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.