Exploration Geophysics - Volume 40, Issue 4, 2009

Kalahari cover greater than 60 m thick precludes indicator sampling from identifying individual kimberlite intrusions in the Kokong kimberlite field of Botswana. Employing concurrent multidisciplinary geophysical surveying methods is the exploration technique for the discovery of kimberlites. Time Domain Electromagnetic (TEM) surveying is an effective technique for the prioritization of aeromagnetic and gravity signatures. Diagnostic discrete conductive TEM signatures are usually returned from kimberlites overlain by up to 120 m of Kalahari cover. Following the success of kimberlite identification by ground TEM, a VTEM helicopter time domain survey was flown over the Kokong field to increase the TEM coverage in this prospective kimberlite terrain. A comparison of responses over covered kimberlites traversed by both TEM survey methods and their comparative effectiveness for kimberlite identification is presented.

The seismic data collected during base metal exploration are often degraded by a variety of non-reflecting signals due to the heterogeneity of ore bodies and host rocks. It is then difficult to generate a high-quality seismic section when conventional reflection seismic processing algorithms are applied to the data. Two seismic records have been simulated from constructed typical ore geological models using the Kirchhoff integration principle. One geological model is the Tongling copper mine in eastern China, and another is the Gejiu (also known as Gegeo) tin mine in the Yunnan Province of south-western China. The simulated seismic data has then been processed using reflection seismic and seismic scatter imaging methods, respectively. The seismic scatter imaging method, which stacked all kinds of reflected/diffracted waves, seems more powerful to image the complexity of the subsurface.

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.

To be reliable, Earth models used for mineral exploration should be consistent with all available geological and geophysical information. During the past several years an important focus of inversion research has been towards advancing the integration of geological data (such as lithology and structure), physical property data (measurements taken on rock samples) and geophysical survey data through appropriate inversion methodologies. We expand the types of geological information that can be incorporated into ‘minimum structure’ type deterministic inversions involving minimisation of an objective function. These include orientation information and physical property trends. We also present an iterative cooperative inversion strategy for combining multiple types of geophysical data and recovering geologically realistic models involving sharp interfaces between rock units. We provide a synthetic example to illustrate our methods.

Multi-scale edge detection was applied to potential field data over the Gawler Province in the central part of South Australia. Also known as worming, the multi-scale edge analysis technique can aid identification of structural controls and depth extents of anomalies. A geological interpretation of the multi-scale edge detection results was then undertaken; integrating drill-hole information, ground mapping and tectonic understanding with geophysical modelling to gain a better comprehension of the dominant structures present.

The multi-scale edge detection process provides potential solutions for the lack of outcrop, particularly that which is representative of three-dimensional architecture. The latter is particularly important in understanding how terrains are juxtaposed or dissected tectonically which, in turn, influences the style of any mineral system that may be present. Moreover, correct identification of structural geometry and cross-cutting relationships allows a more confident assessment of fault kinematics and potential dilatancy. In particular, the degree of uranium mineralisation in iron-oxide-copper-gold systems in the Gawler Province may be dependent on the interconnectivity of fault plumbing in three dimensions to nearby uraniferous Mesoproterozoic granitoids.