ASEG Extended Abstracts - 2nd Australasian Exploration Geoscience Conference: Data to Discovery, 2019

Exploration for targets at depth or targets obscured by conductive overburden have historically been a challenge with airborne EM methods. Although modern systems have been improved with greater primary transmitter moments, noise from receiver coil motion in the Earth’s ambient field limits detection of secondary target signals, especially in late time. The new Helitem² system uses a patented low-noise receiver and a 50% duty cycle square pulse transmitter waveform to achieve increased signal detectability for deep and covered targets.

A series of demonstration surveys were conducted by surveying a known target to compare several helicopterborne time-domain system configurations. Despite having a larger dipole moment, a half sine pulse at standard 30 Hz base frequency was predicted to have lower responses than low base frequency (15 Hz and 7.5 Hz) square pulse operation in a thin-plate nomogram over a wide range of target conductances. At early times, the sharper (quicker) turn off of the square wave results in much more highfrequency energy and therefore better signal for weakly conductive targets and better near-surface resolution. At the other extreme, the response from very conductive targets is determined by the area under the transmitter curve, so the low frequency square waves with 16 and 33 ms widths should produce more than twice the signal as the half sine. Survey line profiles and decay curves over the target and background locations confirmed these predictions for a 400 m deep target and variable overburden.

The combination of pulse width, power, and low noise enabled the system to be effective at low base frequencies, where very late time data is beneficial for detecting strong and deep targets.

Six hundred and twenty nine kilometres of deep crustal reflection data were collected for the Southeast Lachlan 2D seismic survey during March to April 2018. The purpose of the survey was to image the Lachlan Orogen geology, as a key to the geodynamic evolution and mineral potential of Victoria and New South Wales with implications for eastern Australia as well as natural hazard mapping. The project is a collaboration between Geoscience Australia, the Geological Survey of Victoria, the Geological Survey of New South Wales, and AuScope Ltd. The seismic data were collected by Terrex Seismic and processed by Downunder Geosolutions, and posed significant challenges in both phases. The project has been successful in obtaining final images of the whole crust in this area of the Lachlan Orogen and fundamental structural information on the crustal architecture in southeast Australia.

Passive seismic HVSR surveying is increasingly being used for investigating the thickness of soft sedimentary cover deposits sitting over hard and fresh bedrock, and for direct drill targeting of paleochannels for groundwater and brine resources. A detailed passive seismic HVSR survey was carried out at the Chilalo Graphite Project in SE Tanzania to assist with defining the thickness of alluvial deposits on the margins of the Mbewmburu River system. These alluvial sediments have the potential to host significant groundwater supply in paleochannels filled with porous coarse clastic material which can to be used for supporting a graphite ore processing. The HVSR data were acquired by local field operators, and then processed and depth converted using a Vs of 370m/s, with the modelled bedrock surface ranging in depth from <4m to 28m. The gridded bedrock depth highlighted two paleochannel features of variable depth within the river valley. One bore was drilled along the margin of a paleochannel and had encouraging water flow, and several water bores have planned to test the thick paleochannel deposits for groundwater potential. Drilling is ongoing at the time of writing and results are pending.

The objective of this presentation is to demonstrate the successful application of the passive seismic HVSR method in assisting with hydrogeological studies by providing modelled bedrock surfaces which can be used for direct drill targeting, budgeting and volume estimations, and identifying potential aquiclude clay layers which could affect the hydraulic connectivity and recharge of the groundwater resource.

Magnetotellurics (MT) is emerging as a key tool for understanding giant mineral systems. For MT to be effective in mineral exploration we require both an improved understanding of the causes of electrical conductivity anomalies and an increased number of surveys over known mineral deposits.

We present results from a new, lithospheric-scale 3D MT dataset collected in the southern Yilgarn Craton, Western Australia. This 250x250 km survey covers the western Eastern Goldfields Super Terrane and the world-class Kambalda nickel and Kalgoorlie gold deposits. We interpret the electrical conductivity models using experimental electrical conductivity data to link the MT results to a mineral systems analysis.

In the CO2CRC Otway Project, seismic monitoring has evolved from traditional campaign-based seismic acquisition using a large array of seismic receivers and mobile sources, towards the techniques using continuous borehole seismic acquisition based on permanently deployed sources and various receiver and DAS arrays deployed in the wells. Permanent borehole-based reservoir monitoring can minimize the cost and the environmental impact of geophysical surveillance. To this end, Surface Orbital Vibrators were deployed as permanent sources in late 2015 to two locations for real time and on-demand plume imaging. The sources include one large and one small motor. After optimisation of the SOV design in 2016, the existing buried geophone array was used to record continuously over several months. First, surface geophone array feasibility was performed and promising seismic repeatability was obtained that validates the borehole applicability of these vibrators. We then ran a series of VSP trials to test performance of DAS in combination of SOV sources. The objective was to test different fibre types and different sweep parameters. The results of the field trials show that the DAS/SOV combination presents good quality VSP datasets and, when using a more powerful motor with sweeps from 0 to 80 Hz, imaging beyond the injection interval. These results provided confidence in progressing further stages of the Otway Project which will focus on permanent borehole seismic monitoring.

Airborne time domain electromagnetic and high resolution 2D seismic data were acquired in 2018 to map structures of hydrogeological significance in the Peel region; Western Australia. Interpretation of the airborne electromagnetic (AEM) survey was complicated by the presence of major utilities including power lines. We consider methods for removing the impact of these sources of EM noise within processing prior to interpretation. Generally, we found this to be counterproductive as information was unnecessary lost. Imaging from high resolution seismic reflection data is unaffected by EM noise. We show how a strategically located high quality seismic imaging was instrumental in providing an interpretational framework that could be extended to the full AEM survey area. We provide examples of AEM interpretation for many hydrogeological features including: major faults, 3D hydrostratigraphic surfaces, geological dip, saline water interfaces and zones with potential hydraulic connection between shallow and deeper aquifer systems. This work facilitated significant revision of groundwater systems conceptualisation in the Peel region.

There has been extensive exploration in the Mt Woods inlier in the northern Gawler Craton, South Australia since the 1960s, particularly for iron oxide-copper-gold (IOCG) mineralisation since the 1975 super giant Olympic Dam IOCG-U discovery by Western Mining Corporation ~150 km to the SE.

IOCG discoveries in the Gawler Craton have been largely made based on targeted discrete magnetic and/or gravity anomalies since the advent of regional geophysical datasets. Only one significant economic discovery has been made in the Mt Woods inlier – the Prominent Hill metasediment-hosted IOCG deposit in 2001 by Minotaur Resources, with no subsequent economic mineralisation encountered elsewhere in the inlier.

In an effort to accelerate the discovery of a new economic deposit, OZ Minerals opened up more than 2TB of their private geoscience data to external participants as part of the ‘Explorer Challenge’ competition in early 2019. Once registered, participants had 3 months to use the data provided, along with what was already in the public domain, to attempt to predict where the next mineral deposit in the Mt Woods inlier might be.

Integrating a minerals systems approach with machine learning (ML) has allowed the development of a series of conceptual models for targeting a range of deposit types and commodities. Exploration success based on this modern targeting approach will eventually lead to new suitable exploration strategies to discover the next generation of economic mineral deposits, particularly those concealed under cover.

Ocean Bottom Node (OBN) acquisition has been widely used in oil and gas industry to provide improved reservoir imaging when compared to conventional narrow azimuth (NAZ) towed streamer data. Its full azimuth (FAZ), high fold count, high repeatability and broadband character prove to be beneficial especially in areas with complex geological structures and were a driver to record the first OBN survey in Gorgon gas field, NW Australia. These characteristics play an important role in resolving fault shadows that have hindered interpretation on previous towed streamer datasets. Furthermore, the limitations arising from a relatively sparse receiver grid in the OBN acquisition are mitigated by an advanced processing sequence. The benefit of OBN data, and the workflow effectiveness, has delivered a step change improvement in reservoir imaging results over the Gorgon gas field.

Iron Oxide Copper-Gold deposits (IOCGs) are structurally controlled, and typically display zonation of iron oxides and sulphides, which is potentially related to redox zonation. There are three main factors that determine the location and architecture of an IOCG: 1. fluid pathway(s); 2. trap/host, and; 3. plumbing system (i.e., mechanisms for depressurising the system). Generally, these are loosely referred to as structural controls, but they exercise very different functions within the system. In this study we integrate the results of petrophysical property analyses (including, magnetic susceptibility, remanence, radiometrics and conductivity), structural fabric analyses and TIMA scans that provide information on both mineralogy and texture. The results when placed in an oreproximal- distal-background framework, allow us to understand the footprint of the system.

The results show that the initial ductile-brittle metasomatic traps (i.e., the NE-trending shear zones) are highly magnetic due to their relatively reduced (magnetite-albite) mineral assemblage and preserve the pre-existing structural fabric. Conversely, the secondary, brittle, trap (i.e., the breccia) is moderately magnetic and weakly oxidized with a magnetite-hematite-pyrite-chalcopyrite mineralogy, and has randomised magnetic fabric due to brecciation. The least magnetic, and most oxidised zone of the system has a quartz-calcite-chlorite-hematitechalcopyrite mineral assemblage. It overprints the breccia, at the intersection of the trap (NE-trending shear zone(s)) with the fluid pathway (N-S strike-slip fault) and has an NS sub-horizontal AMS lineation.

The different metasomatic alteration assemblages present at Ernest Henry (i.e., sodic, potassic and calcic alteration) may be related to redox zonation within one event, or represent overprinting of metasomatic episodes. However, the zonation of the system is initially controlled by the preexisting architecture, a compressive jog within a N-S trending strike-slip fault. When the trap (the jog) is permeable and/or reactive, the redox gradient lies along the trap, between the fluid pathway and the pressure valves. As conditions become more brittle and the trap becomes impermeable and/or non-reactive, and the system becomes over-pressured, leading to brecciation at the intersection of the trap with the fluid pathway.

The Surat Basin is one of the most prospective onshore basins in Australia for CO2 storage. The Precipice Sandstone and Evergreen Formation have been appraised for their feasibility as a future CO2 storage reservoir-seal pair. Here we will focus on predicted CO2-water-rock reactions. These predictions rely on mineral and porosity data from drill core. Data were obtained from northern and two southern regions of the Basin. The northern region was more data rich. The southern region is more well core and data sparse with the exception of the Moonie oil Field. Additional drill core samples were collected from archived well core of Moonie and other parts of the basin. The core samples were characterised for porosity, mineral, and metal content to build geochemical models to predict local CO2-water-rock reactions and their potential effect on reservoir scaling, changes to porosity and mineral trapping of CO2. For the northern region, our work has predicted low reactivity of the Precipice Sandstone, with mineral trapping in the Evergreen Formation. The Precipice Sandstone sampled in the Moonie field has different mineralogical characteristics to wells in the Northern region. Here, CO2-water-rock predictions indicate minor alteration of plagioclase and K-feldspar to kaolinite, chalcedony and ankerite in cleaner Moonie sandstones, with additionally precipitation of smectite in clay rich sands. Formation water pH was buffered between 5 and 6 by dissolution of calcite or siderite cements. Sampled core has also shown evidence of previous natural CO2 and hydrothermal fluid alteration, fractured quartz grains, and fracture fills with mineral trapping as carbonates. This type of natural analogue data is vital to validate long term predictions. New drill core and data are still required in future for the southern and central Surat Basin region which is most prospective for CO2 injection and storage.

In order to improve exploration success under cover the UNCOVER initiative identified high resolution 3D seismic velocity characterization of the Australian plate as a high priority. To achieve this goal government and academia have united around the Australian passive seismic Array project (AusArray) which aims to obtain a national half degree data coverage and an updateable 3D national velocity model which grows in resolution as data become available.

AusArray unites data collected from the Australian National Seismological Network (ANSN), multiple academic transportable arrays (supported by AuScope and individual grants) as well as the seismometers in schools program. A recent addition has been Geoscience Australia’s Exploring for the Future program (EFTF) which has doubled the national rate of such data collection. Extensive quality-control checks have been applied across this combined dataset to improve the robustness of subsequent tomographic inversion and interpretation. The aim being an updatable national velocity reference model with resolution from depths of a few meters to hundreds of kilometres.

The first stage of lithospheric seismic modelling has been the development of P and S body-wave tomography models. An initial earthquake catalogue of ~26,000 events, dating from 1993 to present, has been developed. This is used to estimate first-arrival times using high-performance routines on the National Computational Infrastructure supercomputer facility. Obtained parametric data were then used in non-linear tomographic inversion with a realistic wave propagation scheme. Resulting P and S tomographic images show a strong correlation with major crustal and lithospheric mantle boundaries. Vp and Vs variation patterns are not always positively correlated thereby providing new insights into the architecture of the Australian plate. Integration with resistivity models derived from magnetotelluric data provide insights into the control of minor phase distribution on the imaged architecture.

Shale oil and gas is of recent interest in Australia, and the Roseneath, Epsilon and Murteree (REM) shales of the Cooper Basin, Australia, are unconventional gas targets. Shales have a high proportion of porosity in the submicron scale and hydraulic stimulation of USA shales has historically employed high volumes of water with dilute HCl, proppants and various chemicals. Supercritical (sc) CO2 has also been used as a fracture fluid in water sensitive formations. In addition, shales may contain high concentrations of potentially toxic or regulated components such as U, As, Pb, or BTEX which may be released to production water. Improper disposal or treatment of waste water has caused recent environmental concerns in the USA.

REM shales were characterised with various techniques including high resolution synchrotron XFM for association of metals with specific minerals. Several metals and As were associated with sulphides in coal pores; Mn with siderite, and Pb with pyrite cements. Shales were reacted with dilute HCl or scCO2-water +/- SO2. The fraction of SANS gas accessible meso-pores was highest in the Epsilon Formation core. Siderite dissolved in HCl reactions, and the fraction of open meso-pores increased. Fe-rich precipitates formed in scCO2 reactions and mesopores partly closed. Mobilised concentrations of Pb, Fe, U, and Na were highest from the reactions with dilute HCl. Understanding the mineral sources of metals and their potential release with different fracture fluids may result in better predictions and mitigation options for production water. Reactions of minerals such as siderite, and sulphides may release regulated metals to production water.

The Geological Survey of New South Wales (GSNSW) is undertaking a ten-year project to increase knowledge of basement and cover geology, mineral systems and hydrogeology in five under-explored regions in NSW. The project is part of the MinEx Cooperative Research Centre’s (MinEx CRC) National Drilling Initiative (NDI). GSNSW will acquire new regional geophysics, hydrogeochemical samples, biogeochemical samples and geological field observations, and analyse legacy data and samples before drilling. All data and mapping will be made available to the public.

Analysing rock micro-structure from micro-computed tomographic images of porous media is vital to understand fluid-flow and estimating petrophysical properties like permeability. The two main approaches of analysing rock micro-structure are (1) through experiments, a timeconsuming process and (2) using numerical simulations which are a part of the standard digital rock physics (DRP) workflow. The standard DRP workflow requires the micro-computed tomographic (micro-CT) images to be segmented into distinct phases (pores and minerals). Segmentation is a user-biased and manual process. It relies heavily on the user to choose a threshold(s) that distinguishes unique phases present in the rock microstructure. Thus, introducing uncertainty in these petrophysical properties. Our approach to resolving this subjectivity and uncertainty in analysing rock microstructure is to directly apply techniques on the micro-CT images or the greyscale images, rather than using segmented images. For this purpose, we use a pattern recognition technique namely the Grey-Level Cooccurrence Matrix (GLCM). The GLCM technique is used to calculate spatial maps that describe features present in the rock micro-structure. Calculating these spatial maps at varying length-scales by using different displacement vectors aid in analysing the grain-sizes, grain-pore interface and pore-sizes. Unlike the histograms which only preserve the frequency of intensity values that represent different features in micro-CT images, GLCM is a secondorder pattern recognition technique that additionally preserves the spatial variation and occurrence of grey-level intensity values. This method of studying the rock microstructure using greyscale images and pattern-recognition techniques provides an advantage over the conventional segmentation techniques because full-information regarding the rock micro-structure captured during microcomputed tomography is preserved and a threshold-less workflow leads to lesser user subjectivity. Lastly, the GLCM based analysis also provides a pathway for automated investigation of rock-microstructure.

Regional scale geophysical data is widely and publicly available in many countries. Traditionally, these data include transects that span many hundreds of line kilometres. More recently it was recognised that imaging deep three dimensional crustal structures requires new 3D data sets. An example of this is the Australia AusLAMP magnetotelluric grid. We consider this data from a different perspective and ask how sparse regional data can be incorporated in mineral exploration workflows which focus on the top few kilometres of the Earth. We select a reference area of about 100,000 square kilometres in central Australia, which includes the 450 kilometre long Yilgarn Craton, Officer Basin, Musgrave Province (YOM) seismic and MT transect. We test and compare a number of 1D and 2D inversion strategies with broadband MT data and then stretch that geo-electrical model to the third dimension with an additional 18 long period MT stations on a 50 km grid in the Officer Basin and Musgrave block. With additional data from just a few sparse stations, such as those from the AusLAMP project, we show that the value and outcome of MT inversions can be enhanced. The search for new tier one mineral deposits is transitioning to the under explored deeper covered areas at basin margins, and we have demonstrated techniques for building 3D geo-electrical frameworks towards more relevant shallower exploration depths from including exceedingly sparse long period 3D MT data.

The Geological Survey of South Australia (GSSA) designed the Gawler Craton Airborne Survey (GCAS) to provide high resolution magnetic, gamma-ray and elevation data covering the northern portion of the Gawler Craton. In total, 1.66 million line km were planned over an area of 295,000 km², covering approximately 30% of the state of South Australia.

The survey design of 200 m spaced lines at a ground clearance of 60 m can be compared with the design of existing regional surveys which generally employed 400 m line spacing and a ground clearance of 80 m. The new survey design results in ~2 x the data coverage and ~25% closer to the ground when compared to previous standards for regional surveys in South Australia.

Due to the enormous scale of the survey, the data were acquired using four contractors who employed ten systems to fly the sixteen blocks.

To standardise the data from the multitude of systems, Geoscience Australia (GA) employed a comprehensive set of technical specifications. As part of these specifications the contractors were required to fly each of the ten systems over a series of test lines termed the “Whyalla Test Lines” (Whyalla).

The final GCAS data provide truly impressive high resolution regional scale products. These will allow more detailed geological interpretation of the prospective Gawler Craton.

A laser altimeter was added to the list of required survey equipment. Deficiencies in the technical specifications relating to laser altimeters were identified. Standards and procedures specific to laser characteristics will need to be considered on future surveys.

Analyses show that weaknesses in current standards and procedures are still evident. The weaknesses identified allow room for improvements to be made for future surveys.

Gamma-ray processing results raised the most serious concerns, with repeatability not achieved. Changes to standard procedures may need to be considered.

Integrated geological-geophysical interpretation offers a robust way to answer geoscientific questions by providing better and faster decisions on where to explore in a regional context, and ultimately on local-scale targets. In particular, regional scale geophysical datasets can be used in areas of extensive cover and limited outcrop to quickly develop three dimensional geological models that can serve different purposes such as targeting or hypothesis testing via forward modelling. To show these concepts, we present a case study from the Curaçá Valley Cu region in Bahia, Brazil. A large geophysical data set including airborne gravity, magnetic and airborne electromagnetics was used and combined with existing 2D regional geology from different sources to produce a 3D integrated model.

Airborne 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.

This study is part of the groundwater investigations of the Ord Bonaparte plains in the East Kimberley region of Western Australia. A key project aim is to establish a spatial hydrostratigraphic framework to better understand the hydrogeology.

To achieve this, AEM data, inverted using 1D SELMA model, were produced as conductivity sections and elevation grids. Interpretation of the AEM data, in conjunction with lithostratigraphic information from three petroleum wells and seven project bores, aided the mapping of hydrostratigraphic units of the Devonian to Permian sequence of the Bonaparte Basin. Mapping results show that the Carboniferous Weaber and Kushill Groups are dipping to the east-northeast and contain laterally continuous stacked aquifers. Within the strata, resistive signatures are associated with sandstone aquifers, slight to moderate conductors are mapped as fine textured aquitard, or as interbedded fine to coarse textured sediment forming semi-confining layers.

A water table elevation map was constructed using surface NMR water content profile and machine learning approach to extrapolate across the study area. Using Archie’s Law, groundwater conductivity was predicted from AEM conductivity and porosity derived from borehole NMR measurements.