ASEG Extended Abstracts - ASEG2004 - 17th Geophysical Conference, 2004

High levels of ambient noise and safety factors often limit the use of "active-source" seismic methods for geotechnical investigations in urban environments. A recent field test in Melbourne demonstrates the ability of the microtremor method using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However this layer of sediments is resolved by the microtremor method and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records.

Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrates that estimates of shear velocities and layer thicknesses are accurate to within approximately 10% to 20% using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity-thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.

Modelling earthquake hazard and risk involves the incorporation of source, attenuation, site response, building damage and financial loss models. Each of these input models has its own uncertainties. Moreover, in most cases there is more than one possible choice for each of the input models. For example, many regions of the world have several attenuation models that can be used in studies of earthquake hazard and risk.

This paper demonstrates the need to incorporate multiple attenuation models when modelling earthquake hazard and risk by illustrating the effect of using three different attenuation models. Results indicate that varying the attenuation model can almost double the earthquake hazard and triple the earthquake risk estimates in the Newcastle and Lake Macquarie region

Seismoelectric methods are based upon physical properties of the earth that produce electrical signals from seismic waves. The electrokinetic sounding (EKS) is one such method that has great potential for hydrogeological studies as it arises from the movement of pore fluids under seismic excitation. In theory, the method should be able to directly map changes in hydraulic permeability, rock porosity, or fluid-chemistry. A number of researchers have recently tried to exploit the phenomenon in groundwater problems where the conventional methods worked poorly. However, publications of successful case histories to support the theory are rare. This may be blamed upon the very weak amplitudes of the electrical signals generated from the seismic wave which are milivolts to nanovolts in magnitude, and the presence of cultural noise which is usually much greater in magnitude.

We demonstrate electrokinetic responses from formations more than 50 metres deep in two test areas in Western Australia. One is over a saline paleochannel and the other over a freshwater aquifer. The data was generated from a sledgehammer source and recorded by a seismic acquisition system. Seismic refraction and reflection data provide seismic velocity information for depth conversion and support the seismoelectric data. The signals were then compared to borehole logs to find what physical contrasts were detected. Significant hydrogeological boundaries were detected up to 50 m deep in saline groundwater conditions, and at least 80 m deep in freshwater aquifers. In addition, we examine some pitfalls in the method and our approach to overcoming these problems.

This paper is concerned with numerical considerations of fluid effects on wave propagation. The focus is on effective elastic properties (i.e. velocities) in different kinds of dry and fluid-saturated fractured media. We apply the so-called rotated staggered finite-difference grid (RSG) technique. Using this modified grid it is possible to simulate the propagation of elastic waves in a 2D or 3D medium containing cracks, pores or free surfaces without explicit boundary conditions and without averaging elastic moduli. Therefore the RSG allows an efficient and precise numerical study of effective velocities in fractured structures. This is also true for structures where theoretically it is only possible to predict upper and lower bounds.

We simulate the propagation of plane P- and S-waves through three kinds of randomly cracked 3D media. Each model realization differs in the porosity of the medium and is performed for dry and fluid-saturated pores. The synthetic results are compared with the predictions of the well known Gassmann equation and the Biot velocity relations. Although we have a very low porosity in our models, the numerical calculations showed that the Gassmann equation cannot be applied for isolated pores (thin penny-shaped cracks). For Fontainebleau sandstone we observe with our dynamic finite-difference approach the exact same elastic properties as with a static finite-element approach. For this case the Gassmann equation can be checked successfully. Additionally, we show that so-called open-cell Gaussian random field models are an useful tool to study wave propagation in fluid-saturated fractured media. For all synthetic models considered in this study the high-frequency limit of the Biot velocity relations is very close to the predictions of the Gassmann equation. However, using synthetic rock models saturated with artificial "heavy" water we can roughly estimate the corresponding tortuosity parameter.

Geo-environmental hazard from near-surface cavities collapsed into sinkholes is alarmingly increasing in Northeastern Thailand. Detailed subsurface studies in the concerned areas are required for identification of sinkhole-prone areas, which may relieve fear of sinkhole hazard and help prevent possible and significant damage from the hazard. Thus, we examined the usefulness and capability of 2-D resistivity imaging technique for mapping and monitoring of surface cavities. We conducted 2-D resistivity survey at a man-made square tunnel, a known subsurface geology, and the nearby sinkhole areas. Synthetic resistivity data of various electrode configurations, including Wenner, Dipole-Dipole, Schlumberger, Pole-Pole, and Pole-Dipole were generated from the model similar to a known geology of the tunnel. We acquired a resistivity survey across the tunnel in a similar way to the synthetic data. Results from the field data show that the Dipole-Dipole and Wenner configurations provide better tunnel imaging. The tunnel appears as a lateral anomaly in a homogenous medium. An anomalous zone of the tunnel is distinguishable by the zone with high resistivity, surrounded by the lower background resistivity. However, location of the tunnel anomaly appears to be misplaced on the field resistivity pseudosection. It is different from the synthetic resistivity pseudosection. This suggests a strong 3-D effect from nearby structures. Results from the nearby sinkhole areas show that a Dipole-Dipole configuration provides a better result for cavity imaging than the Wenner configuration. An anomalous zone of the cavity is distinguished by very low and lowest resistivity zone surrounded by the higher background resistivity. These two results suggest that the 2-D resistivity surveys provide a reasonable basis for mapping a subsurface cavity. In addition, the 2-D resistivity survey was found to be useful for the monitoring of cavity progressive expansion, from similar resistivity survey was acquired at the same location when conducted at different times.

Data processing methods originally developed for the TEMPEST system allow GEOTEM half-sine data to be deconvolved and transformed to GEOTEM square-wave data. The advantages of the transformed square-wave data are that they refer to a standardised waveform that does not vary through a survey. The high-frequency information contained in the data recorded during the transmitter pulse can be readily utilised and the data can be easily corrected for variations in the transmitter loop height, pitch, roll and receiver coil offset. Modelling results from transformed GEOTEM data acquired across the Bull Creek mineralisation indicate that the transformation works well for survey data.

Traditional off-time, single component conductivity - depth modelling of GEOTEM data can be improved by utilizing the full waveform and by inverting multicomponent datasets. In highly conductive terrain, such as above seawater, where system parameters such as the bird position are hard to derive reliably from the time - domain in-phase component as a proxy for the primary field, the joint inversion of multicomponent data helps to correctly resolve layered-earth parameters. Jointly inverting the 3-component on- and off-time data of a GEOTEM bathymetry survey in the Torres Strait showed that the data fit can be greatly improved by allowing the inversion to determine the receiver offset and attitude. This results in greater confidence in the derived conductivity - depth values.

This paper describes a research program at the Australian Resources Research Centre to establish and use an analog model of a turbidite channel reservoir to investigate issues with reservoir simulations and time-lapse seismic monitoring of these complex fields. The project goal is to gain a better understanding of issues relating to uncertainty in reservoir simulations of channelised fields and their seismic expression. The results demonstrate the potential of using carefully scaled analog models to reproduce realistic reservoir phenomena in a controlled laboratory environment for integrated reservoir engineering and time-lapse seismic research.

Two types of filters have been developed for the purpose of enhancing weak magnetic anomalies from near-surface sources while simultaneously enhancing low-amplitude, long-wavelength magnetic anomalies from deep-seated or regional sources. The Edge filter group highlights edges surrounding both shallow and deeper magnetic sources. The results are used to infer the location of the boundaries of magnetised lithologies. The Block filter group has the effect of transforming the data into "zones" which, similar to image classification systems, segregate anomalous zones into apparent lithological categories. Both filter groups change the textural character of a dataset and thereby facilitate interpretation of geological structures.

The effect of each filter is demonstrated using theoretical model studies. The models include both shallow and deep sources with a range of magnetisations. Comparative studies are made with traditional filters using the same theoretical models. In order to simulate real conditions, Gaussian noise has been added to the model response. Techniques for noise reduction and geological signature enhancement are discussed in the paper.

The new approaches are applied to actual magnetic survey data covering part of the Goulburn 1:100 000 scale map sheet area, New South Wales. Some new geological inferences revealed by this process are discussed

Effective stress is a key concept in determining reservoir pore pressures from seismic velocity data. It is usually defined as the difference, ΔP between confining (overburden) pressure, Pc and pore pressure, Pp. However , a more general definition is σ´r = PC - nPp where n is an effective stress coefficient. Seismic velocity is observed both in the field and in laboratory experiments to vary non-linearly with effective stress. Furthermore, a form of hysteresis in velocity is observed when reservoir rocks are subjected to differing pore pressure histories. The resulting hysteresis can be quantified with the effective stress coefficient. This coefficient is frequently derived from laboratory velocity-effective stress data at ultrasonic frequencies. This paper describes some recent laboratory based, rock physics experiments involving pore pressure history induced velocity hysteresis and the implications for reservoir 4D monitoring.

In the Buffalo Head Hills diamond province of Alberta, Canada, kimberlites have intruded a thick sequence of sedimentary rock units with thickness in excess of 300-500 m and resistivities of 5-10 ohm-m. In contrast to the conductive sedimentary rock the kimberlite intrusion is invariably characterised by a much higher electrical resistivities.

These "resistive" kimberlites can be detected by both airborne and ground time domain EM methods. Because of the host sedimentary rock‖s very low resistivity, the resistive kimberlite response in many cases appears counterintuitive to the expectations based on the simpler analysis of the problem which ignores the EM interactions between a 3D body and a conductive host. For similar reasons, successful detection of kimberlites in the Buffalo Head Hills province also required developments of algorithms which facilitate correction of airborne TDEM data for variations in aircraft altitude and pitch. "Anomaly hunting" analysis which decompose the spatial and temporal characteristics of the EM response into a number of components and innovations in the inversion and transformations of ground TDEM data sets was also required. Application of standard Conductivity-Depth-Image (CDI) techniques was not appropriate to the solution to the problem. A new pseudo ID-inversion algorithm was developed to partially assist in the assessment of the data.

A number of areas previously covered by detailed airborne magnetic surveys were re-surveyed with the Geotem airborne TDEM system. Use of the developed concepts and insights has resulted in a number of new diamonds-bearing non-magnetic kimberlite discoveries. New discoveries, for example, include kimberlites K296 (with a surface area of some 500 by 500 m), and K252 (with an estimated mini-bulk sample diamond content of 55.0 cpht), the highest estimated diamond content of all Alberta kimberlites to date.

The Neoproterozoic Officer Basin in Western Australia is an inland frontier area that covers about 300 000 km² in Western Australia and 225 000 km² in South Australia. The present-day structural-stratigraphic framework is dominated by salt deformation and associated features. In the central western Officer Basin, approximately 6500 km of 2D seismic data were recently reinterpreted and integrated with potential field regional datasets. This has led to improved understanding of the halokinetic evolution and the petroleum potential of the region, summarised herein.

Compressional processes, associated with tectonism in the adjacent Paterson Orogen, are probably the key mechanism initiating multi-phase mobilisation of a thick halite-dominated sequence, the Browne Formation, low in the succession. Significant salt redistribution resulted in thickness variations of the Browne Formation and the overlying section, and a variety of halokinetic structures.

Prospective structures caused by salt tectonics include drape folds with a potential for multiple pay, thrust-related anticlinal features, combined traps at diapir flanks and enhanced porosity traps. Most of these traps were in place before the main phases of hydrocarbon generation and could form attractive petroleum exploration targets.

Over the last five years the use of airborne lidar technology as a bathymetric survey tool has increased substantially. The systems are well suited to surveying in clear, coastal water and surveys have been performed for a variety of reasons, including: updating nautical charts, supporting coastal and reef management, and oil and gas exploration and development. Seamlessly and safely surveying shallow, complex and drying areas is possible with an airborne platform and because the swath width of the systems are independent of water depth shallow areas are surveyed with no loss of efficiency. Because airborne lidar systems are able to efficiently operate in areas where acoustic, vessel mounted, systems suffer significant inefficiencies a new segment of the bathymetric survey market has been defined; contract surveys for airborne lidar systems.

In Australia the two users of airborne bathymetric lidar technology have been the Royal Australian Navy and the offshore Oil and Gas Exploration community. Since 1998 several companies involved in offshore exploration including the explorers themselves as well as seismic exploration companies have commissioned airborne bathymetric lidar surveys. Bathymetric surveys have been completed for a variety of purposes; supporting seismic acquisition in poorly charted and dangerous waters, allowing efficient route planning for pipeline construction in complex and environmentally sensitive areas and provided bathymetric and topographic data to allow evaluation of alternative pipeline landfall locations. In each case the ability of airborne lidar systems to efficiently and safely survey large and remote areas when acoustic systems either could not operate or could not meet tight schedules was important to the end user of the data.

We address evaluation of resistivity of a target zone of a C02-sequestration site using a simple measurement made from inside metal-cased boreholes. An approximate analytic expression is derived for the admittance measured between two metal casings in a stratified formation. It is shown that in a highly conductive environment measurement electrodes should be positioned inside the target depth range. Charts and plots are produced for evaluation of the admittance as function of the inter-casing separation and gas saturation in the target layer. The results indicate the possibility of monitoring the target zone using metal-cased borehole separated by distances of several hundreds meters..

The effect of the gravitational attraction of isostatic roots on the Bouguer gravity data for New South Wales is significant. The anomalies caused by isostatic compensation are most pronounced in the eastern third of the state. This is due to the load of the coastal mountain range being compensated, combined with the effect of the crustal thinning at the continental margin. To use the gravity data to interpret regional geological structure it is advisable to remove these large-amplitude long-wavelength anomalies as they tend to obscure the anomalies caused by the geological structures of interest. A variety of methods is available to perform this. The method used here is a relatively simple method that assumes isostatic equilibrium and zero crustal rigidity.

Seamless access and efficient management of vast quantities of resource data is a significant and increasing challenge for both government and industry. CSIRO and Schlumberger embarked on a joint a study of geoscience data exchange between industry and government, based on their complementary domain of expertise and networks of national agencies and industry.

It is clear that rapidly increasing volumes of data; the need for faster, more-efficient business practices; low-price data storage and higher Internet bandwidth will change the way that companies and governments do business. To explore current perceptions and future directions for geoscience data management and data exchange, interviews were conducted with 19 petroleum companies, 9 minerals companies, 11 government groups, 4 software providers and 2 CRCs.

Perceptions in data exchange vary widely between industry and government. However, some common themes emerge: few groups have the resources to properly manage, curate and add-value to data. Much time is spent (and wasted) in moving data around. Companies generally ‘minimally comply’ with regulations for data submission as they see little advantage in putting any more time into data submission than is required by legislation. Finally, data management is usually seen as a financial and administrative burden rather than an enabler to add value.

The vision promoted by most people visited can be summarised as the immediate need for a "Single interface that delivers all validated spatial geoscientific data independent of database structure and location". The need for a multi-agency collaborative virtual databases is clear, and the way forward is as much controlled by policy issues as with technological enablers.

Dryland salinity, a growing problem over much of Australia, can only be mapped and predicted with a thorough understanding of the landscape in three dimensions and the hydrological processes that operate within it. Hydrology is the key for understanding how salt stores are mobilised through the Earth, both in a vertical and horizontal sense.

Mapping techniques have important roles in delineating soils, landforms, water flow and pathways through subsurface topography, as well as detecting or inferring the presence of salt itself. Satellite and airborne remote sensing techniques are useful in delineating existing surface and near-surface salt, and tracking changes over time. Airborne geophysical techniques, combined with ground and borehole control, are important tools in understanding salinity and hydrology at depth, and essential tools for predicting future changes in salinity.

Salt is a hazard when it has the potential to be moved, usually by water, to a location where it can threaten an asset, which may include agriculture, infrastructure, water quality and biodiversity. Salinity risk is a measure of the chance that the salt hazard will cause harm to the asset at some time in the future. Cost-benefit analyses in salinity management should take into consideration total cost and total benefit in context with the value of the asset.

The optimum strategy for salinity hazard and risk mapping depends on the scale (farm, community or catchment) and resources available to the user. The best approach makes use of existing information and then integrates a range of the available mapping methods in such a way to best address the specific question at hand. No one method has primacy, nor is there a ‘magic bullet’ for salinity mapping or prediction. Effective use of mapping methods requires expert knowledge or access to trained personnel.

We present a deterministic methodology for mapping the lithology, pore fluid, and porosity from seismic data. The input is the P- and S-wave data volumes that may come, e.g., from acoustic and elastic impedance inversion or cross-well measurements. The output is the pore fluid type (hydrocarbon versus water), total porosity, and clay content. The key element of this methodology is a site-specific rock physics model that provides the needed transforms from the elastic rock properties to the reservoir properties. The model is established by comparing model-based predictions, such as impedance versus porosity, to the relations present in well log data. Once selected, the model is used to identify the presence of hydrocarbons from a combination of the P-wave impedance and Poisson’s ratio. Then the P-wave impedance is used to map porosity and clay content assuming that a deterministic relation exists between the latter two properties. All deterministic equations are calibrated at a well. These equations subsequently are applied to up-scaled well log data to confirm their validity at the seismic scale.

The use of radiometrics as a regional geological mapping tool is widespread. Ternary, single band, ratio images, profile sections and data fusion techniques were applied to airborne gamma-ray spectrometry data of the Musgrave Province in South Australia. Chemical variations, previously unrecognised were used as a basis for subdividing and mapping key lithologies of Birksgate Giles Complex, and the Kulgera Suite Granites.

Variations in emissivity were found to occur in conjunction with one or more of the following;

• Key tectonic structures
• In some cases differing magnetic susceptibility
• Within distinct tectonic domains

Due to limited geological mapping little is known about the Musgrave Province however, the Giles Complex is considered as a unit of economic significance for nickel sulphides and PGE, so the need to map its extent becomes vital. Groundtruthing results is required to determine causes of emissivity variations which may be due to alteration associated with hydrothermal fluids or mineralogical/lithological variations within geological units.

In September 2002 a medium resolution seismic reflection survey was undertaken at the St Ives gold field, Western Australia. The data were acquired by the Australian National Seismic Imaging Resource (ANSIR) using two Hemi60 vibrators and an ARAM24 acquisition system. The data were processed, and correlated with drillhole geology and drillhole geophysics to facilitate interpretation.

The seismic sections demonstrate that seismic reflection is effective at imaging stratigraphy and structure in Archaean terrains, even at relatively shallow depths (less than 500m). A package of strong reflections is associated with the mafic stratigraphy at St Ives. A complex pattern of reflectors at the Victory Mine is correlated with a fault network interpreted from drilling. Results indicate that seismic reflection could be used for the discovery of new gold orebodies in this mature exploration province.