deposition of the Barremian-Albian Eumeralla Formation), though the extensional transport direction was now oriented NNE-SSW, almost perpendicular to that of the earlier Crayfish Subgroup rift stage. This later rift episode ultimately led to continental breakup at ~96 Ma and produced the 'traditional' normal fault orientations (NW-SE to WNW-ESE) throughout the Otway Basin.
The Ord Bonaparte Plains area is a priority area for irrigated agriculture development as part of the Ord Stage 3 development in the East Kimberley region of Western Australia. Irrigated agriculture in this area will depend on access to groundwater resources in underlying bedrock aquifers. A program of airborne electromagnetics (AEM), drilling, ground and borehole geophysics and hydrogeological investigations is being undertaken to confirm the presence of suitable groundwater resources, map the connectivity between surface and groundwater systems, and identify potential risks to agriculture and water infrastructure including salt stores, groundwater salinity and seawater intrusion.Preliminary analysis shows that the AEM survey has successfully mapped key elements of the groundwater system, including aquifer and aquitard extent, groundwater quality (salinity) distribution, hydraulic properties, compartmentalisation and inter-connectivity, the seawater intrusion (SWI) interface in coastal zones, and key tectonic elements of regional hydrogeological significance. The survey has mapped significant faulting within the Cockatoo Sandstone and Point Springs Sandstone aquifers, while conductivity distributions suggest that faults within and bounding major stratigraphic units display both fault barrier and conduit behaviour. The survey has also found that fresh groundwater in the aquifer system continues offshore as discontinuous lenses.Initial inversions have been used to target drilling, hydrochemical investigations, and a program of ground geophysics (including Surface Nuclear Magnetic Resonance (SNMR)). Further analysis and groundwater modelling is required to determine appropriate development and management of any groundwater resource and the potential risks to agricultural development.
Geoscience Australia is releasing into the public domain software for the inversion of airborne electromagnetic (AEM) data to a 1D conductivity depth structure.The software includes two different algorithms for 1D inversion of AEM data. The first is a gradient based deterministic inversion code for multi-layer (smooth model) and few-layered (blocky-model) inversions. The second is a reversible-jump Markov chain Monte Carlo stochastic inversion algorithm suitable for assessing model uncertainty. A forward modelling program and some other ancillary programs are also included. The code is capable of inverting data from all of the commercial time-domain systems available in Australia today, including dual moment systems.The software is accessible in three forms. As C++ source code, as binary executables for 64 bit Windows® PCs, and as a service on the Virtual Geophysics Laboratory (VGL). The code is fully parallelized for execution on a high performance cluster computer system via MPI or a multi-core shared memory workstation via OpenMP.
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