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

The multi-channel analysis of waves (MASW) is a seismic survey method, in which the seismic data are analysed in the frequency-phase velocity domain to estimate underground S-wave velocity structure. It is used in a variety of engineering projects from large to small at various stages.

An MASW survey was carried out to investigate palaeochannels which is suspected to provide passage to leakage of a dam. For the condition peculiar to the dam site, three different methods of data acquisition were used: spiked geophones; landstreamers and unspiked geophones supported by play dough. These methods of placing geophones provided comparable data quality.

The result was presented in section form along the survey lines and interpolated depth slices in plan view of S-wave velocity distribution. The plan view maps showed a pattern of low S-wave velocity anomaly which is interpreted as drainage. This result was correlated with the other geotechnical tests and used in the subsequent hydrological study.

The presence of orthorhombic anisotropy can severely affect the imaging of multi- and wide-azimuth data which is rapidly developing with the benefit of better illumination, better imaging, and better multiple elimination. Analysis of multi-azimuth (MAZ) data often reveals noticeable fluctuations in moveout between different acquisition directions, preventing constructive summation of MAZ images. Vertical transverse isotropy (VTI) effects can also co-exist causing well misties and higher order moveout. We have developed an approach for imaging in the presence of orthorhombic anisotropy. In this paper, we first describe our approach, including a newly developed orthorhombic imaging method and a newly developed practical method for orthorhombic anisotropy model building. We then demonstrate with both synthetic and real data from offshore Australia that our approach can successfully take into account the coexisting HTI/VTI effects, reduce the structural discrepancies between seismic images built for different azimuths, thereby producing constructive summation of MAZ datasets and resolving well misties to match with geology. The combined effect of these improvements is a step-change in the final seismic image quality.

Seepage from canals and reservoirs can be identified using geo-electric streamers. About 10 kilometres of canal can be surveyed by two people in one day. In electrical conductivity (EC) imagery created, seepage pathways through the substrate reflect the EC of surface water from which seepage was sourced as well as substrate permeability and clay content which enhances EC. Seepage tends not to occur readily through clay and the result, within EC imagery, is generally clear definition of seepage pathways through the substrate. In Australia, canals are typically situated on clayey low flow regime sedimentary deposits where evapotranspiration has concentrated salt in shallow sediment, particularly clays, and seepage has preferentially flushed out this salt resulting in a very clear EC signature regardless of water table depth. In high flow regime environments such as much of New Zealand, seepage pathway anomalies are usually more conductive than the host substrate which is usually cobbles, glacial rock flour and air.

A practical imaging system has been created using a submerged streamer towed behind a floating waterproof equipment capsule housing geo-electric, DGPS, sonar, data logging, and often other instrumentation. The capsule is towed either behind a boat or by two ropes pulled by walkers on each canal bank. Operation is via a wi-fi connection. The capsule is light enough to lift over the numerous obstacles that cross most canals. Imaging is presented in 3D within Google Earth so that water managers can readily handle and use the data.

Systematic geological investigations in Nepal started late in comparison with the start of such studies in other countries. The topographical feature of Nepal varies from 100 meters in south to 8848 meters in altitude in north. Physiographically, Nepal is separated into four zones similar to the geological zones. They are Terai plain (Indo Gangetic plain), Siwalik range (Churia range), lesser Himalaya and higher Himalaya region. The Terai and Siwalik region in the foreland of Nepal Himalaya are known for sedimentary basins with considerable thickness. These regions are targeted for hydrocarbon exploration.

In this paper, we describe the possibility of occurring oil and gas in Siwalik, Surkhet, Gondwana and Lakharpata (Vindhyana) groups of rocks based on the results from field investigation and geochemical analysis of collected samples.

The seismic interpretation of Terai region indicate the evidence of unconformities between Siwalik sediments and the under lying meta sediments group of rock which are equivalent to the oil bearing formation of unnamed formation in the northern India. This group contains potential source and seal rocks. One exploration well drilled though dry gave valuable information to petroleum exploration.

The Onshore Energy Security Program, funded by the Australian Government and conducted by Geoscience Australia, in conjunction with State and Territory geological surveys, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia.

Here, we present the key aspects of the stratigraphy and structural architecture identified using seismic data from the Arckaringa, Officer, Amadeus and Georgina basins on seismic lines 09GA-GA1 and 08GAOM1.

In recent years major coastal desalination plants have been constructed at various locations around Australia. Engineering geophysics has contributed to the successful completion of these major projects. During 2009-10 two of Australia’ largest desalination plants with a combined capacity of 370Megalitres/day (upgradable to 670Ml/day) were commissioned in Sydney, NSW and at the Gold Coast, Queensland at a total cost of approximately $A2.5 billion.

Case studies at these desalination plants discuss aspects of the land, borehole and marine geophysical technologies that were applied during the feasibility and design phases of these projects. These technologies were used to investigate geotechnical conditions at the proposed plant sites and along the land and marine sections of the sea-water intake and brine outlet tunnels.

These studies had a significant impact on the geotechnical risk assessments and the final design of these desalination plants and involved the use of electromagnetic, gravity and seismic methods.

The case studies demonstrate the value of appropriate geophysics to large civil engineering projects.

We introduce a nonlinear constrained inversion technique for 2D interpretation of aeromagnetic data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor (PGGT) derived from gridded magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters horizontal position, depth to the top, dip angle, width and susceptibility contrast can be estimated by nonlinear least squares inversion of the magnetic field data along the flight lines.

We use the Levenberg-Marquardt algorithm together with the trust-region-reflective method which enables users to define inequality constraints on model parameters such that the estimated parameters always lie in a trust region. Assuming that the maximum of the calculated gzz (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centered at the maximum of gzz are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one.

Application of our method is demonstrated on a new aeromagnetic data set from the Sarna area, West Central Sweden. Constraints from laboratory measurements on rock samples from the area are used in the inversion scheme.

Seismic amplitudes within target intervals are often affected by localized absorption anomalies in the overburden. In the North-West Australian shelf and in many other regions, the majority of such anomalies are caused by gas trapped in shallow sediments.

We apply amplitude tomography to build a high resolution 3D absorption model and to take this effect into account in geological settings typical for the North-West Australian shelf. Two approaches have been developed recently to correct for absorption in depth imaging – the first uses frequency independent models and the second based on Q-compensation with the linear frequency dependent assumption. We show how both techniques correct seismic amplitudes but their results are not frequency balanced. In order to achieve a better fit with the real seismic data, we propose and apply a mixed absorption model that combines frequency independent amplitude correction with the linear Q-compensation and reflects the presence of different effects responsible for seismic energy attenuation in real geological media. Prestack depth migration with this model corrects for the overburden effects and produces seismic data with spatially balanced amplitude and spectral characteristics.

The effect of single-phase fluid saturation on a rock’s bulk modulus is well understood using Gassmann’s equation. However when multiple fluids are involved the behaviour is not as well understood. Several fluid mixing averages have been suggested (Voigt, Reuss, Hill), and each apply in certain situations, however it is often not clear which model to select in a specific scenario and in some scenarios none of the models are accurate. The critical factor in deciding which average to use depends on the way the fluids are spatially distributed within the rock. We have applied elastic finite difference computational modelling to many different fluid distribution scenarios and have replicated behaviour described by various theoretical, empirical and lab data results, as well as generating results that span the space between these models. Importantly, our results compare well with observations in lab experiments, without relying on poroelastic or squirt-flow models which require parameters that are difficult to estimate for real reservoirs. Our elastic scattering approach is less computationally expensive than poroelastic modelling and can be more easily applied to actual reservoir rock and fluid distributions. Our results provide us with a powerful new method to analyse and predict the effects of multiple fluids and ‘patchy’ saturation on saturated rock bulk moduli and velocity. They also challenge traditional assumptions about the controlling factors on saturated bulk moduli suggesting it is more dominantly affected by the spatial fluid distribution properties rather than pore-scale fluid flow effects.

The cost of data acquisition in land is becoming a major issue as we strive to cover larger areas with seismic surveys at high resolution. Over sand dunes the problem is compounded by the week coupling obtain using geophones, which often forces us to bury the phone. A major challenge is designing such a land streamer system that combines durability, mobility and the required coupling. We share a couple of such designs and discuss the merits behind such designs and test their capability. The testing includes, the level of coupling, mobility and drag over sand surfaces. For specific designs loose sand can accumulate inside the steamer reducing its mobility. On the other hand, poor coupling will attenuate the high frequencies and cause an effective delay in the signal. The weight of the streamer is also an important factor in both mobility and coupling as it adds to the coupling it reduces the mobility of the streamer. We study the impact of weight and base plate surface area on the seismic signal quality, as well as the friction factor of different designs.

Between 30 and 40% of the Northern Territory is underlain by basalts, mainly the late early Cambrian Kalkarindji Large Igneous Province. These basalts provide a solid floor to mining activities in the overlying strata. They might also provide a buffer of alkalinity and reduction during ore genesis, and should therefore be included in modelling by mineral explorers. A key parameter in the modelling of prospective mineralised zones is the depth to the basalts.

Using the power spectra of open file airborne magnetic data, depths to subsurface basalts can be mapped. A point-and-click package was developed for this purpose, is currently being used to map the depths to the basalts of the Kalkarindji LIP in the Northern Territory.

Open file surveys of magnetic data have varying quality, affecting the ease of extraction of magnetic depths. Although successful depth results can be expected from modern 10 Hz aeromagnetic surveys, the older, 1 Hz proton procession magnetometer data, is more problematic. However some good depth results have been obtained.

With a view to energy security of the world, unconventional energy resources - coalbed methane (CBM), Methane GasHydrate, shale gas, basin centred gas, tight gas, oil shale and heavy oil-exploration and exploitation is pertinent task before geoscientist . Shale gas is natural gas from shale formations which acts as both the source and the reservoir for the natural gas. Each Shale gas reservoir has unique characteristics. Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide permeability . For a given matrix permeability and pressure, gas production are determined by the number and complexity of fractures created, their effective conductivity, and the ability to effectively reduce the pressure throughout the fracture network to initiate gas production. Understanding the relationship between fracture complexity, fracture conductivity, matrix permeability, and gas recovery is a fundamental challenge of shale-gas development. Shale gas reservoirs almost always have two different storage volumes(dual porosity) for hydrocarbons, the rock matrix and the natural fractures .Because of the plastic nature of shale formations, these natural fractures are generally closed due to the pressure of the overburden rock. Consequently, their very low, matrix permeability, usually on the order of hundreds of nanoDarcies (nD), makes unstimulated, conventional production impossible. Almost every well in a shale gas reservoir must be hydraulically stimulated (fractured) to achieve economical production. These hydraulic fracture treatments are believed to reactivate and reconnect the natural fracture matrix .

The Frome airborne electromagnetic (AEM) survey is the largest of three regional AEM surveys flown under the 5-year Onshore Energy Security Program (OESP) by Geoscience Australia (GA). The aim of the survey is to reduce risk and stimulate exploration investment for uranium by providing reliable pre-competitive data. The Frome AEM survey was flown between 22 May and 2 November 2010, is approximately 95 450 km² in area and collected 32 317 line km of new data at an average flying height of 100 m. The Frome AEM survey covers the Marree (pt), Callabonna (pt), Copley (pt), Frome (pt), Parachilna (pt), Curnamona, Olary and Chowilla (pt) 1:250 000 standard map sheets in South Australia and was flown largely at 2.5 km line spacing, with the northern portion flown at 5 km line spacing. GA partnered with, the Department of Primary Industries and Resources South Australia and an industry consortium.

The survey results indicate a depth of investigation (DOI - depth of reliable signal penetration) of up to 400 m in areas of thin cover and resistive basement (e.g., Adelaidean rocks in the Olary Ranges). In Cenozoic – Mesozoic sediments in the Frome Embayment and the Murray Basin the DOI is up to 100-150 m. A range of under-cover features are revealed, including (but not limited to): extensions to known palaeovalley networks in the Frome Embayment; the under-cover extent of the Benagerie Ridge; regional faults in the Frome Embayment and Murray Basin; folded and faulted Neoproterozoic rocks in the Adelaide Fold Belt; Cenozoic – Mesozoic stratigraphy in the Frome Embayment; neotectonic offsets in the Lake Eyre Basin; conductive Neoproterozoic rocks associated with coppergold mineralisation; and, coal-bearing structures in the Leigh Creek area, as well as groundwater features.

A magnetotelluric survey, comprising 40 stations, has been completed in the southern Yilgarn Craton. The preferred resistivity cross section through the crust and upper mantle shows the local lithosphere comprises three distinct units separated by steep boundaries. The central unit, interpreted as equivalent to the Southern Cross Domain has a resistive crust overlying a more conductive mantle. The two units on either side comprise a conductive lower crust overlying a resistive mantle. Dipping narrow zones of increased conductivity in the crustal part of the model correlate with known surface structures. The eastern margin of the Southern Cross Domain as inferred from deep crustal and mantle resistivity occurs about 50 km to the west of the Ida Fault, the margin of the domain at the surface. The three fold subdivision of the local lithosphere is consistent with the geologically and geochemically defined terranes and domains in this part of the Yilgarn.

Current models for regional mineral exploration targeting emphasize the significance of major geological structures and the edges of cratonic blocks as areas of greatest prospectivity. The South Yilgarn MT dataset demonstrate that such features can be located based on variations in the electrical conductivity of the lower crust and mantle, which can be measured in a cost effective manner using the magnetotelluric method.

The on-shore Canning Basin, located in northern Western Australia, has a long and complex depositional and tectonic history spanning almost the entire Phanerozoic. The succession includes several regional unconformities and estimating the amount of uplift with which they are associated can provide important constraints on the geohistory of the basin.

Estimation of uplift based on analysis of sonic slownessand density-depth data is a well-established method but has mostly been applied to Mesozoic basins which have experienced relatively little deformation. Application of the method to the Canning Basin requires careful definition of the ‘reference curves’, with particular attention paid to the geological context of the reference well locations, and the use of a combination of density and slowness data.

Initial results, from the Permian Noonkanbah Formation have produced results which are consistent with the known history of the Canning Basin. The map apparent uplift suggests only a few hundred meters of uplift has occurred and the dominant trend is parallel to the Fitzroy Trough, the main fault controlled depo-centre in the northeast of the basin.

A step-change evolution of airborne gravity gradiometry (AGG) instruments will enable the rapid acquisition of high quality and high resolution geophysical datasets. On top of the advances in the instrument hardware, the nature of the uncertainties that each stage of data processing introduces must be understood so that there is a high level of confidence in the deliverable result. This paper introduces a dataset that aims to facilitate the comparison of terrain correction and 3D inversion packages, using a combination of real and synthetic data from the Kauring airborne gravity test site.

The method by which an airborne gravity gradiometer measures gravity gradients and is stabilised within the aircraft affects the magnitude of the observed gravity gradient due to the nearby masses in the aircraft. To first order, the gravity gradient due to masses within the aircraft can be modelled using point masses. When the centre of mass of the instrument is stationary with respect to the aircraft, self-gradient is caused by rotation of masses about the centre of mass of the instrument resulting in a modest contribution to changes in observed gravity gradient. Movement of the centre of mass of the instrument with respect to the aircraft produces a larger self-gradient signal. In either case, the self-gradient signal correlates well with aircraft motion and can be easily removed from the observations by post-processing without the need for a complex model of the mass distribution within the aircraft.

A towed marine EM acquisition system has been under development for seven years, and two field tests were recently completed in the North Sea. Traditional CSEM technology is based on sparsely spaced receiver stations placed on the seafloor and the source dipole is towed close to the seafloor. The source signal is a square-wave, or a modified square-wave, that is emitted continuously.

In the towed EM system, the source dipole is towed at 10 m below the sea-surface and the receiver cable is towed at a nominal depth of 100 m. The prototype system described here is sufficiently powerful to work in water depths up to 400 m, with a nominal depth penetration of 2,000 m below the seafloor. The signal is a transient signal that can be a modified square-wave, or a PRBS. All aspects of the data acquisition are monitored real-time and pre-processed on-board facilitating quality assurance and optimization of all acquisition parameters.

Two successful tests were conducted over the Peon shallow gas field and the Troll oil and gas field. One of the sail-lines over the Troll field was simultaneously acquiring EM and 2-D seismic data. By keeping the EM source and the seismic streamer separated, the level of induced electrical noise on the streamer was never reaching levels where it would become an issue.

In total 615 line km were acquired over 138 hrs and the data has been successfully processed and inverted to delineate all targets.

Angle-domain common image gathers (CIGs) play an important role in migration-based velocity updating. One popular criterion of migration velocity analysis (MVA) is the flatness of common-angle image gathers. We first revisit the popular schemes of generating angle-domain CIGs by wavefield continuation migration. Then, we present an alternative approach to produce commonangle image gathers. Our method, one of multiple-weight methods, is also for shot-profile migration based on oneway wave equation. To avoid the stability problem of multiple-weight methods, our scheme is implemented in complex number domain for every frequency after imaging. The ability of anti-alias resulted from sparse shot geometry is investigated. Numerical examples show that our strategy is efficient, stable, and easy to implement.

Recent technological advances suggest that we are on the threshold of a new era in applied magnetic surveys, where acquisition of magnetic gradient tensor data will become routine. In the meantime, modern ultrahigh resolution conventional magnetic data can be used, with certain important caveats, to calculate gradient tensor elements from total magnetic intensity (TMI) or TMI gradient surveys. Until the present, not a great deal of attention has been paid to processing and interpretation of gradient tensor data. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for a number of elementary, but useful, models. These include point pole, line of poles, point dipole (sphere), line of dipoles (horizontal cylinder), thin and thick dipping sheets, sloping step and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The scaled source strength, calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over compact sources, 2D sources and contacts, independent of magnetisation direction. New algorithms for uniquely determining the location and magnetic moment of a dipole source from a few irregularly located measurements or single profiles have been developed. Besides the geological applications, these algorithms are readily applicable to the detection, location and classification (DLC) of magnetic objects, such as naval mines, UXO, shipwrecks, archaeological artefacts and buried drums. As an example, some of these new methods are applied to analysis of the magnetic signature of the Mount Leyshon gold-mineralised system, Queensland.

Forced imbibition was performed in a Limestone Savonnieres by injecting water into a dry sample. The injection was monitored with X-ray Computed Tomography (CT) and active ultrasonic measurements so that the time-space distribution of the invading fluid could be simultaneously observed in CT images and quantified through measuring P-wave velocities and water saturation.

The CT scans allowed us to observe a water front advancing away from the area of injection and estimate saturation. Through the evolution of P-wave velocities, we observed a strong influence on the acoustic response with the presence of water and with the changing of injection rates. The approaching of the water front to the monitored position decreased P-wave velocities while the saturation increased continuously. The P-wave velocities decrease occurred for a short period of time and was followed by a sharp increase which happens when the fluid front crossed the monitored position. Decreasing injection rate decreased P-wave velocities and saturation. Increasing injection rate, increased P-wave velocities and saturation, sharply and for a short period of time followed by a slight decrease for P-wave velocities and a continuous increase.

Our experimental data confirms how sensitive acoustic waves are to the presence of water and that changing injection rates promote considerable fluid distribution that is drastically reflected in the acoustic velocities.

This case study compares 3D inversion results from Spectrem Air’s SPECTREM 2000 fixed-wing timedomain airborne electromagnetic (AEM) system, and Geotech’s Z-axis Tipper Electromagnetic (ZTEM) airborne audio-frequency magnetics (AFMAG) system flown over the Pebble Cu-Au-Mo deposit in Alaska. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and with the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The 3D interpretation of both surveys has yielded improved the understanding of the geology, alteration and mineralization of the Pebble system. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation on one system over the other. We do conclude however, that 3D inversions of AEM and ZTEM surveys add significant value to exploration.

To improve mineral exploration success, there is an accepted need to increase the “discovery space” by exploring under cover and to greater depths using 3D geological modelling supported by multiple 3D geophysical inversions. To facilitate this approach, multi-sensor airborne platforms capable of simultaneously measuring electromagnetic, gravity, and magnetic data are now being deployed. The availability of data from such systems poses a significant challenge to the exploration geophysicist: How do you generate a shared earth model that satisfies all data? We address this with a case study from the Reid-Mahaffy test site in Ontario, demonstrating how multiple 3D inversions of MEGATEM time-domain electromagnetic, FALCON gravity gradiometry and TMI data can be analysed by the self-organizing maps (SOM) data mining approach to produce 3D pseudo-lithological models. The results of our analyses are shown to be in agreement with the known geology of the Reid-Mahaffy test site.

We are presenting a technique for laboratory measurements of the velocities and polarisations of compressional and shear waves in rock samples using a laser Doppler interferometer (LDI). Such measurements dramatically improve estimations of anisotropy.

LDI can measure the particle velocity of a small (0.01 mm²) element of the sample’s surface along the direction of the laser beam. By measuring the particle velocity of the same surface element in three independent directions and transforming them to Cartesian coordinates, we obtain three components of the particle velocity vector. Therefore LDI can be used as a localized threes component (3C) receiver of acoustic waves, and, together with a piezoelectric transducer or a pulsed laser as a source, can simulate a 3C seismic experiment in the laboratory. Performing such 3C measurements at various locations on the sample’s surface produces a 3C seismogram, which can be used to separate P and two S waves, and to find polarisations and traveltimes of these waves.

A ‘walk away’ laboratory experiment demonstrates high accuracy of the method. The measured data matches very well with the results from the analytical modeling, From our results, we can conclude that it is possible to characterize elasticity properties of materials from the described measurements. In particular, we are able to determine: 1) the angle between the particle movement and the direction of the wave propagation, i.e. the polarisation, 2) the types of waves and 3) the arrival times of the wave at the point and thus the wave velocities.

A study of the pore fluid effects on the elastic and anelastic properties of sedimentary rocks is important for interpreting seismic data obtained for reservoirs as well as for monitoring the fluid movement during both fluid extraction in producing fields and injection of CO2 for storage purposes. In most sedimentary rocks low intrinsic permeability and, as a consequence, low fluid mobility lead to a situation when relative motion between pore fluid and rock skeleton has significant influence on acoustic wave propagation even at seismic frequencies. Therefore, in many cases the experiments conducted only at seismic frequencies are not sufficient to validate commonly used theoretic models of elastic moduli dispersion and attenuation.

We present data obtained with a new version of lowfrequency laboratory apparatus designed for measurements of Young’s moduli and extensional attenuation of rocks at seismic (1-400 Hz) and teleseismic (≤1Hz) wave frequencies. The apparatus can operate at confining pressures from 0 to 70 MPa. Elastic and anelastic parameters of dry and water-saturated sandstone quarried in Donnybrook, Western Australia, were measured at various confining pressures and room temperature (~20⁰ C). A peak of attenuation in a watersaturated sample with 14.8% porosity and 7.8 mD permeability was found at frequency 0.8 Hz.

The Earth’s upper crust hosts many important economic resources, from minerals to groundwater to energy, but the subsurface structure and associated thermal structure is poorly understood. Internal heat is the driving force behind many of the Earth’s processes and is now being considered as a new form of clean renewable energy. High resolution 3D thermal models, incorporating detailed geological structure and real world data, are effective in assessing thermal structure and provide improved temperature estimates for geothermal exploration. Unlike historical 1D models and extrapolated temperature at depth maps, 3D thermal models are appealing to the geothermal exploration industry as they are not limited by the sparse nature of down-hole temperature measurements or heat flow and the uncertainties of such models can be calculated.

In the Sydney-Gunnedah-Bowen Basin (SGBB) system, Australia’s energy rich sedimentary basin, the thermal structure is poorly understood resulting in its geothermal potential largely being ignored. Thermal modelling using optimised parameters shows estimated temperatures at 5km below the surface, the economic limit of drilling, to range from 120°C to 240°C, with highest temperatures under thick sediments with multiple insulating coal layers. Using the 150°C temperature contour as in indicator for potential geothermal prospectivity, the most potential basins are in the Bowen and Sydney.

The general aim of seismic surveying is to map the subsurface in as much detail as possible, subject to economic constraints. As technology has developed so has the ability to acquire and process larger amounts of data. This often translates to acquisition of larger offsets, potentially increasing the fold and signal-to-noise ratio of the stack.

Far-offset traces are subject to non-hyperbolic NMO, which may be handled by incorporating higher-order anisotropic terms. However, even if far-offset nonhyperbolic events can be flattened, they are likely to suffer from NMO stretch. This can result in a serious reduction in dominant frequency, and hence in vertical resolution. Several techniques have been published which apply NMO to P-wave data without introducing stretch. We have focused on extending one of these techniques through analysis of modelled and production data.

We have also extended the analysis to include convertedwave (PS) data, where NMO stretch can have even greater impact. For PS surveys, reflections on the near offsets have lower amplitude and are often swamped by noise. Therefore, most of the data contributing to the stack are from the mid to far offsets, particularly at the shallower coal scale. The dominant frequency of PS data can be significantly reduced by NMO stretch. This may be one of the factors that contribute to the poorer than expected resolution observed on some PS imagery.

Both the P and PS non-stretch NMO algorithms developed in this investigation successfully incorporate anisotropic parameters to accommodate non-hyperbolic NMO effects.

A new approach for the 1D inversion of AEM data has been developed. We use a reversible jump Markov Chain Monte Carlo method to perform Bayesian inference. The Earth is partitioned by a variable number of nonoverlapping cells defined by a 1D Voronoi tessellation. A cell is equivalent to a layer in conventional AEM inversion and has a corresponding conductivity value. The number and the position of the cells defining the geometry of the structure with depth, as well as their conductivities, are unknowns in the inversion.

The inversion is carried out with a fully non-linear parameter search method based on a transdimensional Markov chain. Many conductivity models, with variable numbers of layers, are generated via the Markov chain and information is extracted from the ensemble as a whole. The variability of the individual models in the ensemble represents the posterior distribution. Spatially averaging results is a form of ‘data-driven’ smoothing, without the need to impose a specific number of layers, an explicit smoothing function, or choose regularization parameters. The ensemble can also be examined to ascertain the most probable depths of the layer interfaces in the vertical structure.

The method is demonstrated with synthetic time-domain AEM data. The results show that an attractive feature of this method over conventional approaches is that rigorous information about the non-uniqueness and uncertainty of the solution is obtained. We also conclude that the method will also have utility for AEM system selection and investigation of calibration problems.

Data from a VTEM™ airborne electromagnetic survey over resistive terrain is examined. Forward modelling and analysis of high-altitude lines shows that the amplitudes of random noise, bucking error, processing corrections and geological signals can be large compared to the geological signal in the resistive terrain. The negative impacts of the low geological signal to noise ratio on conductivity estimates generated by layered-earth inversion and conductivity transformations are demonstrated. The reader is alerted to the degree of uncertainty and non-uniqueness that is inherent in conductivity estimates generated from similar datasets.

Airborne electromagnetic data (AEM) are used in many and diverse applications such as mineral and energy exploration, groundwater investigations, natural hazard assessment, agriculture, city planning and defence. Unfortunately, many users do not have access to a simple workflow for assessing the quality of the data that they are using. This poster outlines the main quality assurance and quality control (QA-QC) procedures used by Geoscience Australia (GA) for our 2008-11 AEM surveys.

Geoscience Australia is embarking on a project to upgrade the National Airborne Geophysical Database to better manage the data from major AEM surveys. This will better preserve the data and associated documentation to allow users to access and take advantage of the data well into the future. The quality of historical data included in this endeavour will unfortunately be variable and dependent on the QA-QC standards of the time.

Awareness and implementation of standardised QA-QC techniques ensure future generations of AEM data captured in the National Airborne Geophysical Database will be to a higher standard.

The Amplitude Variations with Offset (AVO) technique, and the related technique of pre-stack seismic inversion, has grown to include a multitude of sub-techniques, each with its own assumptions. This vast array of techniques makes it difficult for the interpreting geophysicist to understand how they are all related, and which method should be used in a particular exploration area. In this talk I will present a framework from which all of the current AVO and pre-stack seismic inversion methods can be understood. This will involve looking at the concept of seismic reflectivity in its various forms, as well as inverted reflectivity, or impedance. To illustrate the various AVO and pre-stack inversion techniques, I will use a gas sand example from central Alberta, Canada.

Magnetic field data are of fundamental importance in many areas of geophysical exploration with 3D voxel inversion being a common aid to their interpretation. In the majority of voxel based inversions it is assumed that the magnetic response arises entirely from magnetic induction. However, in the last decade, several studies have found that remanent magnetization is far more prevalent than previously thought. Our experience with numerous minerals exploration projects confirms that the presence of non-induced magnetization is the rule rather than the exception in base metals exploration.

In this work we show that failure to accommodate for remanent magnetization in 3D voxel-based inversion can lead to misleading interpretations. We present a technique we call Magnetization Vector Inversion (MVI), which incorporates both remanent and induced magnetization without prior knowledge of the direction or strength of remanent magnetization. We demonstrate our inversion using model studies and field data. Successful application to numerous minerals exploration surveys confirms that incorporating remanent magnetization is essential for the correct interpretation of magnetic field data.

Knowledge of the pressure dependencies of rock properties in unconsolidated sands is important for accurate time-lapse feasibility studies, pore pressure prediction, and reservoir characterization. A key problem that arises in determining such pressure dependencies is an accurate model at low effective stress. We propose a double exponential model to describe the pressure sensitivity of the bulk modulus (K) or shear modulus (G) for unconsolidated sands. The physical basis for our model relies on observed porosity-depth trends in unconsolidated sands, and the concept of critical porosity. Our new model matches laboratory measurements on unsaturated sand samples that have a range of grain size distributions and compositions. Grain size distribution data is first used to estimate critical porosity, which is then used as a zero effective pressure constraint in the data fitting process. We show that our new model more accurately predicts pressure sensitivity near zero-effective pressure compared to current methods, and is thus more accurate for situations in which core measurements at low effective stresses are not available.

A 2D Occam inversion algorithm for modeling ZTEM data is described that takes into account topography and EM receiver terrain clearance. The responses of hills and depressions are shown for synthetic models. These results show that hills and depressions produce responses that could be confused with the responses of conductors and resistors, respectively.

The analysis of ZTEM data acquired at different flightline directions at the Forrestania test site, WA, shows great consistency and repeatability of the ZTEM data. 2D inversion results indicate that the derivation of pseudo tipper profiles, perpendicular to the survey flightline direction, from across-line data contain valuable information, especially where the local strike is not perpendicular to the flight-line direction.

We have analysed CT scans of core plugs obtained from high permeability sandstones in the Wanneroo Sandstone member of the Leederville formation in the Perth Basin.

Plugs taken from drill core at representative sections of aquifer horizons have been scanned in a SkyScan CT scanner and the resulting greyscale image stacks analysed to estimate hydraulic transport parameters of the aquifer horizon. These parameters are compared with laboratory measured porosity and permeability values obtained from standard physical tests.

The analysis of the CT data provides support for understanding parameters derived from standard core plug analysis and wire line logging. However it also allows for a localised study of different zones within the core plug volume that is not possible with more ‘holistic’ laboratory measurement. Also, the mechanical framework of the grain and pore structure can be extracted as 3D geometric models for additional types of analysis and numerical modelling.

We estimate values for porosity and permeability for distinct zones within the core plugs and for the full width of the core plug. The full width values are compared with the equivalent laboratory values and for calibration. In addition, the possible impact of millimetre to centimetre zonation for grain size and shape distribution is considered with reference to anisotropy in larger scale physical measurements from wire-line logging.

Offshore exploration for hydrocarbons in increasingly challenging environments often requires more advanced acquisition methods than conventional 3D narrowazimuth towed streamer to better image the sub-surface for AVO analysis and reservoir characterization. Multi- Azimuth (MAZ), Wide-Azimuth (WAZ) or Full-Azimuth (FAZ) seismic acquisition overcomes the limitations of the conventional acquisition in better illuminating the sub-surface, suppressing the multiple and enhancing signal to noise (S/N) ratio. Nevertheless, to realize the added value of multi-azimuth data, the data need to be combined in a way that will overcome the issues such as time-shift and amplitude difference due to varied illumination between the surveys.

This paper describes a method that can be used for combining MAZ pre-stack data to generate AVOpreserving common image gathers (CIGs) in the presence of poor illumination. Based on the concept of crosscorrelation, MAZ CIGs are first flattened to account for any inaccuracy in the velocity model and imaging process so as to align the events to a pilot. Repeating the crosscorrelation process, weights are then derived from the correlation coefficients and applied to individual offsets that take into account the AVO behaviour. With this post-migration processing, any anomaly in AVO resulted from poor illumination can be mitigated. Applying it on MAZ post-stack data, the method can also provide optimal stacking for obtaining higher S/N images.

We demonstrate, through synthetic and real data examples, that clearer images with high AVO fidelity can be obtained from MAZ data using our optimal stacking method.

Gravity surveys have become an accepted part of the geophysical exploration process for hydrocarbons and minerals. Both scalar gravity and gravity gradient surveys are commonly conducted on land, sea, and in the air. The positive reception to these capabilities provides the backdrop for looking at other applications for emerging needs. Scenarios where gravity and gradiometry may have significant benefit include the detection and characterization of aquifers, monitoring of CO2 storage sites, geothermal exploration, and enhanced oil recovery monitoring. This paper will review modeling and analysis scenarios that evaluate the use of gravity in non-traditional applications.

Reservoir rocks are often subjected to anomalous vertical and/or horizontal stress conditions and may also contain complex physical attributes such as fracture sets. These rock properties are not easy to detect and map directly away from the borehole, but are sometimes indirectly evident in seismic data as azimuthal anisotropy. Thus, an analysis of anisotropic rock physics seismic attributes can be important for estimation of stress orientations and magnitudes, useful for reservoir evaluation, reducing drilling risks, and enhancing reservoir injection and production methods.

The Stybarrow field, located offshore NW Australia, is an example where seismic data exhibit strong azimuthal anisotropy, likely due to the effects of large horizontal tectonic stresses applied to the Carnarvon sedimentary basin. We find evidence for azimuthal anisotropy in AVO reflection amplitude difference maps and cross plots from two repeated 3D seismic surveys acquired at different azimuths, as well as in dipole shear logs and borehole breakout data. We model azimuthal AVO responses using Ruger’s horizontal transverse isotropy (HTI) AVO equation, using the anisotropy parameters derived from dipole shear logs, and compare the results with AVO gathers from the two 3D seismic surveys with different acquisition azimuths. We use a least squares method to find the coordinate rotation and scaling factor that optimally matches the real seismic data to the modelled data predicted by Ruger’s theory.

The Geoscience Australia Geomagnetism Program monitors the changing geomagnetic field using a network of nine geomagnetic observatories and fifteen repeat stations. The observatories collect calibrated vector data with 1-second sample interval and scalar data with 10- second samples which are sent to Geoscience Australia in near real-time. Among their many uses, these data have excellent application as base station data for magnetic surveys or remote reference data for magnetotelluric (MT) surveys, and for monitoring geomagnetically induced currents (GIC).

These data also contain the subtle signal of the slowly changing main magnetic field that originates in Earth’s outer core. An understanding of the scalar main field is essential for main-field removal from magnetic survey data sets. Geomagnetic inclination and declination are required for reduction to the pole techniques. Across Australia, the scalar field is 40% stronger in southern Australia than in the north, the angle of inclination is 50% steeper in the south than the north, and the declination angle varies from about -3 degrees in Western Australia to +15 degrees in Tasmania. Additionally, the rates of change of these field components also differ across the country.

This temporal and spatial dependence of the main field is represented in Australian and international geomagnetic reference field models. In Australia, both the Australian Geomagnetic Reference Field model (AGRF), produced by Geoscience Australia, and International Geomagnetic Reference Field model (IGRF), produced by an international group of modellers, are available to users. Each has its pros and cons and users may select the model most appropriate to their needs.

Detennining thè location of mining-induced seismic events is strongly dependent on having an acctirate velocity model. However, such a model is seldom available. This paper describes thè determination of a velocity model for seismic event location, using thè seismic events themselves as sources whose location is to be determined along witli tlie parameters of tlie velocity model (a simultaneous inversion of event locations and velocity struchire). Seismic monitoring of a mine in Colorado is used as an example, witli an array of geophones mstalled both on thè surface and in tuidergrotuid roadways. Velocity models of increasing complexity are considered, starting with a homogeneous velocity, movmg to a (slightly dipping) layered-earth model, and eventually hicludmg static time sliifts to accotuit for tlie effects of a weathered, near-surface, low- velocity layer on arrivai times at geophones motuited on tlie surface. This series of increasingly complex models obviously shows increasingly better fits to thè data, but also shows more plausible event locations, and with more realistic elevatimi spans. Examination of spadai patterns hi tlie residuals indicates tliat tliere are likely mining- induced changes in velocity tliat are not accotuited for hi tlie model.

The two-dimensional impedance method is used to calculate the electromagnetic surface impedance above subsurface structures at very low frequencies (VLF). The method was derived from Faraday’s and Ampere’s Laws and results in a single matrix equation where the right hand side corresponds to the source field introduced into the model as a fixed magnetic value. The left hand side corresponds to the impedance matrix determined by discretizing the solution space into pixels bounded by lumped impedance elements with values determined by the electromagnetic properties of the local media. Due to matrix sparsity and very large matrix dimensions, an iterative solver with a preconditioning technique was used to improve the speed, size and convergence of the solution. The improved method has been applied to the analysis of a coal seam with various structural anomalies and line of oxidation (LOX) along a line defined by 500m with 0.5m resolution. This paper reports a number of likely coal-seam scenarios relevant to surface mining operations.

In this paper we will first review the industry-standard continuous simultaneous inversion methods and point out some shortcomings. We will then introduce our new joint categorical/continuous simultaneous inversion technology, which reformulates the problem to address these issues. It casts the inversion as a Bayesian problem, which needs to be solved iteratively, as the inversion to categorical and continuous properties cannot be written down in closed form. We present some examples and conclude that the new inversion is able to overcome the shortcomings mentioned.

The problem of shallow, highly absorptive zones negatively affecting the quality of the seismic image at target depth is widespread across the Malay Basin. Whilst developments in processing regimes have lead to considerable improvements in the compensation of amplitudes below these areas of high absorption, the issue of improving the seismic image, and subsequently, our confidence in exploration decisions within affected fields remains a key challenge for operators in this region.

This study investigates the impact that acquisition geometry and design can have on the problem of imaging below shallow gas. Using advanced ray-based modeling techniques, we take a detailed look at the improvements in sub-gas amplitudes afforded by adopting nonconventional acquisition geometries; including, but not limited to, the large-offset (approx. 14 km), full-azimuth, dual coil shooting approach recently used in the Gulf of Mexico and broad-bandwidth acquisition techniques.

In addition to the illumination and amplitude characteristics associated with each design, we also investigate the improvements in resolution by simulating 3D prestack depth migration (PSDM) cubes at key areas of interest to try to understand the additional benefits afforded by adopting a non-conventional approach to acquisition.

Through the use of forward modeling, we provide a comprehensive insight into whether non-conventional acquisition techniques have a role to play in addressing the problem of imaging below zones of high absorption located in the overburden.

Recent advances in crosswell seismic technologies have lead to the successful development and release of a new downhole seismic source that imparts greater energy into the formation by sweeping frequencies in the range of 30-800 Hz. The result is, for the first time, a powerful, efficient and cost effective vibrator for use in the borehole allowing for enhanced seismic imaging between wells up to 2km apart.

In addition, the patented design and operation of the source allows for both compressional and shear wave energy to be acquired simultaneously along with the flexibility of being able to acquire data in reverse vertical seismic profile configuration.

The deterioration of ground such as rock delamination and fracturing can cause considerable safety concerns and result in significant interruption to production. There is a change in seismic resonant frequency when the strata are prone to collapse. A repeated seismic survey is an efficient way to diagnose the deterioration process and predict impending hazards. In this paper, a laser vibrometer method is proposed to sense rock condition in areas where normal seismic sensors cannot be deployed. A laboratory experiment using a laser vibrometer PDV-100 along with eleven geophones was carried out on a block of rock sample. The results have shown that the seismic waveforms obtained by non-contact laser vibrametry and the geophones are very consistent. Significant changes in seismic characteristics are evident when sensing across a rock surface with solid and hole sections behind it. It is also found that significant changes occur to the seismic signals when the holes of the rock sample are filled with different materials. The experiment has demonstrated great potential for laser vibrometer in sensing rock weakening process associated with vibration frequencies which is not resolvable using geophones.

Internal multiples due to a series of subsurface impedance contrasts are commonly observed in seismic data acquired in various places such as the Gippsland Basin of Australia and the Santos Basin of Brazil. The origin of these impedance contrasts can be due to coal seams as in the case of the Gippsland Basin or geological unconformities as in the Santos Basin. Regardless of how they are generated, internal multiple reflections often pose problems to the interpretation of geological structures. They are not easily handled by conventional de-multiple methods such as Radon Transform because internal multiples often have little difference in moveout than their corresponding primary events, or predictive deconvolution because the multiple generators, and hence the multiple period, are often not known.

In this paper, we present a case study of applying inverse scattering series (ISS) based method for attenuating internal multiples. The ISS method is a data-driven approach that can predict all internal multiples of a given order without any priori knowledge of the multiple generators. Moreover, it does not require any information about the subsurface velocity field. We discuss the application of this method for handling internal multiples in Santos Basin data, and present results on both the synthetic and field data from the Tupi oilfield. The results show that the internal multiples are well predicted and, with a suitably constrained subtraction technique, the migration artifacts caused by these multiples are greatly suppressed resulting in much clearer migration image for interpretation.

Work over the last decade on seismic azimuthal anisotropy has identified a link between fracture density and orientation observed by well logs and the intensity and orientation of the observed anisotropy. Recent work has correlated these measurements to provide quantitative estimates of fracture density from 3D wide-azimuth seismic data for tight gas sands. The work highlights the impact of advanced seismic processing in successfully recovering reliable fracture estimates which correlate well with borehole observations. These kind of areal, quantitative estimates of fracture density provide a valuable tool to guide drilling and completion programs in tight reservoirs.

Building upon this work and considering Coal Seam Gas plays in particular we need to consider some additional reservoir quality parameters whilst trying to impose the same quantitative approach on the interpretation of seismic data and correlation with borehole logging observations. The characterization of CSG plays involves the understanding of the reservoir matrix properties as well as the in-situ stresses and fracturing that will determine optimal producing zones.

Pre-stack seismic data and azimuthal WAZ (wide azimuth) seismic processing can help in the identification of sweet spots in CSG resource plays through detailed reservoir-oriented gather conditioning followed by prestack seismic inversion and multi-attribute analysis. This analysis provides rock property estimates such Poisson’s ratio, and Young’s modulus, amongst others. These properties are in turn related to quantitative reservoir properties such as porosity and brittleness. In this presentation we show an integrated approach based on pre-stack azimuthal seismic data analysis and well log information to identify sweet spots, estimate geomechanical properties and in situ principal stresses.

With the advent of high-channel count recording systems, one of the major hurdles for increasing spatial sampling density has been overcome. We are able to deploy dense receiver geometries with small group intervals and compact arrays or even point receivers. This allows us to record unaliased signal and noise and therefore do a much better job with noise attenuation during processing. We can then reap the full benefits of long offsets and wide azimuths for processing, imaging and reservoir. There is a need to match the increase in receiver density on the source side. To accomplish this in 3D land seismic we need a significant increase in source productivity while decreasing the source array size. Such productivity improvements can be created by spending less time per source point and by utilizing alternative source methodologies such as slip-sweep and blended acquisition.

The following challenge to deliver a clearer image and improved reservoir characterization is to emit and record broadband signals which offer better penetration and resolution. Our solution is to “performance-tune” the sweep to the vibrator’s mechanical and hydraulic limits. It extends bandwidth for a desired target output spectrum where low frequencies are enhanced, mid frequencies are unaffected and high frequencies retained or extended. In this presentation, we describe our successive technological leaps towards point source-point receiver seismic acquisition and illustrate it with recording and processing case studies from different regions of the world.

Standard techniques for inverting magnetic field data are marginalized when the susceptibility is high and when the magnetized bodies have considerable structure. A common example is a Banded Iron Formation where the causative body is highly elongated, folded, and has susceptibility greater than unity. In such cases the effects of self-demagnetization must be included in the inversion, which can be accomplished by working with the full Maxwell’s equations for magnetostatic fields. This problem has previously been addressed in the literature but there are still challenges with respect to obtaining a numerically robust and efficient inversion algorithm. In our paper we use a finite volume discretization of the equations and an adaptive octree mesh. The octree mesh greatly reduces the number of active cells compared to a regular mesh, which leads to a decrease of the storage requirement as well as a substantial speed up of the inversion. Synthetic and field examples are presented to illustrate the effectiveness of our method.

The Pedirka Basin of central Australia remains a frontier for petroleum exploration. Cost saving methods for exploration exists with publicly available high quality magnetics and regional gravity datasets. This data has been combined with available seismic sections and well logs to assist in gravity 2D section modelling and magnetic 3D geophysical inversions. The resulting magnetic susceptibility voxel when clipped to 0.012 SI units suggests a top depth to basement of ~5500m on the selected Simpson gravity modelled geology section.

The basement of the Pedirka basin region shows a significant heterogeneous nature in surface and well samples and in the modelled densities and magnetic susceptibilities. The heterogeneity is suggestive of Proterozoic granite systems having undergone convective fractionation. Thermal gradients point to a break down in magnetic minerals at depths greater than 15km. Outcropping of the Mereenie sandstone has been utilised to confirm the negligible influence of sedimentary units on magnetic signatures below 600m depth.

As part of the recently completed Onshore Energy Security Program, Geoscience Australia has delivered new types of pre-competitive data to as well as the more familiar government pre-competitive data types such as aeromagnetics. These new data provide industry with additional information for assessing the geological prospectivity of a region and making targeting decisions.

The visionary AWAGS continental airborne magnetic and radiometric survey enabled the first ever national merging of all radiometric surveys to produce the Radiometric Map of Australia. The rigorously calibrated data supports quantitative normalisation of uranium anomalies against rock-type, and hence identification of zones of uranium enrichment, at either a prospect or regional scale.

Geoscience Australia also undertook three large AEM surveys (>50,000km2) to map regional scale geological structures important for uranium deposition. These surveys were effective in imaging depth of regolith, geological unconformities, shale units and fault zones. Often these features are not apparent in regional magnetic or gravity data.

MT data are now acquired along all deep crustal seismic transects to image the deep conductivity structure. These data are highly complementary to the structural information interpreted from the seismic sections. MT can also detect conductivity features in the non-reflective crust below the Moho.

A program described by Sugeng [1998] which calculates the full domain 3D EM response of an earth modelled by hexahedral finite elements was extended to invert ground survey data using the damped eigenparameter method described by Jupp and Vozoff [1975].

The program is demonstrated using a small numerical model of a SIROTEM-like fixed-loop survey consisting of three surface traverses and a downhole line. Results suggest that provided data are not too noisy, a reasonable model can be recovered if the starting model is quite close to the true model. This suggests that the program might best be used to inverse-model particular features of interest rather than as a starting point for interpretation.

The Carrapateena iron-oxide copper-gold deposit is located 160km north of Port Augusta, within the eastern margin of the Gawler Craton, South Australia. The deposit was discovered, in 2005, by RMG Services Pty. Ltd. Subsequent exploration by joint venture partner, Teck Australia Pty. Ltd., demonstrated strong similarities with Olympic Dam, albeit at a much smaller scale. In April 2011, Oz Minerals purchased Carrapateena and soon afterwards released an inferred resource for the southern portion of the deposit of 203Mt @ 1.31% Cu, 0.56g/t Au, 270ppm U3O8 and 6g/t Ag.

Carrapateena lies beneath approximately 470m of moderately conductive Stuart Shelf sediments, presenting significant technical challenges to exploration. Therefore, geophysical surveys played an important role in both discovery of the deposit and subsequent delineation. Work undertaken by Teck comprised laboratory petrophysical measurements and gravity, aeromagnetic, IP/resistivity/MT, EM and down hole surveys.

Results show that Carrapateena lies on the south-western margin of a broad magnetic anomaly of moderate amplitude, being associated with a weak, discrete, ellipsoidal magnetic response, and near-coincident, weak, bullseye gravity high. The mineralised system is also associated with a distinct conductivity anomaly. However, chargeability data are less convincing, being strongly impacted by the thick and conductive nature of the cover sequences. EM surveys did not provide any responses attributable to bedrock conductors and are not recommended for this style of deposit. The observed geophysical responses at Carrapateena are dominated by the presence of Fe-oxides, particularly hematite, with sulphide mineralisation playing a lesser role.

Aquitards surround many aquifers. Due to their comparatively high storage capacity, they will control the hydraulics of aquifers in the long term. Long term management of aquifers therefore requires a good understanding of aquitard characteristics. To quantify water storage changes in an alluvial sequence a micro gravity station has been set up adjacent to an agricultural field in the Liverpool Plains in NSW, Australia. Gravity changes over time will provide an integrated signal of the total water storage changes at the site. Piezometers at the site will allow to determine at which depth the storage change is occurring. To derive water storage changes from the water level changes in the aquitards and confined aquifers at the site, the specific storage (Ss) of the formation needs to be known. Two insitu methods to derive Ss are applied. Firstly, Ss is derived based on the bulk modulus, which is obtained in a cross hole seismic survey and secondly Ss is derived based on pressure analysis in the lysimeters. Ss values derived from the pressure analysis in the lysimeters are one magnitude higher than those based on the cross hole seismic. This difference needs to be further investigated and might be due to the thick unsaturated zone (20 m) at the site, which appears to interfere with the pressure analysis in the lysimeters.

Seismic images of the earth’s interior can be significantly distorted by complex wave propagation effects arising from 3D structural velocity variations, combined with the presence of azimuthal velocity anisotropy within some of the rock layers. Most image processing techniques attempt to separate and compensate for both of these phenomena sequentially; they rarely address both simultaneously. These approaches implicitly assume that the effects of 3D structural velocity and azimuthal anisotropy are separable, when in fact both effects are coupled together in the seismic data. The presence of strong azimuthal velocity anisotropy can lead to significant errors in estimated seismic velocity and degraded quality of subsurface images, especially for large source-receiver offsets, wide azimuths and steep geologic dips. Such imaging errors can greatly increase the uncertainty associated with exploring, characterizing, developing and monitoring subsurface geologic features for hydrocarbons, geothermal energy, CO2 sequestration, and other important geophysical imaging applications. Our approach is to simultaneously address velocity structure and azimuthal anisotropy by development of an elliptic dip moveout (DMO) operator. We combine the structural-velocity insensitivity of isotropic DMO with elliptic moveout representative of azimuthal velocity anisotropy. Forward and adjoint elliptical DMO operators are then cascaded together to form a single elliptical moveout (EMO) operation, which has a skewed saddlelike impulse response that resembles an isotropic azimuthal moveout operator. The EMO operator can be used as a prestack data conditioner, to estimate azimuthal anisotropy in a domain that is relatively insensitive to 3D velocity structure, or to compensate and map the data back to its original prestack domain in its approximately equivalent isotropic wavefield form. We show that EMO can reduce structural dip image errors of 10-20º or more for realistic azimuthal anisotropy values at far offsets.

The Coorong is a shallow (typically 1 - 2 m) narrow coastal lagoon extending approximately 110 km parallel to the coastline, and forms an extensive wetland area of international significance. It is divided into two lagoons, the North and South lagoons. The northern lagoon section opens into the mouth of the Murray River and the southern lagoon section is closed. During periods of extended drought where there is no flooding to flush the lagoon system, hypersalinisation gradually increases, especially in the southern lagoon section where salinity may be in excess of four times that of seawater. A helicopter time-domain EM (TEM) system was flown along the Coorong, as extensive flood waters from Queensland (2010) were reaching the North Lagoon lowering the salinity. The derived bathymetry from TEM data was shown to be in good agreement with known bathymetry in areas of high salinity. The conductivity of the saline water in the North Lagoon and underlying sediment was estimated from inversion of TEM data using the known water depth as a fixed parameter. The derived conductivity varied from ~1.6 S/m in the north of the North Lagoon to ~8 - 9 S/m at its southern end, underestimating the gradient (~0.6 to ~13 S/m respectively) observed from a sparse distribution of fixed conductivity meters located in the Coorong. These results show that AEM has the potential to remotely map shallow water depths, and water conductivity gradients using known bathymetry to monitor hypersalinisation in these wetlands where changes in the ecology have been linked to high salinity.

Depth estimation procedures for potential field data are well recognised techniques. Both Euler and Werner methodologies are typically used as a series of automated steps and applied to both gridded and profile data. The Tilt Derivative Depth method works on gridded data and has been used extensively on magnetic data. Its advantage is its ability to produce a focused set of solutions and is now being commonly adopted for potential field data.

This paper describes an Adaptive Tilt Angle method for depth estimating Full Tensor Gravity data. The method is an adaptation of the Tilt Derivative depth estimation procedure adopted for magnetic data.

The procedure works on 4 of the independently measured Tensor components and produces sets of solutions that are more easily interpreted. The tilt angle method is defined as a ratio of the Tensor components in each of the X, Y and Z directions and assumes a vertical contact geological setting. The implementation of a scaling factor allows the technique to work on horizontal contacts. The scaling factor is essentially similar to the concept of a Structural Index as used with Euler depth estimation methods.

The technique was tested successfully on an Air-FTG® survey data set over a shallow salt feature onshore USA and is now being routinely deployed. The benefits of the direct depth estimation technique are immense in that it not only provides constraint on other interpretative processing techniques, but quickly establishes a starting depth model for any detailed forward/inverse modelling exercises.

New compilations of levelled marine and onshore gravity and magnetic data are facilitating structural and geological interpretations of the offshore northern Perth Basin. Multi-scale edge detection applied to these data helps the mapping of structural trends within the basin and complements interpretations based on seismic reflection data. Together with edge detection, magnetic source polygons determined from tilt angle aid in extrapolating exposed basement under sedimentary basins and, therefore, assist in the mapping of basement terranes. Three-dimensional gravity modelling of crustal structure indicates deeper Moho beneath the onshore and inboard parts of the Perth Basin and that crustal thinning is pronounced only under the outboard parts of the basin (Zeewcyk Sub-basin).

A common technique in seismic inversion for estimating near surface velocity model is Turning Ray Tomography (TRT) which would employ the available 2D seismic reflection data in conjunctions with set of mathematical equations to calculate the elastic constants and interpret the lithology of the subsurface of the earth. In addition, knowing the elastic constants is not only an important issue in civil engineering, but also in mining engineering. Thus, it is necessary to describe the mathematics of the medium’s elasticity, taking into consideration homogeneous and isotropic media. In this technique, the velocity model was computed from a 2D seismic reflection data. This resulted in the reconstruction of the velocity model accurately as the seismic waves travel. TRT result showed only a P-wave velocity model. However, this is insufficient to calculate the elastic constants and predict the lithology, which also require an S-wave velocity and a density. The S-wave velocity was calculated by using Castagna’s equation, while the density was computed by applying Gardner’s equation. Furthermore, the remaining elastic constants were calculated by using their relationships with P-wave velocity, S-wave velocity and the density. Consequently, the results showed that the nearsurface velocity model derived from TRT was capable of computing the elastic constants.

Seismic fracture prediction is becoming a more important exploration and development problem with the growing focus on unconventional reservoirs. A non-linear inversion technique is presented to estimate layer-based fracture parameters and velocities based on azimuthal reflectivity data. The earth model assumes a single set of vertical fractures per layer parameterized in terms of linear slip parameters - the normal and tangential fracture weaknesses - and fracture strike. In addition, the background P-wave and S-wave impedances are estimated. Either the exact Zoeppritz equation or some linearization thereof is used in a convolutional modelling scheme to estimate seismic amplitude data. The inverse problem is solved in a nonlinear fashion using simulated annealing.

The new technique has several advantages over performing azimuthal amplitude versus angle analysis (AVAz). The reflectivity calculation used in the new technique is more theoretically correct, allowing for the symmetry plane to change as a function of layer. The 90 degree ambiguity in estimating the symmetry plane typical of the near offset approximation also disappears. Further, there is an improvement in the isotropic parameter estimates compared to isotropic inversion since the bias introduced by neglecting anisotropy has been removed by incorporating it into the forward model. The azimuthal inversion is demonstrated on both synthetic and real seismic data.

As the number of near surface deposits decreases, it becomes increasingly important to develop geophysical techniques to image at depth. Because of the penetration advantage of plane wave natural sources, these techniques are ideal to answer questions about the deep subsurface to the earth. A ZTEM survey is an airborne electromagnetic survey which records the vertical magnetic field that result from natural sources. The data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground. While the airborne nature of the survey means that large survey areas can be surveyed quickly and economically, the high number of cells required to discretize the entire survey area at reasonable resolution can make the computational costs of inverting the entire data set all at once prohibitively expensive. Here we present a workflow methodology that can be used to invert large natural source surveys by decomposing the large inverse problem into smaller more manageable problems before combining the tiles into a final inversion result. We use the procedure to invert synthetic ZTEM data for the Noranda mining camp as well as a field data example. Both of these data sets were far too large to solve on a single grid even with multiple processors at our disposal.

Cosmic rays producing muons shower the Earth daily. These natural, high-energy particles decay as they pass through matter and are directly affected by density. Recently, sensors have been placed in existing tunnels and mine shafts that observe muon flux in a brown-field mining scenario. We have developed an algorithm to invert these data individually, or jointly with gravity data, to recover a 3D distribution of density. Muon and gravity data are both linear functionals of density but the associated sensitivity functions are substantially different. These differences in physics between muon ray paths and gravity data provide a unique insight into the subsurface. This is illustrated through synthetic examples. Inversion of a set of field data, obtained at a mine site in south-west British Columbia, Canada, illustrates the potential benefits and challenges for the technique to be used in field surveys.

In this paper we show that 3D inversion of large airborne time domain EM data, which is traditionally considered impractical, can be rapidly carried out by using a thoughtful workflow. In our 3D inversion algorithm, the number of cells in the mesh and the number of soundings are two factors that slow down the inversion. Therefore, we develop a strategy of adaptive mesh and sounding refinement to minimize the number of cells and the number of soundings required by the inversion. At the beginning, a coarse mesh and a few soundings are used to quickly build up a large-scale model. Then the mesh is refined and more soundings are added based upon their data misfit. At each iteration of the inversion, a certain number of soundings are randomly selected, and we change the data selection from iteration to iteration. This allows us to down-sample the field data without much loss of information. Once the large-scale model is obtained, we carry out some tile inversions that focus on smaller areas with a locally refined mesh to better resolve the small-scale features. The workflow is demonstrated by a synthetic example with 2121 transmitters that takes about 10 hours to be solved compared to about 150 hours if we had started the inversion on a fine mesh and used all of the transmitters. The methodology of speeding up the inversion by adaptive mesh and data refinement can also be applied to other EM surveys.

Magnetic field inversions are non-unique but a realistic goal is to find a causative earth structure that is compatible with the geophysical data, the petrophysical constraints, and with geology. Invariably the inversion results are improved as the number and diversity of constraints are increased. In this paper we concentrate upon the inclusion of geologic structural information. Geologic structural modelling programs can import faults, boundaries, and strike and dips of geologic units and interpolate this sparse information in space. When provided with a 3D voxel mesh, they can compute a strike, dip, and plunge for each cell. Following previous work, structural geologic information is incorporated into the inversion as a weak constraint by encapsulating it into the model objective function. The model objective function is formed such that each prism has its own set of rotated vectors to enforce smoothness along the direction of the geology. User-controlled parameters specify the degree of smoothness throughout the 3D volume and thus allow additional geologic insight to be directly incorporated. In addition to structural geology, the inversion algorithm utilizes reference models and bound constraints that help us realize our goal of incorporating all available information. The efficacy of the inversion is demonstrated through a synthetic and a field example.

The distribution of groundwater salinity is a key input for management of water resources. Estimates of the three dimensional distribution of groundwater salinity below areas spanning thousands of square kilometres may be required. Airborne transient electromagnetic methods provide the possibility of recovering first pass large scale solute concentration distributions provided lithological influences on electrical conductivity distribution are not dominant. The Allanooka airborne TEM survey is located in the northern most portion of the Perth Basin in Western Australia. We use data from this Airborne TEM survey combined with data recovered from monitoring wells to highlight the steps used to construct a first pass large scale solute distribution model. We provide a method for converting airborne TEM datasets to estimates of solute concentration distribution for sandstone dominated sediments at a basin scale. For the Allanooka monitoring well network, base line empirical relationships are developed between laboratory derived total dissolved solids and formation conductivity derived from wire line logs. This relationship is then extended to include airborne TEM derived formation conductivities. Appropriate layer discretisation of input seed models for inversion of the airborne TEM data set are based on analysis of resistivities derived from wire-line logs. The interpretation of the inverted airborne TEM was assisted by geological constraints and high resolution seismic reflection transects. Selected inversion statistics were also mapped throughout the 3D volume to provide a quick method for assessing the “importance” of particular layers to the outcome of the inversion.

An approximate volume of low solute concentration sandstone dominated formation below the regional water table was extracted from the airborne TEM data. Our first pass basin-scale Airborne TEM derived 3D solute concentration provides a starting point for more detailed interpretation to commence.

Elang is a large porphyry Cu-Au deposit situated about 70 km east of Batu Hijau on Sumbawa Island, Indonesia. The deposit is associated with a series of tonalite porphyry intrusions that are hosted by andesitic volcanics. Mineralisation is associated with potassic alteration (chlorite-magnetite±biotite) that produces a discrete magnetic high of about 700 nT. Forward and inverse modelling have been used to define this zone and the results have been used to direct drilling.

Very strong chargeabilities are associated with the porphyry alteration and resistivity from pole-dipole IP and airborne EM shows the extent of the alteration system. The Elang alteration system clearly shows up as a conductive zone in a relatively resistive background. There is an advanced argillic lithocap, up to 200 m thick, covering much of the deposit and is highly resistive and well defined with resistivity.

Elang is typical of a number of Cu-Au porphyry systems in that magnetite is associated with mineralisation and produces a strong discrete magnetic anomaly. It has a larger potassic zone than most systems, which may be due to more than one porphyry centre. The Elang system is more pyrite rich than many, leading to very strong chargeabilities, both in the ore zone and the pyrite halo. There is a broad resistive low due to clay alteration and sulphide veining similar to some other porphyry systems

Multiple geophysical data collected over the same area but based on fundamentally different physics usually contain complementary information about the subsurface. Joint inversion combines the complementary information by integrating all the geophysical data into a single inversion scheme. Thus, models resulting from joint inversion are more likely to represent the subsurface better than models derived from a single type of data. In this study, we consider joint inversion of seismic traveltimes and gravity data, and present a new joint inversion algorithm that uses petrophysical information as constraints. Using a synthetic example, we show that this new method can effectively build the available petrophysical information into inversion and improve the definition of both structure and physical properties. We also show that this method can deal with the situation where only partial petrophysical information about the subsurface is available. An important component of our method is applying fuzzy c-means (FCM) clustering algorithm to the recovered physical property distribution to generate a lithology map that is consistent with both the observed geophysical data and the a priori petrophysical information.

Traditional Bouguer corrections assume that the Earth behaves as a flat slab (Bouguer reduction), an assumption that is incorrect. A more accurate method of removing the effects of an Earth model is to take into account the terrain information, and assume a curved Earth. Terrain corrections are becoming more commonplace in individual surveys as computer power increases. We have produced a regional (state-wide) scale terraincorrected gravity grid, using a 1-second DEM to calculate the correction. The correction adds a subtle change to the state wide gravity, removing some regional lows. It can be used as a simple visual tool to determine where terrain-corrections are more appropriately undertaken.

Simultaneous (blended) sources have attracted a great deal of attention recently because of their potential to increase significantly the rate at which seismic data can be acquired. The viability of the method was previously demonstrated through the use of small-scale tests on synthetic and field data. In this paper, we present a case history from Australia of the first field-development-scale use of this technology in the world.

Concept studies involving simulations of simultaneoussource data from conventional data indicated that the proposed survey design would yield data that were separable into components for each source. The resultant data set contains twice as many traces as its conventional equivalent, and provides improved sampling for important processing steps such as coherent noise attenuation.

Simultaneous-source acquisition requires quality control methods that are specific to the technique to ensure that the data are acquired as planned. New QC methods were developed specifically for this project, and showed that no problems related to the simultaneous-source technique were encountered.

Data processing involved source separation at an early stage, after which a conventional processing sequence could be used on the resultant, densely-sampled data set. Separation was performed using a sparse inversion technique, which proved very effective. Very little signal leakage was observed, and the interference was almost completely suppressed.

Through this case history, we demonstrate the viability of simultaneous sources as an effective marine seismic acquisition method.

Geoscientific data interpretation is a highly subjective task as human intuition and biases play a significant role. Based on these interpretations, however, mining and petroleum industries make decisions with paramount financial implications. The aim of this study is to better understand variability in geophysical data interpretation between and within individuals. We examine the data observation pattern during interpretation using an eye tracker that captures the interpreter’s eye gaze motion. Two preliminary experiments were conducted to analyse how individuals approached the task of identifying prescribed ‘targets’ in magnetic and seismic data respectively. Each experiment used five participants who have varying degrees of experience in these tasks.

The first experiment involved identifying responses from porphyry-style intrusive systems in magnetic data of an area from Reko Diq, Pakistan. The target responses are sub-circular positive magnetic anomalies surrounded by annular lows. The second experiment was to spot unconformities and faults in a seismic image using data from the Mentelle Basin in Western Australia.

The results show a significant variation in data observation patterns between interpreters. Some key findings include: a direct correlation between a higher target spotting success rate and a more systematic data search pattern; significant inconsistency in target spotting results when viewing a data in a different orientation; and a significant variation in the amount of time spent on noise dominated region.

Automated image analysis techniques can be effectively used to detect discontinuities (e.g. faults, pinchouts, channels, etc.) within seismic data in a non-subjective manner. Conventional image processing techniques, such as the coherency cube, typically locate discontinuities by finding regions of sharp intensity shifts and are thereby sensitive to contrast variations and noise. Here, we present a phase-based technique that offers contrast-invariant and noise-robust feature characterisation through local phase and orientation information.

Phase congruency is an edge-detection algorithm that differs from traditional approaches by defining edges as points where the Fourier components of a signal are maximally in phase. Applying 2D phase congruency to horizontal time slices extracted from a 3D seismic volume is problematic, though, because horizons are rarely parallel to horizontal time slices, causing horizon boundaries to appear artificially discontinuous. To better detect 3D seismic discontinuities, we extend phase congruency to a 3D algorithm using conic spread filters that provides a localised, multi-scale and dip-independent feature detector.

Preliminary results show that 3D phase congruency is capable of detecting velocity anomalies, but has some limitations in identifying fault boundaries in seismic data. However, it can provide an increased level of feature detail over conventional coherency cube processing. More importantly, these results indicate the potential for using multidimensional phase-based algorithms in 3D/4D seismic processing and imaging workflows, with particular applications in image denoising, image registration, feature detection, and velocity model verification.

Demand for water in the Perth Metropolitan Area, Western Australia, is increasing and new water supply options need to be considered. Aquifer replenishment by injection through wells is seen as a part of the solution however before any large scale implementation of an injection well field is considered several trials are being completed. Time lapse induction and temperature logging have been completed as part of two aquifer replenishment trials in the Perth Metropolitan area. The intention of the time lapse logging is to detail the movement of water away from the injector well into the Leederville formation. A hydrothermal computer model constrained by time lapse wireline logging induction and temperature results has been created to understand the movement of water and heat during injection into the Wanneroo sandstone formation. As with most practical numerical modelling, a level of non-uniqueness in the model parameters selected will exist. It is demonstrated that the calibration to time lapse logging results provides an important constraint on the range of flow, solute transport and heat parameters that can be used to build a reasonable hydrothermal computer model. First, the flow and solute transport model is constrained with time lapse electrical conductivity distributions at the monitoring wells. Next, the model is expanded to include heat transport. Results of our modelling provide the first field scale estimates of heat parameters in the Leederville Aquifer in Perth.

We present a new survey methodology to map the distribution of chargeable material in the subsurface using inductive electromagnetic sources and observations of the magnetic fields in the frequency domain. An accompanying inversion algorithm is developed, and the technique is tested on synthetic data.

Statistical de-noising and compressive inversion methods based on Principal Component Analysis can reduce random noise, separate desired signals from correlated noise, and improve the efficiency and results of airborne EM inversions. However, inversion of PCA-processed data with standard kernels produces inaccurate results due to the improper forward mapping operators used. These inversions must incorporate the PCA rotation in the inversion process for accurate results. In order to appropriately apply these operators to the inversion kernels, the statistical distribution of the noise before and after processing and its effect on the data misfit must be understood. We can then develop compressive inversion techniques utilising PCA.

In this presentation, we demonstrate the need for incorporation of rotation into the inversion kernels through linear examples and show the utility of principal component analysis in compressive inversion. We then examine the statistical distribution of TEM data and noise and show that the noise follows a multivariate tdistribution both before and after processing with PCA. We conclude by introducing a compressive inversion technique formulated in the principal component domain.

We demonstrate a robust workflow for time-lapse gravity modeling in reservoir sequestration/production monitoring applications. This systematic approach outlines a reliable methodology to understanding the value and limitations of 4D gravity at a particular site, for both pre-acquisition decision making, and as a guide for post-data acquisition interpretation. To demonstrate, we present a multi-faceted feasibility study for monitoring CO2 injection into a reservoir at various injection times using 4D micro-gravity method. The simulations are performed for a currently active CO2-EOR site, the Louisiana Delhi Field in the United State. We construct an accurate representation of the field directly from current seismic data, followed by application of binary inversion technology adapted to the time-lapse gravity problem and tailored to the specific site. Finally, we illustrate a method of resolution analysis to demonstrate the decreased recoverability of fluid movement at the site in the presence of varying data noise.

Two helicopter TEM systems (HoistEM and RepTEM) were flown over waters in Backstairs Passage, South Australia, in 2003 and 2010 respectively to test the bathymetric accuracy and hence the ability to resolve seafloor structure in shallow and deeper waters (extending to ~40m depth) that contain interesting seafloor topography. The topography that forms a rock peak (South Page) in the form of a mini-mountain that barely rises above the water surface was accurately delineated along its ridge from the start of its base (where the seafloor is relatively flat) in ~ 30 m water depth to its quasi-submerged peak. A much smaller submerged peak (Threshold Bank) of ~ 9 m peak height located in waters of 35 to 40 m depth was also accurately delineated. These observations when checked against known water depths showed that the two airborne TEM systems were operating correctly. The third component of the survey was flown over a series of quasi-parallel seafloor ridges (resembling large sand waves rising up to ~ 20 m from the seafloor) that branch out and gradually decrease in height as the ridges spread out across the seafloor. These features provide an interesting topography because the interpreted water depths obtained from 1D inversion of TEM data highlight the effect of the EM footprint in resolving both the separation between the ridges and the height of individual ridges, and possibly also the limitations of assuming a 1D model in areas where the topography is quasi-2D.

A deep seismic reflection survey was carried out over the Meso-Neoarchean Youanmi Terrane, Yilgarn Craton in Western Australia, including 3 seismic lines across the Windimurra Igneous Complex. This gabbroic intrusion is the largest exposed single mafic-ultramafic intrusion in Australia, currently mined for magnetite-hosted vanadium in its upper zone. Acquisition was carried out with 3 Hemi 60 vibrators, 80 or 40 m VP interval, 40 m group interval, and a 300 channel symmetric split spread, resulting in 75 to 150 fold high quality data. Key processing steps included refraction and residual automatic statics, spectral equalisation, detailed stacking velocity analysis, dip moveout (DMO) correction and post stack Kirchhoff migration. The Windimurra Igneous Complex exhibits high seismic velocity (approximately 6.5 km/s), both in interval velocity from stacking velocity and bedrock velocity from refraction statics analysis. Seismic attenuation is low, resulting in high frequency and good resolution to the base of the complex, seen at a maximum two way time of around 2.5 seconds. Gross structure on the seismic sections is also consistent with the gravity and magnetic data. An inward dipping conical structure is evident and consistent with surface geological observations. The base of the complex is marked by distinctive high amplitude multi-layered reflectors, which may represent significant thicknesses of ultramafic zone material unexposed at the surface. This zone has been a long sought after target for Cr-PGE exploration. The complex internal structure of the intrusion, possibly including discordant igneous layering features, has been well imaged up to the base of regolith.

A new numerical approach, called the “sub-domain Chebyshev spectral method”, has been developed to calculate differentiations in a curved coordinate system, which may be employed for 2D/3D geophysical forward modelling. The new method utilises non-linear transformations defined by the free-surface topography and subsurface interfaces and incorporates cubic-spline interpolations to convert the global domain into subdomains, and applies Chebyshev points in the model discretisation and computation of the spatial derivatives. Such effort makes the numerical differentiations have “spectral accuracy” inside the subdomains whose boundaries match the free-surface topography and subsurface interfaces.

2D and 3D synthetic experiments have been performed with two geological models, both having different free-surface topographies and sub-surface interfaces. The computational errors of the new approach were compared with traditional finite-difference schemes, and the results show that the sub-domain Chebyshev spectral method is superior to traditional finite-difference method in its accuracy and applicable for all of the geophysical forward modelling problems.

Most seismoelectric surveys to date have been acquired on a small scale in temperate regions. Our objective was to establish if seismoelectric data could be acquired on a large scale in an arid environment.

In April 2011, we acquired over 21,000 traces of 2D seismoelectric data at an arid site in Abu Dhabi, United Arab Emirates. The test also included seismic measurements made using WesternGeco’s UniQ singlesensor acquisition system. The source used for the acquisition was an 80,000 lb tracked Desert Explorer vibrator, the largest hydraulic Vibroseis source ever used for seismoelectric acquisition. This large source was used to attempt to overcome the low signal to noise issues inherent in seismoelectric acquisition that have been exacerbated in the past by the use of low energy sources.

We successfully acquired high quality data with coseismic signal present to the limits of our acquisition (420 m offset and 2 s record length). Our current equipment is, however, ill-suited to rapid deployment, having far too many components.

The acquisition of large seismoelectric datasets, such as that described here, enables the data to be viewed in the common receiver domain enhancing data processing and bad trace identification.

Time-lapse analysis of seismic data acquired at different stages of hydrocarbon production or fluid/gas injection has been very successful at capturing detailed reservoir changes (e.g., pressure, saturation, fluid flow). Conventional 4D analysis is performed in the time domain assuming a constant baseline model; however, this procedure becomes difficult when the subsurface is significantly altered by production/injection and large time anomalies and complex 4D coda are recorded. We argue that a more robust 4D analysis procedure in these situations requires iterative wave-equation depth imaging and time-lapse velocity analysis.

Wave-equation depth migration requires accurate knowledge of the velocity field usually obtained by one of two ways. First, data-space methods are where recorded data are matched to those calculated through a background velocity model. Differences between the two datasets are used in tomographic backprojections to generate velocity model updates. Alternatively, imagespace methods are where discrepancies between migrated images (non-flat gathers) are backprojected to estimate velocity model updates. These types of approaches are termed migration velocity analysis (MVA).

This abstract focuses on extending 3D wave-equation MVA (WEMVA) approaches to time-lapse velocity analysis. We discuss the differences between 3D and 4D WEMVA inversion goals, and how we leverage the locality of 4D image perturbations to provide highresolution velocity model updates. We demonstrate the utility of 4D WEMVA analysis in a synthetic CO2 geosequestration experiment by successfully inverting for a velocity perturbation corresponding to a thin layer (<20m) of injected CO2 in a typical North Sea reservoir.

Check-shot survey measures the first arrival time with a known depth receiver in borehole to assess formation velocity. This information can be used in correlation with sonic log and surface seismic products for adjustment of interpretation. Check-shot survey can also be implemented with seismic-while-drilling using drill bit noise as the source. This differs from usual check-shot survey as source is in the borehole. It provides a real time, cost saving, and safe measurement.

Check-shot survey needs a known receiver depth, thus velocity can be obtained by fixed wave travel path and the measured first arrival time. However, in seismicwhile-drilling (SWD), drill bit position can vary a lot from vertical drilling to deviated drilling. To address this issue, we present a method that finds the location of the source and estimates the velocity of the formation at the same time. Using a synthetic model, with medium receiver offsets, this method shows good estimation of the drill bit depth location and formation velocity in a layered Earth model.

The West Musgrave Province preserves a geological history spanning much of the Proterozoic, including two major Grenville-aged orogenic events, ca. 1345-1120 Ma. These were followed by the intraplate Giles Event, ca. 1080-1050 Ma, which is characterised by 1- the voluminous mafic intrusions of the Giles Complex and 2-the deposition of a thick sequence of bimodal volcanic rocks and sedimentary rocks (the Bentley Supergroup). The architecture resulting from these events was subsequently overprinted by later events, the ca. 600-520 Ma Petermann Orogeny, and the ca. 450-350 Ma Alice Springs Orogeny. Here we use aeromagnetic, gravity and magnetotelluric data, constrained by geological mapping and petrophysical data, to characterise the 3D architecture of the region, and to unscramble its structural evolution. Early deformation events are well preserved in places, although they do not permit a robust interpretation of the regional architecture at that time. The architecture of the Giles Event is extremely well preserved, and a complex polyphase history is conserved. A series of discrete to semi-continuous deformation events is recorded, with a dominant ESE-WNW trend supplemented by additional NE-SW and N-S trends, each of which was active at several times during the event. Later events are manifested primarily as reactivations of earlier structures. Using this architectural framework along with a variety of other spatial datasets (geology, geochemistry etc) GIS-based prospectivity analysis was applied using a mineral systems approach, targeting Ni, Cu, PGEs and orogenic gold. The conceptual method used compares spatial distributions of various targeting criteria, represented by predictor maps. Each predictor map is then related to metal source, fluid pathways, and chemical and physical trap zones. The output prospectivity maps are used as decision-support tools for exploration and reinforce the idea that the area is highly prospective for magmatic nickel-copper and PGE’s deposits.

In 2011, several Bowen Basin coal exploration projects in central Queensland were surveyed with the SkyTEM airborne electromagnetic system. The aim of these surveys was to trial the effectiveness of the method in for coal applications, with specific focus on mapping the key aspects of the weathering, overburden and hydrology that are most important for effective development of a coal project.

The first survey, which was over an area near Moranbah with 150m of Tertiary overburden with extensive basalt flows, mapped crucial hydrological and geological features important to efficient further development of the project: These included a) a convoluted network of basalt-filled palaeochannels, some of which extend through and into the coal measures b) location, extent and thickness of major saline aquifers, c) depth to base of Tertiary.

Subsequent surveys over different parts of the Bowen basin, showed that SkyTEM was a very effective tool for mapping the thickness of weathering, a parameter very important for drilling out coal resources. The results are surprising in that they not only provide a detailed map of depth of weathering (including volumes of lateritic sediments requiring excavation), but also map more subtle geological features such as synclines (previously only loosely inferred from surface mapping), major faults, previously unknown dykes, dip and strike of the main stratigraphic units, fresh water aquifers and even possibly also various coal seams (generally because they are saline aquifers).

These surveys represent, at least to the authors’ knowledge, the first example of modern helicopter-borne transient EM in the Australian coal fields. The results from this project and subsequent projects are surprising in that they have had unexpected applications above and beyond those which were originally forecast. Thus, with informed and thorough use, airborne EM promises to deliver valuable geological and geotechnical information to aid in the further development of Australia’s coal resources.

The Telfer region in the Paterson Province consists of Proterozoic metasediments of the Yeneena Group, of lower-upper Greenschist facies. Sediment lithologies vary from siltstones to quartzites and carbonates. These are intruded by intrusive suites ranging from felsic to mafic, plutonic to hyperbyssal.

Known world-class assets include the Telfer Gold operation, and the O’Callaghan’s Tungsten resource (both 100% Newcrest owned).

The sedimentary and intrusive lithologies have considerable density contrast. These factors, combined with steeply dipping on-edge stratigraphy make the Telfer region ideal for gravity surveying.

Previous gravity coverage in the region has been a combination of small prospect scale surveys, and 1980s vintage 2 x 4 km regional helicopter surveys with barometric levelling control.

In order to get systematic gravity coverage of good resolution, a FALCON survey of 250m line spacing, 60m flight height, was flown in 2010.

The data show a number of blind felsic plutons, some intersected in drilling, some interpreted – especially associated with the O’Callaghan’s tungsten resource, and the Trotman’s Stockwork tungsten prospect. The data also outline much more clearly the NE over SW thrust faults in the region, and the late north-south transfer faults, the most well known of which is the Graben Fault through Telfer Dome.

Three dimensional gravity inversions performed over the O’Callaghan’s and Trotman’s Stockwork tungsten areas in Gocad/VPMG software have defined extra felsic intrusive targets for tungsten and gold mineralisation. Further diamond drilling will increase inversion constraints.

A JORC inferred resource of 1.4 billion tonnes at a magnetite Davis Tube Recovered Grade of 15.5% was defined at The Hawsons Iron Project near Broken Hill in western New South Wales making it the largest magnetite iron ore resource identified to date in eastern Australia.

Reconnaissance surface exploration in 2009 identified the area’s potential for magnetite iron ore fifty years after it was last considered an iron ore play prior to the discovery of Mt Tom Price.

Neoproterozoic Braemar Ironstone Facies sediments host the magnetite mineralisation which generates aeromagnetic anomaly amplitudes greater than 6000 nT. Five target areas for magnetite iron ore were identified from the open-file aeromagnetics.

A realistic Exploration Target was estimated from selective 2.5D magnetic modelling and image processing of the airborne aeromagnetic data. The inferred resource was established within 18 months of commencing the exploration program drilling 72 holes into the five aeromagnetic targets.

The monotonous sedimentary host sequence of siltstones and sandstones with few marker beds makes detailed geological interpretation extremely difficult. A magnetic stratigraphy was constructed from recognizable patterns in wireline magnetic susceptibility traces and correlations of the mineralisation could then be established down dip and across strike.

It is often desirable to extract meaningful lithologic information directly from geophysical data. Here, we describe a multi-faceted approach that combines the known geologic sections of a site from borehole data with recovered density and magnetic susceptibility distributions from 3D inversion of airborne gravity gradient and magnetic data. The technique can produce end-member solutions based on average property values extracted from literature and well records; or, it can be implemented using the geologic sections to extract an appropriate range of property values to address the model smoothing common to modern 3D generalized inversions. To demonstrate our approach, we present results utilizing magnetic and gravity gradient data over part of the Gandarela Syncline iron formation in the Quadrilátero Ferrífero, Brazil.

Knowledge of the depth of cover is poor across large areas of Australia. The spectral method is an efficient method of producing reliable depth to magnetic basement estimates across large regions of the continent.

A semi-automated work-flow has been created that enables the generation of depth to magnetic source estimates from windowed magnetic data using the Spector and Grant method. The work-flow allows for the correction of the power spectra, prior to the picking of straight-line segments, to account for the fractal distribution of magnetic sources. The fractal parameter (β) varies with depth and was determined by picking multiple depth estimates in regions of outcropping magnetic basement which have been upward continued to different levels in order to simulate different amounts of burial beneath non-magnetic sediments. A power law function best approximates the decay of β with depth.

An iterative schema used to determine the optimum β where the depths of magnetic sources are unknown, has been incorporated into the workflow. Preliminary testing in a region of known magnetic basement depth has produced encouraging results, although further testing is required. The decrease of β with increasing depth suggests that the fractal distribution of magnetisation becomes less correlated, or fractal, over larger volumes of observation.

The effect of water and rock loading on seismic velocities and consequently on interpreted geometries is often underestimated in offshore studies. Direct comparative analysis of interval velocity patterns between areas of significantly different water depth and thickness of rock overburden requires various pressure related changes in velocity to be accounted for. Presentation of velocity models as a function of pressure rather than two-way time, or depth, emerges as a possible solution. An accurate velocity model is essential for meaningful timeto-depth conversion of interpreted seismic horizons. Ideally, it should be based on integration of seismic velocities from well log measurements, refraction seismic surveys and from stacking of multi-channel marine reflection data. In some cases velocities derived from stacking of high quality long streamer marine reflection seismic data correlate reasonably with well log measurements and velocities derived from refraction seismic studies, and provide clues to reasonable depth conversion and lithology interpretation.

Symmetric acquisition geometry consisting of identical sampling of shots and receivers, can maintain the spatial continuity of the wavefield automatically, according to symmetric sampling theory. However, asymmetric geometry is often adopted in practical seismic exploration applications. Such geometry can cause uneven sampling and is necessary to be assessed for its sampling performance prior to acquisition. In conventional survey design, based on the common mid-point (CMP) analysis for a horizontally layered earth or common reflection point (CRP) analysis for a complex subsurface structure, the quality of acquisition geometry is generally judged by such bin properties as effective fold, offset scalar and azimuth distributions. However, these conventional approaches are limited by an incomplete understanding of the offset-vector sampling. Therefore, we propose a new method for quantitatively evaluating the continuity of offset-vector sampling including four spatial coordinates of shot and receiver. On the basis of physical potential energy and force-balance principle, it analyzes the regularity coefficient of offset-vector sampling as a whole using potential function model and takes into account fold, offset-scalar and azimuth distribution factors. The combination of regularity coefficients of every bin can produce spatial continuity distribution of offset-vector sampling. Similar to symmetric sampling, this approach emphasize the spatial relationships between adjacent bins rather than single bin attribute, since it aims to maintain the spatial continuity of the wavefield which allows the faithful reconstruction of the underlying continuous wavefield. Using this method, we can quantitatively compare spatial continuity distribution for different seismic acquisition geometries, and then choose the better acquisition scheme.

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16 m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18 m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.

The first stage of a major port expansion project called for an approach to rapidly and cost effectively screen a broad area to assist with determining the most appropriate location for additional berths, turning pockets and associated channels. A combination of marine geophysical techniques were used to achieve this goal without the need for drilling which, although essential for detailed assessment and planning of the final location, would have resulted in significant delays and extra costs at this stage.

Seismic refraction and continuous seismic profiling (CSP) were used together to characterise the upper 20 m below the seafloor. Sidescan sonar, swath bathymetry and magnetometry were used to check for possible seafloor hazards.

The results showed a region with a deeper sequence of low velocity material and a high seismic velocity ridge. Since the high velocity materials represent a significant impediment to dredging the surveys highlighted the areas more and less suitable for use in the expansion.

In these surveys no unknown seafloor hazards were identified.

The results at the site demonstrate the applicability of marine geophysical surveys to efficiently provide an indication of subsurface conditions over a broad area. This ability makes them a valuable tool for port development investigations.

This paper critically evaluates the utility of seismic data to assist in the interpretation of bedrock. Four highresolution seismic reflection profiles were carried out to provide estimates of the depth to the bedrock and to detect any geological faults present in the area, which may affect the hydro-geological system at Wadi Al Dawasir region, 690 km south of Riyadh city. The bedrock reflection is clearly present in all sites. The depth of the bedrock at site 1 was approximately 750 m, 800 m at site 2, 700 m at site 3, and 950 m at site 4. The bedrock depth obtained from the seismic agreed with the well information. We developed a clear understanding of the bedrock in the study area and mapped its depth as well as mapped some of the clear fault locations.

This abstract will detail Quantec’s 3D system, a proprietary 3D DC/IP and MT acquisition developed by Quantec Geoscience Ltd., Canada by means of a case study over the Hidden Hill Deposit in central Nevada, USA.

An integrated data acquisition, modelling and interpretation of the DC, IP, and MT surveys over a complex geological setting embedding a known epithermal gold and silver-bearing deposit are presented. The survey and interpretation are based on a true 3D acquisition and inversion of the geophysical data.

The results show a very close correlation between the geophysical resistivity and chargeability signatures and the location of the known mineralization. The extremely large data volume collected in an omni-directional current source proved to be essential for a detailed and reliable imaging of the subsurface.

In the last decade converted-wave (PS-wave) seismic reflection studies have successfully demonstrated that a more complete geological interpretation can be obtained by integrated interpretation of P-wave and S-wave information, at both the petroleum and coal scales. Full 3D implementation of PS reflection presents particular challenges at the coal scale, because relative offsets and dominant frequencies are both large compared to petroleum-scale reflection.

One of the most difficult steps in the PS processing sequence is estimation of the S-wave receiver statics. Statics are time delays caused by variations in the weathering layer, and changes in source and receiver elevation. These time errors can significantly degrade CMP stack quality, and the final geological interpretation of seismic images. Static errors tend to be much more significant in PS surveys since the S wave travels more slowly through the weathering layer and is therefore more likely to be affected by variations within this layer.

In this presentation we evaluate of a number of different approaches for estimating 3D PS statics solutions. These include a surface-consistent inversion algorithm (analogous to the residual-statics method used in conventional P-wave processing), a so-called 'robuststatistical' method, and PPS refraction analysis. The methods are evaluated using a coal-scale 3D-3C survey acquired in the Bowen Basin. This has been used to examine the comparative performance, and the influence of various algorithmic and geological factors.

The results indicate that the surface-consistent inversion method can fail under some weathering conditions. When this occurs the refraction based method or a robust statistical method are preferred.

3D P-wave seismic surveys can exhibit significant azimuthal variation in stacking velocities, and failure to allow for such variations can introduce smearing into the stacked volume. The problem is likely to be worse in the case of converted-wave (PS) reflection. Firstly, S-waves have lower velocities such that time variations are amplified. Secondly, PS rays are asymmetrical, such that anomalous features may be traversed by different wavetypes (P or S), depending on the direction of travel.

Recently, a coal-scale 3D-PS trial was recorded in the Bowen Basin with the aim of providing a detailed investigation of such azimuthal variation. The survey was designed with extremely high fold (>500). This allowed good quality images to be constructed for data subsets having restricted ray azimuths. Target structures interpreted using different ray-azimuths exhibit significant timing variations (up to 30ms). The observed azimuthal variations may not necessarily indicate true azimuthal anisotropy. They can result from poor statics solutions, and PS imagery is notorious for difficult statics.

On the other hand, the observed variations may be indicative of true geological anisotropy. Based on shearwave splitting models, our PS velocity variations have been modelled in terms of elliptical variation with azimuth, and this approach predicts the orientation of the horizontal stress field. It is interesting that a majority of our data zones indicate a consistent stress orientation. Furthermore, the interpreted horizontal-stress orientation is consistent with observed reverse faulting in the area.

Over the past decades, airborne electromagnetic (AEM) surveys have mostly been used in connection with mineral exploration and a variety of issues in hydrogeophysical mapping. However, increasingly, AEM is used for a wide range of geotechnical purposes: pollution mapping, geotechnical assessment on road and freeway alignments, bathymetry, and depth to bedrock.

We present an investigation using a helicopterborne transient electromagnetic system along the planned trace of a gas pipeline. Oil and gas pipelines are often buried at a depth of a few meters and the cost of construction depends critically on whether the subsurface is composed of soft sediments that can be easily excavated or hard rock formations that require much heavier equipment and possibly have to be blasted. The aim of the AEM survey was to distinguish between the soft, relatively conductive sediments and the hard, relatively resistive bedrock in the upper few meters of the subsurface.

Data were collected with a rather small transmitter moment, but a high repetition frequency that simultaneously allowed high acquisition speed, and reliable data quality. Measurements were inverted with 1D models with both vertical and lateral constraints to produce model sections along flight lines. A novel method of statistical analysis of the set of equivalent models for each inverted model, calibrated against boreholes, improved the estimates of the presence of hard rock along the flight lines.

The amplitude of a seismic refraction event is determined by the properties of rocks through which the seismic waves travel, the amplitude of the shot and the offset at which the refraction is recorded. A surface-consistent, non-linear inversion scheme, which uses the Levenberg- Marquardt algorithm, has been developed which aims to extract near-surface rock properties from the measured refraction amplitudes.

Perhaps the most important challenge to extracting a unique, geologically plausible solution is in determining initial values of control parameters which dictate how the solution is allowed to progress. If these control parameters are not tailored specifically to the problem in question, convergence to a solution can be very slow and even fail to get off the ground in some cases.

Comparison between results obtained using default control parameters, compared to those obtained using control parameters specifically tailored to the problem, shows a reduction in the error between the true observations and the model-generated observations, while retaining fidelity in the solution. A reformulation of the problem significantly reduces the error but fidelity is apparently compromised. Comments are made on how to achieve an optimal middle ground.

In this paper, I propose the algorithm that the location of a magnetic dipole can be detected from the magnetic gradient tensor. I derive the location vector of a vertically magnetizated dipole from magnetic gradient tensor. Deficit of magnetic moment of magnetic dipole makes the induced location information incomplete. However if the observation of magnetic gradient tensor would be collected on one more points, the algorithm is able to point the location of magnetic dipole by clustering the solution of the proposed method. For example, I show that magnetic gradient tensor can be converted as the source location successively by picking common solution area in synthetic case of borehole observation.

The goal of geophysical inversion of electromagnetic (EM) data is to recover a model of the geoelectrical properties of the sub-surface. The standard practice for 1D inversion of marine controlled source EM (CSEM) data is to generate smooth conductivity models using least squares (L2-norm) methods. However, sedimentary geology is stratified and piece-wise continuous. As such, smooth resistivity models cannot represent this character. In response to this inconsistency, a means was sought to generate more geologically plausible, piece-wise continuous models.

The common approach in the literature when generating piece-wise continuous inversion models is to regularize L2-norm methods in such a manner as to induce blocky behaviour. Although effective, these techniques are selfconflictive; forcing non-smooth behaviour from an implicitly smooth algorithm. In contrast, L1-norm inversion inherently produces piece-wise continuous models. To investigate the possible utility of this approach, a L1-norm inversion algorithm has been developed and tested on synthetic and real datasets. The L1-norm results were compared with those generated using an industry standard L2-norm algorithm.

The synthetic inversions focused on previously published examples. The real data inversions focused on electric and magnetic field measurements recorded over the main reservoir sand of the Pluto gas field in block WA350-P, North West Shelf, WA.

The L1-norm inversions recovered, to within the resolution limits of the CSEM method, the depth, thickness and resistivity of the synthetic geological models and the Pluto-1 resistivity well log, whilst fitting the input data to within noise. When compared against the L2-norm profiles, the L1-norm inversion more closely represented the stratified character of the sedimentary sequence. It was therefore concluded that L1-norm inversion is an attractive alternative to smooth L2-norm methods when blocky inversion models are desired.