ASEG Extended Abstracts - ASEG2012 - 22nd Geophysical Conference, 2012

The Xade Complex, situated in central Botswana, is completely buried under younger cover. It was identified during the first regional aeromagnetic survey of the country in 1976. This paper presents the results of an exploration venture which has been in progress since 2005.

The Complex, as previously published, is evidenced by coincident magnetic and gravity anomalies, occurring on the margins of the Kaapvaal and Zimbabwe Cratons. It is shown here that the Complex comprises two lobes - a southern lobe (SL), and an hitherto unidentified northern lobe (NL). Forward modelling of both data sets over the SL indicates it to be a lopolithic feature. The NL is deeply buried in the northwest beneath the Neoproterozoic Passarge Basin, but its southern and eastern margins partially suboutcrop beneath Karoo sediments. Inversion depths range from 250 to 900 m beneath Kalahari and Karoo sediments, and well in excess of this beneath the Passarge Basin.

One recent and two historic boreholes in the SL reveal a sequence of basaltic lavas with subordinate gabbro. A further single historic and two recent boreholes drilled into the NL margins reveal a package of heterogeneous gabbronorites. An U-Pb zircon age of 1109.0±1.3 Ma has been published for a gabbro unit, which is coeval with the Umkondo Igneous Province.

The combined extent of both lobes of the Xade Complex is approximately one-third the size of the Bushveld Complex, making it a very large differentiated magmatic system, with Ni, Cu and PGE potential.

Thermal infrared spectroscopy (TIR) (6000 – 14500nm) has recently become available through HyLogger technology. Framework silicates such as quartz, feldspar, garnet, pyroxene and olivine have primary molecular responses at these wavelengths. With this new suite of minerals visible to the automated scanner, studies using semi-quantitative methods will be able to map key alteration vectors with which geologists are familiar. The Haylands Prospect, south of Morgan, South Australia offered the opportunity to demonstrate the capabilities of the new system mapping albite, quartz, microcline and carbonate in addition to chlorite and sericite offered by the shortwave infrared detectors. TIR spectroscopy can assist in delineating regional alteration systems, such as albitisation and K-feldspar alteration. Such alteration styles are a feature of many mineral systems, including regional alteration associated with iron oxide-copper-gold systems.

Complex near surface is one of the main challenges for onshore seismic data processing. Refraction tomography is becoming a common way to estimate an accurate near surface velocity model. One of the problems with refraction tomography is the low signal to noise ration in far offset data.

To improve, we propose using super-virtual refraction interferometry to enhance the weak energy at far offsets.

We use Interferometric Green’s functions to redatum sources by cross-correlating two traces recorded at receiver stations, A and B, from a source at location W. The result is a redatumed trace with a virtual source at A and a receiver at B, which can also be obtained by correlating two traces recorded at A and B from different shots. Stacking them would enhance the signal-to-noise ratio of this “virtual” trace.

We next augment redatuming with convolution and stacking. The trace recorded at B from a virtual source at A is convolved with the original trace recorded at A from a source at W. The result is a “super-virtual” trace at B in the far-offset from a source at W. Stacking N traces gives a √N-improvement.

We applied our method to noisy synthetic and field data recorded over a complex near-surface and we could pick more traces at far offsets. It was possible to accommodate more picks resulting in a better subsurface coverage.

Time Domain Airborne Electromagnetic (TDEM) systems are defined by a set of technical specifications, which include dipole moment, bandwidth, transmitter waveform and transmitter-receiver geometry.

Comprehensive analysis of these specifications is fundamental in understanding how they define the target response. For example, a system optimised for mapping deep, discrete ore bodies is not necessarily the ideal solution for mapping regolith where good vertical resolution may be required.

Data acquired by three TDEM systems developed by Fugro Airborne Surveys are used to demonstrate the effects that different system specifications have on the response of an exploration target.

The breakup of Australian landmass from Greater India, part of the late dispersal of Gondwana, started around 136 Ma. Concurrent with this breakup was the eruption of significant volumes of volcanics, on both the continental margins of Australia and India, including the North-West shelf, and in the ocean basin separating the two. Later submarine volcanism occurred within the Christmas Island Seamount Province (ChrISP), or which Christmas and Cocos (Keeling) Islands are two subareal examples. These volcanic events significantly impacted the thermal evolution of these margins, but the ultimate cause for this disparate volcanism, and the relationship between margin volcanics and later submarine volcanic events, remains unclear. The study tries to establish a relation between the volcanic activities and look for any evidence of these volcanic episodes. The study uses gravity, magnetic, and subsidence modelling to attempt to constrain the structure and ages of seamount volcanism within the Christmas Island Seamount Province, including the Wharton Basin and Argo Abyssal Plain. Gravity modelling helps to determine the crustal structure and constrains the depth of limestone cap, which further helps in calculating when the seamount was sub aerially exposed and approximates the time since it was last exposure. The relationship between the volcanism observed in the ChrISP (Late Cretaceous to Eocene) and that recorded earlier on the NW shelf continental margin, remains ambiguous, and will be better constrained with forthcoming geochemical analysis. But the results presented here point to a rich and complex history of volcanism within the Indo-Australian plate.

The Muglad basin, Sudan, is a good example of polyphase rifting with at least three major phases of basin development. Each phase has resulted in the generation of source rock, reservoir and seal geology with structural traps often closely linked to basement highs. In this contribution we investigate the tectonic processes that have contributed to basin development at both macro and micro scales.

The macro perspective investigates the basin’s evolution as part of an Africa wide rift system and how it is intimately linked to global plate tectonics and to changes in plate interactions. These changes in plate interactions have caused significant modifications in the orientation and magnitude of the African stress field which in turn has controlled the development of the rift system. On the micro basin scale, the methods used to investigate structure include the compilation of structural maps for different time periods of the individual rift basins into regional structural maps, the role and importance of stratigraphic unconformities within basins and geophysical mapping of the basement morphology of the Muglad basin using an integrated interpretation approach.

Susceptibility and density volumes, derived from the inversion of airborne magnetic and ground-based gravity measurements for a region of the Mt Isa inlier, Queensland, Australia, were analysed using a selforganizing map (SOM) approach. Three-dimensional sub-surface voxel distributions of susceptibility and density were derived from the inversion of magnetic and gravity data using the University of British Columbia (UBC) codes. These petrophysical volumes are often difficult to interpret because of their nebulous nature, with subtle differences between adjacent volume elements. As the SOM approach uses vector quantization, it is an ideal tool to identify subtle relationships in such volumes of disparate data. The CSIRO data-mining SOM tool (SiroSOM) was used here as it was designed specifically for the analysis of such spatially-located, diverse exploration data. Our SOM analysis of the petrophysical voxels has identified (1) some structural features that are evident on the previously constructed Geological Survey of Queensland (GSQ) model of the area; (2) anomalous voxels that form coherent patterns, which may be related to mineralisation and hence be exploration targets; and (3) explicit domains that relate to lithological packages. Further work is needed to validate the SOM results; however, our analysis has shown the value of the SOM approach for analysis of such data. By using SOM, we have been able to assess petrophysical volumes to extract information related to structure and lithological packages in addition to identifying geophysical targets with potential for mineralisation.

The capability to perform laboratory measurements with both low-frequency forced-oscillation and high-frequency wave-propagation methods, under conditions of independently controlled confining and pore-fluid pressure, offers the prospect of new insight into the frequency-dependent seismic properties expected of cracked and fluid-saturated rocks of the Earths upper crust. An important step in the development of such broad-band capability has been the modification of existing laboratory equipment to newly allow flexural, as well as torsional, forced-oscillation testing of cylindrical rock specimens. Flexural oscillation tests on an experimental assembly containing a fused silica control specimen yield results indistinguishable from those of numerical modelling with both finite-difference and finite-element methods – demonstrating the viability of the method. Both torsional and flexural oscillation methods along with complementary high-frequency wave propagation methods have been applied to specimens of dense polycrystalline alumina and quartzite, each thermally cracked to generate an interconnected network of cracks of low aspect ratio, and tested dry, and saturated with either argon or water. The shear and flexural moduli vary systematically with effective pressure – providing clear evidence of pressure-induced crack closure. Similarities and differences between effective moduli measured under different conditions of pore-fluid saturation are tentatively interpreted in terms of the timescales for stress-induced redistribution of pore fluid.

High resolution X-ray micro CT has become a very powerful tool for 3D analysis of core and rock samples. The results can be obtained in a non-destructive way and yields to conclusions for a better understanding of rock structures, mineral composition and fluid flow in the pore space just to name a few.

Tendencies of modellers to analyse rock properties at scales going down to μm and even nm forces the industry to develop appropriate equipment for sub-μ applications. The examples chosen in this paper emphasize the capability of the used X-ray CT systems to fulfil modellers’ requirements and cover different fields of investigation like reservoir rocks, volcanology and palaeontology with resolutions down below 1 μm. The described method enables geo-scientists to receive 3D models of their rock samples at sub-μ scale with extraordinary contrast resolution and opens them not only new possibilities in modelling research but also can speed up all further rock analysis by properly choosing slices for thin sectioning as an example. Furthermore not only palaeontologists will appreciate that they are able to virtually cut their valuable samples for visualization without any physical destruction at all.

Azimuthal Shear Wave Anisotropy Analysis is based on angular energy contributions from on and off axis dipole tool components. Depending on tool geometry, the number of the receivers that participate in computations varies from 6 to 11. Since the fast shear azimuth is usually only weakly variant with depth, the cross correlation of the angular energy distributions from multiple stations can be computed. This allows one to estimate standard deviation and measure correlation of the angular energy distributions. Other quality measures are also available.

Seismic acquisition in environmentally sensitive areas is governed by exacting Commonwealth and State regulatory frameworks. These are designed amongst other things to (1) minimise risks of adverse impacts to marine biota, and (2) minimise disturbance to other users of the marine environment. Seismic surveys today require an adaptive approach to survey design and operations to ensure exploration and environmental objectives are realised.

This paper describes seismic survey design, operational practices and implications on signal processing of Gazelle 3D, a shallow water marine seismic survey off the coast of Exmouth in exploration permit WA-399-P.

Survey design and operational practices were modified to meet regulatory charters. We describe the effects these had on signal processing and the techniques used to mitigate acquisition signatures on resultant seismic images.

In thè Browse Basin, as in many areas of thè world, complex seafloor topography can cause problems with seismic imaging. This paper compares ways in which imaging can be improved under seafloor channels, using both time and depth domain processing.

In the time domain, to improve on the standard PSTM we applied removable seafloor statics in order to reduce the push down effect under seafloor channels prior to migration. This allows for better event continuity in the seismic imaging below. However this approach does not fully tackle the problem, still giving sub-optimal imaging, leaving amplitude shadows, and structural distortion. Only depth domain processing with a migration algorithm that honours the paths of the seismic energy as well as a detailed velocity model can provide good imaging under these seafloor channels, and give confidence in the structural components of the exploration targets in this area. We therefore performed depth velocity model building followed by PSDM, and produced a much improved result..

The Cambrian Ponto Group of the Koonenberry Belt is a suite of fore-arc rocks of the Mt Wright Arc, and includes basalts that are associated with copper mineralisation. Magnetic derivative maps allow the Ponto Group to be followed southwest under shallow cover, where they define the magnetic texture of the Kars Zone that flanks the southeastern margin of the Curnamona Craton. Shallow-sourced positive magnetic anomalies in the Kars Zone are superimposed on a longer-wavelength magnetic low; this contrasts with their equivalents along the Koonenberry Belt, which sit on broad magnetic highs. Gravity images of the Koonenberry Belt suggest that shallow dense sources associated with the Ponto Group overlie deeper mass excesses, while equivalent shallow and deep mass excesses appear to be uncoupled in the Kars Zone. Joint magnetic and gravity modelling of a profile across the Kars Zone suggests that highsusceptibility, dense bodies representing the Ponto Group have been displaced to the northwest, over-riding in their path deep magnetic sources below the Menindee Trough. Similar deep magnetic sources below the Bancannia Trough have been interpreted as very large intrusive features forming the base of the Mt Wright Arc. Together, the potential field imagery and modelling suggests very large scale, low-angle thrusting of the Mt Wright fore-arc over a rigid basement of the Mt Wright Arc.

Accurately delineating sub-vertical graphitic thrust faults and shear systems is one of the major strategies followed in West African gold exploration. Due to the dense vegetation and limited outcrops, airborne electromagnetic (AEM) surveys have been routinely flown for geological mapping. We present a case study for practical 3D inversion of VTEM data acquired in Ghana. The geology is very complex, and consists of carbonaceous shear zones containing multiple thrust faults. Certain types of gold mineralization are directly associated with the shear zones which manifest themselves as sub-vertical conductors. We compare results from 3D AEM inversion to those obtained from traditional AEM interpretation methods, including conductivity depth imaging, decay constant analysis, profile anomaly picking, Fraser filtering, and plate modelling. We show that even with no a priori information, 3D AEM inversion accurately delineates the sub-vertical conductors, and recovers the most accurate representation of the geology. Interpretations based on CDIs are shown to be unreliable.

The main problem in demining process is to locate and characterize the landmines in the ground, as their different types and sizes make them difficult to be detected using a single technique. In this paper we present results of an experimental study of integration between Neutron Backscattering and magnetic gradiometer techniques. The experiment was established in a test site located in Cairo using different types of landmines buried at different depths. The Neutron Backscattering technique provided information about the hydrogen content for the buried object and was successful to detect almost 85% of the used objects. The magnetic gradiometer detected most of the objects with ferro-metallic contents. Integration of both techniques increases the detectability to reach 100%. Such integration is very effective in decreasing the high false alarm rate resulting from the magnetometers sensitivity to any metal debris and detects landmines of relatively deeper depths which are not sensed by Neutron Backscattering sensor. The results suggest multi-sensors detection approach of landmines would help greatly in the demining process.

The Speculant 3D Transition Zone (3DTZ) Seismic Survey was acquired by Origin Energy in the Otway Basin, approximately 30km east of Warrnambool, Victoria during November and December 2010. The objective of the survey was to fill a data gap between an existing Marine 3D Seismic Survey (3DMSS) and Land 3DSS, with the target located offshore in the shallow waters of the coastal zone. Although the survey covered a small surface area, the coastal location combined with the economic and environmental significance of the region dictated numerous exclusion zones. To achieve coverage of the target under these exclusion zones, the survey was designed to simultaneously use two independent recording systems; a cable free system on land and an Ocean Bottom Cable (OBC) system offshore, two different sources (GI air gun offshore and Vibroseis on land) and three different receiver types. The final survey design was an orthogonal geometry with a high density of marine shots and a high density of land receivers, with large holes due to access restrictions. The field data were a collection of six different source - receiver combinations, each with unique noise characteristics and statics, covering separate but overlapping CMP’s. Combining these disparate data sets was the key challenge of the processing. The lessons learnt during planning, acquisition and processing are shared in this paper along with examples of the effect that survey design, field conditions and equipment use and limitations can have on field records and processed data.

Located in northern Sweden, the Skellefte mining district has been subject to several geological and geophysical investigations, as it is hosting abundant volcanic-hosted massive sulfide deposits. The importance of mineral exploration at greater depths in the Skellefte district has been increased since most of mineralization at shallow depths are already discovered and exploited. Therefore, geophysical methods become particularly important as they can improve our knowledge about spatial relationship between geological features at the depth. In the first part (local-scale), we used resistivity/IP data to map the subsurface geometry down to 430m depth. Furthermore, the results of the resistivity/IP studies were constrained with potential field data down to 1.5 km depth. In the second part (Regional-scale), potential field data were used to constrain the interpretation of the reflection-seismic data down to 5 km depth. The result from the first part indicated a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The volcanosedimentary contact between the Skellefte and Vargfors group rocks and three proposed locations for sulphide mineralization were identified along the resistivity/IP profile. In second part, the gravity and magnetic data were investigated to better understand the contact between the Skellefte group, volcanic rocks and the Bothnian Basin sedimentary rocks. Furthermore the data was used to constrain the geometry of late-orogenic gabbro-diorite and granite intrusions, which occur along inferred shear zones that are only poorly indicated, or not visible at all in the reflection-seismic data. As the main outcome, the proposed integrated 3D model of the central Skellefte district (CSD) revealed crucial information about the spatial relationship between key lithologies, which will be further used to understand the evolution of CSD in the 4th dimension, time.

The central-loop TEM technology has been widely used in geological exploration. For efficient working, a square (or rectangular) loop is generally used instead of the circular one. One may define a range of one ninth of central part (can be approximately regarded as uniform) as the central survey location, which can be referred as a modified central-loop configuration.

However, the obtained field parameters at such noncentral positions usually calculated by the central-loop formula result in decreased accuracy in the explanation. The large-fixed loop has advantage to calculate the induction potential at any point inside or outside the loop. We in this study introduced the formula for the largerfixed loop into the above modified central-loop system and solve the problem, in which electromagnetic response of any field point has been obtained by using an electric dipole integration method. Both theoretical modeling and real application indicate that such combination not only improves the accuracy for the TEM survey, but also enlarges the exploration depth due to a large loop is used in the deployment.