Thermal Map from Assessed Proxies (TherMAP): a pilot study to estimate subsurface temperatures for the Australian continent
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A new approach for developing a 3D temperature map of the Australian continent is currently being developed that relies on combining available proxy data using high-performance computing and large continental-scale datasets. The new modelling approach brings together the current national-scale knowledge contained in datasets collected by Geoscience Australia and others, including AusMoho, OZTemp, OzSeebase, OZCHEM, surface temperature, the Surface Geology of Australia, sedimentary basins’ thermal conductivity and the National Gravity Map of Australia. Bringing together such a range of datasets provides a geoscientific basis by which to estimate temperature in regions where direct observations are not available. Furthermore, the performance of computing facilities, such as the National Computational Infrastructure, is enabling insights into the nature of Australia’s geothermal resources which had not been previously available. This should include developing an understanding of the errors involved in such a study through the quantification of uncertainties. Currently the new approach is being run as a pilot study however, initial results are encouraging. The pilot study has been able to reproduce many observed temperature trends without using direct bore-hole temperature observations as an input into the modelling process. Furthermore, a number of regional areas have now been identified which may warrant further study.Keywords:
Proxy (statistics)
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Mapping the three-dimensional distribution of rock properties from potential field data is a difficult and arduous task, with inherent ambiguity remaining a major problem. We apply a combination of automated interpretation procedures, based on multiscale wavelet analysis and three- dimensional visualisation methods, in an attempt to extract geometrical information from potential field datasets, and display this information in an easily understandable and intuitive way. The resulting visualisations are similar to 'worm' maps commonly produced by interpretation of aeromagnetic data, but are defined in three dimensions.The techniques are tested on a series of synthetic and observed datasets, of varying complexity and geographical scale. The tests show both the effectiveness of the technique as an aid to the geological interpretation of potential field maps and in its use for providing constraints on the three-dimensional geology.The results of the testing on synthetic datasets show that for certain geometries there is an intuitive relationship between 3D edge location and shape, and subsurface geometries. Such relationships prove particularly robust even under 'noisy' conditions where surface features (e.g. the response of laterites in magnetic datasets) mask the coarser scale features that characterise the broader geological picture.Three real datasets at different geographical scales have been analysed. These include the Western Australian gravity dataset (representing the continent-scale), aeromagnetic data covering a 1:100,000 scale map sheet from central Victoria (representing the district-scale), and a prospect- scale aeromagnetic dataset from a mineralised greenstone terrane in Western Australia. The results produce different information at the different scales. At the continent-scale the multiscale edges allow discrimination of different tectonic styles, and comparison of the significance of crustal-scale structures. At the district- to prospect-scale, the edges can be used for geological mapping purposes such as to map subtle changes in sedimentary sequences, map alteration patterns, and constrain pluton geometries at depth.
Potential field
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Australia, via the efforts of the Government Geological surveys, has a program of releasing ever bigger, higher resolution, continental-scale datasets. The recently released isostatically corrected gravity data imagesmany deep and large-scale crustal features. This is a key dataset for understanding the primary structure of the deep crust across thousands of kilometres. Direct "inversion" of this dataset to a consistent 3D fault surfaces network explains more than 50% of the primary information. The method of choice relies on multi-scale edge detection or "worming". This continues to enjoy increasing popularity in the regional mapping domain. Large-scale minerals and oil exploration mapping often make use of this technique. With the current shift to 3D geology modelling, issues arise to improve/generalise the worming technology and get 3D contacts that can be interpreted, particularly the sub-set that indicates a primary fault network. Methods to rapidly compute a consistent 3Dfault network for the entire Australian continent, linking the dominant 20 km deep features back to the surface, are described. If measured gravity curvature gradients are available an even better, more detailed use of these methods at the prospect scale is now available.
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The Australasian Code for Reporting of Mineral Resources and Ore Reserves, December 2004 (the JORC Code) deals with reporting exploration target size and type, which are expressed as ranges in order to properly convey the high level of uncertainty that exists early in the project evaluation process. To inform investors of the significant potential mineralisation in BHP Billiton’s Pilbara tenements and to enable strategic planning, a rigorous modelling approach has been devised for such early stage projects. An innovative approach to modelling scenarios that capture the uncertainty range has been developed. The approach integrates drill holes, mapping, geophysical surveys and the geologist’s interpretation. The modelling workflow consists of two parts: geology modelling, and grade modelling. An implicit 3D modelling approach is utilised to rapidly generate multiple regional models of stratigraphy. Mineralisation envelopes and weathering horizons, providing further controls on the target range, are also modelled using fit-for-purpose tools and data structures. Grades are estimated with a recoverable resource estimation approach. The estimation accounts for uncertainty of local grade distributions and provides multi-variate grade models that honour correlations between variables. The final product delivers a rigorous approach to the estimation of tonnage and grade ranges for reporting the exploration target that captures the uncertainty of the estimate.
Tonnage
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Abstract Modelling of surface and shallow subsurface data is getting more and more advanced and is demonstrated mostly for onshore (hydro)geological applications. Three-dimensional (3D) modelling techniques are used increasingly, and now include voxel modelling that often employs stochastic or probabilistic methods to assess model uncertainty. This paper presents an adapted methodological workflow for the 3D modelling of offshore sand deposits and aims at demonstrating the improvement of the estimations of lithological properties after incorporation of more geological layers in the modelling process. Importantly, this process is driven by new geological insight from the combined interpretation of seismic and borehole data. Applying 3D modelling techniques is challenging given that offshore environments may be heavily reworked through time, often leading to thin and discontinuous deposits. Since voxel and stochastic modelling allow in-depth analyses of a multitude of properties (and their associated uncertainties) that define a lithological layer, they are ideal for use in an aggregate resource exploitation context. The voxel model is now the backbone of a decision support system for long-term sand extraction on the Belgian Continental Shelf.
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Several countries have acquired, over the past decades, large amounts of area covering Airborne Electromagnetic data. Contribution of airborne geophysics has dramatically increased for both groundwater resource mapping and management proving how those systems are appropriate for large-scale and efficient groundwater surveying. We start with processing and inversion of two AEM dataset from two different systems collected over the Spiritwood Valley Aquifer area, Manitoba, Canada respectively, the AeroTEM III (commissioned by the Geological Survey of Canada in 2010) and the “Full waveform VTEM” dataset, collected and tested over the same survey area, during the fall 2011. We demonstrate that in the presence of multiple datasets, either AEM and ground data, due processing, inversion, post-processing, data integration and data calibration is the proper approach capable of providing reliable and consistent resistivity models. Our approach can be of interest to many end users, ranging from Geological Surveys, Universities to Private Companies, which are often proprietary of large geophysical databases to be interpreted for geological and\or hydrogeological purposes. In this study we deeply investigate the role of integration of several complimentary types of geophysical data collected over the same survey area. We show that data integration can improve inversions, reduce ambiguity and deliver high resolution results. We further attempt to use the final, most reliable output resistivity models as a solid basis for building a knowledge-driven 3D geological voxel-based model. A voxel approach allows a quantitative understanding of the hydrogeological setting of the area, and it can be further used to estimate the aquifers volumes (i.e. potential amount of groundwater resources) as well as hydrogeological flow model prediction. In addition, we investigated the impact of an AEM dataset towards hydrogeological mapping and 3D hydrogeological modeling, comparing it to having only a ground based TEM dataset and\or to having only boreholes data.
Data Processing
Electrical Resistivity Tomography
Geological survey
Ground truth
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Summary Europe is in need of fresh aluminum for its vast variety of developments and Greece has the potential to deliver. Delphi Distomon S.A is one of the largest bauxite producers in Greece and is interested to explore new deposits in new unexploited areas. Logistics, accessibility, environmental issues and high cost are key obstacles in the application of a high-definition 3D active seismic survey. Hence, an alternative integrated method of exploration will be carried out based on gravity, magnetotelluric and passive seismic methods. As a preliminary step for an optimized acquisition scheme, a dynamical approach is followed that utilizes a lithology model created by available drilling data, its transformation to a density one, the forward modelling and the comparison of the synthetic data with a previous gravity study in the area. The preliminary results of the analysis gave the chance to identify the vulnerabilities of the lithology and the equivalent geophysical model. As it was observed, they should be enriched during the survey with additional geological information and in situ observations. The emerged models can contribute remarkable to the stage of the processing of the different geophysical methods as well as the final stage of the integrated interpretation.
Magnetotellurics
Lithology
Exploration geophysics
Seismic survey
Delphi
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In this contribution, we aim to draw on the wealth of information that now exists across several Earth Sciences
disciplines and relates to the structure of the lithosphere and asthenosphere of Antarctica. Geological terranes
that are well constrained in continents that were neighbours of Antarctica prior to the break-up of Gondwana
(South America, Africa, India and Australia) are represented in three dimensions. Extrapolation into the interior
of Antarctica is constrained by extensive remote sensing and geophysical datasets. We also incorporate direct
information on the Antarctic continent which has substantially improved in both quality and coverage following
extensive field programs of several nations in association with the 2007-2008 International Polar Year. Where
several contrasting models remain possible, we construct multiple models that allow such alternatives to be readily
compared.
The models that we construct are of an appropriate resolution for continent scale rheological and seismological
simulations. They consist of spatial coordinates including depth, material property values, and also
metadata which provide for nominal uncertainty estimates and provenance information for the model values. This
approach enables a variety of information to be included in a single model, and well and less-well constrained
parts of the model to be handled with rigor. The combination of multiple models, and model uncertainty metadata,
into model suites is a liberating one.We maximise the inclusion of information across the disciplines of geoscience
such that inaccurate, insufficient and inconsistent data may be evaluated.
Applications of the new models include large-scale ice sheet modelling, including glacial isostatic adjustment
studies. They can also be applied to sensitivity testing with respect to new instrumental deployments in
Antarctica such as large scale passive seismic experiments. As the international community progresses from
reconnaissance studies to understanding the more detailed implications of lithospheric and asthenospheric heterogeneity,
in the continent beneath Antarctica, such simulations have two-fold potential. Firstly, in illuminating the
relationship between observable and inferable physical properties. Secondly, in optimising the locations of future
deployments for the purpose of distinguishing between candidate deep Earth structures. Model suites will be made
available for the use of the research community in interoperable data formats.
Asthenosphere
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Insights and Lessons from 3D Geological and Geophysical Modeling of Mineralized Terranes in Tasmania
Over the last two decades, Mineral Resources Tasmania has been developing regional 3D geological and geophysical models for prospective terranes at a range of scales and extents as part of its suite of precompetitive geoscience products. These have evolved in conjunction with developments in 3D modeling technology over that time. Commencing with a jurisdiction-wide 3D model in 2002, subsequent modeling projects have explored a range of approaches to the development of 3D models as a vehicle for the better synthesis and understanding of controls on ore-forming processes and prospectivity. These models are built on high-quality potential field data sets. Assignment of bulk properties derived from previous well-constrained geophysical modeling and an extensive rock property database has enabled the identification of anomalous features that have been targeted for follow-up mineral exploration. An aspect of this effort has been the generation of uncertainty estimates for model features. Our experience is that this process can be hindered by models that are too large or too detailed to be interrogated easily, especially when modeling techniques do not readily permit significant geometric changes. The most effective 3D modeling workflow for insights into mineral exploration is that which facilitates the rapid hypothesis testing of a wide range of scenarios whilst satisfying the constraints of observed data.
Prospectivity mapping
Mineral exploration
Mineral resource classification
Identification
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Within any construction project the most significant factor in controlling the cost and feasibility is the
subsurface ground conditions. This is particularly the case in underground construction. Geological modelling
in three-dimensions (3D) can provide a detailed definition of sub-surface conditions. Such modelling
requires the extension of traditional GIS methods to handle the volumetric representations. Over the
past two decades, a series of sophisticated 3D modelling technologies have been developed to address this
need. However, the adoption of these techniques in the geotechnical industry has lagged behind technological
advances.
Two contrasting approaches to 3D geological modelling are presented: a) the Thames Gateway Development
Zone (TGDZ) in London, UK and b) subsurface characterisation studies in Boston, USA. The
TGDZ studies used ‘GSI3D’ software, while the Boston studies involved geostatistical evaluations of the
field data and the Environmental Visualization System (EVS) for model creation and visualisation. Both
studies have created 3D geological models attributed with physical and mechanical property data, but this
has been achieved in two different ways. The TGDZ study provides a single uniform property attribution
to individual geological units, whereas the Boston studies used geostatistical methods (kriging) to interpolate
borehole sample data onto a 3D structured mesh. This 'discretisation' allowed the development of
volumetric models that quantified the variability of the data used to build the property model.
These different modelling methods provide solutions to two very different problems. In the TGDZ, the
requirement was for regional scale information for ground investigation design, for assessing water management
strategies, and as a tool for communicating information to non-geo-specialists. In this situation,
the best approach was a system for model building that did not require a specialist modeller, the use of
bulk attribution, and the ability for modelling to be carried out quickly using a desktop computer. However,
in Boston, a more specialist solution was required to provide a detailed understanding of the natural
variability of the complex geology, thus discretisation and spatial interpolation of sample data values was
necessary.
The 2001 EuroConference in Spa, Belgium, that addressed characterisation of the shallow subsurface,
identified four major constraints on the use of 3D digital geological data: This paper shows that these
constraints are being overcome with the use of new modelling software and techniques and, more importantly,
with an understanding of the needs of the client.
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Petrophysical rock properties are key to populate local and/or regional numerical models and for the interpretation of many geophysical investigation methods. To date, large numbers of datasets are available in numerous sources. This diversification often causes time-consuming literature research with often limited success due to unwanted generalization of the dataset. A lack of detailed information on the sample location, petrography, stratigraphy, measuring method and measurement conditions makes it difficult to use these data for specific locations or reservoir units.
An open-access database is currently under development within the scope of the EC funded project IMAGE (Integrated Methods for Advanced Geothermal Exploration, grant agreement No. 608553). Similar to the World Stress Map (Heidbach et al. 2010), the IMAGE database aims at providing information on published data – here petrophysical properties – in one single compilation. The data-base is designed to allow for an easy access to data relevant for geothermal exploration and reservoir characterization. Collected data include petrophysical properties, thermophysical properties, mechanical properties, additionally electrical resistivity and magnetic susceptibility. Each data point or sample is identified by an unambiguous sample ID, based on the name of the first author of the original publication, the publication year and a running number. Furthermore, sample information like the type of sample location, geographic coordinates, elevation and sampling depth are documented and linked with the stratigraphy and an as detailed as possible petrographic description following a hierarchical petrographic catalogue for each sample. In addition, information on the experimental set-up of the different laboratory measurements are given for quality control. This also allows analyzing whether the properties are dependent on temperature, pressure or degree of saturation or type of saturating fluid and thus will enable the definition of specific parameters.
Up to now, most of the data that entered the database are either from published data collections or from laboratory measurements performed at TU Darmstadt. So far, more than 38,000 data points from all over the world were collected. It is planned to set up a publicly accessible database through a web-based interface to allow external users and scientists to add own research data. Using this provided data and own measurements the web-based database will be updated continuously.
The collected data will help researchers and users in the early stages of new geothermal projects to make a first assessment of the geothermal rock properties and can be used for the planning of future exploration and in areas where the existing data density is sufficient even for exploitation projects. Additionally, the database helps improving local and regional geoscientific models of many different purposes.
Petrophysics
Sample (material)
Data set
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