ASEG Extended Abstracts - 1st Australasian Exploration Geoscience Conference – Exploration Innovation Integration, 2018

Exploring inversion solution space is the process of mapping out the domain of inversion models which have a predicted response in satisfactory agreement with the observed data. This is particularly important in mineral exploration geophysics because the associated inverse problems are highly ill-posed. An inversion case study over a Cu-Ag deposit in the Kalahari copper belt provides a good demonstration of the process: in the case study 3D IP field data show a clear indication of chargeability so it is no surprise that a chargeability anomaly is recovered from a standard 3D IP inversion. Extensive drilling delineated a disseminated sulphide zone however the drilling delineates a shallow target while the inversion indicates an interesting deeper target. The immediate exploration question is whether to invest in a deep drillhole to test the deeper target? Or is the deeper target merely an artefact of non-uniqueness associated with the IP method?

Exploring the inversion solution space produces a suite of models all of which fit the data satisfactorily. In particular, exploring the solution depth produces a suite of models with a variety of depths, including some models with a shallow chargeability anomaly. This significantly weakens expectations associated with the initial deep inversion model but improves exploration decision making.

Exploring inversion solution space is achieved computationally by systematically modifying the regularization term in the traditional Tikhonov formulation. With an inversion workbench the process is easy, the results are remarkably helpful, and the outcomes surprisingly quickly become a natural part of the exploration team’s interpretation process.

In January 2017 the largest airborne magnetic, radiometric and elevation survey in South Australia’s history began in the Gawler Craton. The aim of the South Australian Government is to use the survey as an opportunity to achieve best practice in relation to the coordination, landholder liaison, reporting and quality control of the survey, in tandem with collaborative partners at Geoscience Australia. Some of the outcomes include a series of test lines flown by all survey aircraft to assist in back-calibration of radiometric data; landholder and stakeholder information website to maximise public value; subscriber email updates; close liaison with field capture teams; timely delivery of survey data and results and a proactive approach in ensuring the data captured is of consistent, high quality across the entire survey region. The survey is being performed in three stages and each stage provides an opportunity to assess approaches and fine tune requirements. It is expected that the end result will set a benchmark for other jurisdictions performing similar work.

Embankments are common features in mine sites necessary for tailings storage, surface water management or general infrastructure such as dewatering ponds. Differing construction methodologies, from loosely placed waste material to engineered embankments with individually compacted lifts, will achieve varying density, strength and permeability. Conventional construction quality assurance is however not always possible without causing significant interruptions to the construction program. Estimating levees’ bulk shear wave velocities via passive seismic HVSR surveying as a proxy for stiffness is a practical, continuous and non-invasive method that can be carried out with limited construction interruption over all types of structures. This also provides a continuous dataset throughout the embankment as opposed to discrete observations using conventional geotechnical methods.

Field data acquired over the length of several embankment types demonstrate the very good correlation between estimated shear wave velocities and the levees’ degree of compaction. As a result, alternative construction methodologies can be quantitatively benchmarked against a bulk density spectrum with fully engineered embankments and loose waste dumps as end-members. Collection of repeated measurements over time also discriminates stable embankments from altering ones, and constitutes a cost-effective way to identify possible zones of weakness before hazardous failure.

To provide a “no major surprises” guarantee of coal seam conditions, reflection seismic surveying is often used for delineation of faults and dykes that have the potential to disrupt underground coal mining operations. Although reflection seismic methods are usually effective for locating faults with throws greater than 5-10 m for 2D and 2-5 m for 3D seismic data, detection of faults with smaller throws, shears and dykes with widths of a few metres remains a challenge to seismic methods.

Instead of ignoring or suppressing diffractions by conventional seismic data processing, it has been demonstrated that diffractions contain valuable information, which can be used for identification of subtle coal seam structures. In this paper, we describe a moving average error filter (MAEF) applied in the neighbouring traces to extract diffractions from post-stack reflection seismic data. The filter estimates the reflections with the average values of the neighbouring traces along the reflection direction or dip, which can be computed by the gradients of seismic data. The difference (or error) between the original data and the estimated reflections, yields the diffractions. By identifying diffractions, small faults and other minor features that are difficult to detect using conventional seismic reflection processing can be detected. Numerical and real data examples are used to illustrate the effectiveness of the proposed method in coal seam structure detection by extracting diffractions from reflection seismic data in a relatively complex geological environment.

The Tres Hombres structure is a large mid-crustal structural feature that that underlies the Permian - Late Jurassic mega-sequences of the Northern Beagle Sub-basin, Western Australia. Originally identified on regional 2D seismic lines, the Tres Hombres structure has now, for the first time, been fully imaged by high quality, deep record modern 3D seismic data. The area is also covered by gravity and magnetic datasets which were acquired together with the 3D seismic survey. Seismic mapping reveals a dome-like structure with a diameter of more than 30km, and with vertical relief of over 5km. This paper integrates seismic and potential fields datasets to explore the origins of this intriguing structure.

Mechanisms considered for the emplacement of this feature include; basement cored compression, reactivated extensional basement faulting, remnant Palaeozoic topographic relief, salt-related diapirism, or plutonic/igneous intrusive activity. The actual mechanisms responsible for the evolution of Tres Hombres have obvious implications for adjacent and overlying petroleum systems within the Beagle Sub-basin.

Detailed mapping of new 3D seismic datasets enables structural and stratigraphic restorations to be generated, which provide valuable insights into the timing of the Tres Hombres feature. Variations in the thicknesses of overlying sequences show the influence that this structure had on the stratigraphic evolution of the basin. Gravity and magnetic datasets have also been integrated into this study, and provide valuable controls on potential lithologies within the core of the Tres Hombres feature. These in turn have important implications as to the origin of this structure, and relationships to the tectonic evolution of the Beagle Basin.

We show how clustering algorithms can ensure that the core intervals that are pertinent to specific objectives of a sampling campaign are actually sampled. We also show how clusters can be validated prior to sampling with auxiliary data not used for the cluster analysis.

We chose to target our core sampling to ensure that both clay poor and clay rich intervals of the Springbok Sandstone are sampled. The clay phases in the Jurassic Springbok Sandstone generally do not exhibit a prominent gamma ray signature and are therefore poorly defined in wireline logs. Similar, hydrogeological properties of the Springbok Sandstone are not well defined through wireline logs. This introduces uncertainty to groundwater models of the Springbok Sandstone. Hence, a better understanding of the clay distribution is thought to be a key to improve the definition of the hydrogeological properties of the Springbok Sandstone. We applied our sample targeting approach to five study wells from the Surat Basin in Queensland. We tailored the application of the cluster analysis to our working hypothesis that the variability of hydrogeological properties of the Springbok Sandstone is controlled by the presence and type of clays, rather than compaction. This informed our choice of wireline logs to include in the clustering (nuclear logs) and of logs to be used for control purpose (resistivity logs, spontaneous potential).

We show that identification of five clusters was the most useful number towards our sampling objectives. This allowed for example to exclude coal and siderite layers from sampling for clay analysis and to focus on the differentiation of the clastic sediments in the formation. Further, we show that certain clusters correlate with resistivity and spontaneous potential log signatures. The correlation between the categorical clusters based on nuclear logs and continuous wireline logs not used in the cluster analysis allowed us to interpret the meaning of the clusters in the context of our project and target our sampling to ensure that all clusters are represented in our sample set.

The McArthur basin/EMU fault study has a classic 2D fault feature and a buried conductor with an off-end effect with other 2D/3D effects away from the EMU fault. The collected AEM data has demonstrable AIP effects. This has stimulated an investigation of a simple 2D geology cross-section of a dipping fault with a strong conductor on one side of the fault.

A forward model of the predicted response near the EMU fault represents a synthetic observed signal from the cross-section in agreement with the AEM data. Our modelling shows that the 1D inversion gives results which do not reproduce the survey data whereas 2.5D performs well reconciling the inverted section with the observed EM response. Away from the 2D geology region and other 2D/3D EM effects, 1D does perform well as expected. Therefore 2.5D gets it right in significantly more situations by honouring the information in the observed data raising questions about the use of 1D.

Emerging AEM systems can provide estimates of economic rock unit thicknesses, dips, faults and anticline/syncline definition at an accuracy that mitigates the need for pattern drilling. The use of 2.5D allows marker beds of more conductive material to stand out at a depth of 500 m or more on sections created beneath individual flight lines. Routine treatment of all survey data is now possible without supercomputing capability.

CSIRO has also recently undertaken comparative studies of the available AEM 1D, 2.5D and 3D inversion codes. Their work raises some stark reminders of what is different in the methodologies and how the progression to higher-order geophysics methods requires not just careful test work but also effective education of the user community.

We explain the fundamental differences between 1D and 2.5D and point out issues with the 1D forward modelling and inversion technology. Importantly, Maxwell’s equations are used to constrain 2.5D whilst empirical methods are commonly used in 1D. This leads to the situation where a near zero average misfit using stitched 1D models can be achieved with families of 1D inversions, whilst incorrectly predicting the geology. Therefore a low misfit does not necessarily indicate a good solution for 1D. The 2.5D method is a least-squares best fit of the observations and so the quoted misfit for 2.5D is a very different measure than for 1D. The study demonstrates that 2.5D yields a much more satisfactory geology section and a better reconciliation with information contained in the survey data.

The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is an eventual continent-wide deployment of long-period (10-10 000 s) magnetotelluric instruments in a half-degree interval (~55 km) grid across Australia to map the electrical resistivity structure of the continental lithosphere. AusLAMP aims to provide constraints on the tectonic evolution of the continent and the mineral exploration potential as part of the UNCOVER initiative. The coverage of sites in South Australia is nearing completion with about 350 out of 400 sites acquired to date. The survey has already provided new insights into the South Australian subsurface, and will continue to do so as the final stages of data are collected in the APY lands and modelling continues. Areas of economic potential or interest covered by the survey include the Mesoproterozoic Coompana Province, the mineral-rich Archean-Proterozoic Gawler Craton beneath cover of the Neoproterozoic Stuart Shelf, extending across to the east to cover the Neoproterozoic Ikara-Flinders Ranges and Paleo-Mesoproterozoic Curnamona Province. The central Gawler Craton is imaged as a resistive zone with conductive margins surrounding the core of the cratonic block at shallow upper mantle depths. Seismic tomography models across the almost-cratonic Curnamona Province show a fast velocity structure however very low resistivities in the crust indicate an enrichment in carbon and/or hydrogen. The most recent acquisition covers the NE of the state in the Cooper Basin and the Simpson Desert, an area that has minimal coverage by any deep-probing geophysical techniques. Preliminary results indicate the presence of a north-south trending conductor, with final modelling results presented at the AEGC 2018. The results of the inversions of the AusLAMP data highlight the correlative significance with other geochemical data and points towards MT as a geophysical fertility vector for mineral discovery.

Acquiring seismic data has typically been an expensive pursuit due to the high price of the acquisition systems. Such systems are also typically not easily adaptable to suit different acquisition scenarios. In this paper we detail how you can build your own simple, low-cost (~$60/channel), seismic acquisition system. Data recorded using such systems is comparable to that obtained using a far more expensive commercial seismograph.

Seismic sources are similarly expensive with the only low-cost option being a sledgehammer. In this paper we also describe how to manufacture a small vibroseis unit from easily available components at a cost of less than $3,000. This unit has a wider, more controllable, bandwidth than an impact source and can be easily adapted to create a shear wave source for MASW surveys.

The primary variable of interest in a coal resource study is the volume of coal as estimated from the coal thicknesses in each drill hole. It is therefore essential to accurately determine, down to the centimetre level, the thickness of each seam. To attain this accuracy, each drill hole is geophysically logged as these logs provide a considerably more accurate indicator of seam boundary depths than the geologist’s log. Currently, coal geologists spend a large amount of their time integrating their logs with depth information from the geophysical logs. They do this by displaying the two logs next to each other and then manually changing the depths in their logs. Most of this process is relatively routine and thus rather tedious and boring but like many seemingly simple cognitive tasks, not easily transformed into a computer algorithm. The manual method also suffers from being subjective and often non-repeatable.

Previous methods to automate this process have used multivariate statistical techniques to assign lithologies down the hole based on the geophysical values at each reading depth. However, despite these methods having been developed and publicized for over thirty years they still have not been widely adopted as they still do not integrate the information from the two types of data. Following the generation of a lithology log from the geophysics, geologists still need to manually integrate it with their log.

This current study has successfully managed to develop algorithms to automatically determine both coal/non-coal and clayey/non-clayey boundaries based on the gradients and inflection points of the geophysical logs and then integrate this information with the geologist’s log.

The Exmouth Plateau, part of the Northern Carnarvon Basin, has experienced a multi-phase extensional history, which is associated with regional-scale uplift, as well the uplift and erosion of individual footwall blocks. Detailed interpretation of 3D seismic surveys over the area shows that fault activity began in in the latest Triassic, mainly on NNE-SSW and NE-SW trending faults.

Rotation of Triassic fault blocks initiated in the Upper Triassic and continued during the Jurassic. Erosion of pre-rift Triassic sediment occurred during the latest Triassic and the Jurassic. In the latest Jurassic deposition infilled half-grabens and deposition onto highs was limited in the west as the area was starved of sediment. A significant change in sediment supply in the early Cretaceous associated with progradation of the Barrow Delta resulted in the infilling of previously starved half-grabens. Fault activity had decreased by the mid-Cretaceous, with limited activity confined to major faults. Later Cretaceous sediment distribution in the study area was largely controlled by remnant topography.

High-quality 3D seismic data allows a detailed examination of the way in which rift-related fault activity affects sediment distribution. In addition to creating fault block traps in pre-rift Triassic sediments, understanding syn-extensional sediment patterns and fault reactivation has implications for syn-rift plays and seal integrity.

An understanding of fault zone structure and transmissibility can have significant implications for reservoir appraisal and development within petroleum systems. Studies tend to focus on low porosity host rocks that have experienced simple tectonic histories while the influence of complex fault systems (that have undergone multiple phases of deformation) on porous rocks within fault damage zones have not been investigated. We present results from a detailed mineralogical and geomechanical investigation of the Castle Cove Fault within the Otway Basin at Castle Cove, southeast Australia. Castle Cove provides excellent exposures of the Lower Cretaceous Eumeralla Formation, which is a fine-grained volcanogenic sandstone with moderate to high porosity (up to 27%) and low permeability (mostly <1 mD). The Castle Cove Fault originated as a normal fault during the late Cretaceous and was reverse-reactivated during Miocene-Pliocene compression. Core plugs were sampled at distances between 0.5 to 225 m from the fault and were orientated with respect to the fault plane. We show that closer to the fault (within 75 m), porosity increases by nearly 10% (from approximately 17% to 24%) and permeability increases by two orders of magnitude (from 0.04 mD to 2.92 mD). Microstructural investigations from thin sections show an increase in microfracture intensities closer to the fault. Also observed is a change in the morphology of pore-lining chlorite, from well-structured away from the fault to broken up and disaggregated adjacent to the fault. This study highlights the importance of detailed mineralogical and petrophysical analyses when attempting to understand the reservoir properties of high porosity and low permeability sandstones.

The Temora project is a key exploration target for Sandfire to discover an economic copper-gold deposit in the Lachlan Fold Belt, NSW. The district contains an Ordovician Volcanic centre that has been explored over many years and hosts a number of defined porphyry and epithermal deposits. The availability of historical drill holes meant pulps could be re-analysed for multi element geochemical data and Short Wave Infrared analysis (SWIR) data to be collected across the district.

Spectral data from the white micas defined the areas of higher temperature alteration associated with mineralisation across the district. Analysis, logging and re-interpretation of the data led to the definition of a number of new targets where the systems were poorly or untested. A high priority target was defined at the Donnington prospect to the north of a small known porphyry resource on the margins of the Rain Hill intrusive complex. The target showed a potential porphyry zonation systems in the interpreted geology, alteration, SWIR analysis, geochemistry and geophysics.

Drilling in the first field season intersected 120m @ 0.3 % Cu and 0.5 g/t Au from 250m with follow up drilling intersecting 77m @ 0.3 % Cu and 0.5 g/t Au from 250m around a small porphyry intrusive. Mineralisation is hosted in quartz-magnetite-feldspar-pyrite-chalcopyrite veins associated with a chlorite-magnetite-carbonate alteration. A later quartz-sericite-pyrite alteration postdates the mineralisation causing a demagnetised zone surrounding a weak magnetic high that has been used in defining the mineralisation. The mineralisation is still open and further drilling is required.

An efficient, accurate, multi-grid algorithm has been implemented for the modeling of airborne, land, and marine controlled source electromagnetic data, providing accurate 3D depth inversions of frequency and time domain data with cost-effective compute timelines. This is achieved by decoupling the inversion grid from the modeling grid used in the finite difference simulation of the fields. The approach helps also when inverting data from different methods jointly.

The model grid consists of columns of prisms that can be arbitrarily dimensioned. This helps to discretize in particular the topography and other interfaces without densely discretizing the upper part of the resistivity model. By setting the horizontal smoothing accordingly, the general geological setting of the survey area can be easily taken into account.

Depending on the specifics of the implementation, other structural information will impact the chosen discretization.

The assumption of continuity of mineralisation between sampling points, as stated in the JORC Code, requires a “confident interpretation of the geological framework”. The elements of relevance to a geological framework vary greatly depending on th e commodity and style of mineralisation. In general terms, at least two elements must be considered to underpin a geological framework: space and time.

The geometry and location of a mineralised body are controlled by physical and/or chemical elements, which can be unravelled by detailed geological mapping, adequate geochemical (including a quality analysis-quality control program) and structural interpretations, and by 3D geological modelling. These elements may involve, among other, aspects of stratigraphy, chemical or physical properties of the rocks (e.g. texture, grain size) and structural features such as faults, fractures and folds.

Mineralisation events that lead to economic deposits are often relatively short-lived periods of focused fluid transfer and element-exchange, which result in mobilisation and deposition of metals in well-defined areas. Understanding the temporal framework and interaction of structural elements and mineralising events (determining genetic relationships, e.g. pre-, syn- and post-mineralisation) results in the development of more accurate geological models and can lead to predictive capabilities and new discoveries.

We present case studies in regional metamorphic, igneous, sedimentary and surficial geological environments, demonstrating how understanding the mineralisation system not only results in increased confidence in the resource, but also facilitates reduction of exploration risks.

Co-located seismic and magnetotelluric (MT) profiles provide fundamental geophysical data sets to image the crust of Australia. Despite their overlapping nature, the data are processed and interpreted separately based on legacy workflows. We qualitatively compare 2D resistivity inversion models derived from MT and uninterpreted seismic reflection profiles across Proterozoic Australia to address the long-standing cross-cutting nature of interpreted seismic faults and low resistivity zones derived from MT. We find that a good correlation exists between high/low reflectivity in seismic sections and low resistivity in MT sections. These relationships elucidate signatures of past magmatic and fluid-related events and constrain zones of weakened rheology in the crust. Depending on their characteristics, these signatures may signify fossil melting of the crust due to underplating or magmatic invasion into the crust or reworking associated with redistribution of fluids along newly developed faults. These findings have implications for constraining mineral deposit genesis and location.

Multichannel Analysis of Surface Waves is a seismic technique used to define the near-surface structures and rock properties. It has been commonly used for both geotechnical engineering as well as seismic exploration purposes with active sources. It can also provide information about regolith heterogeneity that is of relevance to reflection seismic data processing. However active surface wave investigations are not always possible due to site restrictions and environmental constraints. In this research, we studied the feasibility of passive seismic for the analysis of surface waves caused by different type of ambient noise and ground motion. The example presented comes from a data set collected over a hard-rock environment. We showed that the achieved results from passive data have a considerable correlation with the results from active data of the same acquisition survey.

Evaluating gas content in coals has significant commercial and operational importance. In coal seam gas exploration and development, gas contained in coal seams is the resource of interest. In coal mining, quantifying gas content and evaluating the effectiveness of degassing is essential to safe mining operations.

Traditional approaches to saturation evaluation in conventional oil and gas reservoirs rely on relationships between resistivity and water saturation. These relationships are challenging to apply in coals due to complexities in their pore systems and gas trapping mechanisms. Therefore, geophysical log-based methods are not commonly employed for saturation evaluation, and core canister desorption measurements are the standard approach for gas content evaluation. Desorption measurements present their own challenge due to the unknown and variable volume of gas lost during core recovery, so an in-situ measurement of gas content is desirable.

Advanced magnetic resonance measurements are one method of resistivity-independent saturation evaluation that have been employed in the oil and gas industry for the past approximately fifteen years. However, previous approaches to these types of measurements have focused on the evaluation of conventional reservoirs and hence free gas and oil volumes, and have lacked sensitivity to quantify adsorbed gas, which has a different magnetic resonance response. A novel magnetic resonance acquisition scheme has been developed that provides sensitivity to both adsorbed and free gas, as well as water, allowing for the complete evaluation of fluid content in coal seams. This measurement has been employed in evaluating coal gas content for mining optimisation with encouraging results.

The Lower Barrow Group (LBG; Latest Tithonian - Early Valanginian) is a shelf-margin that prograded during a late phase of rifting under various subsidence regimes and supply-dominated conditions. A 3D semi-automatic, full-volume seismic interpretation method allow identifying high-order clinothems presenting an estimated cyclicity of -40,000 yrs, in which a quantitative analysis of the shelf-margin architecture and shorelines processes was conducted. Overall, three and four main types of hydrodynamic regimes and deep-water systems were identified, respectively. Falling to flat shelf-edge trajectories are associated with sediment bypass, whereas rising shelf-edge trajectories are linked with increasing sediment storage on the shelf. While fluvial to wave processes can be dominant in all A/S conditions, results show that fluvial-dominated coastlines are associated with steep high-angle slope clinoforms and short to longer run-out turbidites. Conversely, wave-dominated coastlines are linked to low-angle slope clinoforms and poor turbidite system development (occasional sheet sand and MTDs). The short and longer run-out turbidite systems present a tripartite architecture (canyon / slope valley; channel; lobes) which mostly appear as short-lived, vertically / laterally stacked elements fed my multiple small rivers forming linear ramp systems. Due to the shallow configuration of the margin (<500m), the presence of short slopes and overall high sand-to-mud ratio, the turbidite systems are smaller scale (<50 km) and probably shorter lived than most modern turbidite systems (100-1000 km). This study sheds new lights on the significant role of shelf-margin architecture (slope gradient, hydrodynamic regime) in predicting the deep-water sediment delivery behavior (sediment partitioning, type of deep-water system).