ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

Geoscience Australia (a.k.a. BMR and AGSO) pioneered the acquisition of regional gravity and magnetic data to aid geological mapping. These data revealed for the first time the extent and nature of the major tectonic elements of the Australian continent. In the 1980’s, airborne survey and major exploration companies extended this concept to higher resolution at the province scale, further bringing the geology into focus. Qualitative interpretation of this type of information in 2D plan view has proved invaluable. Thoughts turned to 3D modeling and interpretation. Despite an array of software tools to perform the modeling, we are yet to feel that it has really met expectations. As we move into the future, the grand challenge for us all will be to inject more geological knowledge (“prior information”) into the modeling. Technology in the form of better geophysical data acquisition capabilities, improved software tools, High Performance Computing facilities, and novel ways to integrate interpretations and visualize 3D spaces will all contribute to the solution. However, user input will remain the key ingredient for success. Injecting geological knowledge into the modeling process and understanding the results that modeling provides will enable us to reveal more detail of the 3D subsurface structure and to identify and manage the resources that are hidden therein.

The Pilbara region of North West Australia is one of the world’s major iron ore provinces. Geophysical techniques have been applied routinely for exploration of iron ore. The first known application of a geophysical technique in mineral exploration was, in fact, the use of a magnetic method in the search for iron.

This presentation summarises the use of geophysical techniques in iron ore exploration over the last half century. Magnetic methods have been the most favoured, followed by gravity. EM has been applied in niche areas, such as in the exploration for CID. However it is time to venture beyond conventional techniques and start focusing on future developments in geophysics that promise yet greater benefits to mining generally and iron ore exploration in particular.

Recent Geological Survey of Victoria work has confirmed the presence of the Miga Arc, a buried Andean-type Cambrian arc system in western Victoria.

Geophysical data sets, particularly magnetics and gravity, were interpreted to characterise the regional tectonic setting, and to gain a deeper understanding of poorly exposed bedrock of the southern Miga Arc, within the Geological Survey of Victoria’s Willaura Cu Porphyry project area.

Geophysical interpretation and modelling, and field mapping of Cambrian bedrock has identified new regional Late Silurian dextral faults which are important in understanding the distribution of Miga Arc rocks in western Victoria. The updated bedrock interpretation together with new geochemical results has significantly expanded the potential exploration fairway for Cu porphyries in rocks associated with the buried Miga Arc.

The Tropicana Gold Mine is located 330km east-northeast of Kalgoorlie, Western Australia. Discovered in August 2005 the deposit is the first world-class gold resource discovered in high-metamorphic grade gneissic rocks, in an Archean terrane not previously thought to be prospective for gold.

The contrast in petrophysical properties of host rocks observed across the Tropicana gold mine enable geophysical methods to assist in mapping the deposit. Geophysical methods applied at Tropicana include; regional aeromagnetics and gravity, high-resolution airborne magnetics, gradient array Induced Polarisation (IP), pole-dipole IP, detailed gravity, MIMDAS IP, 2D seismic reflection, 3D seismic reflection, helicopter TEM, and SPECTREM.

Initial gradient array IP combined with geochemical analysis of aircore drilling samples provided the most cost effective method to direct early diamond and RC drill testing of auger and soil anomalies. Integration of all available data within a 3D Common Earth Model (CEM) facilitates lithology constrained 3D potential field and 3D IP inversions. When combined with the lithological packages, structural architecture, alteration assemblages and zonation, and geochemical signatures the 3D CEM provides a powerful means of delineating ore positions and exploration targets.

The discovery of the Atlantida Cu-Au-Mo porphyry deposit is a recent example of exploration success under cover in a traditional mining jurisdiction. Early and appropriate acquisition of geophysics was a key tool in the discovery and in guiding resource definition drilling through the lifecycle of the project. Close review of the geophysical response of the deposit with respect to its lithological distribution and petrophysical properties has allowed it to be fully characterised despite no mineralisation being exposed at surface. Data acquired over the project includes induced polarisation, ground and airborne magnetics, gravimetry and petrophysics.

The distribution of the key lithologies is demonstrated to be readily defined via a combined application of susceptibility and density properties, which agree well with geophysical data acquired at surface. This is in contrast to the electrical properties which instead map the extent of mineralisation associated with the hydrothermal system via chargeability, and the location of copper bearing sulphides via resistivity

In combination these characteristics can be used to infer depth to exploration targets and potential for high grade mineralisation in a geological context. Future exploration will be increasingly reliant on the understanding of the surface manifestations of buried deposits in remotely acquired data. This review summarises the application and results of these principles at the Atlantida project.

Remanent magnetisation is an important consideration in magnetic interpretation. In some cases failure to properly account for remanence can lead to completely erroneous interpretations. In general the strength and orientation of remanence are unknown. Two main strategies have been pursued for “unconstrained” inversion of large data sets. One strategy is to invert quantities, such as total magnetic gradient (3D analytic signal), which are insensitive to magnetisation direction. The inverted property is then magnetisation amplitude. Another strategy is to invert for the magnetisation vector, allowing its three components to vary freely. These approaches are useful, but the resulting magnetisation models are highly non-unique.

When interpreting magnetic data in tandem with geological modelling there is greater potential to infer remanence parameters. Non-uniqueness is reduced if the shape of magnetic domains is constrained, especially if the susceptibility is known and if remanence can be assumed uniform. Accordingly, inverting for the remanent magnetisation of individual homogeneous geological units of arbitrary 3D shape is the subject of this paper. Our remanent magnetisation inversion (RMI) approach can be regarded as a generalisation of parametric inversion of simple geometric bodies.

If susceptibility is known, the optimal remanent magnetisation vector within each selected unit is determined via iterative inversion. Sensitivity to change in magnetisation is determined in the x-, y-, and z-directions, and the perturbation vector is found via the method of steepest descent. If the susceptibility is unknown, the optimal susceptibility of each unit (subject to bounds) can be determined via a similar inversion procedure. The geological units can carry remanent magnetisation, but it is fixed during this stage. The susceptibility and/or remanence inversions can be repeated, if necessary, to refine the magnetic parameters. Self-demagnetisation and interactions are taken into account when susceptibilities are high.

The application of the RMI algorithm is illustrated in examples for both known and unknown susceptibility.

High-resolution, low-level aeromagnetic surveys of the Teetulpa gold field in the Nackara Arc, South Australia, map the distribution of magnetic minerals in the alluvial cover, in the form of linear anomalies with a dendritic pattern typical of drainage systems. These anomalies are not evident in the regional aeromagnetic data flown at wider line spacing and higher elevation.Combination of the high resolution magnetic field data with mapping of present day drainage is an important input to gold exploration of the area. The magnetic anomalies can be modelled and inverted, and this might provide quantitative information to indirectly target and evaluate gold resources. Sampling and statistical analysis of relationships between the gold and magnetic minerals within the alluvium are required to form the basis for any such study.

The Signum transform is a simple derivative-based method for qualitative and quantitative interpretation of magnetic anomalies from discrete sources. The methodology is based on the normalization of a filtering function, which is a derivative of the anomalous field or function of this, by its absolute value. The filtered anomalies have only two values (+1 or -1) and the causative sources are represented by the positive values. The transform has been applied to three different functions, namely the first order vertical derivative of the magnetic anomaly, the first-order vertical derivative minus total horizontal derivative and second-order vertical derivative

For a vertical magnetisation the edges of the sources can be recognised from the locations where one or more of the spatial derivatives change its sign: the zero crossover point. The zero cross over point and actual source edge are separated by an amount which depends on the dykes depth and the type of data being transformed. Thus, actual edge locations are easily computed from the Signum transformed data.

The method performs well when closely spaced sources cause anomalies to overlap. Imagery based on the Signum transformation of first and second-order derivative based transforms of the magnetic data combines the advantages of the resolution of the second-order transform with the greater stability of the first-order transform.

The Camelwood and Musket nickel sulphide deposits are significant recent discoveries, located within the Mt Fisher Greenstone Belt, in the northern goldfields region of Western Australia. Camelwood was the first deposit to be discovered, in December 2012, from a reverse circulation (RC) drilling campaign designed to test a coincident airborne electromagnetic (AEM) and geochemical anomaly.

The original objective of the AEM surveys was to detect massive sulphides known to be associated with gold mineralization at the old Mt Fisher gold mine. However, a number of discrete, late-time EM anomalies were identified along an interpreted ultramafic sequence on the eastern boundary of the greenstone belt. The EM anomalies represented classic nickel sulphide mineralisation targets.

Ground time-domain electromagnetic (TEM) surveys, down-hole TEM (DHTEM) surveys, and extensive drilling have been carried out since then, resulting in a JORC compliant resource at Camelwood (1.6Mt @ 2.2% Ni) and the discovery of the Musket deposit.

The application of the AEM method was instrumental in the discovery of the Camelwood nickel deposit. Systematic use of ground and down hole geophysical methods has been valuable in delineating the resource at Camelwood and in the discovery of the Musket deposit. The discovery of Camelwood and Musket proves the potential of the Mt Fisher Greenstone belt to host significant nickel sulphide mineralisation.

In 2008 Geotech flew a regional scale 24,675 line-km survey covering a 25,000 km² area (1 km line spacing) in the Selwyn Basin. The survey footprint straddles east-central Yukon and overlaps into the western Northwest Territories. In March 2013 Yukon Geological Survey purchased the survey data, and in November 2013, released the data publicly. The Selwyn Basin area is prospective for SEDEX-style Pb-Zn-Ag mineralization and the ZTEM survey data provide insights into regional structures and plutons in the region. The Howard’s Pass SEDEX deposits at the southeastern edge of the Selwyn Basin survey area host a combined ~250 million tonne resource with ~4.5% Zn and ~1.5% Pb.

Major NW-SE to ESE and minor NNW-SSE linear conductive trends correlate with known regional geologic, structural and inferred mineral trends. Circular conductive anomalies surrounding resistivity highs reflect po-rich hornfels surrounding intrusive plutons. 2D-3D computer inversions reveal a correlation between enhanced conductivity along strike and the clustering of deposits at Howard’s Pass.

Geological models that represent the structure of the subsurface, are becoming a regular product of geological surveys. It is widely accepted that the sparse data at depth and the ambiguity of structural interpretations of geophysical data lead to inherent model uncertainties . The analysis and visualisation of model uncertainties is therefore the scope of current research. We here apply a recently developed method to estimate uncertainties to a 3D model of the Sandstone Greenstone Belt in the Archean Yilgarn Craton in Western Australia. On the basis of errors in geological parameters, a suite of probable models is generated and analysed. Our results show that visualisations of unit probability and information entropy provide suitable methods analyse uncertainties in this geological model.

The seismic reflection method is a high resolution technique that can be used in many exploration environments including mineral exploration. However, mountainous terrain, depth of burial and the steepness of ore bearing structures pose a challenge to the application of surface seismic in mineral exploration. The cross-hole seismic method may present an alternative approach under such conditions. Presented here is a synthetic study examining the capability of the cross-hole seismic method to delineate a volcanogenic massive sulphide ore body in a shale hosted environment.

A simple model typical for volcanogenic massive deposits in Tasmania has been considered. There, an elongated steeply dipping volcanogenic massive sulphide deposit with an average thickness of 10 m is seated within a shale rock. The primary aim of the modelling is to test the capability of the technique to delineate relatively medium sized, steeply dipping volcanogenic massive sulphide lens in shale hosted environment. A second objective is to use the technique to prospect for extensions to mineralization along steeply dipping reflectors.

Synthetic cross-hole seismic records were generated using a 120 Hz energy source. Kirchhoff VSP migration was applied to wavefield separated shot records and Pre-stacked Depth Migrated images created. The resulting migrated images correlate well with the position and dip of the ore body demonstrating the potential of the cross-hole reflection technique to delineate steeply dipping ore structures in challenging environments.

During 2012 and 2013 FQML through its Zambian exploration team acquired about 300 line kilometres of Audio magneto telluric (AMT) survey for copper and nickel exploration in the north western Zambian copper belt. Kansanshi mine and the new $2bn Kalumbila (Enterprise Nickel deposit) development project are the survey locations (see Figure1). Hence this paper is a case study discussing major aspects of the projects including but not limited to major outcomes, difficulties encountered as well as solutions for futures data acquisition in similar geological settings. Stunning results were achieved whereby basement fault and many interesting geological features were identified (See Figure 2, Tm mode section along Line 19) over Enterprise and doming structures in Kansanshi (See figure 3).However limiting factors such as mine equipment noise, accessibility, swamps, tropical rain forest and wild life were some downside during the survey.

A major geophysical experiment has begun in the Capricorn Orogen in Western Australia. Orogen-scale passive seismic and magnetotelluric surveys are on-going and preliminary results suggest have successfully delineated the base of the crust and major structures and tectonic boundaries. Airborne electromagnetic data have successfully mapped features in the near-surface such as palaeovalleys.

The integration of the different geophysical datasets with each other and with parallel geological studies are intended to lead to a better understanding the Capricorn Orogen and develop exploration approaches and appropriate toolkits that significantly improve our ability to prospect under cover.

Geoscience Australia is releasing into the public domain software for the inversion of airborne electromagnetic (AEM) data to a 1D conductivity depth structure.

The software includes two different algorithms for 1D inversion of AEM data. The first is a gradient based deterministic inversion code for multi-layer (smooth model) and few-layered (blocky-model) inversions. The second is a reversible-jump Markov chain Monte Carlo stochastic inversion algorithm suitable for assessing model uncertainty. A forward modelling program and some other ancillary programs are also included. The code is capable of inverting data from all of the commercial time-domain systems available in Australia today, including dual moment systems.

The software is accessible in three forms. As C++ source code, as binary executables for 64 bit Windows® PCs, and as a service on the Virtual Geophysics Laboratory (VGL). The code is fully parallelized for execution on a high performance cluster computer system via MPI or a multi-core shared memory workstation via OpenMP.

The Granites Tanami Orogen (GTO) in central Australia is a significant gold producing province. Future exploration will be facilitated by determining the structural controls on mineralisation and crustal evolution of the orogen. A whole of crust model has been generated through the multi-scale integration and interpretation of geophysical, geological and remote sensing data.

The architecture of the orogen is that of a basin that has been inverted, deformed and intruded during: a) the collision between the Kimberley and Tanami basins along the Halls Creek Orogen to form the North Australian Craton; and b) during the amalgamation of the North Australian Craton with the Central Australia Craton. These continent-continent collisions have resulted in a complex structural framework, which is further complicated by deep weathering and extensive regolith across the region. Reconnaissance style outcrop mapping coupled with potential field interpretation has identified two main phases of deformation. The first regional deformation event resulted in north- to northeast-trending isoclinal fold trains of wavelengths ~10 km or greater. These folds are recognised through the interpretation of joint gravity and magnetic anomalies and are confirmed in outcrop. Gold mineralisation within the GTO is coincident with the second regional deformation event, which is recognised in regional aeromagnetic data as polyphase deformational interference patterns caused by the refolding of earlier folds around axes trending E to ESE.

This defining of upper crustal architecture to structural features observable in sparse outcrop coverage could not be sufficiently identified without this combined geology and geophysical approach.

Regional geophysical datasets are critical to the task of uncovering the basement geology of the southern Thomson Orogen in far western New South Wales. As part of a National Collaborative Framework project, aeromagnetic, gravity and seismic data have being processed and interpreted to construct the structural framework. Subdivision into structural domains has been validated and constrained by geological information, relying on observations and measurements from sparse drill holes and outcrops.

Boundaries between structural domains are complex and poorly understood. This study aimed to recognise major faults and, where possible, define their displacements, depth extent, and understand their dynamics and timing. Analysis of available company and government seismic surveys provided details for some of the major fault systems such as the Olepoloko Fault, Culgoa Lineament, and also for many newly recognised fault trends

The seismic interpretations were reconciled with deep sourced aeromagnetic and gravity gradients that were enhanced by multiscale edge analysis. The structural framework will underpin geochronology and mineral systems studies as the Southern Thomson Orogen project continues.

Little is known about the basement of the Hay-Booligal Zone (located in NSW). Magnetic modelling of long wavelength anomalies within the Hay-Booligal Zone indicates that the Hay-Booligal basement consists of serpentinised ultramafic material at depths of 6 to 12 km. This supports interpretation that the Hay-Booligal basement could be similar to the Selwyn Block in Victoria and rather than a crystalline microcontinent.

Also, interpretation of short wavelength semi-parallel TMI anomalies near the previous boundary of the Hay-Booligal Zone has lead to the adjusted the boundary of the Hay-Booligal and Bendigo zones.

Interpretation methods and tools for geophysics datasets continue to evolve. Advances in clustering algorithms, the use of implicit functions to create 3D surfaces, new algorithms to estimate source depths and dips, and the availability of clever computational geometry libraries, contribute to the discipline of potential field interpretation techniques, allowing for a much more explicit statement of implied 3D description. While traditional scalar measures of potential fields have benefited from applying new ideas, perhaps more exciting is the reduction in ambiguity imposed from gradient measurement when used as the basis for field interpretation. Full tensor gravity gradiometry in particular, allows for 2D fault dip and throw calculations. Direct detection of high density bodies and faults via state-of-the-art gravity gradiometry is now a reality. Bodies greater than 200m in lateral extent are detectable. Implicit 3D structural geology modelling techniques derived from gravity curvature attributes of the observed gravity field present a leading edge technique for defining structurally controlled near surface geology geometry. A demonstration from the Bathurst camp dataset is given.