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

Since 2000 the .Aarhus Workbench (aarhusgeo, 2008) is constantly developed to meet the research needs of the HydroGeophysics Group (HGG) at the University of Aarhus. It allows you to handle, process, invert and visualize electric and electromagnetic (EM) data on a common GIS platform. Geological data can be shown on the GIS map and on cross-sections for comparison with the geophysical results.

The basic idea of the Aarhus Workbench was to develop a single and integrated software platform for handling a number of different data types. The Workbench uses an open-source client server database to manage data and settings. The benefits of using a databases compared to flat ASCII column files should not be underestimated. Firstly, user-handled input/output is nearly eliminated, thus minimizing the chance of human errors. Secondly, data are stored in a well described and documented format which is well suited for both exchange and storage of data.

The Workbench allows the user to present the output of inversions as point themes or as color contoured thematic maps, such as mean resistivity slices, depth to a conductor etc. Models can also be shown on sections which are linked both to the GIS and to displays of data and forward data. The sections can contain numerous layers representing different data types.

Over the years HGG has developed stable processing and inversion algorithms for airborne and ground-based EM data. The inversion is known as Laterally Constrained inversion (LCI) for quasi 2-D modeling and Spatial Constrained inversion (SCI) for quasi 3-D inversion. The Workbench implements a user friendly interface to these algorithms enabling non-geophysicists to carry out inversion of complicated airborne data sets without having in-depth knowledge about how the algorithm actually works. Just as important is an extensive system for evaluation of inversion results with plots of results like resistivity models, flight height, pitch, roll, residuals etc.

One hundred meter square in-loop ground TEM soundings were an effective way to screen aeromagnetic, ground gravity, and Falcon® gravity gradiometer signatures caused by kimberlite intrusions overlain by 40-120 meters of transported Kalahari sedimentary cover in the Kokong kimberlite field of Botswana (Figure 1). Ground TEM’s effectiveness in identifying kimberlite pipes led to the flying of the VTEM aeroTEM system over selected areas at Kokong. Ten kimberlites previously covered by ground TEM surveys were over flown by the VTEM survey (Figure 2). A comparison of the ground TEM and VTEM responses show that the VTEM effectively drill-screened nine out of the ten kimberlite magnetic or gravity signatures, whereas the ground TEM systems effectively screened all 10 kimberlite signatures (Table 1). KS40, the largest kimberlite at Kokong and the one kimberlite less well mapped by the VTEM was, at 10-30 milliseconds (msec), a later-time conductor that best responded after the VTEM’s last channel 27 at 7.5 msec. KS40’s kimberlite crater was mapped as a good late-time conductor (later than 7.5 msec) by both ground TEM systems. The ground TEM systems employed were a Geonics TEM57 transmitter and PROTEM receiver, and two Zonge ZT-30 transmitters and GDP32 receivers; their specifications are compared with the VTEM in Table 2 below.

The conductive overburden found on the African and Australian continents are normally considered a challenge for airborne EM surveys, severely limiting the depth of investigation and reducing anomaly amplitudes of basement conductors. However, interpretation of airborne TDEM data acquired with the VTEM system has led to the discovery of a number of base metal deposits in Africa and Australia over the last three years. These successes are ascribed to a combination of the VTEM’s high signal to noise ratio and interpretation skills of experienced geophysicists. Examples of these discoveries include Bertram NiS and Sunchaser VMS (Fox Resources) and Balia Balia (Straits Whim Creek Copper) in Australia as well as a discovery in Zambia for Zambezi Resources Limited. These will be discussed in terms of their geological setting, TDEM response signatures and interpretation procedures which were followed.

The HeliGEOTEM system was introduced in 2005 to provide greater operational flexibility and improved lateral resolution compared with a fixed-wing system (Fountain et al., 2005). The system, described in more detail by Fountain et al. (2005), is a vertical-axis dipole transmitter towed below and behind a helicopter. The receiver, also attached to the tow cable is about 15 m in front and 35 m above the transmitter. The system measures the response in the time domain when a half-sine current pulse excites the ground. The dB/dt and B-field responses are measured in the x, y and z orientations. The geometry of the system and the coordinate system are shown on Figure 1. Compared with fixed-wing systems, the helicopter systems have their transmitter / receiver closer to the ground surface, which is why the spatial resolution is greater (the response is much sharper); also, the response of shallow bodies is much larger. However, when the bodies are deeper, the responses are more comparable.

The Tusker gold deposit is located in the Lake Victoria Goldfields of Tanzania. The deposit contains total estimated resources of 123.27 Mt at 1.15g/t Au, as at 5^th September, 2006. Geophysical techniques trialled over the deposit include down hole measurements, airborne time domain electromagnetics, dipole-dipole induced polarisation and resistivity and airborne magnetics.

Petrophysical measurements suggest the deposit is associated with a conductive and chargeable signature. This is confirmed by airborne EM and dipole-dipole IP and resistivity data. Magnetic data map the umnineralised magnetic mudstone package overlying the deposit, and show that stratigraphy is deformed.

In the last five years the Munali nickel-copper deposit has been subject to a systematic assessment using a variety of airborne and ground geophysical techniques. These results are reviewed here to assess the efficacy of the techniques and define a target model that could be used in the search for similar deposits. 3D modelling and display have been used to aid in this assessment.

An airborne time domain electromagnetic and magnetic survey (Geotech YTEM system; VTEM is a helicopter-borne Time Domain Electromagnetic (TDEM) system with concentric loop transmitter receiver geometry.) was flown over the Kedia mountain and Mhaoudat Intrusive in Mauritania. Hematite has been mined in both these areas for many years. The objectives of this survey were to determine if additional ore bodies existed below old mined areas and to find and delineate possible new ore bodies.

A detailed structural interpretation, using the magnetic and EM data, was done with the assumption that major faults could be a controlling factor on the occurrence of iron ore. Decay curve analysis was done of the VTEM data as a fast, first pass delineation of conductive areas. Good correlation with existing mines was found. CDI’s were consequently calculated for all flight lines. These were used to calculate conductivity contour maps at various depth intervals below the surface. Conductive areas were correlated with the magnetic data as well as the known geology. In several areas known iron ore was associated with the edges of observed anomalies. These anomalies continued at depth and distance, indicating possible extensions of the ore. Further anomalies occurred in a sand covered area south of Mhaoudat, leading to the possibility of new iron ore deposits.

Geoscience Australia (GA) is acquiring regional airborne electromagnetic (AEM) data in areas of Australia considered prospective for energy commodities as part of the Onshore Energy Security Program (OESP) and to encourage industry exploration activity. The aim of the AEM surveys under the OESP is to release pre-competitive regional data and conductivity-depth information to the public. As part of this initiative, private exploration companies have collaborated with GA by subscribing to infill selected areas at closer line spacings. In return, for favourable financial conditions and management of the survey process, companies will allow GA to release the infill data to the public at the end of a twelve month moratorium period. The large volumes of data acquired under the OESP will be publicly available from GA once internal QA-QC procedures are completed.

The Paterson Region in Western Australia (WA) (Figure 1) is the first area surveyed under the AEM acquisition program of the OESP. The main aim of this survey is to promote exploration for unconformity-related uranium deposits in previously under-explored and predominantly undercover areas. The uranium systems in this area are believed to be related to the unconformity between the Coolbro Sandstone and the underlying Rudall Complex (McKay and Miezitis, 2001). They may also be related to the contact between the Broadhurst Formation and the Coolbro Sandstone, and are also likely to be controlled by the location of basement faults. Other types of uranium mineralisation that may be prospective in the survey area are surficial calcrete-style uranium systems and sandstone-hosted uranium systems that may be active within buried palaeovalleys.

The drive to explore under cover necessitates methods of geological or geochemical mapping that can “see through” the masking cover rocks. Magnetic data are routinely collected for this type of structural mapping in various terranes. In Mali, magnetic data have been of very limited use to Resolute Mining’s mineral exploration programs - magnetically featureless cover rocks hide almost all structural expression in the basement. .Airborne EM has proven invaluable in tracing lithological contacts, subtle stratigraphic horizons, and intrusive bodies through conductive weathering up to 100m deep. Helicopter time domain systems with high transmitter moments penetrate deep enough to escape contamination by surficial conductivity features such as drainage patterns. The results under basin cover in Mali are particularly impressive in their ability to provide a clean picture of basement geological structure.

AeroTEM is a helicopter-borne time-domain electromagnetic (HTEM) system developed and operated worldwide by Aeroquest Limited. The AeroTEM II, AeroTEM III and AeroTEM IV systems share the same basic system configuration and data characteristics, but offer varying system power and field performance characteristics. We introduce the general specifications of the system and provide examples to highlight the various aspects of the system response.

This paper describes the survey design, data collection and results from a geophysically driven exploration program in Northern Australia. The exploration project was a joint venture between Falcon Minerals NL and Anglo American. It was focussed on finding massive to net textured nickel sulphides associated with a mafic intrusive body. The extent of the intrusive body was inferred from interpretation of a significant 14mgal gravity anomaly. Limited previous exploration indicated any drill targets were likely to be covered by 400 to 500m of □esozoic and recent sediments. These sediments are estimated to have a conductance of 200Siemens, making it difficult to use conventional electrical geophysics for effective exploration.

Anglo American, together with its research partner IPHT of Germany have developed a low temperature superconducting quantum interference device (LTS) that can be used as a sensor in transient electromagnetic (TEM) surveys. The LTS is a "B field" sensor with significant advantages in noise levels over other "B field" sensors and the ability to detect targets with time constants up to 3 orders of magnitude larger than conventional dB/dt coils (Le Roux et al 2007). The LTS sensor is particularly suited to finding highly conductive targets beneath conductive cover.

After forward modelling and survey design, the JV completed some 270km of reconnaissance moving loop ground electromagnetic surveys. Selected anomalies identified with this survey were then followed up with more detailed moving loop and some fixed loop surveys. These results were then modelled using a variety of software packages to try and fit the late time TEM field data.

Drilling of selected targets and some downhole TEM surveys are in progress.

A VTEM survey was conducted in the search for magmatic nickel-copper sulphide mineralization in the poorly explored Irindina Province of the Eastern Arunta Region, NT . The VTEM data was analysed for prospective targets and selected priority targets were followed up with B Field moving loop ground EM.

The ground EM followup survey over the highest priority target showed that the VTEM data is biased toward the shallower conductors and was not able to identify the deeper higher conductance targets. Drilltesting and subsequent downhole EM surveys confirmed the VTEM and ground EM targets were caused by disseminated, stringer and semi massive sulphides (pyrrhotite and pyrite with subordinate chalcopyrite) interpreted to be derived from a skam style of mineralizing event.

Geoscience Australia and State and Territory Geological Surveys have systematically surveyed most of the Australian continent over the past 40 years using airborne gamma-ray spectrometry to map potassium, uranium and thorium elemental concentrations at the Earth’s surface. The quality of the radiometric data acquired over this period varies markedly. Early surveys (prior to about 1990) were flown with a line spacing of about 1500 m and a 16 litre detector at a flying height of 150 m agl. Later surveys have been flown with a detector volume of 32 litres and a line spacing of 500 m, or closer, and flying heights of 100 m, or better. The use of high-performance survey aircraft have now enabled flying heights to be lowered to heights of 60-80m for regional surveys and even lower for detailed surveys.

The extent of the digital airborne gamma-ray spectrometric coverage of Australia is shown in Figure 1. For early surveys (shown in blue in Figure 1), the results were usually reported in units of counts per second. Thus the magnitudes of these data values depend on both the instrumentation used in the survey (such as crystal volume) and the survey parameters (such as nominal flying height). This means that the results from early surveys that used different instrumentation and survey parameters are not directly comparable. Also, even where survey acquisition systems were calibrated to report results as equivalent concentrations of the radioelements, limitations in the calibration of these instruments and temporal variations in radiation output from the earth often result in mis-matches between surveys along their common boundaries.

These problems limit the usefulness of the gamma-ray spectrometric data, as surveys are not easily combined into regional compilations, and quantitative comparisons between radiometric signatures from different surveys are difficult. The solution is to adjust all of Australia’s public-domain gamma-ray spectrometric data to a common datum. This will enable surveys to be easily merged into larger regional compilations, and thus facilitate the recognition and interpretation of broad-scale regional features in the data.

This paper describes the adjustment of Australia’s National Radioelement Database to a common datum. We have used an Australia-wide Airborne Geophysical Survey (AWAGS) to adjust all the public-domain radiometric surveys in Australia to the International Atomic Energy Agency’s (IAEA) Global Radioelement Datum. The levelled database has been used to produce the first "Radiometric Map of Australia" - levelled and merged composite potassium (% K), uranium (ppm eU) and thorium (ppm eTh) grids over Australia at 100 m resolution. Interpreters can use these grids to reliably compare the radiometric signatures observed over different parts of Australia. This enables the assessment of key mineralogical and geochemical properties of bedrock and regolith materials from different geological provinces and regions across the continent.

Continental-scale merges of Australian airborne magnetic data (Figure 1) are only accurate for wavelengths less than about 100 km, due to limitations of survey size (Figure 2(b)) and data processing. Wavelengths greater than about 400 km are available from satellite data; thus, there is a "gap" in the spectrum for intermediate wavelengths between 100 and 400 km (Milligan and Franklin, 2004; Milligan et a!., 2004). Geoscience Australia has filled this gap by acquiring new airborne magnetic data as part of the Australian Government’s Onshore Energy Security Program. Intermediate wavelengths are important, for example, for better definition of sedimentary basins for petroleum prospectivity evaluation, for interpreted depths to bottom of magnetic sources in attempts to define the Curie Point isotherm, and for long wavelength regional removal in magnetic modelling.

The Council for Geoscience, South Africa, has embarked on a program of flying the country with high resolution - high density airborne magnetic and radiometric data to generate new exploration targets. It was during the seventies and the eighties of the past century when the existing regional coverage was flown with flight lines 1000 m apart at a nominal flight height of 120 m a.g.l. This kind of first generation data is not any longer satisfying the requirements of highest possible spatial resolution set up by the exploration and environmental industries. This demand was recently acknowledged by the South African Government and has been materialized by launching a new high resolution airborne survey program with 200 m line spacing and nominal flight height of 80 m a.g.l.

Airborne geophysics, particularly aeromagnetic and gamma-ray spectrometer (radiometric) surveys, forms a critical component of geological mapping and mineral resource inventory programs in many African countries. In the 60’s and 70’s, regional aeromagnetic surveys were fairly widespread over much of the continent, in both sedimentary and hard rock terrains (Barritt, 1993). In the 80’s and 90’s, higher resolution surveys, incorporating radiometrics, were carried out in certain countries, particularly in southern Africa. In the last decade, a number of national initiatives (e.g. Madagascar, Mozambique, Namibia, Morocco, Mauritania, Nigeria, Ghana, etc.) have seen the high-resolution geophysical coverage greatly improve. The surveys form part of larger initiatives to improve the geological knowledge of a country or region, with the ultimate objective of increasing mineral investment and developing a sustainable mining industry. These geoscience programs are typically accompanied by reforms in the mining law to promote such investment. They contribute to tectonic reconstruction, groundwater and environmental applications, and petroleum exploration, all of which ultimately assist societal development (Reeves, 2005). International funding agencies such as the World Bank, European Community and African Development Bank have seen the value in such programs, and ensure that airborne geophysics receive a large share of project budgets. In jurisdictions throughout the world, it has been demonstrated that high-quality geophysical coverage leads directly to increased and more focused exploration. A trend in the last few years has been the inclusion of an airborne electromagnetic follow-up component to the airborne program.

This paper provides current examples from two countries. In Uganda, more than 600,000 line-km of magnetic and radiometric data are being acquired over most of the country. In addition, eight blocks with high mineral potential are being flown with electromagnetic systems (Tempest and heliGeotem). In Senegal, the entire hard rock region has been covered by 130,000 line-km of magnetic and radiometric data, followed by a Tempest survey over three large blocks.

Pre-competitive geoscience is a strategic asset for assessing and reducing risk in energy exploration in Australia. Exploration differs from other business processes because the outcomes are highly uncertain. Investment in exploration carries a high degree of risk which is often exacerbated by a lack of information and knowledge pertaining to areas being considered for targeting. Perceived prospectivity is usually the main factor influencing the decision to explore a certain country or region, and this can only be assessed by a robust scientific understanding of the geological processes that may have been active in the area (Powell, 2007). Such understanding is generally based upon thorough analysis and interpretation of good quality data-sets.

Mining and energy companies cannot capture the value of this type of geoscientific analysis until they have taken up exploration permits or tenements. Thus companies will not usually commit large amounts of exploration dollars and geologist’s time to regional-scale data acquisition and interpretation for area selection (Powell, 2007). This is especially true for under-explored ‘frontier areas’ which have sparse data and are often poorly constrained geologically. Exploration is now an international business with companies taking a global perspective when deciding where they will obtain best return for their exploration dollars. Provision of high-quality pre-competitive geoscience data by Australian Governments allows these companies to assess the risk in regional prospectivity without committing to expensive data acquisition themselves. It also promotes Australia as a sound exploration destination in a highly competitive global market.

In addition, exploration is a cyclical business which is often driven by commodity prices, and is iterative in nature. It is rarely the first company that takes up tenements over an area that discovers and develops a profitable resource. New data and results can radically change the assessment of an area, while new technologies and geological ideas may provide new pathways to exploration success. Governments must maintain pre-competitive data-sets throughout these cycles and develop them over the long term (20 years plus). Data need to be updated and improved as scientific and technological advances are made, and they must be readily accessible and vigorously promoted to the global exploration industry (Powell, 2007). By attracting exploration investment, governments ensure effective exploration and development of energy resources for the benefit of all Australians.

The magnitude 8.0 Ms Wenchuan earthquake of 12 May 2008 occurred along the Longmen Shan fault zone, southwest of China, is a huge hazard. Why the earthquake is happened along the NE-SW tending and in an inactive orogenic area? What is the geodynamics caused the earthquake erupting abruptly? Usage of the results of the processing and interpretation of the magnetotelluric data observed before the Wenchuan earthquake from Luqu-Zhongjiang passed through the Qinling tectonic zone, Songpan-Garze block and Longmen Shan as well as Sichaun basin, this paper reveals crustal architecture of 50 km depth of the Eastern Qinghai-Tibet plateau and Western Sichuan foreland basin and the relationship between them, with the characteristics of eastern pressing of the Qinghai-Tibet plateau forcing Songpan-Garze block thrusting upon the Yangzi continent block, later obstructing the eastern movement of the Qinghai-Tibet Plateau. The Longmen Shan is located at the joint of two block and has been found there is a west inclined low-conductivity layer in the crust of 10-20 km depth of Longmen Shan, it is deduced as the deep conductivity characteristics of thrustbelt structure in Longmen Shan. The huge earthquake has caused about 300 kilometers long rupture zone it happened mainly in faults of Beichuan-Yingxiu and Pengxian-Guanxian of Longmen Shan faults setup, which is going down with the north-western inclined along the thrustbelt in the Longmen Shan. The two faults of Beichuan-Yingxiu and Pengxian-Guanxian seem going to converge together at the deep but they are probably separated with the Maoxian-Wenchuan fault in the Longmen Shan structure zone. There are a little different characters for the Sichuan basin and the Songpan-Garze foldbelts located at the two side of the Longmen Shan, for Sichuan basin has the characteristics of the upper thick low resistance sedimentary, and the under stable high resistance basement, while Songpan-Garze block has high resisitivity cover of upper crust with continuous low resistance bed of the crust. Resulting from the Songpan-Garze block thrust upper the Sichuan Basin stability block, the Longmen Shan structure zone formed three layer of geoelectrical structure which is a high resistance in the upper and lower part of high resistance basin basement, and mingling with low resistance thrust fault zone in the middle. As explanation above, the conclusion could be focused on (1) there is a western inclined thrusting zone range from surface to depth 20km or more under Longmen Shan and Songpan Block, which caused the earthquake happen as it be activated; (2) the different action of surface breaking of Longmen Shan thrusting faults depend on the deep structure characteristics; (3)the earthquake is caused by the mutually promotive with Eurasian plate, Indian plate and Pacific plate in a broad sense. So the research on the MT profile is important for both continent dynamics in the orogenic zone and deep mechanism of Wenchuan earthquake at Longmeshan fault zone in the east of the Qinghai-Tibet plateau.

Mineralisation of economic importance typically occurs in hard rock environments that tests the resolution limits of surface seismic techniques. Complex geology, steeply dipping structures and thick highly heterogeneous regolith cover hinder collection, processing and interpretation of seismic data. Hard rock units in contact often exhibit similar elastic properties with small velocity and density contrasts. In addition, reactivated, altered and highly fractured zones cause significant scattering of seismic energy. All these factors can combine to make it very difficult to produce interpretable images from seismic reflection surveys. Borehole seismic methods reduce the effects of wavefield scattering by placing either the receiver or source within the bedrock. The result is higher signal to noise ratios, higher frequency content and greater accuracy in velocity observations, ultimately giving higher resolution images.

To test the viability of borehole seismology in typical hard rock exploration environments we have conducted a number of modelling tests. These tests are aimed to demonstrate the potential of recording and producing high resolution images from borehole seismic measurements. We also offer potential Vertical Seismic Profile (VSP) methodologies to assist in hard rock seismic exploration.