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

The Radiometric Map of Australia shows the distribution of potassium (% K), uranium (ppm eU) and thorium (ppm eTh) over Australia. A suite of image enhancement and data integration techniques can be used to enhance the value of these data for both mineral exploration and environmental mapping.

Gradient-enhanced ternary and pseudo-colour image enhancements are now routinely used for the presentation and interpretation of gridded radioelement data. Where digital elevation data are available, these colour representations can be draped over the elevation data to form 3D perspective views, or hill-shaded derivatives of the DEM can be embedded into the ternary imagery as an intensity component. These are useful because the radioelement response can then be interpreted within the context of the relative position of anomalous features in the landscape. However, subtle variations in the concentrations of K, U, and Th are best interpreted using the ratios of the radioelements. The U/Th and U²/Th ratios are important indicators of uranium mineralization. The Th/K ratio is widely used for the detection of several styles of mineralization associated with K alteration.

For the interpretation of map data, classification and clustering methods can be used to assist pattern recognition and are useful for the rapid assessment of large multivariate datasets. Automatic edge detection procedures can be used to speed up the annotation of unit boundaries. Residual modelling techniques can be used to highlight potential anomalies in the data.

Magnetotelluric (MT) measurements were undertaken at 40 broadband (0.01s – 500s) and 12 long-period (10s-10000s) stations across the central-eastern Eyre Peninsula, South Australia, using Auscope MT equipment. Typical site spacing is of the order of 15km in between sites for the long-period stations and and 3-10km for the broadband stations. This ensures sufficient coverage to map the upper crustal to upper mantle structures underneath central Eyre Peninsula.

The profile extends south of the Gawler Range Volcanics and crosses the Archaean Sleaford Complex, the Hutchinson Group and the Donington Suite from west to east. The 2D MT profile also crosses the location of the postulated Eyre Peninsula Anomaly (White and Milligan, 1984, Kusi et al., 1998, Thiel et al., 2005) as well as the Kalinjala Shear Zone in the eastern part of the profile (Vassallo and Wilson, 2002, Thiel et al., 2005).

Dimensionality analysis and strike determination has been carried out using phase tensor analysis (Caldwell et al., 2004). Subsequently, data influenced by 3D effects could be discarded and the remaining data were inverted for 2D structure using a code by Rodi and Mackie (2001). 2D inverse modelling of the broadband and long-period data indicates a resistive crust in the western part of the profile representing the Archaean Sleaford Complex. Along its eastern margin extends a major conductive crustal boundary in the central part of the profile.

Technical Area: Minerals exploration, magnetotellurics

PRESENTER PROFILE (100 words in sentence format):

Stephan Thiel is a IMER Research Fellow at the Center of Tectonics, Resources and Exploration (TraX) at University of Adelaide. Stephan completed his Masters at the Freiberg University of Mining and Technology, Germany and obtained his PhD at the University of Adelaide. Stephan’s speciality is the analysis and modelling of electromagnetic data to define large-scale lithospheric structures, mineral systems and geothermal areas. Email:[email protected]

Riemannian wavefield extrapolation, a wave-equation imaging method for efficient one-way propagation of 3D wavefields, is extended to 3D seismic surveys. I formulate an inline delayed-shot migration procedure in 3D tilted elliptical-cylindrical (TEC) coordinate systems. When inline coordinate tilt angles are well matched to the inline source ray parameters, the TEC coordinate extension affords accurate propagation of both steep-dip and turning-wave components important for successfully imaging complex geologic structure. I show that wavefield extrapolation in TEC coordinates is no more complicated than propagation in elliptically anisotropic media, which is easily handled by current finitedifference practice. I apply this approach to a realistic 3D wide-azimuth synthetic derived from a field Gulf of Mexico data set. The resulting images demonstrate the imaging advantages made possible through 3D RWE implementations, including the improved imaging of steeply dipping salt flanks, at reduced computational cost.

Hungary is promising for utilization of low temperature (< 150°C) geothermal energy because of its high thermal gradient, reaching almost 50° C/km over most of the country. This high gradient is mainly caused by a relatively thin layer of the Earth’s crust in that area and partly due to the non permeable lower Pannonian sediment layer that covers a large part of the country. Our geothermal evaluation project in Hungary has, since 2007, yielded over 30 possible well sites for geothermal energy production and utilization. The first drilled well was recently successful.

The correlation between resistivity and temperature is associated with the local degree of hydrothermal alteration. Most high-temperature hydrothermal systems are indicated by a low resistivity layer over the geothermal reservoir which is caused by clay mineral alteration. Electrical methods provide information about rock properties, temperature, and the degree of hydrothermal alteration. This information can be used to determine the geometry of hydrothermal reservoirs, its depth, location of fracture zones, and the permeability distribution. To complement the electromagnetic method of choice (magnetotellurics or MT), gravity surveys were acquired along the MT survey lines with higher density spacing to assist in detecting fault systems. Gravity data may be used to interpret the subsurface and to aid in locating prospective heat sources. Integrating the MT and gravity data reduces the ambiguity of either dataset and produces a more robust interpretation.

The distribution characteristics of the fault zones with relatively low resistivity and with boundaries outlined by cooperative constrained inversion of MT and gravity data indicate that the prospective zones for potential geothermal reservoirs in the survey area are along the midnorthern part of the AMT/MT survey line 1 and the middle part of AMT/MT survey line 2.

Based on integrated processing and interpretation of electromagnetic, gravimetric and seismic combined with stratigraphic information; the position of the first geothermal well site was selected and drilled in the Szentlőrinc survey area. Hot water with temperatures in excess of 85°C, estimated to have a peak heating capacity of 4 MW, was found at depths of 1,620 to 1,790 meters. This discovery was possible due to the utilization of different geophysical and geological methods to determine the best well location.

The integrated approach which uses different datasets has proven to be a very effective method for locating the most promising areas for geothermal exploration. Utilizing this method in Hungary, with the goal of supplying 700,000 homes with geothermal energy within the next decade, is readily possible.

In this paper, we propose and present a seismic stratigraphic interpretation, based on two-dimensional seismic data, of units attributed to the Upper Cambrian to Middle Ordovician Scropes Range Formation within the northwestern portion of the Blantyre Sub-basin in the central part of the Palaeozoic Darling Basin, western New South Wales. Data from only two shallow wells of the 831 km² study area dealt with in this paper were available to support the seismic investigations. Six seismic reflection horizons are defined and used to separate five seismic sequences named Units A, B, C, D and E. These features are identified in three seismic lines and demonstrate the continuity of the Scropes Range Formation throughout the northwestern part of Blantyre Sub-basin. All seismic sequence boundaries are based on good continuous markers, with strong amplitudes throughout the study area. The acoustic basement beneath the Scropes Range Formation, was almost certainly metamorphosed in the Delamerian Orogeny (around 500 Ma) within the study area. In addition, acoustic facies recognized in the three seismic lines can be correlated with sedimentary facies observed in outcrop in the Scropes Range, 130 km west of the study area, and are compared with acoustic facies in modern non-marine (fluvial) sandstone units associated with local braided delta depositional systems. The seismic facies identified from the seismic data show features such as low-angle and high-angle clinoforms, parallel and sub-parallel reflections and a few hummocky/wavy reflections indicating shallow channel-fills and small-scale fluvial channels. A combination of horizon mapping, seismic facies analysis and seismic attribute mapping defines the sequences. Complex geological features suggest many potential structural and stratigraphic hydrocarbon traps. A high-resolution seismic survey could lead to the definition of new exploration plays in this underexplored region.

Borehole logging for uranium was much studied in the 1970s and early 1980s. Calibration required building large concrete calibration pits. Codes for gamma-ray transport modeling were then the specialty of large nuclear research institutions. Today, building concrete pits is difficult but sophisticated modeling codes are freely available. For example, the GEANT code, used in this study and available from CERN, enables gamma-ray problems with complex geometry to be evaluated by modeling rather than experiment. This paper illustrates three examples of recent applications of the modeling approach to examine deconvolution, the effect of high U grades on total count logging tools and the applicability of the cerium-doped, lanthanum bromide (BrilLanCe) detectors to direct U logging using the 1.00 MeV peak from ^234mPa.

In 2008-2009 Geoscience Australia, contracted Fugro Airborne Surveys and Geotech Airborne, to respectively acquire TEMPEST and VTEM airborne electromagnetic (AEM) data with broad line spacings covering more than 71 000 km² in the Pine Creek region, Northern Territory.

Near surface geophysical methods are used in archaeology to estimate the distribution, depth, form and physical properties of the buried features before digging. For studying the burial mounds from the Histria necropolis was chosen the micromagnetic method conducted with a proton procession magnetometer in rectangular panels with a sampler interval of 2 m, resulting 16 profiles for each mound. Histria necropolis is located north-west from Histria citadel and is formed from elevated burial mounds with different characteristics and structures. Previous archaeological studies encountered several burial rites, including pyre incineration. These studies revealed even metallic remains of the actual pyre and a burnt shaly horizon with an increased magnetic susceptibility. The magnetic maps of the two studied burial mounds indicate a very complex archaeological situation, also distinguished in the actual archaeological diggings. There are some anomalous aspects that could correspond to some ceramic fragments, to greenshists used to mound building and to some metallic objects such as shields or darts. The presence of metallic objects and ceramics within the burial mounds was strongly connected to afterlife belief.

Detection of highly conductive ore bodies with airborne TEM is difficult or impossible as a result of the response of the target being largely in-phase with the transmitter signal. Measurement of magnetic field spatial gradients, in addition to the usual field measurements is proposed as a means to allow in-phase measurements in a non-rigid TEM system. With in-phase measurements the most conductive targets are as detectable as any geometrically equivalent target with lower time constant, greatly extending the utility of TEM.

A simplified, proof-of-concept, analytical system model, in which only coaxial transmitter, receiver and target are considered, is used to demonstrate the compensation concept and to quantify the expected performance of such a system. The model demonstrates that a compensated signal preserving (or even enhancing) the target response can be generated.

An assessment of noise in this model shows that noise from the gradient measurements significantly increases the system noise, over that of an equivalent receiver, by ~ 100-fold. However this does not extinguish the benefit of in-phase measurement and only results in halving of the depth of detection for the most conductive target, in comparison with an equivalent target with optimum time constant, in a system measuring the off-time response.

A numerical model of a system with 7 degrees of freedom is then used to confirm the results can be extended to physically realistic TEM systems.

Conventional models of gas transport in the earth centres on diffusion, convection and advection. Anecdotal and experimental evidence suggests that sometimes gas transport can be orders of magnitude faster. A model of microbubble transport with vertical velocities of the order of 1 - 1000 m/day, dependant on microfracture size, provides a mechanism for these observations. Because the force acting on a microbubble is buoyancy, the flow will be vertical with little dispersion. This presentation will review this theory and some of the tests conducted to confirm such rapid vertical velocities. Fast gas transport also has applications. For example, radon (²²²Rn, t½ = 3.8 days) is often used for locating uranium but can only work with rapid movement of a carrier gas. Radon has also been used for fault location. Measurement of radon gas flux on the surface has also been used to locate in-situ fires in coal seams in the Hunter Valley, fires which are located at 300 – 450m depth. Whether the radon is from the source (burning coal) or picked up by the flowing gas (CO2), the surface measurements of radon show an anomaly located vertically over the fires and which may then be used to locate such events.

The occurrence of rapid gas movement gives cause for concern over geosequestration of CO2 as zones of high flux must be mapped and avoided.

Detecting of spherical anomalies is an important case in exploration of some mineral bulks and cavity exploration in runways. The effect of a spherical anomaly, located in a half space is studied by different authors.

In this paper, the potential of a sphere buried in two layered earth is obtained by theoretical calculation as a forward modeling. Both of layers are assumed homogeneous and isotropic and the sphere is located into the first layer. Two current electrodes are positioned in far distance with the anomaly in a symmetric array. The electric flux, produce a uniform electric field into the earth.

The electric field polarize the buried sphere as induced electric charges situate in two sides of sphere and orient opposite the primary uniform field. In practice the electrical charges on two sides of sphere will be equivalent with an electric dipole. The potential of a buried sphere into a half space earth is obtained by solving the Laplace equation with applying boundary conditions and considering an electrical image of sphere in relation to the earth's surface.

Using these calculation and considering large number of electrical images, yield an explicit expression for total potential. Desired potential in various situations with parametric variations of problem is calculated by computer programming as a numerical analysis. At last apparent resistivity of structure is computed in different conditions and compared with each other in plotting modes.

Based on examination and re-interpretation of 2D seismic and well data, three groups of faults were recognized in the Abrolhos Sub-basin: a) major normal faults transecting the Permian, Triassic and Jurassic, b) small normal faults developed in the Early Permian, and c) small normal faults developed in the Triassic and Jurassic.

The major faults usually have large offsets and strike north-northwest. Most of these are at intra-basin boundaries, subdividing the area into an eastern margin, an eastern graben, an eastern horst, a central depocentre, a western horst and a western graben. Mostly these trend north-north-west, except for the eastern horst, which trends north-south and extends southward to join the western horst.

Some small faults developed in the Early Permian. They usually dip east and show strong syn-depositional character, indicating block rotation. These faults terminate at the Late Permian Unconformity. Other small faults developed in the Triassic and Jurassic. They usually dip west. These late faults and the major faults terminate at the Early Cretaceous Unconformity.

The Abrolhos Sub-Basin has undergone two main rifting stages. In the Early Permian the basin was initiated under east-northeast – west-southwest extension, resulting in development of north-northwest striking normal faults. During the Jurassic, northwest-southeast extension commenced. The oblique extension resulted in re-activation of some pre-existing northnorthwest striking faults and generation of new strike-slip faults. Each rifting stage was followed by basin inversion due to the break-up of Gondwana. This resulted in regional uplift and formation of the two key regional geological boundaries: the Late Permian Unconformity and the Early Cretaceous Unconformity.

A case study of using migration of duplex waves to trace zones of near-vertical fracturing in coal formation is described. As a rule, coal-bed methane traps are connected with these zones, however, identification and tracing of them based on the standard seismic exploration techniques are practically impossible. It is shown that the joint analysis of structural interpretation and seismic facies investigation data and of the results of duplex wave migration improves efficiency of coalbed methane prediction. The work has been carried out based on 3D seismic survey conducted at one of the coalmines in the Donets Basin (Ukraine).

Predictive 3D geological models of the subsurface can be developed using a range of available tools. Each tool is suited to slightly different problems and datasets. The method described here, using the UBC-GIF inversions algorithms, allows rapid development of models using an objective, automated procedure. It has flexibility to include as little or as much geological information as is available, making it ideal for greenfields exploration or mapping programs.

The steps involved are: 1) develop a solid understanding of the expected physical properties; 2) convert geological observations into physical property constraints; 3) perform geologically-constrained inversions; 4) apply geological classifier to recovered 3D physical property models.

The procedure is demonstrated for the southern Agnew-Wiluna greenstone belt in WA, a highly mineralised region with a high proportion of surface cover. The predictive 3D lithology models developed for the area are particular effective at mapping the extent of dense mafic and magnetic ultramafic rocks, and provide new insights about their distribution at depth.

The Moran nickel orebody was discovered in 2008 when underground drill-hole LSU-152 intersected a 12.3 m thick interval of mineralisation with an average grade of 6.0% Ni. The discovery drill-hole had been designed to test a strong conductor detected by transient electromagnetic surveys read in drill-holes that had passed above and below Moran.

Moran is located approximately 1.1 km south of the Long orebody. Both orebodies are interpreted to be hosted within the same komatiite lava channel. The exploration program was premised on the observation that fertile komatiite lava channels are often mineralised over significant distances. Diamond drill-holes were drilled from underground mine workings in order to test the target lava channel to the south of Long over 1.7 km of strike. Recognition of potential ore-hosting environments, integrated with conductor models developed from DHTEM data, were keys to success.

The Australian National Gravity Database (ANGD) contains over 1.8 million gravity observations from over 2,000 surveys conducted in Australia over the last 80 years. Three processes are required to correct these observations for the effects of the surrounding topography: (a) a Bouguer correction (Bullard A), which approximates the topography as an infinite horizontal slab; (b) a correction to that horizontal slab for the curvature of the Earth (Bullard B); and (c) a terrain correction (Bullard C), which accounts for the undulations in the surrounding topography. These three corrections together produce complete Bouguer anomalies.

Since February 2008, a spherical cap Bouguer anomaly calculation has been applied to data extracted from the ANGD. This calculation applies the Bullard A and Bullard B corrections. Terrain corrections, Bullard C, have now been calculated for all terrestrial gravity observations in the ANGD allowing the calculation of complete Bouguer anomalies. These terrain corrections were calculated using the Shuttle Radar Topography Mission 3 arc-second digital elevation data and Geoscience Australia’s 2009 Bathymetry and Topography Grid.

The complete Bouguer anomalies calculated for the ANGD provide users of the data with a more accurate representation of crustal density variations through the application of a more accurate Earth model to the gravity observations.

Iran holds the second largest gas reserves in the word with over 27.5 trillion cubic meters (TCM) of natural gas. Due to lack of geological surveys in certain geographical regions in Iran. It is likely to explore further reserves in the future. Hence, for utilizing this energy carrier, it is essential to have comprehensive and explicit planning knowledge. The study of gas industry development policies is indicative of certain barriers in utilizing prospective opportunities. Iran is one of the largest gas rich countries in the world that production capacity exceeds domestic consumption and gas injection requirements. Gas can be utilized as feed stock in petrochemical plants and refineries or exported through pipeline or LNG. Through reinjection of gas to oil reservoirs, while increasing the oil recovery ratios, the produced gases from fields shared with other countries could be stored into domestic gas fields.

Gas consumption in domestic markets and its substitution with oil products, in addition to providing environmental benefits, will also result in optimum consumption of these products and relieving the government from the heavy burden of existing and heavy expenditures of importing these products from foreign countries.

The need for energy sources in the world along with the enormous natural gas reserves in the country opens broad economical/political dialogue scene towards us and contributes an outstanding strategic significance to our gas resources. Supplying gas requirements, proper and timely production and operation of joint reservoirs such as the south pars with the intention of supplying gas requirements and providing balance of supply and demand as well as maximum utilization of our share in these fields are other development requirements of this significant industry.

For planning and policy making regarding the development of gas industry, it is essential to manage all aspects of gas from exploration and production to consumption, injection and exports and etc. through a sole administrative institution so that prearranged plans could be implemented without becoming subjected to such problems as lack of coordination parallel activities and organization al problems. Gas fuels consumption occupies one half of Iran's energy consumption, and to increment this share, the government should increase investment in the gas sector.

Natural gas while being utilized as a clean fuel in domestic markets can become a device to implement gas injection to oil production from the countries reserves as well as gas exports towards presence in international venues and commercializing gas globally.

Prediction indicates that natural gas being the favorable fuel of the present century, will enjoy the largest growth among items within the energy basket and during the next twenty years as well, the growth of natural gas demand in the world will exceed other conventional energy sources. In the future, due to various factors such as vaster accessible sources and reserves, developed technologies which in effect reduce project expenditure and construction periods and consequently improve the economy of developing gas transmission projects, as well as global endeavors to curtail emission of green house gases, are the major reasons for gas consumption growth.

Iran, in respect to her geographical and political strategic situation, can play a leading role in global gas supply and act in Europe and Asia. Natural gas still maintains the fastest consumption growth rate among the world's primary energies and is realized to have the highest consumption growth among the developing countries.

Gas has been recognizing as the fuel for the future. This is entirely evident as a result of decreasing global resources due to environmental considerations. According to IEA estimates, in year 2015, oil demand will reach 93 million barrels and natural gas demand will reach 64 million barrels of oil equivalent. This in effect is a 20% increase for oil and 45% for gas in regard to current figures in conclusion. It could be stated that more than ever before, gas has gained significance and is the leading basis for modern services in energy, and in the long term is considered a bridge towards a hydrogen resource based economy.

Gas as a new fuel, provides the modern technology of fuel cell construction for vehicles possible. To portray the enormity of Iran gas reserves it is enough to consider that Iran's natural gas reserves alone exceeds the total volume of natural gas reserves in USA, Canada, Europe and the entire Asia pacific.

A new method has been developed to determine the depth to magnetic basement. This method uses an efficient automated approach that produces depth estimates via inverse modelling the azimuthally averaged log of the power spectra of gridded airborne magnetic data. The model consists of a horizontal slab possessing band limited fractal magnetisation. The sensitivity and uncertainty of the depth estimates were analysed based on the magnetic slab’s fractal property using Bayesian analysis via Markov chain Monte Carlo and Jeffery’s prior. A sliding window technique with an appropriate amount of overlap to the adjacent data window has been applied to a magnetic grid of the Gawler Craton region. The window size selected was optimised to produce depth estimates for relatively shallow magnetic sources due to the relatively thin cover material in the region The resulting depth estimates were combined with drill-hole data and seismic data and then gridded using the minimum curvature technique to produce a depth to basement map of the region. The new automated depth method was developed within Geoscience Australia’s Geodynamic Framework Project, part of the Onshore Energy Security Program.

We noticed during depth processing on real seismic data with fault shadows that:

- Appearance of fault shadows and the convergence speed of the tomographic inversion depend on the acquisition direction;

- Tomographic depth-velocity modelling usually produces models that closely follow geology; but sometimes the models contain odd looking nongeological anomalies; in both cases the depth migration delivers distortion free seismic images;

- Anisotropy in faulted areas creates additional image distortions.

To examine these effects and optimize our depthprocessing workflow, we created several synthetic seismic datasets for different types of velocity anomalies associated with faults in isotropic and anisotropic media and different acquisition directions.