ASEG Extended Abstracts - ASEG2006 - 18th Geophysical Conference, 2006

Radiometrics is a successful natural resource tool; however technology captive projects of the past need to progress to on-ground decisions. This paper has focussed on developing a strategy to bridge this gap. The data to decision strategy involves a four stage process. (1) Be aware of the strengths and limitations of the technology; (2) match the data to the application, (3) target research with an application focus; and (4) evaluate the value of the geophysical information in decision making. This approach was used to bridge the gap between the gamma radiometrie technology and ‘on-ground’ decisions in plant based industries.

A fundamental problem in earth sciences is how landscapes respond to the climatic and meteorological processes imposed upon them. Sedimentary features such as alluvial fans are commonly used to reconstruct paleoclimatic / paleo-meterological conditions. Alluvial fans in the central Flinders Ranges provide a Late Pleistocene to Holocene record of cyclic sedimentation and incision that may be interpreted to reflect climatic oscillations and episodic large magnitude flooding events. Debris flow deposits ranging in depositional age from ~71 to 55 thousand years before present (ka) are interpreted to reflect high-energy deposition of sediment derived from soil-mantled catchments, implying more humid climates and higher soil production rates than present. These deposits are capped by a ~30 ka paleosol and incised by a series of clast-supported fluvial gravels, indicating a change to bedrock dominated landscapes and more arid climates during early stages of the Last Glacial Maximum (LGM). Although localized aeolian deposition / sediment remobilisation occurred on distal portions of the fans, fanhead trenches were deeply eroded and incised during this time (~30 to 18 ka). A series of alluvial terraces were deposited at ~ 1000 year intervals in the middle to late Holocene, interpreted to reflect infrequent, large magnitude flood events that periodically flush bedrock dominated catchment systems. We suggest that alluvial fan sequences in the Central Flinders Ranges provide meaningful information about Late Pleistocene to Holocene catchment paleogeography, climate change and return intervals of large magnitude floods.

The stratigraphy and structure of a Tertiary sub-basaltic sedimentary succession near Streatham in western Victoria have been studied as part of a groundwater project linking the geology to the salinity distribution. Using palynological ages, airborne geophysics and an extensive bore log record from exploration and groundwater observation bores, we have modelled the evolution and the extent of this palaeodrainage system. This palaeodrainage/deep lead system covers an area of more than 2000km² and consists of several tributaries which fuse into one central incised valley. It contains up to 140m of fluvial gravel, sand, ligneous clays and coal often with a clear fining upwards trend. Palynological ages conducted on sediments from selected bores within the pallaeovalley indicate that deposition within this complex system took place in at least three major phases ranging from Palaeocene to Late Miocene. Using airborne geophysics (magnetics and gravity) and a digital elevation model we were able to identify the faults affecting the deposition in the Streatham Sub-basin and a comparison with the basement geology map shows that the faults are all reactivated older structures between Palaeozoic units or even tectonic zone boundaries (as in case of the Moyston Fault).

We present petrographic, geochemical (major, traces and REE) and isotopie (Sm-Nd and Rb-Sr) data from ca. 1685 Ma mafic rocks of the Willyama Supergroup in the Broken Hill Inlier of western NSW, Australia. The mafic rocks occur throughout the lower Willyama Supergroup stratigraphy and are interpreted here as shallowly emplaced sills that were metamorphosed to upper amphibolite and granulite facies during the Olarian Orogeny (ca. 1600-1580 Ma). Our data indicate that the metabasites originated by variable degrees of partial melting of a depleted mantle source, only weakly more enriched in incompatible elements compared to present day N-type MORB. This was followed by simple crystal fractionation or by an AFC process involving only small degrees of crustal assimilation (r = 0.05-0.2). Crystal fractionation proceeded along a tholeiitic trend of extreme primary iron and titanium enrichment, leading to melts with up to 25 wt % of total iron as Fe2O3 and 4.2 wt % of TiO2. Some intermediate rocks were derived from this fractionation, but the bulk of the contemporary felsic magmatic rocks (Alma, Rasp Ridge and Potasi Gneisses) are not linked by fractional crystallization to the mafic melt that produced the meta-igneous amphibolites, and are products of anatexis of crustal material from the Willyama sedimentary pile. Based on the occurrence of bi-modal magmatism, a depleted mantle source, partial melting modelling and minimal crustal contamination of the mafic rocks, we infer that the Broken Hill Inlier (ca. 1685 Ma) was the extensional axis and depositional centre of an advanced stage intra-cratonic rift with relatively thin crust and lithosphere. Data from the neighbouring Olary Inlier, in contrast, that imply smaller degrees of partial melting, relatively thicker lithosphere and more crustal contamination, are consistent with placing this Inlier on a rift margin. Active faulting during the rift stage coupled with submarine sedimentation and an anomalous geothermal gradient driven by lithospheric thinning, provide an ideal theoretical environment for the formation of the Broken Hill Pb-Zn-Ag orebody.

Over the last decade, southeast Australia has been the focus of several teleseismic imaging studies that have mapped arrival time residual information from distant earthquakes as 3-D variations in seismic wavespeed in the upper mantle. Two of the most recent experiments are TIGGER and SEAL, which respectively involved the deployment of dense arrays of seismometers in northern Tasmania in 2002, and southern New South Wales and northern Victoria in 2004-2005.

Tomographic imaging results from TIGGER show that the lithosphere beneath Tasmania is broadly characterized by (1) a marked transition from lower velocities in the west to higher velocities in the east. Significantly, the Tamar Fracture System does not overlie the narrow transition from lower to higher velocities; (2) an easterly dipping zone of relatively high velocity material beneath the Rocky Cape Group which may be related to remnant subduction of oceanic lithosphere. Tomographic imaging results from SEAL clearly reveal the west to east transition from Proterozoic to Palaeozoic lithosphere beneath the Murray Basin as a change from higher to lower velocities. Elevated velocities further east beneath the Wagga-Omeo Complex imply that the lithosphere beneath the central subprovince of the Lachlan Orogen is seismically distinct from that beneath the western subprovince.

Our long term goal in southeast Australia is to combine the TIGGER and SEAL datasets with those from past, current and planned passive seismic arrays in order to construct a unified tomographic model of the entire region.

Dryland salinity is a serious threat to the sustainable management of land and water in many parts of Victoria. However, the onset of dry climate conditions has reduced its priority as a natural resource management issue in recent years. Apart from the elevated need for water taking the spotlight, the dry conditions reduce the risk of salinity, temporarily at least. Questions are naturally posed about what impact that these conditions have on salinity incidence. Does salinity retreat to some or any extent? How significant is the recession or improvement, if any? Obviously, a number of other factors besides climate influence whether, and to what degree, a salt affected area alters. This paper concentrates on investigating the climate variation impacts on several ground water flow systems to gain some further insight into salinity risk in time and space, as well as management of this risk.

Close observation of the landscape and analysis of the ground water level record improves knowledge about the ground water flow systems and their responsiveness to changes in climate and landscape management. A review of observation bore data has revealed considerable declines in ground water levels across northern Victoria since 1996. The dry climate conditions during this time are hypothesised to have been largely responsible for these declines despite a number of landscape factors. The declines have been accompanied by marked changes in the occurrence and spatial distribution of recharge. In upland salt-affected areas of north central Victoria, cleared crest and upper slope landscape positions, which have typically had a high potential recharge status, have recorded very little recharge since 1996. During the same time, many lower slope and discharge positions in these areas have continued to record some seasonal recharge in most years. This recharge is argued to be due more to local infiltration than aquifer throughflow and has significant implications for salinity management priorities.

We present an efficient and stable method for wave equation prestack depth migration (PSDM) in laterally varying VTI media. The method accurately handles large lateral variations in the Thomsen anisotropy parameters, ε and δ, as well as vertical velocity. It performs wavefield extrapolation using a constrained explicit operator that we originally proposed and successfully applied for isotropie wave equation PSDM. The constrained operator has a reduced number of independent coefficients, leading to improved migration efficiency whilst retaining accuracy. The optimization process for the operator design ensures that we can quickly design a large number of well-behaved operators for different combinations of velocity and anisotropy parameters. Model and field data examples show that the migration method proposed is able to produce well focused and correctly positioned images in areas of complex geology.

The SSI 1/2004 research voyage of the Marine National Facility to the northern Tonga Arc and adjacent Fonualei Rifts (backarc) discovered several new submarine, basalt-to-rhyolite volcanic centres (stratovolcanoes and massive caldera systems) in the Arc. Numerous sector and caldera collapses were likely tsunamigenic. Fonualei Rifts lavas are a global backarc end-member with markedly low Na contents reflecting a highly refractory mantle wedge source coupled with high degrees of partial melting. The southern Rifts have captured the entire supra-subduction zone magma flux over about 150 km of strike; adjacent Arc volcanoes are inactive and carbonate capped.

Traditionally, geoscientific data collected by GeoScience Victoria (GSV) were held in databases focussed on providing the content for maps and their accompanying legends. A wealth of information would be stored in free-text legend fields or geological reports. Analysis of these data using other software is difficult and this approach can lead to inconsistent or contradictory systems.

GSV has redesigned its geological information systems, based on a data model that describes true geological entities and concepts. Consequently, information stored by GSV can be more readily used for a variety of purposes and by a variety of users, and the information model is not constrained by the implementation requirements of a particular vendor.

Research was conducted to identify models generated by other groups or agencies before adopting the North American Geologic Map Data Model - Conceptual Model 1.0. This model provided the foundation for a logical model explicitly defining the geoscientific concepts with respect to GSV’s business requirements. The geoscientific application of the Geography Mark-up Language .GeoSciML, was used as a reference during the logical modelling.

An extensible pair of aspatial and spatial databases was constructed. Web-services, including Open GIS Consortium (OGC) compliant web map services and web feature services, will be used to deliver data to client applications. These databases will enable GSV to conduct it’s own work and deliver its data to external parties using international standard exchange languages such as GeoSciML.

An automated technique for optimized placement of gravity stations is presented that takes into account both the spatial distribution of pre-existing data and also variations in the form of the gravity surface. The new method involves calculation of a weighted normalized grid incorporating a point-distance function, a point-density function and information on horizontal gravity gradients determined from pre-existing data. The maximum in the normalized grid is taken as the optimum position for a new gravity station. The positions for multiple new stations can then be generated from successive maxima following iterative recalculation of the point-distance and point-density functions. Gravity grids generated from optimized data distributions require significantly less new data points to achieve the same relative accuracy when compared to regular, systematic infill patterns. Although other factors, such as difficult access, may in some cases limit the application of this technique, there are many circumstances where significant reductions in survey costs can be achieved by application of this new method.

The majority of earthquake location techniques, even modern techniques such as double difference, use only a small component of the seismic waveform or seismogram. A new technique to aid earthquake location is coda wave interferometry (CWI). In this method the cross correlation of coda waves, the later arriving waves in a seismogram, are used to directly estimate separations between earthquake sources.

An extension of the existing CWI theory is derived to consider the effect of changes in source mechanism. Preliminary testing of the existing and extended CWI is presented. Plans are outlined to use CWI source separation estimates to complement travel time information and constrain earthquake location inversion for groups of earthquakes.

The last 20 years have seen major re-appraisal in the understanding of metamorphic processes operating at very low-grade metamorphic conditions. Identification of the increase in crystallite size as the cause of change in the illite crystallinty index was a major step on the road to consideration of processes in terms of reaction progress. In addition, the analysis of the ubiquitous layer-silicate minerals in terms of reaction progress has provided new insights into documenting the metamorphic process and identifying the mainly kinetic controlling factors. This style of metamorphic process provides a contrast and different perspective to the “normal” metamorphic style at higher grades.

Petrogenetic grid analysis, and mineral projections of VLGM assemblages in metabasites have rationalised the documentation of the relationships between whole rock and P-T controls on mineral distributions and facies designation. This work has shown that recognition of the prehnite-pumpellyite, prehnite-actinolite and pumpellyite-actinolite facies directly overlap in P-T space. As such, to use these facies to identify differing P-T conditions is unjustified and it is better to use sub-greenschist facies if it wished to designate the P-T conditions between the zeolite, blueschist and greenschist facies.

New methods of dating utilising highly characterized, neoformed minerals (e.g. illite and monazite) are now able to provide precise ages of VLGM thermal events. This, coupled with the new understanding of very low grade metamorphic processes, has great potential to constrain timings of discrete erogenic events and rates of tectonic processes in regions peripheral to main areas of orogenic activity.

Seepage detection and shallow gas distribution are used as a tool for hydrocarbon exploration around Australia. The integration of remote sensing (SAR, Landsat), seismic data (3D and 2D lines), sub-bottom profile, multi-beam, side-scan sonar, echo-sounder, sea surface fluorescence data and geochemical analyses of seabed and the water column samples was used to test a combination of tools and techniques in an area of inferred seepage on the Yampi Shelf prior deployment of tools for seepage detection and characterisation in more poorly understood regions and frontier basins, in the Arafura Sea and Rowley Sub-basin.

Active present-day hydrocarbon seepage has been imaged on the tropical carbonate Yampi Shelf, in 50 and 90 m water depth. Seepage is evidenced by gas plumes in the water column that are associated with seabed features, such as clusters of reflective blocks, hard-grounds, pockmark fields, and mounds. Seepage activity and intensity seems to vary with changes in pressure related to macro-tidal cycles. The seabed features coincide with sub-surface features such as areas of seismic signal attenuation under high amplitude reflectors, seismic discontinuities and bright spots. Hydrocarbon migration-seepage pathways appear to be controlled by the reactivation of pre-existing fractures and dykes within the basement.

The experience gained during the Yampi Shelf survey was used to identify and sample locations of potential active hydrocarbon seepage in the Arafura Sea and Rowley Sub-basin. These integrated seepage surveys represent the initial work to identify new exploration opportunities and reduce risks for future petroleum exploration in these frontier regions.

The coal plays an important economic role in the energy equation of India as it provides 70% of power requirements for one of the fastest growing economy in Asia. The Permian coal represents 99% of thermal and coking coal of the country, and it is expected to dominate as the low cost and reliable energy source in comparison to rising price of the imported oil and gas. The geological setting, macerai distribution and trace elements distributions of major and moderate concerns are presented in the paper.

Late Cretaceous to Cenozoic reconstructions of the Southwest Pacific region show a large degree of discrepancy. Moreover, some do not incorporate the geology of the New Caledonia region, most do not incorporate the geology from Northland, New Zealand, and none incorporates the lithospheric-scale structure from the d’Entrecasteaux zone. A new tectonic model is proposed that incorporates the regional geology from these three regions into the large-scale and well-defined tectonic framework of relative motion between the Australian plate, Pacific plate and Lord Howe Rise microplate. In the new model, the d’Entrecasteaux zone is interpreted as a dextral transform fault. The New Caledonia and Northland regions are interpreted to bear the hallmarks of a former northeast dipping subduction zone with an east dipping subduction segment in between, located west of the Three Kings Ridge. The subduction zone formed at 50-45 Ma, a period when Australia–Pacific relative motion changed from strike-slip to oblique convergence. Subduction was primarily realized by east to southeast-directed anticlockwise slab rollback of the South Loyalty Basin lithosphere and tearing of the slab at the surface along the d’Entrecasteaux zone. The subduction rollback process resulted in formation of the North Loyalty-South Fiji back-arc Basin and induced arc volcanism along the d’Entrecasteaux-Loyalty–Three Kings Ridge-Northland Plateau seamount chain. The final stage of the subduction process resulted in southwest-directed obduction of ophiolites in New Caledonia at 38-34 Ma and in Northland at 25-22 Ma.

Since the advent of plate tectonics different reference frames have been proposed to describe the “absolute” motion of plates. The most prominent ones include the hot spot reference frame and the no-net-rotation reference frame. The difference in absolute plate motion between different reference frames can be substantial (up to 4 cm/yr). These differences are truly significant, considering that the velocity of the fast plates is only 6-12 cm/yr in such reference frames. It is thus of primary importance to extract which of these reference frames is most likely to describe the absolute motion of plates. Here we propose that the best candidate representing an absolute reference frame is the one, which minimizes the cumulative amount of trench migration for all subduction zones. From the absolute reference frames proposed thus far, it is found that the cumulative amount of trench migration is lowest in the Indo-Atlantic hot spot reference frame. This implies that the Indo-Atlantic hot spot reference frame is currently the best approximation of an absolute reference frame on Earth.

While the routine measurement of the magnetic gradient tensor is some way off, in certain circumstances it is possible to calculate the gradient tensor from total magnetic intensity (TMI) information. Such circumstances include anomalies being no more than about 20 percent of the local Earth’s field, where the field departs from being a true potential field, and adjacent lines being well levelled.

Euler deconvolution requires solving at least four simultaneous homogeneous equations to yield the location in 3D and the Euler structural index of the source. Combining two or more adjacent tensors provides an over-determined system which allows the covariance to be estimated and gives a measure of uncertainty.

This method extracts a wealth of information on the location and geometry of magnetic sources. However, the method is sensitive to departures of the TMI from being a true potential field, for very strong anomalies, in which case conversion to a true potential can be performed using an iterative method involving calculating components from the TMI and projecting the components onto the local field direction.

A systematic relationship between the distribution grossly stratiform Cu-deposits, basin architecture, and facies associations in the Zambian Copperbelt (ZCB) suggests the deposits formed at sites of former significant accumulations of both mobile and/or in situ hydrocarbons. Deposits occur within a 200 m stratigraphic window containing the first laterally-extensive, fine-grained, originally organic-rich strata within the host Katangan Supergroup. Arenite-hosted deposits occur within, or immediately overlie confined or peripheral portions of sub-basins formed during the initial stages of sedimentation and are overlain by relatively impermeable strata; sites with classical hydrocarbon trap geometries. Direct evidence for the former presence of hydrocarbons (e.g. pyrobitumen) is lacking in the ZCB, however anomalously light carbon isotopie signatures for both vein and matrix carbonate, spatially associated with the ores, are interpreted to record the oxidation of organic matter synchronous with ore formation.

Antithetic distributions of (early diagenetic) wall-rock anhydrite and sulfides at many ZCB deposits suggest sulfur in the deposits was largely sourced and reduced in situ. A local source for migrated hydrocarbons and evidence that extensive in situ thermochemical sulfate reduction accompanied or preceded ore formation requires pre-mineralisation burial of the host rocks to depths approaching or exceeding the maximum preserved thickness of the Katangan succession in the ZCB. This indicates Cu-mineralisation occurred late in the depositional history, perhaps during initial stages of basin inversion at the onset of onset of Lufilian orogenesis after ~600 Ma, but prior to significant folding of the host succession.