ASEG Extended Abstracts - ASEG2009 - 20th Geophysical Conference, 2009

In certain geological situations, improved characterisation of the subsurface can be achieved through the integration of conventional P-wave seismic reflection and PS-wave (converted, or C-wave) reflection. Initial noteworthy successes for this technology related to offshore petroleum targets, which were difficult to image with standard P-wave methods (e.g. Barkved et al., 1999; Hanson et al., 1999). More recently, integrated interpretation of P and PS imagery has been shown to yield improved geological interpretation of coal targets, including detection of new fault structures, improved determination of fault geometries, and superior imaging of top-of-coal for shallow open-cut targets (e.g. Hearn, 2004; Velseis, 2003, 2007). Application of PS-wave techniques is arguably more challenging in the onshore situation, due partly to severe S-wave receiver statics. These statics are more problematic with shallow high-resolution data, where maintenance of high frequencies is critical.

In the P-wave arena, there is an increasing trend toward 3D surveys since these produce a better spatial interpretation. It would be reasonable to assume that 3D PS-wave surveys would also lead to an improved geological interpretation. However, 2D PS-wave surveys have identified a number of issues that will make 3D PS exploration more challenging than the conventional case. For example, different geological interpretations may result from images created with positive (forward shooting) or negative (back shooting) offsets. Additionally, the asymmetry of PS-wave paths means that it is more difficult to achieve regular azimuthal and offset distributions when designing 3D grids. Associated with this is the fact that PS offsets may be more restricted due to phase effects. PS path asymmetry is generally more pronounced at the shallower depths appropriate to coal exploration (50-500m). There is also a greater tendency to use rays having higher incidence angles. Hence we believe that the problems noted here are likely to be more severe at coal depths than at petroleum depths.

In this paper, the discussion of these issues is largely based on prior 2D experience, and numerical modelling of 3D effects. It represents the design phase of a 3D converted-wave field experiment to be carried out in late 2008 (ACARP, 2008). Although we emphasise the peculiarities of the shallow coal exploration environment, the discussion is of more general relevance.

Formalisation of guidelines for interpreting radiometric images of the regolith in the NT has been made possible with the recent AW AGS calibration flights by GA, following a series of calibration measurements on test sites across the NT using spectrometer at ground level.

In the first instance, objects dominated by potassium can be firstly considered as clay. Image objects dominated by thorium can firstly considered as duricrust. Similarly, image objects dominated by uranium can be firstly considered as carbonaceous ground.

Demonstrating these guidelines on the the radiometric image of the NT gives immediate meaning to much of the area. Interpreters are invited to apply them any over radiometric image of the regolith.

This paper will review the history of and recent progress in the development of a new airborne gravity gradiometer based on an innovative design originating at the University of Western Australia (UWA). Current work is being carried at the UWA as a collaborative project with Rio Tinto.

With the advent of new potential field full tensor gradient instrumentation, new methods have been developed to denoise and process these curvature gradients. Traditional Fourier Domain and Minimum Squares least squares residual of the linear differential relationships have been adapted. This leads to levelling, gridding and grid filtering innovations. The result is a Full Tensor Grid representation of the curvature gradients that is coherent and compliant with the physics at all points in the grid. All of the observed data is thus honoured in the Tensor grid. Superior anomaly interpretation and inferences can then be made. A case study showing the improvement that can be obtained is presented. Special attention is warranted for the Full Tensor Magnetic gradient signal. Multiple surveys of this quantity have been made recently in South Africa.

Carrapateena is a new Olympic Dam - style, iron-oxide copper-gold (IOCG) deposit, located approximately 160 km north of Port Augusta and 100 km south-east of Olympic Dam, South Australia (Figure 1). Carrapateena was discovered in June 2005, when significant copper- gold mineralisation was intersected by RMG Services Pty. Ltd. (RMGS), within drill hole CAR002. CAR002 was targeted on near - coincident gravity and magnetic anomalies and ended in mineralised hematite breccia, returning an intercept of 178.2m at 1.83% Cu, 0.64g/t Au, 0.21% Ce, 0.13% La and 59ppm U, from 476m, including a high grade top of 73m at 2.89% Cu and 0.4g/t Au (Vella and Cawood, 2006).

Jaguar is an Archaean volcanogenic massive sulphide (VMS) deposit is located approximately 250km NNW of Kalgoorlie, and about 4 km south of the historic Teutonic Bore Cu-Zn-Ag mine (Figure 1). The current mining reserve is 1.6mt @ 3.1% Cu, 11.7% Zn, 0.72% Pb and 120g/t Ag. Jabiru Metals Limited began mining the deposit in May 2007 via a 1.8 km long decline that reaches the top of the orebody, which is 300m below the surface. The underground operation has a planned life of five years.

The Canadian company Inmet Mining Pty Ltd entered a joint venture with Jabiru’s predecessor, Pilbara Mines Ltd, to explore for VMS deposits in 2001 and discovered the Jaguar deposit in February 2002. Their exploration programme included a large FLEM survey that covered most of their tenure. The discovery drillhole, TBD-202, was the second of two holes drilled into an 1800m long FLEM conductor. The initial holes were planned 600m apart as Inmet Mining were chasing large targets. Following the technical success provided by the FLEM survey a range of electrical geophysical techniques were trialled over the Jaguar deposit. The results of these surveys are reviewed here.

In 2004, Gold Fields Limited and Curtin University of Technology undertook a project to assess the feasibility of the application of high resolution reflection seismic for gold exploration in Western Australia. A large scale regional survey several years before had indicated that deeper structures can be successfully imaged with course seismic acquisition techniques. These surveys provided images on a regional and camp- scale only while shallower structures of direct interest to exploration remained unresolved. The acquisition of high resolution seismic intended to detect economically viable targets for mineral exploration, turned out to be not a trivial endeavour. Seismic lines which traversed over and along busy mine site roads, around restricted areas, and through irregular, jagged terrain result in crooked-line geometry, often saturated with ambient noise, and running in an unfavourable direction with respect to the dominant trend of the major structures. This creates out-of-plane reflections and limits the effectiveness of pre-stack and post- stack imaging techniques and severely affects the calibration with sonic logs. While mine-sites generally have an abundance of borehole drilling data, sonic logs are in sparse supply and often restricted to shallow depths by hydrocarbon standards (200-900 meters), which presents a difficulty for seismic data inversion and subsequent lithological interpretation.

Ghana has been a producer of gold since the 16th century and today boasts one of the largest and richest reserves of the precious metal in the world. The principal gold producing areas of Ghana occur within Palaeoproterozoic Birimian meta-volcanic and meta-sedimentary rocks, and within the marginally younger, overlying Tarkwaian meta-sedimentary succession. The giant 40+ million ounce (Moz) Ashanti deposit at Obuasi and 38+ Moz deposit at Tarkwa are 2 monstrous examples located in Ghana. Other deposits include Prestea/Bogoso (7 Moz), Konongo (2 Moz), Damang/Aboso (3Moz), Bibiani (5 Moz), and Chirano (2Moz).

Gold exploration in the past was primarily conventional stream sediment and soil sampling, followed by trenching and drilling. This methodology was used to discover a majority of the deposits. However, completely unexplored grounds in Ghana, where cursory, first-pass reconnaissance surface sampling methods lead to a major discovery, are virtually nonexistent. The modem phase of exploration calls for a more interdisciplinary approach involving the use of geophysics, geochemistry, and regolith mapping, as well as detailed structural and geologic observations. Newmont is one such company that integrates geophysics extensively in its gold exploration programmes in Ghana.

Some of the geophysical techniques used are airborne magnetics, radiometrics and electromagnetics. Ground based geophysical tools include gravity, magnetics and IP/Resistivity. Geophysical data are used to help with ground selection, direct drill targeting, and to help produce interpreted geology and regolith maps. Newmont Ghana has trained Ghanaian nationals to conduct a majority of the data collection, processing and interpretation. Several example data sets are shown.

IP, DC Resistivity and MT data have been acquired using three-dimensional arrays at the Green Dam Prospect, located approximately 120 km east-northeast of Kalgoorlie. The 3D acquisition has enabled data interpretation using both two and three-dimensional inversion methods, leading to increased confidence in results and an improved understanding of the variation in mineralization along strike.

The results in the case study will show the IP data clearly map the dominant disseminated Ni- Cu-PGM sulphide mineralization. The data have also mapped the semi-massive to massive Ni-Cu-PGM sulphides and clearly show a high degree of correlation with previously acquired moving loop TEM (MLTEM) and downhole TEM (DHTEM) data.

In the last few years, many companies have purchased abandoned mines. This gives ready access to economic mineralization that was either "missed" with previous generations of geoscience technologies and methods, or that represents ground not yet evaluated. Today, new deep geophysical technologies are helping with investigations "in the shadow of headframes" – assisting not only in exploration, but also in ore delineation and mine development (ground condemnation).

However, brownfield work is not easy. Cultural noise, scheduling, electrical noise, remoteness and resistance to new technologies are some of the traditional obstacles that have been overcome through deep electrical imaging and Distributed Acquisition Systems. DAS technologies have a large multi-channel, fixed receiver array; sensitive electronics; advanced processing and noise removal; and other characteristics that result in improved depth of penetration, data quality and detectability. Numerous brownfield sites have been surveyed over the past 5 years.

In this paper, we review the components and capabilities of DAS systems, and specifically, Titan 24 Deep Earth Imaging for brownfield work, including near mine and minesite applications. Three case studies are presented, including two from porphyry copper environments in western Canada as well as a gold project from Bulgaria. These case studies represent the state-of-the art in geophysics for brownfield work and are a unique and novel application for today’s DAS technologies.

The NSW government’s New Frontiers exploration initiative program is currently focused on under explored areas of the state, under cover. As part of this program, production of geophysical/geological interpretations for Ana Branch, Pooncarie, Booligal, Balranald, Hay and Deniliquin 1:250 000 map sheet areas, has commenced. The aim is to encourage exploration to frontier areas of NSW by extrapolating the geology beneath covered areas using regional aeromagnetic, gravity, radiometric, Landsat7, seismic and borehole stratigraphy datasets.

To date, both the Hay and Balranald 1:250 000 map sheet areas have been completed using Total Magnetic Intensity (TMI) data and 1VD TMI data. Outcomes of this work include:

An improved understanding of the tectonic setting of the area, such as:

o The Stawell Zone extension into NSW

o Silurian-Devonian granites that terminate at the Bendigo Zone

o NNW-SSE trending, weakly magnetic, dykes

An enhanced understanding of the areas mineral potential, pertaining to:

o Heavy mineral sands that are magnetically detectable

o Diatreme-pipe-like magnetic sources with gemstone potential, of a possible Carboniferous–early Cretaceous age

o Orogenic gold; related to Stawell Zone extension

The Thomson Orogen, which underlies the Channel Country in far northwest New South Wales, is a major, largely unknown region overlain by the Mesozoic Eromanga Basin and Cainozoic sediments. The region has potential for arc- and ocean-crust-related gold and base metal deposits while the northern part of the Lachlan Orogen immediately to the south of the orogen, may have potential for Mississippi Valley style zinc and lead deposits. The prospective targets are obscured by variable thicknesses of Mesozoic and Cainozoic sedimentary units.

To develop a better understanding of the tectonic setting and mineral potential of the Thomson Orogen an integrated geoscience program has included high resolution aeromagnetic and radioelement data; deep seismic data; new gravity stations; regolith mapping through classification of satellite data; review of past and present company data; baseline geochemistry; stratigraphic drilling; and studies of lithofacies, age data, geochemistry and petrology from selected drill core acquired by exploration companies. The data are the basis for the new Thomson Orogen GIS.

The first stage of this integrated study has been completed and indicates that the Thomson Orogen in NSW has many similarities to the mineral-rich Lachlan Orogen.

In conjunction with CRCLEME, an Explorers’ Guide has been developed to assist mineral exploration in this regolith dominated terrane.

The Silurian–Middle Devonian history of the Lachlan Orogen is characterised by the formation of rift basins and the emplacement of large amounts of granite. Many rift basins contain felsic or mixed felsic and mafic volcanic rocks, indicative of crustal as well as mantle melting being involved in lithospheric extension. However, there are several large rift basins that are filled by siliciclastic sedimentary rocks in which volcanics occupy ≪ 1 % of the basin fill and may be buried. For the latter basins the question is: was rifting amagmatic, or are products of melting present at depth below the surface, either in deep basin sediments or in basement below the basin.

In this paper, we attempt to address this question for sedimentary basins in the Cobar–Louth region of western New South Wales. For the Late Silurian–Early Devonian Cobar Basin, we use 1989 explosion–generated seismic reflection data that have been reprocessed using a new semblance filtering technique to improve the data quality. For the Nelyambo Trough, part of the Devonian Darling Basin in western NSW, we used Vibroseis deep seismic reflection data recently acquired in cooperation with Geoscience Australia and the Predictive Mineral Discovery Cooperative Research Centre. Gravity profiles were acquired along the Cobar lines. For the Nelyambo Basin, gravity data were extracted from a statewide dataset to match the seismic lines. The combined seismic and gravity data sets suggest that bright reflectors in the seismic sections represent mafic volcanics. These reflectors lie within inferred rift fill near the base of the Nelyambo Trough, but also occur in basement under the southwestern margin of the Cobar Basin.

The Vredefort Structure is a deeply eroded, complex impact structure, located near the centre of the Kaapvaal Craton with an age of 2.02 Ga. Estimates of the original diameter of the impact structure range from 250-380 km, making it the largest known terrestrial impact. The Vredefort Dome, the central uplift, has a diameter of approximately 80 km and consists of a core of uplifted Archaean migmatites and granulites of 3.1-3,2 Ga, with an 20 km wide outer collar zone of supracrustals of the Archaean to Palaeoproterozoic Witwatersrand, Ventersdorp and Transvaal Supergroups. In the northwest, there is good outcrop and the collar sequence can be seen to dip steeply towards the center of the dome or are overturned. To the south and east, overlying Karoo sediments obscure the geology. The morphology of the central uplift has been modified by post-impact deformation

The Archaean core consists of an outer annulus of heterogenous amphibolite facies migmatites of the Outer Granite Gneiss (OGG) around the central Inlandsee Leucogranofels (ILG) terrain of granulite facies metamorphic grade. Stepto (1990) recognizes a threefold concentric zoning, with the Steynkraal Formation between the OGG and ILG.

Previous work (Corner at al (1990), Muundjua et al, 2007, Stepto, 1990) have mainly analysed aeromagnetic and gravity data separately. Henkel and Reimold (1998) carried out profile modelling of magnetic and gravity data. Antoine et al (1990) imaged aeromagnetic and gravity data. We have investigated the remanent magnetization problem and used similarity images and cross grey level coocurrence matrix texture transforms to analyse and compare aeromagnetic, gravity and SRTM DEM data to provide a clearer picture of the Vredefort Dome.

During 2007 the NSW Department of Primary Industries contracted the Australian National Research Facility for Earth Sounding (ANSIR) to conduct the SEAL2 (South East Australia Linkage experiment, part 2) teleseismic program in southwestern New South Wales, to supplement earlier teleseismic acquisition in Victoria and southeastern South Australia (Figure 1). Interpretation of the results of the earlier SEAL1 program (Rawlinson et al., 2006), acquired with a seismic array in the southern Murray basin, appeared to conflict with recent interpretations of upper-crustal tectonic trends inferred primarily from aeromagnetic surveys in southwestern NSW (Hallett et al., 2005), which suggested that the Stawell Zone of western Victoria continued northwards into NSW. Instead, the SEAL1 tomographic model suggested a break in upper mantle velocities near the state border; the model implied that the Stawell Zone in Victoria is underlain by high vp upper mantle, consistent with lithospheric basement of Proterozoic age, while the putative extension of the Stawell Zone into NSW is underlain by low vp mantle, corresponding to Palaeozoic basement.

The northern Gawler Craton, South Australia holds the key to understanding the Palaeo- to Mesoproterozoic evolution of Australia and the geodynamic setting and significance of major mineralizing systems active at that time, particularly the iron-oxide copper gold (IOCG) mineralization of the Olympic Domain (~1580 Ma). Unfortunately, these basement rocks are buried beneath significant thicknesses of Neoproterozoic and younger sediments, such that that our understanding of their architecture, composition and evolution has to be derived from analysis of limited basement penetrating drill-holes and regional geophysical data. Here, we present an interpretation of the basement geology of the Marla Region as constrained by available drill-hole, seismic and potential field data.

Large areas of prospective North-West and North Queensland have been surveyed by airborne hyperspectral sensor, HyMap^®, and airborne geophysics as part of the " Smart Exploration" and Smart Mining" initiatives of the Department of Mines and Energy Queensland. In particular, 25000 km² of hyperspectral mineral and compositional map products, at 4.5 m spatial resolution, have been generated and made available via the internet (http://www.em.csiro.au/NGMM/; http://www.dme.qld.gov.au/mines/hyperspectral.cfm) (Figure 1). In addition, more than 130 ASTER scenes were processed and merged to produce broad scale mapping of mineral groups (Figure 1) (Thomas et al, 2008). Province-scale, accurate maps of mineral abundances and mineral chemistries were generated for North Queensland as a result of a 2 year project starting in July 2006 involving CSIRO Exploration and Mining (www.csiro.au/science/psl6a.htrnl), the Geological Survey of Queensland (GSQ) (www.dme.qld.gov.au/mines/hyperspectral.cfm), Geoscience Australia (www.ga.gov.au). James Cook University, and Curtin University. This project also involved CSIRO’s Minerals Down Under National Research Flagship (MDU - www.csiro.au/science/MineralsDownUnder.html), the Cooperative Research Centres for Predictive Mineral Discovery (pmd*CRC) and Landscape Environment and Mineral Exploration (LEME), and HyVista Corporation (www.hvvista.corn).

Before the advent of the CSIRO HyLogger™ technology (Huntington et al, 2004), semi- quantitative mineralogy was beyond the realms of practical application being expensive and time consuming. With a high density, high volume, spectral dataset, down hole mineralogy can be mapped, not only by presence, but also by abundance and chemical gradient. (Mauger, 2007) Four main mineral suites are presented here: White Mica/Clays (Al(OH)), Chlorite (Fe,Mg(OH)), Carbonate (CO3) and Fe Oxide.

One of Australia’s National Research Priorities (www.dest.gov.au/sectors/researchsector/policies_issues_reviews/key_issues/national_research_priorities/default.htm) for achieving an "Environmentally Sustainable Australia" is tackling the problem of acid sulfate soils (ASS) which affect many of our most desirable lands for urban and agricultural development. Fitzpatrick et al. (1998) stated that currently ‘There is no consistent standard for mapping their [sic ASS] extent or severity." To help address this problem a national atlas on ASS has been established (www.clw.csiro.au/acidsulfatesoils/index.html) which has been incorporated into the ‘Australian Soil Resource Information System’ website (ASRIS - www.asris.csiro.au/index_ie.html). However, these current GIS map products are based on interpolation of isolated ground sample points and the interpretation of remote sensing data that is not optimum for detection of ASS related mineralogy. In addition, these map products, especially for the large area assessments such as the Murray-Darling Basin, are not suitable for monitoring purposes except for those (few) sites where ground data is being routinely collected.

Hyperspectral sensing technologies from both aircraft and satellites have the potential to provide accurate mapping and monitoring of ASS through its ability to measure often subtle but diagnostic compositional information of surface materials (Clark & Roush, 1984; Clark et al, 1990a; 1990b). This includes the measurement of the abundances, compositions and crystallinity of the specific minerals, including those associated with ASS such as jarosite, goethite, hematite, kaolinite and gypsum. Typical ASS minerals include iron oxyhydroxides (goethite), oxyhydroxyl-sulphates (jarosite) and sulphates minerals (barite and gypsum) (Fanning et al., 2002). Fitzpatrick & Self (1997) found that minerals found in acid sulphate soil environments are similar to those analogous to acid mine drainage. These environments consisted of ferrihydrite, schwertmannite, goethite, lepidocrocite and jarosite. Although there have been a number of studies of the application of hyperspectral for mapping and monitoring acid conditions associated with mined environments (e.g. Swayze et al., 2000; Crowley et al., 2003 and Ong et al., 2003) there has been little work on its use for mapping acid sulphate soils (e.g. Lau et al., 2006) despite its potential value for large area (catchment-scale) mapping and monitoring of ASS (http://www.dec.wa.gov.au/news/department-of-environment-and-conservation/new-technology-identifies-ancient-acidification-risk.html; www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/218489/ASSAY-44.pdf).

The main aim of this study is to assess whether airborne hyperspectral imagery can be a useful tool for mapping surface mineralogy potentially associated with ASS in coastal areas undergoing urban development. A collaborative project was established in 2007 between the Department of Environment and Conservation (DEC) and CSIRO to investigate the ability of the airborne hyperspectral imagery to help identify potential "hot-spot" sites of ASS contamination in the South Yunderup area of the Peel region of WA.