Peter Milligan

The Radiometric Map of Australia

Geoscience Australia and the Australian 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. However, the individual surveys that comprise the national gamma-ray spectrometric radioelement database are not all registered to the same datum. This limits the usefulness of the database as it is not possible to easily combine surveys into regional compilations or make accurate comparisons between radiometric signatures in different survey areas. To solve these problems, Geoscience Australia has undertaken an Australia-Wide Airborne Geophysical Survey (AWAGS), funded under the Australian Government’s Onshore Energy Security Program, to serve as a radioelement baseline for all current and future airborne gamma-ray spectrometric surveys in Australia. The AWAGS survey has been back-calibrated to the International Atomic Energy Agency’s (IAEA) radioelement datum. We have used the AWAGS data to level the national radioelement database by estimating survey correction factors that, once applied, minimise both the differences in radioelement estimates between surveys (where these surveys overlap) and the differences between the surveys and the AWAGS traverses. The database is thus effectively levelled to the IAEA 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 the map 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 with contrasting landscape histories.

Merging airborne magnetic surveys into continental‐scale compilations

Regional compilations of airborne magnetic data are becoming more common as national databases grow. Grids of the magnetic survey data are joined together to form geological province‐scale or even continental‐scale compilations. The advantage of these compilations is that large tectonic features and geological provinces can be better mapped and interpreted.We take a holistic approach to the joining of survey grids. The leveling of the grids into a regional compilation is treated as a single inverse problem. We use the weighted least‐squares method to find the best adjustment for each survey grid such that the data value differences in the grid overlap areas are minimized. The method spreads any inconsistencies between grids among all of the grid overlap areas and minimizes the introduction of long‐wavelength errors into the composite grid. This is an improvement on the conventional approach of joining grids sequentially.A comparison of leveled data over Western Australia with diurnally‐corrected long aero...

High definition geomagnetic models: A new perspective for improved wellbore positioning

Earths gravity and magnetic fields are used as natural reference frames in directional drilling. The azimuth of the bottomhole assembly is inferred by comparing the magnetic field measured-while-drilling (MWD) with a geomagnetic reference model. To provide a reference of sufficient quality for accurate well placement, the US National Geophysical Data Center (NGDC), in partnership with industry, has developed high-definition geomagnetic models (HDGM), updated regularly using the latest satellite, airborne and marine measurements of the Earths magnetic field. Standard geomagnetic reference models represent the main magnetic field originating in the Earths liquid core, but the new models additionally account for crustal magnetic anomalies, which constitute a significant source of error in directional drilling. NGDC maintains a public archive of global ship and airborne magnetic field measurements. These are compiled into a global magnetic anomaly grid and expanded into ellipsoidal harmonics. The harmonic expansion coefficients are then included in the high-definition models to accurately represent the direction and strength of the local geomagnetic field. The latest global model to degree and order 720 resolves magnetic anomalies down to 28 km half-wavelength, achieving more than an order-of-magnitude improvement over previous models. A side-by-side comparison of different onand off-shore regions shows the high level of local detail represented in the new model. Accounting for a larger waveband of the geomagnetic spectrum significantly improves the accuracy of the reference field. This directly benefits the reliability of the well azimuth determination. We further demonstrate that model accuracy is a prerequisite for applying drill string interference corrections. Finally, an accurate reference model facilitates the validation of MWD surveys by keeping the field acceptance criteria centered on the true downhole magnetic field. Together, these factors improve well placement, prevent and mitigate the danger of collision with existing wellbores and enable real-time steering to save rig-time and reduce drilling costs. Introduction Magnetic field sensors are widely used in navigation systems and in determining the orientation of devices such as satellites, solar panels and antennas. Such electronic compasses play a particular important part below the sea and earth surface where the global positioning system (GPS) is unavailable. Measurement while drilling (MWD) employs a combination of gravity and magnetic field sensors to determine the inclination and azimuth of the bottom hole assembly (BHA). Conversion from magnetic azimuth to true azimuth requires knowledge of the direction and strength of the ambient magnetic field, which is provided by a geomagnetic reference model. Such a model specifies the declination angle (measured in degrees positive east of true north), the dip angle (measured positive downward), and the total field strength (measured in nT). Geomagnetic reference models are empirical models produced from global magnetic field measurements by satellite, aircraft and ships. The US National Geophysical Data Center (NGDC) has a long term commitment to collect, archive and 2 IADC/SPE-151436-PP disseminate geomagnetic data and reference models for use by academia, industry and the general public. Here, we first provide some background on the three data types and then describe how they are blended into a global reference model. The final section shows the benefits of using High Definition Geomagnetic Models (HDGM) in directional drilling. Satellite magnetic measurements Uniform global coverage of the geomagnetic field is provided by polar-orbiting satellites. They are particularly suited for monitoring the secular variation of the main field originating in the Earth’s liquid outer core. Low-orbiting satellites further enable mapping the long-wavelength portion of the crustal magnetic field, caused by ferrous minerals such as magnetite. The smallest wavelengths resolved by a satellite roughly correspond to its orbital altitude. The recently completed German CHAMP mission (CHAllenging Microsatellite Payload, Reigber, 2002) was launched in July 2000 into an orbit at an initial altitude of 450 km, which gradually decayed to 250 km before re-entry in September 2010. Due to its low altitude and advanced instrumentation, CHAMP was the primary satellite data source for global geomagnetic reference models, such as the World Magnetic Model (Maus et al., 2010), the International Geomagnetic Reference Field (Finlay et al., 2010) and NGDC’s HDGM (http://www.ngdc.noaa.gov/geomag/hdgm.shtml). CHAMP further enabled the production of a series of crustal magnetic field models. The latest being MF7 (http://geomag.org/models/MF7.html) with a full wavelength resolution of 300 km. The corresponding grid resolution or half-wavelength resolution is 150 km. A snapshot of MF7 is shown in Fig.1. The only dedicated magnetic satellite presently in orbit is the Danish Ørsted satellite. It was launched before CHAMP in February 1999 into a higher-altitude orbit at 850 km. After the star cameras failed in 2003, it now only provides measurements of the strength of the magnetic field. Furthermore, due to the weakness of the batteries, the data coverage has become somewhat sparse. These measurements do not completely determine the global magnetic field. However, they are nevertheless sufficient to extend geomagnetic field models for a few years, bridging the present gap in satellite magnetic coverage. The next geomagnetic satellite mission, scheduled for launch in the second half of 2012, is the European Space Agency’s Swarm constellation mission. It consists of three identical satellites in low altitude orbits, which will monitor the geomagnetic field over a mission life time of at least four years. Swarm’s accurate magnetometers and its design to measure gradients between side-by-side flying satellites will offer unprecedented opportunities to improve the specification of the long wavelength crustal magnetic field. Fig. 1–Vertical component of the Earths magnetic field as seen by the CHAMP satellite. The large anomaly on the upper right is due to the iron ore deposits at Kursk, Russia. Snapshot of animation by Maus and Rother, 2011. IADC/SPE SPE-151436-PP 3 Aeromagnetic compilations Due to their high altitude, satellites can only be used to infer the long wavelengths (> 300 km) of the geomagnetic field. Over land, the most effective means of surveying smaller-scale features is by using aircraft. Such surveys have been conducted for more than 60 years. They typically cover areas with sidelengths of the order of tens to hundreds of kilometers. Significant efforts are being made to stitch these surveys together into continental-scale magnetic compilations (Fairhead et al., 1997, Minty et al., 2003). The primary challenge in producing continental scale compilations is to accurately represent the intermediate to long wavelengths. The global satellite magnetic missions have contributed significantly to overcoming these difficulties. Nevertheless, there is still considerable uncertainty in the intermediate wavelengths of about 50 km to 300 km. To address this issue, Geoscience Australia flew a mesh of long-range aeromagnetic profiles which were used to correct the long wavelengths for the 5 edition of the Australian magnetic anomaly map (Milligan et al, 2010). A similar correction of long wavelengths by long-range aeromagnetic profiles was carried out for North America (Ravat, 2009). For NGDC’s global magnetic reference field models, the continental scale compilations, together with some isolated smaller surveys, were merged into a common global grid, displayed in Fig. 2. Fig. 2–Global grid of merged continental-scale aeromagnetic compilations Marine and airborne magnetic trackline data The second source of near-surface geomagnetic survey information is from ship and aircraft trackline data. Areas far from the shore are usually surveyed by ships on marine scientific cruises. The US Naval Research Laboratory has further flown extensive aeromagnetic surveys under the Project Magnet program, which was conducted for several decades. NGDC maintains a global Geophysical Data System (GEODAS) archive of marine trackline data. This archive will be augmented with the available aeromagnetic trackline data. Both trackline data types are shown in Fig. 3. 4 IADC/SPE-151436-PP Fig. 3–GEODAS trackline magnetic data (blue) and Naval Research Lab aeromagnetic trackline data (green) Earth magnetic anomaly grid The continental-scale compilations were merged with the ship and airborne trackline data into a common Earth Magnetic Anomaly Grid (EMAG2), with a grid cell size of 2 arc minutes (Maus et al., 2009). The trackline data were first line leveled onto the combined continental-scale compilations. Then, the data were merged using least-squares collocation, also sometimes referred to as kriging. In order to better represent linear sea-floor spreading anomalies, an anisotropic covariance model was employed over the oceans. The direction of strike was inferred from isochrones of the age of the oceanic crust by Müller et al. (2008). This directional gridding technique proved particularly successful in filling data gaps in the southern oceans by interpolation and extrapolation. After merging the near-surface data, the long wavelengths were substituted with the MF6 model (Maus et al., 2009) from CHAMP satellite measurements. EMAG2 is available in digital form as grid and GeoTiff at http://geomag.org/models/emag2.html. A version for visualization in NASA World Wind can be downloaded from http://www.getech.com/downloads/EMAG2.htm. Further implementations are available for Google Maps (http://ngdc.noaa.gov/geomag/data/Google_Maps/EMAG2/index.html) and as a KMZ file for Google Earth (http://bbs.keyhole.com/ubb/ubbthreads.php?ubb=showflat&Number=1205597). A Mercator projectio

Structural analysis of extended Australian continental crust: Capel and Faust basins, Lord Howe Rise

Abstract The Capel and Faust basins (northern Lord Howe Rise) are located in the SW Pacific between Australia, New Zealand and New Caledonia. New seismic, gravity, magnetic and bathymetry data and rock samples have enabled the construction of a three-dimensional geological model providing insights into the crustal architecture and basin stratigraphy. Multiple large depocentres up to 150 km long and 40 km wide, containing over 6 km of sediment, have been identified. These basins probably evolved through two major Early Cretaceous rifting episodes leading to the final break-up of the eastern Gondwanan margin. Pre-break-up plate restorations and potential field data suggest that pre-rift basement is a collage of several discrete terranes, including a Palaeozoic orogen, pre-rift sedimentary basins and rift-precursor igneous rocks. It is likely that a pre-existing NW-trending basement fabric, inherited from the New England Orogen (onshore eastern Australia), had a strong influence on the evolution of basin architecture. This basement fabric was subjected to oblique rifting along an east–west vector in the ?Early Cretaceous to Cenomanian and NE–SW-oriented orthogonal rifting in the ?Cenomanian to Campanian. This has resulted in three structural provinces in the study area: Eastern Flank, Central Belt and Western Flank.

Integrating Euler solutions into 3D geological models ‐ automated mapping of depth to magnetic basement

A large proportion of older basement regions of the Australian continent is covered with a variable thickness of sedimentary and regolith material, which obscures most direct geological signatures of the basement architecture (Figure 1). Interpretations of magnetic and gravity potential field survey data provide an indirect insight into the underlying geology, and the continent is now well covered by such data (Milligan et al., 2003). Mineral exploration continues to spread from areas of outcropping and subcropping basement into areas with greater thickness of cover. One of the most useful inferences from potential field data is the depth of crystalline basement beneath the regolith. Many of the exposed/near-exposed mineral resources have probably now been found, and the next target areas are in basement regions underlying shallow cover.

Carpentaria Conductivity Anomaly revisited with preliminary magnetotelluric results from the SE Mt Isa Survey 2014

The region to the east of Mt Isa has complex electrical conductivity, with conductive basin sediments overlying the deeper Carpentaria Conductivity Anomaly (CCA). Early magnetotelluric (MT) model results show alignment of the CCA with aeromagnetic, gravity and seismic features, together implying that they define the major structural edge of the Mt Isa Block. Profile MT data acquired during the previous 20 years have helped refine the position and depth of the CCA. New MT and deep seismic reflection data have recently been acquired in 2014 along a NW to SE profile, funded by the Geological Survey of Queensland’s Greenfields 2020 Program in conjunction with Geoscience Australia. These new data provide further evidence of the complex nature of the crustal conductivity in this region. Induction vectors indicate that the CCA itself is braided into several zones which may define deep-seated fracture systems.