ASEG Extended Abstracts - 25th International Conference and Exhibition – Interpreting the Past, Discovering the Future, 2016

In an incised channel lacustrine shoreface environment the thickness of the incised channels is unknown away from well control. The thickness often varies over short distances and is the predominant control on reservoir quality, as the best reservoir is in the channels and not the shoreface. A common practice in reservoir modelling is to use an empirical relationship between channel thicknesses to derive width ratios. However, this cannot indicate where other channel bodies are in the area of interest. This study created a relationship between waveform classifications and thickness. The absence of an upper peak-trough within the seismic trace was considered to be indicative of where the upper reservoir unit, incised channels, were present. A relationship was defined whereby the more prominent the upper peak-trough, the more shoreface preserved. Using this method, it is proposed that erosion, and subsequent channel fill, controls the presence of the upper peak-trough. Therefore, waveform classification schemes can be used as a probability map in the static model to control the channel thickness and distribution. The resultant models matched thickness of the upper incised channels at the wells and provide realistically geological models which are able to predict thickness away from well control. At the present, work is ongoing to understand what the waveform represents at wells with a poorer match.

When investigating an anthropogenic fluid migration event within a given hydrogeological setting, information relating to hydraulic transmissivity is typically evaluated using a set of observation wells. Due to high production costs observation wells are often scant in numbers; additionally their intrusive nature bares further disturbances to natural aquifer conditions. Now more than ever, there is an indisputable need for low-cost, non-intrusive and reliable geophysical methods sensitive to these groundwater flows.

Ground water flows are known to generate electrokinetic signals that can be measured passively at the ground surface, and these ‘self-potential’ signals generated can be used to measure and estimate patterns of groundwater flow.

Two pump programs were conducted in fractured rock aquifer systems in the Adelaide Hills Region, South Australia. The predominant purpose of these programs was to quantitatively investigate the self-potential responses of these systems, this included gathering of complimentary geophysical data to support conclusions.

Seismic facies classification has been used to reduce risk in laterally heterogeneous reservoir prediction. Studies were focused on the Barrolka/Coolah/Durham Downs Trend in the south-west Queensland sector of the Cooper Basin. The primary reservoir targets are the fluvial channel sediments of the Toolachee Formation. Historically, well success rates across the area have been low, with highly variable reservoir development and connectivity identified as the limiting factors influencing well performance.

Conventional seismic attribute analysis has typically yielded inconclusive results, often associated to the presence of thick coals that dominate the seismic response. However, recent drilling campaigns utilised seismic waveform classification mapping, which resulted in an increase in technical success rate of wells. This study aims to investigate the concepts behind the success of the waveform classification method and to determine alternate techniques to further delineate reservoir presence.

Key outcomes from rock physics studies indicate subtle variances in the seismic wave shape could be attributed to changing reservoir thickness underlying coal formations. Cross correlation of the wave shape against well results confirmed the concept of dimming seismic amplitude response to be related to increased reservoir thickness. In an attempt to capture the lateral extent of these variances, three adjacent 3D seismic volumes, covering majority of the complex, were subject to a variety of attribute analysis methods. Unsupervised waveform classification was found to be the most efficient and effective method for capturing the dimming seismic reflector, and thus defining the channel trends. The strong correlation between waveform class and reservoir thickness measured in wells enabled the generation of risk-segment maps for the reservoir units.

Observed changes in wave shape on full stack seismic data have been related to lithological variations based on rock physics studies. These learnings have been used to select attribute extraction techniques that can best highlight the wave shape variations. Using seismic facies mapping to enhance reservoir prediction capabilities has reduced reservoir risk and improved the technical success rate of drill targets. The output maps from this study have been used to locate future opportunities in the region.

Access to water is identified a key infrastructure need for mining, energy and industry development. In Western Australia, the scale of planned developments linked to current mineral exploration and mining is set to generate significant economic value for the State, but its realisation is dependent on ensuring access to groundwater. To address these issues, The WA Government Department of Water (DoW) has embarked on a series of groundwater investigations to identify and establish long-term water resources in regional areas where agriculture and mining opportunities have the potential for development. The Murchison in northern WA was identified as one of six key priority areas for this initiative. With numerous known mineral deposits having potential for development, locating and securing an adequate, sustainable long-term water supply is a critical consideration if these mineral resources are to be developed further. While it is known that there are significant groundwater resources in the region, at present these are generally poorly understood.

Of particular importance are the palaeovalley aquifers which are known (locally) to contain a significant resource, but which are relatively poorly characterised. To aid an understanding of their extent an airborne electromagnetic (AEM) survey was commissioned and flown in the Murchison extending over an area in excess of ~ 106 000 km². Prior studies at a local scale had indicated that airborne EM would be very effective at defining the location and thickness of palaeovalleys in the region. Pilot investigations also determined the most appropriate AEM system to use for acquisition. These studies suggested that the buried palaeovalleys were most likely to be near-coincident with contemporary valley systems developed in a granite/gneiss-greenstone basement. Covering such a large region required a novel approach to survey design to maximize the information relating to their expected spatial variability. Therefore a terrain index (MrVBF) was used with the SRTM 1sec DEM to define the extent of contemporary valleys, and the extent of the AEM survey area. This approach allowed survey acquisition costs to be kept to less than half that of flying a more “traditional” survey over the entire area. It also allowed for the acquisition of data with a closer line spacing than would have been possible otherwise, therefore capturing more of the spatial variability associated with the palaeovalley systems. The results have demonstrated the validity of the strategy adopted and have shown that in the absence of conventional hydrogeological information, geophysical methods are demonstrably a cost and time effective approach to upscaling local hydrogeological information, thereby fast tracking groundwater resource assessments that would otherwise take decades to complete.

As exposed portions of premier Zn belts reach exploration maturity, targets become deeper and more challenging to explore. Explorationists are faced with having limited methods for effective deep exploration. Seismic reflection is one method which can be used effectively to characterise geological targets at depth. Teck has applied seismic reflection at an early stage on several zinc exploration projects, including the greenfields Yalco Zn project.

Geophysical methods have been used to assess the structural architecture of the remote Yalco area, located within the highly prospective Emu Fault corridor. 2D Seismic reflection lines have been used in addition to potential field methods and magnetotellurics. The superior resolution of seismic reflection has allowed us to characterise the stratigraphy and map key structures in detail in order to validate a conceptual geological target. The preliminary interpretation of the seismic sections has confirmed favourable depths to host, and resulted in a re-evaluation of target locations.

Use of seismic reflection surveys at an early stage has the potential to reduce technical risk and increase the effectiveness of target testing.

Terrain corrections for determination of the complete Bouguer anomaly are empirically evaluated with respect to a number of different techniques, parameters and digital terrain model data sets, for areas in western and northern Tasmania.

For the most part, while terrain corrections calculated from very high resolution terrain models (1.2 metres or better) are presumed to deliver the most accurate results, those computed for the same area using only a Statewide 25 metre-cell digital terrain model to within two metres of gravity stations correspond remarkably well. Internally consistent comprehensive terrain correction of acceptable yet maximal accuracy can therefore be calculated for all Tasmanian gravity stations, even if very high resolution DTMs are unavailable.

Fully automatic terrain correction computation from two metres to 167 kilometres from gravity stations will result in significantly improved removal of topographic effects over extant manual corrections, which were limited to 22 kilometres.

Interpreting inversions and modelling airborne electromagnetic (AEM) data is ambiguous. Assessment on the degree of certainty of how representative a selected model is, always reassuring. Geophysicists assessing AEM models are often faced with the conundrum of determining a single ‘best’, ‘right’ and geologically sensible model from of all the possible solutions. This paper explores the characteristics of several acceptable models, without being concerned with details of any particular one.

Geoscience Australia’s reversible jump (trans-dimensional) Markov chain Monte Carlo (rj-MCMC) is a stochastic algorithm which has enabled the sampling of thousands of plausible models that fit the data at each individual location. Through the statistical analysis of these ensembles of models, a measure of uncertainty and a probable distribution of conductivities at that depth can be derived.

On most occasions, single ‘best’ solutions fromm deterministic inversions are found to be reasonable representations of the whole suite of models recovered by the MCMC. But the importance of exploring multiples models and their limitations resides on trying to extract what information can actually be determined from the data, information which often cannot be given by a single best model.

The Bight Basin on the southern margin of Australia is nearly 2000 km wide from west to east and overlies a number of different basement terranes (Figure 1). Major basement terrane divisions occur between the basement of the Ceduna delta and the Eyre and Bremer sub-basins, resulting in changes in structural styles in the overlying basin successions.

The Eyre Sub-Basin overlies the boundaries of the Proterozoic Madura and Coompana basement provinces, which are separated by the Mundrabilla shear zone. The shear zone is a N-S trending, continent-wide structure visible in magnetic data which appears to extend offshore in the Eyre Sub-Basin and is also visible as a north-trending present-day fault scarp in the onshore Eucla Basin. Seismic data interpretation suggests that the shear zone steps to the east in the region of the Jerboa-1 well. Differential movement across the shear zone during Jurassic-Cretaceous rifting may have influenced the location of depocentres within the Eyre Sub-Basin.

Overlying the Albany Fraser Orogen’s northern foreland, the Bremer Sub-Basin is dominated by WSW-ENE trending half graben structures and large rollover anticlines associated with Jurassic-Cretaceous rifting. The basin is divided by a N-S trending basement structure, visible in gravity data, and in the overlying sedimentary succession as a broad zone of subsidence with several periods of reactivation. Similarities between this structure and the shear zone in the Eyre Sub-Basin suggest they may have a similar origin.

Stratigraphy of the Hamersley Province in Western Australia, featuring alternating units of banded iron-formations and shales with contrasting electrical properties in a mostly gently undulating shallow dipping layered geometry, is particularly favourable to airborne time-domain electromagnetic mapping techniques. Manipulated vertical cross-sections of modelled conductivity obtained from laterally constrained 1D inversion of SkyTEM³⁰⁴ data enable the exploration geologist to interpret weathering profiles, shallow dipping stratigraphy and steep structures, all of which are crucial aspects of bedded iron ore deposits genesis models. Five potential interpretation pitfalls have nevertheless been encountered. Occasional obvious artefacts can be present in inverted models but their causative sources should be easily identifiable in the measured channel data. The inverted models coherence is generally compromised when acquisition system terrain clearance cannot be maintained below approximately 80 m. The in-loop setup and processing routines have inherent limitations over steep stratigraphy. Complex stratigraphy geometries can result in off line responses to be artificially incorporated in the 1D inverted conductivity cross-sections. Lastly, lateral interfaces between fresh and altered rocks could easily be misinterpreted as fault contacts.

The minerals exploration industry’s demand for a highly precise airborne gravity gradiometer has driven development of the VK1^™ Airborne Gravity Gradiometer, a collaborative effort by Rio Tinto and the University of Western Australia. VK1^™ aims to provide gravity gradient data with lower uncertainty and higher spatial resolution than current commercial systems.

In the recent years of VK1^™ development, there have been significant improvements in hardware, signal processing and data processing which have combined to result in a complete AGG system that is approaching competitive survey-ready status. This paper focuses on recent improvements. Milestone-achieving data from recent lab-based and moving-platform trials will be presented and discussed, along with details of some advanced data processing techniques that are required to make the most use of the data.

Recently seismic reflection has been gaining prominence as a tool for the minerals industry for orebody delineation and exploration in known mineralised terrains particularly where mineralisation extends to depths beyond a few hundred metres. We believe it can also play an important role in establishing the structural architecture of an area due to its unrivalled ability to map the orientation of structures at depth. In this paper we present recent results from an exploration program in the Tennant Creek Mineral Field (TCMF) in the Northern Territory of Australia for both of these objectives.

The program included the completion of 2 x 4km seismic lines close to the Gecko mine and a 60km N-S seismic line centred on Tennant Creek together with borehole full waveform sonic and vertical seismic profile data.

The borehole measurements confirm that the ironstone bodies and associated alteration that host the mineralisation provide a strong acoustic impedance contrast within the Warramunga sediments that otherwise show relatively little acoustic impedance variation.

The 4km seismic lines have generated two targets both at approximately 1km depth. Shallow imaging was hampered by strong surface wave noise but it is the targets that are not detectable by gravity and magnetics that motivated the use of seismic at Gecko and Chariot.

The regional survey was acquired to improve the understanding of the mineralising systems and underlying structural architecture around Tennant Creek. The survey mapped major structures that control mineralisation, such as the Southern Shear Zone, Mary Lane Fault and Gecko Fault. The seismic survey also showed that these structures were north verging and identified similar structural positions that lack surface expression but show many of the characteristics of the mineralised structures.

The broadband Kraken 3D Marine Seismic Survey was acquired during 2013 in the outer Browse Basin exploration permit WA-314-P with the specific goal of assessing risk and volumes at the Elvie prospect. The survey was acquired over a highly rugose sea floor, comprised of deep slump canyons that overlie a steeply prograding Miocene carbonate sequence.

Multiple attempts at processing the seismic data have already been made; including a post-stack time migration (a fast-track volume), pre-stack time and pre-stack depth migration. Conventional processing and pre-stack depth migration approaches were unable to fully resolve short-wavelength velocity anomalies below the sea floor that cause obvious residual imaging problems and impact upon depth conversion and seismic amplitude interpretation. A geomechanical pre-stack depth migration now underway to hopefully address the remaining imaging concerns.

Overall, the Kraken 3D is considered to be a significant improvement over the pre-existing 2D seismic. Interpretation was performed largely in the depth domain, although ties to nearby wells were made in the time domain using legacy 2D and 3D seismic. Mapping has further matured the Elvie prospect, which is a robust 4-way dip closure located on the divide between the Caswell and Seringapatam Sub-basins. The survey provides strong evidence for a thick top seal in the form of deep-marine muds of Miocene age, although there is evidence of minor seepage through a thin flank of the sealing unit. These shallow amplitude indicators, nearby surface seeps and pockmarks near the sea floor provide additional support for a working petroleum system. The Elvie structure appears to be draped by potentially high quality turbidite reservoirs of most-likely Paleocene age.

A recently published method of automatically finding magnetic depths to magnetic layers is demonstrated, finding depths to significant details in the magnetic basement under Melville Island, Northern Territory.

By using a method that simulates a magnetic basement as a series of layers of random dipoles, the depths to basement are satisfactorily obtained. Multiple layers appear in the results. However the inversion method used has coarse horizontal resolution, and the layers may be separated horizontally within the sample.

To resolve the ambiguity a forward model, also composed of layers of dipoles, is built on the information obtained from the inversions. Forward modelling requires Fourier convolution for speed. The cycle of analysis is logically completed by comparing the synthetic depth profile with the depth profile obtained by inverting the survey data.

The change in seismic reflectivity from a reservoir - during in situ bitumen recovery processes such as SAGD - can be substantial due to the combined effects of increased temperature, pore pressure and effective stress changes, and the substitution of bitumen with water and steam. These physical property changes can be observed with time lapse seismic monitoring e.g. 4D seismology. The proper interpretation of geophysical observations, however, requires a solid understanding of the saturated reservoir rock’s behavior and pore fluid’s properties under changing conditions. Our first suite of ultrasonic measurements with bitumen saturated carbonate show the P- and S-wave velocities decrease by -11.5 % and ~8.5 % for a temperature increase from 10°C to 102°C, respectively at a constant effective pressure of 5 MPa. In the next effort, direct measurements with bitumen show -29% decrease of P-wave velocity for 10°C to 130°C change in temperature. Different slopes in velocity versus temperature plot may also indicate the possible states of quasi solid and liquid in bitumen. The change in fluid bulk modulus with temperature drives the drop in the P-wave velocities. We may also infer that the decline in S-velocity in core sample is due to greatly lowered viscosity of the fluid with temperature.

We also attempt to simulate numerically the ultrasonic pulse-transmission through rock saturated with viscous fluid, which exhibits similar trends of P-wave velocity drop with temperature. However, the decrease in velocity is not quite large as experimental studies with bitumen saturated carbonates; may be due to the difference in pore structure and fluid properties. In addition, the dynamic moduli of saturated rocks at seismic frequencies on core scale using the strain-stress method indicate its strong dependency on viscosity of pore fluid and (or) the frequency.

The generation of electric fields with electric bipole transmitters are applicable at a wide range physical scales and for many subsurface exploration endeavors. Increase in transmitter power for a wide range of waveforms combined with receiver sensitivity has led to deeper exploration with electromagnetic methods. We investigate the optimal design of grounded bipole EM system for generation of electromagnetic fields over a basin-scale fault in Perth, W.A. The technical objective is to recover detailed electrical conductivity distribution proximal to and within a large fault system to depths of as much as 1000m below the surface. The ultimate geological imperative for the exercise is reveal possible change in solute concentration or hydraulic across these large fault systems. For example, imaging of the difference composition of fault core zone would be a valuable outcome. We investigate various combinations of receiving and transmitting antenna geometries in preparation for a field campaign intended to resolve electrical parameters and structures of this large fault system.

Under the UNCOVER initiative it is generally accepted that construction of accurate cover-thickness maps is the most tractable and urgent means of facilitating resource exploration under cover. To meet this goal we have been undertaking benchmarking of various geophysical techniques, constructing a national database of Estimates of Geological and Geophysical Surfaces (EGGS) to store legacy estimates and developing machine learning algorithms to interpolate between these estimates. Benchmarking magnetic top estimates to ~700 drill sites across the Murray Basin highlights the importance of performing estimates using profile data as opposed to grids. Inversion of horizontal to vertical spectral ratio data, derived from single broadband seismometers, reveals surprisingly robust cover-thickness estimates. While these insights are directly relevant in supporting drilling programs they can also be used to constrain deep Earth architecture and processes. We show that inversion of basin subsidence data can be used to constrain lithospheric thickness and mapped chrono-stratigraphic surfaces can be used to test models of uplift of the Australian continent related to convective flow within the Earth’s mantle.

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.

Over the past decade, advances in new satellite and airborne sensor technologies provide an opportunity for rapid multi-scale mapping, measurement and monitoring of the physical state of the crust, including resolution of key elements of surface and sub-surface hydrological systems. These advances have been mirrored by the development in advanced computational research infrastructure which is now giving the groundwater research community access to high-resolution (spatial and temporal) biophysical datasets (e.g. climate, ecology, geoscience and geospatial) relevant to broader hydrological systems understanding. This infrastructure facilitates integration of multiple datasets and rapid and improved signal processing, inversion, and sophisticated analysis. These datasets provide a catalyst for collaboration, with inter-disciplinary approaches enabling new discovery science in a ‘big data’ environment, and enabling the qualitative and quantitative analysis and modelling of landscape and hydrological system processes.

In Australian landscapes, airborne electromagnetics (AEM) is widely used in near-surface (<200m) groundwater investigations due to the ability to acquire consistent, spatially coherent information of high quality using calibrated systems, in very short timeframes. This study reports on an evolving inter-disciplinary approach to AEM survey design for groundwater exploration. Recent investigations have employed time series analysis of surface water availability (using the Australian Geoscience Data Cube (AGDC)) combined with morphotectonic analysis of digital elevation datasets, tectonic analysis, and geomorphic analysis of satellite optical data, to help predict preferential recharge zones and shallow groundwater resources. This novel approach has been used successfully for groundwater exploration in the western Murray Basin and Kimberley Region of northern Australia.