ASEG Extended Abstracts - ASEG2010 - 21st Geophysical Conference, 2010

A marine continuous seismic (CSP) profiling study and a resistivity study of vibrocore samples of shallow marine sands were undertaken in Jervis Bay, NSW, Australia, to characterise the seabed. The CSP study also included Crookhaven Bight, adjacent to Jervis Bay facing northeast to the ocean. The results of the CSP studies show variable geological conditions below the seabed, indicating very dense sands and variably weathered sandstones. The bedrock surface was also highly irregular in places suggesting cyclic erosional pulses under rapidly falling sea levels at various times in the past. Bedrock was found to form the seafloor in some locations and deeply incised paleochannels extending to -62 m below sea level in the Crookhaven Bight and bay entrance areas.

The sampled sands had very low magnetic susceptibilities, thus corroborating the visual impression of low clay content. However, a variety of grainsizes, grainshapes, colours and cohesions were noted. These factors, together with variations in inferred porosity, gave rise to a range of resistivities, which, when temperature corrected, plotted with some scatter about an appropriate Archie Equation curve. Resistivity values recorded ranged from ~ 1.0 to 0.8 Ω m (20°C) for the samples obtained from 11 sites.

A marine continuous seismic (CSP) profiling study and a resistivity study of vibrocore samples of shallow marine sands were undertaken in Broken Bay, NSW, Australia, to characterise the seabed. The acoustic impedance contrast between deeper sediments and the anticipated sandstone was variable suggesting very dense sands, evident as layered sediments owing to regression and transgression of sea level, and variably weathered sandstones. Interpreted bedrock levels vary considerably across the survey area and represent drowned river valleys of the inner continental shelf. A broad deep channel representing a high-energy paleo-fluvial drainage system in the Hawkesbury outreaches is identified, extending to ~ -80 m below sea level (BSL). Another area is revealed with a dentritic fluvial pattern extending to ~ -70 m BSL. A moderately narrow paleovalley extending to -90 m BSL either side of the Palm Beach tombolo is clearly identified.

The sampled sands had very low magnetic susceptibilities, thus corroborating the visual impression of low ultrafines content. However, a variety of grainsizes, grainshapes, colours and cohesions were noted. These factors, together with variations in inferred porosity, gave rise to a range of resistivities, which, when temperature corrected, plotted with some scatter about appropriate Archie Equation curves. Resistivity values recorded ranged from 1.3 to 0.6 Ω m (20°C) for 64 samples obtained from 17 sites.

Magnetotelluric data have been recorded along two traverses in southern Western Australia. The survey locations were chosen so as to define conductivity structure in the deep crust and upper mantle across fundamental geological boundaries within and at the margin of the Yilgarn Craton.

Data across the suture zone between the Archean Yilgarn Craton and the adjacent Proterozoic Albany-Fraser Orogen suggest major structures and crustal blocks have been successfully mapped based on their conductivity characteristics. The data are interpreted as showing the orogen is largely allochthonous and the suture zone may contain remanent oceanic crust and suspect terranes of continental affinity.

Preliminary results from a survey across the southern Yilgarn Craton from the Southern Cross to Eastern Goldfields greenstone terrains suggest major fault structures, such as the Koolyanobbing shear zone, extend in to the deep crust and dip to the east. A zone of anomalously conductive mantle occurs beneath the Lake Johnston greenstone belt. This belt contains nickel sulphide deposits but the origin and significance of the mantle feature and any link to the mineralisation remains equivocal.

The use of three-dimensional (3-D) seismic attributes to predict reservoir properties is becoming widespread in much area, one of the most underutilized aspects of the methodology is that the property prediction maps can help geoscientists understand depositional and post depositional controls on reservoir development.

Seismic data are built by various seismic attributes such as amplitude, frequency, phase, time and their derivative. Multiattribute analysis is a technique using geostatistics approach to define log properties from seismic data. In simple way seismic attribute shape is used to estimate the nature of log properties shape such as porosity, density and other well logs properties.

We illustrate this point via a case study that examines an Early Miocened-aged carbonate built-up of Tuban Formation in East Java Basin. In the first attempt, seismic analysis is carried out to characterize carbonate facies by utilizing seismic attributes such as Instantaneous Frequency and Reflection then made positive average maps for 200 ms above and below interpreted Turban Carbonate interval. This approach is carried out for 3-D Sukowati Seismic.

The target log of this multiattributes study is porosity log in order to construct porosity cube controlled by nine wells log data which then utilized for reservoir mapping based on the distribution of carbonate reef built-up.

Accurate terrain corrections are important for all gravity surveying. In airborne surveys, a digital model of the terrain is constructed and terrain corrections are calculated at each airborne measurement point. Airborne gravity gradiometry is of high spatial resolution and is particularly sensitive to nearby topographic variations, placing particular requirements on the terrain corrections. A combination of mathematical analysis and simulation studies has led to quantification of the requirements: current on-shore, low-level gradiometer surveys require sub-metre accuracy in navigation and in digital terrain model heights; cell sizes (and therefore also topographic sampling) in the terrain model should be about one-third of the ground clearance. The choice of terrain correction density depends on the application and it is important that the interpreter of the corrected gravity data has the ability to test the impact of changes in this density.

Accurate calculation of the gravity gradient field at the airborne sampling points may be achieved by a wide variety of either spatial or harmonic domain methods. Calculation in the harmonic domain is fast but assumes the data represent a periodic function on a planar surface. Padding methods for periodic extension and piecewise continuation away from a plane both add error and slow the calculation. Spatial domain methods are slower but can be sped up by the use of various approximations. In both cases, a clear understanding of accuracy requirements is essential for making an appropriate tradeoff between accuracy and speed.

Identifying prospects within a large study area is traditionally a time consuming process. Schlumberger Reservoir Seismic Services have produced an efficient prospect screening tool in the form of a well log calibrated gas sand probability cube derived from AVO inversion attributes. Pre-stack simultaneous AVO inversion was performed on a 5048 sq/km multi-client seismic survey to compute seismic elastic attributes (acoustic impedance, Poisson’s ratio and density) which can be used to generate lithology and fluid properties. The survey was acquired in 2008 and is located offshore between the giant Io/Jansz and Scarborough gas fields on Australia’s NW shelf, with potential gas reservoir’s situated at multiple levels within a complex geological setting.

Well log data from within the survey area were calibrated to the seismic, from which wavelets were estimated for each angle stack using the angle-dependant reflectivity logs. Low frequency models were derived using log data from wells both inside and outside the survey extents, in conjunction with seismic velocities and guided by interpreted horizons. Elastic attribute logs were generated from the wells and used to produce non-Gaussian lithology probability density functions (PDF), which in turn are utilised to generate a 3D gas sand probability cube from acoustic impedance and Poisson's ratio volumes. The gas sand probability cube can be loaded into a 3D visualisation software package where techniques such as volume rendering and opacity are applied to quickly and efficiently screen for potential prospects from which gas sand geobodies can be extracted for volumetric analysis and further reservoir characterisation.

Soda-metasomatised and metasediments hosted uranium deposits constitute 18% of the world’s uranium resources. In India, such deposits are identified along the albitite line in the environs of North Delhi Fold Belts in the state of Rajasthan. These are structurally controlled with association of albitisation and pyroxenisation of metasediments, metallic sulphides and carbonaceous phyllites. Uraninite is the dominant mineral phase along with copper, molybdenum and sulphides. The low resistivity of the fractures associated with metallic minerals produces a significant contrast with the host rock, which can be located with electromagnetic methods. High-resolution heliborne geophysical surveys comprising of VTEM, magnetic and gamma-ray spectrometric were conducted to identify uranium deposits in the vicinity of albitite zone. This paper demonstrates the results of heliborne and ground followup geophysical surveys to prioritize targets for uranium exploration. For the first time in India, a ground based Time Domain Electromagnetic survey employing Geonics EM37 system was conducted over one of the target areas delineated based on the VTEM surveys. Interpreted results from ground data correlate well with the spatial locations of the EM conductors delineated from heliborne surveys. Additionally modelling of heliborne and ground data produces comparable physical parameters. The results obtained are extremely useful in planning the boreholes in ongoing exploration programme.

SoPC technology based on FPGA is a flexible and efficient countermeasure for system-on-chip, which provides a new way to develop high-precision geoelectric data acquisition devices. This paper introduces the application of SoPC technology to research and design of an A/D conversion controller which complies with the time sequence of CS5372, a module converting the serial data to parallel one. a resampling FIR digital filter using Matlab and DSP Bulider, a NIOS II Soft IP Core integrated by Quartus II and a PC software designed with LabVIEW. Under the application of equipments mentioned above, Cyclone chip, which replaces expensive CS5376, can be used to realize the acquisition of the high-precision geoelectric data. As the result shown, the use of the technology of SoPC can greatly improve the cost-effectiveness of high-precision geoelectric data acquisition devices and also can integrate the digital parts of geoelectric data acquisition devices into a chip, which also provide an effective way to solve problems under the complex geoelectric conditions.

As near-surface ore bodies are depleted, the exploration for economic minerals requires information from deeper depths. The magnetotelluric method has the necessary depth capability, unlike many of the controlled-source electromagnetic prospecting techniques traditionally used. The geological setting of ore deposits is usually complex, requiring two-dimensional or three-dimensional Earth models for their representation. An example of the applicability of two-dimensional inversion of magnetotelluric data to mineral exploration is presented here. The magnetotelluric inverse problem is ill-posed and the inverse results are unstable and non-unique. It means that different geo-electrical model could fit the observed data with the same accuracy. A stable solution of the ill-posed inverse problem can be obtained by utilizing the regularization methods in the objective function. Solving large scale linear equation of inverse problem, the damped Gauss-Newton algorithm was adopted, which can improve local convergence of Gauss-Newton method. On the one hand, inversion of TE-mode data is more sensitive for the low abnormal body and has poor resolution for the high abnormal body. On the other hand, inversion of TM-mode data has better resolution for the high abnormal body. Jointed two-mode data inversion is able to achieve better model and stack quality in considerably fewer iterations. In order to better inversion results, TE- and TM-mode magnetotelluric data are jointed. Through two-dimensional regularization inversion of the field data, the Sanjiang River copper deposit is located.

Geothermal exploration programs require accurate subsurface temperature information and currently this information primarily comes from temperature maps created from shallow extrapolated down-hole temperature measurements. These extrapolations, often taken from measurements made in non-equilibrated boreholes, do not account for variations in geological structure or thermal conductivity. Here we explore temperature maps at 5km for extrapolated equilibrated and non-equilibrated bores and for modelled basin temperatures. Modelled temperatures are derived from finite element models using 3D basin geology and defined thermal properties.

When 3D thermal modelling is compared to extrapolated temperatures at both shallow (500m) and deep (5km) intervals it can be shown that shallow surface temperature variations (from extrapolated measurements) are often not seen at depth. Depending on whether equilibrated or non-equilibrated temperature measurements are extrapolated, the positions of the potential target anomalies shift. In general both equilibrated and nonequilibrated extrapolated measurements either overestimate or underestimate temperatures at 5km. These extrapolated down-hole temperature measurements may even create false target anomalies, which significantly impacts on exploration.

This study presents the result of an interpretation of heliborne electromagnetic (VTEM) and magnetic data over a base metal exploration tenement in the Central African Copperbelt in Zambia. The interpretation was based on several processed VTEM and magnetic grids, regional scale geological maps, geochemical data on soils and Landsat imagery. The electromagnetic responses in the VTEM or magnetic anomalies could not be ascribed directly to mineralisation. As a result, the identification and ranking of targets was based on the recognition of suitable geological conditions for base metal mineralisation rather than direct geophysical targeting.

Renewed interest in the thermal structure of the upper crust has led to the development of a new generation of tools capable of modelling the 3D heat conduction problem, incorporating the complex geology and physical properties of the crust. The numerical and resolution requirements of such models has necessitated a highperformance performance computing approach, utilising massively parallel machine architecture to obtain ~10m resolution of models over a basin-scale.

Here we demonstrate the application of the StGermain/Underworld geodynamic modelling framework to basin-scale geodynamic problems. The default code has been modified to incorporate importing of geometrically complex geological units, each with its own conductivity and heat production, and temperaturedependent thermal conductivity. Thermal models demonstrate the importance of incorporating 2 & 3D geometries, with large lateral variations in the subsurface temperature field and heat flow as a result of the heterogenous basin architecture.

We explore the effect of lithological resolution, and identify a critical level of detail required to adequately represent basin-scale temperature variations. The regional temperature field play a dominant role in determining tenement-level subsurface temperatures, and highlight the importance of understanding regional temperature variations in geothermal exploration.

A comprehensive investigation of the exchange between surficial waters and groundwater in transitional environments, and hence the mixing of waters characterized by a different salt concentration, is an issue of paramount importance considering the ecological, cultural, and socio-economic relevance of transitional environments. Acquiring information, which can improve the process understanding, is often logistically challenging, and generally expensive and slow in these areas. This applies both for punctual, invasive borehole measurements as well as for ground-based non invasive geophysical surveys. Here we investigate the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy. The quasi-3D modelling of the SkyTEM data by the spatially constrained inversion (SCI) methodology allows to accurately distinguish several hydrogeological features, both underneath the lagoon and the farmland beside it. Hydrogeological features resolved are, for example, the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom, and areas of probable submarine groundwater discharge. While the AEM data show a high degree of coherence with available ancillary information, both from wells and from other geophysical techniques such as high resolution off-shore seismic, they improve greatly the understanding of the hydrogeology in the Venice lagoon, both at large scale and in detail. The research highlights the great potential of AEM to improve significantly the hydrogeological characterization of subsurface processes in lagoons, wetlands, and deltas worldwide.

The Amadeus Basin, a large Proterozoic basin located in central Australia, is the least explored onshore petroleumbearing basin with proven reserves in Australia. The size and remoteness of the Amadeus Basin makes ground exploration expensive, but airborne gravity and magnetic surveys have been shown to be capable of resolving intra-basin structures in sufficient detail to allow prospective areas to be identified.

In the western part of the basin the Gillen Petroleum System is considered most significant: This system has the important characteristic that the source is stratigraphically higher than the reservoir. Thin skinned deformation is expected at the source level and above, with detachments at evaporitic horizons, but deformation of the reservoir is expected to be thick-skinned. This model can form the basis for predicting potential field responses. The most prospective areas are where (i) gravity suggests basement (and reservoir) is shallow, (ii) magnetics maps fold-thrust complexes (structural trap), (iii) these features occur adjacent to gravity lows, indicative of significant thicknesses of basin fill (source at depth and below reservoir). Faults at the margins of the depocentre (mapped using magnetic data) provide a possible migration path for the hydrocarbons.

Compilation of the first South Australian state radiometric image was commenced almost a decade ago. Vastly differing vintage and quality of data between surveys provided a significant problem with the levelling of surveys to one datum. A back calibration method was adopted to derive the true ground concentrations of the three radio-elements, U, TH and K.

The significant time required to back-calibrate one survey via ground-based points was a major obstacle, therefore, a vehicular mounted system has been developed that incorporates a constant streaming back calibration method. Airborne datasets that have been levelled using this method have been subsequently merged to create a suite of much improved State radiometric images.

The Australia Wide Airborne Geophysical Survey (AWAGS2), recently completed by Geoscience Australia (GA) consisted of a number of magnetic and radiometric tie-lines flown over the Australian continent, and around the coast. Constant streaming has been used to collect ground data coincident with the AWAGS2 lines. A comparison between the ground data and the AWAGS2 data back calibrated to ground level, exhibits good consistency, thereby validating the use of constant streaming for back calibration. Numerous other acquisition tests aimed at assessing quality and consistency have been completed, including a direct comparison with airborne data, a ground based grid, dose rate monitoring at Radium Hill, and several dirt versus bitumen and acquisition speed tests.

Future applications will tend towards regional and detailed surveys as an aid to mapping of project areas.

Increasing demand for hydrocarbons is prompting increasing exploration in poorly understood and geologically complex basins. These include basins affected by exhumation, where there is often uncertainty regarding the maximum burial depths of source, reservoir and seal horizons. This uncertainty can be reduced by quantifying exhumation magnitudes using techniques such as sonic velocity analysis. The Cretaceous-Cenozoic Otway Basin of the southern Australian margin is an important oil and gas province whose subsidence history was interrupted by exhumation events during the mid-Cretaceous, mid-Eocene and late Miocene. We have used sonic velocity data from 135 wells to quantify the magnitude and extent of exhumation during these events. Here we present estimates of net exhumation based on analyses of Lower Cretaceous Eumeralla Formation shales, which are important source rocks within the basin. Our approach involves measuring the velocities of overcompacted intra-formational shale units identified from gamma ray logs, and determining their displacement on the depth axis from an empirically derived normal compaction trend based on data from wells where the Eumeralla Formation is at maximum burial at presentday. Our results indicate significant values of mid-Cretaceous net exhumation in the Otway Ranges and Colac Trough, with estimates for Olangolah-1 (~2700 m), Stonyford-1 (~1100 m), Tirrengowa-1 (~860 m) and Ingleby-1 (~1000 m) in good agreement with independent estimates of mid-Cretaceous exhumation for these wells based on palaeothermal data. There is also moderate net exhumation around the Merino High (~700 m) but minor (0-200m) net exhumation around the basin’s northern margins and Penola Trough area and moderate (200-600 m) around the Port Campbell Embayment gas fields. Mid-Cretaceous exhumation is interpreted to have been caused by NW-SE directed compression and inversion of the basin. Our results indicate that potential Eumeralla Formation source rocks in wells where large values of mid-Cretaceous exhumation are recorded are most likely over-mature for hydrocarbon generation, in agreement with previous studies of the thermal history of the basin.

Seismic refraction traveltime data can be inverted to obtain a depth model of the weathered layer in terms of the seismic velocities. Furthermore, the head wave amplitudes can be inverted to generate a model of the scaled density ratios in the near surface.

These seismic refraction models of the near-surface can be used in two modes. The first is to derive a representative model of the in-situ rock densities in the sub-weathering. The second is to fully compensate for the weathered layer, in order to detect any prospective gravity anomalies originating in the sub-weathering.

The extensive sets of seismic data recorded for regional investigations by Geoscience Australia are well suited to deriving detailed models of the depth of weathering and scaled density contrasts. These models can be usefully employed as starting models for model-based inversion of coincident gravity data.

Seismic attributes are any measure that helps to better visualize or quantify features of interest in seismic data. Refraction attributes can be readily computed from multifold seismic data using the standard algorithms of the generalized reciprocal method (GRM), a common offset refraction (COR) modification of the GRM, and the refraction convolution section (RCS).

The COR GRM algorithms are applied prior to the parameterization of the traveltime data into individual layers. The COR GRM presentations facilitate the efficient quality control and interpretation of large sets of multi-fold data. Furthermore, the COR GRM algorithms can be readily applied to single ended refraction data, which are routinely recorded with marine and land streamers.

The time model attribute obtained with the COR GRM algorithm is of comparable accuracy to that computed with the standard multi-fold GRM. However, the COR GRM seismic velocity model is a smoothed version of the fully optimized standard GRM model.

The refraction attributes computed from the head wave amplitudes include the multi-fold head coefficient, and a COR equivalent with the amplitude product. Although the COR amplitude product for each offset is of comparable accuracy to the multi-fold head coefficient, it also includes a residual geometric spreading component.

Another refraction attribute computed from the head wave amplitudes is the geometric spreading coefficient, which is generally assumed to be two in the far field for plane homogeneous refractors. This parameter is consistent over a wide range of offsets, but varies considerably from less than one to up to four along the seismic traverse

The attribute derived from the RCS is bandwidth in octaves.

It is proposed that refraction attributes can be employed to derive more useful models of the regolith using multivariate geostatistics. It is concluded that, refraction attributes offer the opportunity of extending the refraction method to new applications.

Standard implementations of refraction tomography routinely use low resolution starting models. Tomograms obtained from these starting models can frequently fail to detect even major lateral variations in seismic velocities, such as a 50 m wide low velocity shear zone at Mt Bulga.

By contrast, the successful detection of the shear zone is unequivocal in tomograms generated with the generalized reciprocal method (GRM). Further improvements in resolution, which facilitate the definition of additional zones with moderate reductions in seismic velocity, are achieved with a novel application of the Hilbert transform to the GRM refractor velocity analysis algorithm. However, the improved resolution also requires the use of a lower average vertical seismic velocity which accommodates a velocity reversal in the weathering. The lower seismic velocity is derived with the GRM, whereas most refraction tomography programs assume vertical velocity gradients as the default.

Although all of the tomograms are consistent with the traveltime data, the resolution of each tomogram is comparable only with that of the starting model. Refraction tomography rarely, if ever, extracts additional detail which is not apparent in the starting model: it essentially smoothes unrealistic or inconsistent starting models. Therefore, it can be concluded that the major effect of refraction tomography is largely cosmetic.

A comparison of the errors of tomographic inversion does not “prove” that a given result is either “correct” or even geologically reasonable. This study proposes that the three tomograms generated with detailed GRM time and velocity models for the optimum XY value and ± half the station spacing, are a more useful measure of the uncertainties with refraction inversion. Non-uniqueness is most conveniently resolved with refraction attributes derived from the digital field data, before the implementation of more expensive drilling programs.