ASEG Extended Abstracts - ASEG2007 - 19th Geophysical Conference, 2007

I use a simple rock physics model, based on linear combinations of the properties of unaltered and altered rocks, to predict the physical properties of altered rocks at the St Ives Gold Mine, Western Australia. Previous studies of mineral systems demonstrate that alteration can produce physical property contrasts with respect to unaltered host rocks and thus produce geophysical signatures in various datasets such as gravity, magnetics and seismic.

When viewed on a scatter plot, the majority of samples at St Ives plot within a limited field, representing the properties of unaltered host rocks. Samples plotting outside this field are inferred to be altered. As host rocks have a restricted range of physical properties, there are a range of paths (alteration trajectories) which altered samples can follow on bivariate plots. These trajectories define a cone shaped field, the alteration cone. The open end of the cone encompasses the expected physical properties of unaltered samples, and the focus of the cone lies on the physical properties of the alteration assemblage. Samples plotting inside an alteration cone are inferred to result from alteration of a host lithology and contain some proportion of that alteration assemblage. The distance a sample occurs along the cone is proportional to the amount of alteration the sample has undergone.

This model accounts for the physical properties of samples which are known to be altered, by comparison with HyLogger core logging results and the St Ives drillhole database. This model can also be used to predict alteration within other datasets, e.g. gravity-magnetic inversion results.

Technical Area: Case Studies in Western Australia

The Paterson Airborne Electromagnetic (AEM) survey is a major component of the Northern Western Australia (WA) regional project in the Onshore Energy Security Program (OESP) at Geoscience Australia. During 2007 Geoscience Australia, through Fugro Airborne Surveys, will acquire a TEMPEST survey with broad line spacings (up to 2 km) in the Paterson Province, WA. The area to be covered comprises sections of the Yarrie, Nullagine, Paterson Range, Balfour Downs, Rudall, Tabletop, Gunanya and Runton 1:250 000 map sheets. The survey results will help to improve our understanding of the area’s geology and mineral potential by mapping the conductivities of different geological and hydrogeological units under cover. The dataset will contribute to interpretations regarding the presence of graphitic units in the Rudall Complex; the location of major structures; and the extent of Permian palaeovalleys and other regolith features. This poster outlines the survey specifications and objectives, and describes some of the geophysical modelling and processing methods being developed by Geoscience Australia.

Technical Area: AEM, Minerals, Energy.

In June 2007 a ground radiometric survey utilising quad bikes, was conducted over the Radium Hill townsite, former minesite area and processing plant. An eight litre crystal pack was used along with an Exploranium GR-320 gamma ray spectrometer set to continuous (streaming) mode. A total of 148.8 line kilometres of radiometric data was collected over NS lines spaced at 100m intervals. Average ground speeds of ten kilometres per hour combined with the large crystal pack enabled good quality, highly detailed data to be collected on the ground over rough terrains. Data was continuously sampled at one second intervals and the entire survey was conducted in 10 days including mobilisation and demobilisation. The terrain was not suitable for large vehicles due to the danger of below surface mine adits, hence, the only alternative to surveying on quad bikes would be to survey on foot. It is estimated both that to get the same quality of data over this size survey area would require two operators 60 days, and also the use of such a large crystal pack would result in detailed data comparable with that derived from airborne surveys.

Technical Area: Mine site environmental and engineering geophysics

Denoising aerial gamma-ray surveying makes possible the extraction of previously hidden detail. Conventional methods for denoising spectral data make strong assumptions about the levels and type of noise which reduces their efficiency. The proposed methodology cast the problem as manifold learning followed by non-linear regression. Non-linear dimensionality reduction (NLDR) is employed to compute the underlying structure of the data. By calculating the intrinsic dimensionality of the spectra, the algorithm selects dimensions that are more representative of the data while eliminating dimensions with noise. The most representative dimensions are employed to learn a mixture of linear models through Expectation Maximization. Non-linear regression is then performed using these mixtures to recover the denoised spectra from the low dimensional representation. Thus, the model makes no assumptions about the level and type of noise.

Tests performed with a synthetic survey demonstrate that data denoised with NLDR show much clearer detail in images involving uranium but only slight improvements for K and Th channel data. This has been confirmed with real surveys where subtle features involving U has been found using NLDR denoising. The NLDR method offers particular advantages in the search for uranium where combinations such as U*U/Th can be used to highlight areas of coincident high U and high U/Th ratios if cleaned data is available.

A combined heli-borne ground gravity acquisition program of approximately 17,000 gravity stations was acquired over the Northern G2 region within South Australia, as part of The Plan for Accelerating Exploration (PACE) program. The resolution of the gravity data was dramatically enhanced from the existing gravity coverage with a station spacing of approximately 7 kilometres to a 1.5 kilometre grid within the PACE survey. There have been several discoveries in the south of this region, leading to this area becoming a focus for data acquisition. Acquisition of geophysical data, especially gravity on a regional scale is a key strategy of the PACE program. It is expected that the new gravity data will provide exploration companies with many geophysical targets ready to drill.

Technical Area: Geophysics in Government Surveys

Recently there have been suggestions that changes in the earth’s magnetic field may influence climate on millennial, centennial and decadal scales. Continuous wavelet transforms are used to compare time series of geomagnetic and climatic variables.

Continuous wavelet transforms resolve a time series into its time and frequency components, meaning temporal variation in signal power at a particular frequency can be detected. This allows both continuous and intermittent periodic correlations to be found.

Continuous geomagnetic field monitoring in Australia began in 1840 at the Rossbank observatory. Since then the observatory has been re-located to Melbourne, Toolangi and finally Canberra, with all data corrected to the Canberra observatory reference. In this study monthly mean geomagnetic data for the period 1949-2006 were compared using wavelet transforms with temperature and rainfall data recorded at Canberra Airport and cloud data from Sydney Airport. Sunspot number data were also compared.

Wavelet analysis shows peaks in geomagnetic power at annual, 9-14 year and 45 year periods. Temperature, rainfall and cloudiness results show high power annually, at 2-5 years and at 10-15 years. Cloud and magnetic data both show a spike in annual power at 1990. Sunspot power is dominant at 11 years.

Annual variations are seasonal, while 9-15 year variations appear to be related to the 11 year sunspot cycle. The link between magnetic field variations and sunspot activity is well established. Current research suggests that solar activity may also affect climate, perhaps modulated by the Earth’s magnetic field. Other correlations are not currently explained and do not necessarily imply causality. Wavelet analysis has shown potential for further research into geomagnetic-climatic relationships.

Time-lapse (4D) seismic quantitative interpretation is based mainly on measurements of how saturation and pressure changes affect seismic velocities. Effect of saturation can be modeled using Gassmann equations. Pressure effect is usually obtained by laboratory measurements, which can be affected by core damage. In order to assess the adequacy of the core measurements to the properties of the intact reservoir rocks, it is necessary to compare them to in situ measurements. We present a study to assess the adequacy of ultrasonic measurements on core samples by comparing measured ultrasonic velocities at reservoir pressures with sonic log data from two wells located in an oil field in Campos Basin, offshore Brazil. The analysis is performed for these densely cored wells: more than 50 samples were extracted from a turbidite reservoir. We use Gassmann fluid substitution to obtain lowfrequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. Comparisons of these computed velocities with sonic logs measurements show very good agreement. This confirms that for those particular regions the effect of core damage on ultrasonic measurements is below the measurement error. Consequently stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.

Technical Area: Time-lapse seismic

A spherical wave AVO response is investigated by measuring ultrasonic reflection amplitudes from a water/Plexiglas interface. The experimental results show substantial deviation from the plane-wave reflection coefficients at large angles. However there is an excellent agreement between experimental data and full-wave numerical simulations performed with the reflectivity algorithm. By comparing the spherical-wave AVO response, modelled with different frequencies, to the plane-wave response, we show that the differences between the two are of such magnitude that three-term AVO inversion based on the AVA curvature can be erroneous. We then propose an alternative approach to use critical angle information extracted from AVA curves, and show that this leads to a significant improvement of the estimation of elastic parameters. Azimuthal variation of the AVO response of a vertically fractured model also shows good agreement with anisotropic reflectivity simulations, especially in terms of extracted critical angles.

Technical Area: AVO

As part of the New Frontiers exploration initiative, a new series of geophysical–geological interpretation of 1:250 000 map sheet areas is being completed. Interpretation of the Cobham Lake and Milparinka map sheets in the far northwest of NSW has been completed and highlights and extends the following major domains:

• Proterozoic units of the eastern Curnamona Province are present in the west of Cobham Lake sheet where rock types exposed in the Broken Hill area extend northward under relatively thin cover.

• The Bancannia Trough (thick sequences of Cambrian to Devonian age) and Quinyambie Trough (Cretaceous sediments) share a northwesterly trend.

• The Koonenberry Belt comprises a complex structural zone where Neoproterozoic to Cambro-Ordovician units have been affected by a series of orogenic events.

• Intrusions of Silurian age (such as the Tibooburra Granodiorite), possible Cambro- Ordovician age, and Permian diatremes are interpreted.

• The contact zone is drawn between the Koonenberry Belt and the western extent of the Thomson Orogen.

Aeromagnetic data (1VD and TMI imagery) were used to create a framework plot of anomaly sources, assisting recognition of discontinuities, fractures, and textural domains. Previous interpretations, gravity data, mapping, drilling, seismic, and regional studies were integrated to allocate appropriate stratigraphy. Accompanying reports document aspects such as unit characteristics, major structures, magnetic susceptibility data, anomalous features, mineralisation, and depth to basement information. The Cobham Lake and Milparinka map sheets both have extensive cover of Cretaceous, Tertiary and Quaternary sediments and sedimentary rocks but offer exploration targets at shallow depths.

Technical Area: Geophysics in Government Surveys

Cross-dipole borehole acoustic logging has been used for identifying vertical or deviated fractures, evaluating formation stresses, and monitoring hydraulic fracturing effects. It also can be used for predicting horizontal transverse isotropy of a formation that apparently generates shear wave separations. This characteristic is used for identifying formation anisotropy distributions near the boreholes, so that formation stress and fracture in a real formation can be evaluated. However, in practice, because of complicated logging environments, such as formation borehole condition variations, and some times there existing abnormal formations, it has a limited applications. Therefore, a comprehensive procedure should be used for detailed data interpretation, with the help of the other logging data.

In this work, firstly, a 3-dimensional staggered finitedifference method is used to simulate borehole acoustic wave propagation excited by a cross-dipole source in a pre-stressed formation. The wave equations of motion are based on nonlinear acoustoelastic theory. The effects of formation pre-stress on borehole flexural modes are discussed. Then, based on the numerical modelling results, the cross-dipole acoustic logging data in 400 wells from the Daqing oilfield in China are used for fracture identifications in deep formations with complex lithology. Also, the metamorphic formations outside of the main area in Daqing oilfield are studied. Hydraulic fracturing efficiency, formation stress distributions in a damaged casing area are investigated together with other well logging. Finally, the results are validated indirectly with other local formation stress detections, deformed casing analysis, and water injection fracturing data.

The 1 km² studied area is located in Sukchun-ri, Hwasung-koon, the southern part of kyeonggi province of Korea. In order to determine the extent of seawater contamination and a preferred channel of the seawater intrusion, DC resistivity and TEM surveys were performed. According to the resistivity map obtained from geophysical surveys, the study area is divided into two districts as relatively low (less than 30 ohm-m) and high (more than 30 ohm) areas. The distribution of the low resistive area is consistent with the distribution of the layer of composed pf clay minerals, and the resistivity of the clay miner layer decreases slowly as approaching to the old seashore. Hydrogeological analysis shows that the clay layer within a distance of about 200 m from the seashore has been already contaminated by sea water, and its electric conductivity is 8 times higher than that of the sand layer covered by the clay layer. According to the results of the 2-dimentional DC resistivity surveys with a dipole-dipole array, there are two preferred channels of seawater intrusions in the site, and both channels are in NW-SE direction from the old seashore. The DC resistivity and TEM monitorings were carried out along the preferred channel which has low resistivity zone extended to a depth of 80 m. The time series measured by those two methods fluctuates with a period of 12 hours. These observations show that the sea water intrusion caused by tidal action is still in progress along the preferred channel interpreted by the geophysical surveys.

Technical Area: Environmental

Proper terrain correction in airborne and seaborne gravity gradiometry surveys requires detailed digital elevation models (DEMs). However, due to standard low-pass filter techniques applied during acquisition, the required resolution of the DEMs is altered in commercial surveys.

We quantify the ideal resolution for an example dataset with a new and practical method for performing terrain corrections based on Parker's 1972 Fourier domain calculation. In the same vein, we quantify the required spatial extents of DEMs for nominal cases. With these parameters quantified, we can optimize the terrain correction to improve efficiency in both gravity gradiometry forward modeling as well as terrain corrections.

In an ideal case, we find that the required spatial extent of the DEM is roughly nine times the greatest relief in the terrain outside the survey area. For the specific case, we find that 20 meter resolution is suitable.

There is potential for a new oil province in Australia’s expansive and geological diverse EEZ with multiple oil fields having a value to the nation measured in $billion. To screen this large area requires appropriate technologies such as direct detection of leaking hydrocarbons or inferring the nature of petroleum systems from dredge samples or old exploratory wells. A common risk is that hydrocarbon charge and petroleum systems are not proven. Inclusions of oil in sedimentary grains detect migratory oil and can be used to deduce key parameters of petroleum systems that are critical for predicting the fluid type.

Fluorescence spectra of oil inclusions broadly correlate with API gravity. Spectra with computed CIE chromaticity coordinates in the blue part of the visible spectrum are associated with about 42 degrees API gravity oil such as from the Jabiru oil field. Those in the yellow part of the visible spectrum, such as from the Cliff Head oil field, are associated with about 30 degrees API gravity oil.

Seismic data from the Bremer Basin, in which no exploratory wells have been drilled, indicate reservoirs in anticlines and fault blocks with potential for hundreds of millions of barrels, but the risk is that hydrocarbon charge is not proven. Samples dredged from incised canyons contain oil inclusions in low abundance indicating migratory oil. Samples from the Bremer Basin with blue fluorescence and high GOR indicate a paraffinic or mature source.

An important petroleum system in the onshore Perth Basin has a source in the basal Kockatea Shale, but there is uncertainty about its extent in the offshore basin. Calibration of the fluorescence attributes of the oil at the Cliff Head oil field, and mapping of palaeo-oil zones in reservoir intersections, showed this petroleum system was effective for oil accumulation in much of the offshore basin.

Technical Area: General – Radar and Remote Sensing

Pycnoclines (layers within ocean waters defined by a rapid change in density, either due to a change in temperature and/or salinity) have been found to produce reflections that appear in seismic data collected on the Amery Ice Shelf (AIS), East Antarctica. The pycnoclines, present due to an ice pump mechanism under the AIS, are clearly visible within the ocean water column beneath the ice shelf. This is evidenced in a 3 km common depth point (CDP) reflection survey undertaken on the eastern side of the AIS. The reflections are unmistakably primary in nature, producing their own multiples in the seismic record. Further processing of other AIS seismic data also reveals reflections at similar arrival times in the ocean water column. While the reflection coefficient (RC) of the ice-water and water-seafloor boundaries are approximately 600 times greater in magnitude than that of these pycnoclines, the pycnocline RCs are 40 times greater in magnitude than the surrounding ocean waters – sufficient to produce a reflection. The pycnocline reflections correlate well to changes at density interfaces observed by in-situ CTD (conductivity-temperature-depth) data collected under the AIS through a borehole in the ice. Seismic surveys carried out over different field seasons, spanning three years, and observed at different times during the summer seem to indicate that there is a variation in the depth and thickness of the pycnocline layers. One possible reason for this variation is the presence of internal waves at the interface between the density layers.

Technical Area: General - Other

The Arbroath field is located in UKCS block 22/17 and produces 38o API oil from the Forties Sandstone turbidite reservoir at approximately 2500m TVDSS. Production began in 1990 and the field still produces significant oil volumes today.

In 2006, a re-appraisal of 1993 and 2000 vintage 4D seismic data over the field was initiated to de-risk infill drilling targets. This included a petrophysical and petroacoustic review and 4D seismic re-processing:

- Petrophysical log database QC and repair, facies modelling and shear log prediction using clustering to provide a quality log set for all key wells.

- Quantifying the seismic response to reservoir sand presence and fluid fill. Quantifying the dynamic seismic response associated with changes in OWC, oil saturation and fluid pressure due to continued production using seismic forward modelling techniques. Selecting optimum seismic elastic inversion products to highlight produced zones.

- Improved 4D seismic images by using state-of-the-art time lapse processing technology.

- Applying enhanced seismic processing to improve signal levels in seismic AVO volumes, leading to improved relative fluid impedance volumes for better delineation of by-passed oil.

- Predicting continuous rock strength profiles along infill well paths, performing sand stability and in-situ stress analysis to optimise preferred perforation placement and orientation.

This poster presents the integrated progression from seismic data review with petrophysical and petroacoustic analysis leading to identification of un-swept oil, and assisting infill well placement and design. We highlight the innovative technology used during each of the stages.

Continental-scale merges of Australian airborne magnetic data are only accurate for wavelengths less than about 100 km, due to limitations of survey size and data processing. Wavelengths greater than 400 km are available from satellite data; thus, there is a “gap” in the spectrum for intermediate wavelengths between 100 km and 400 km. Geoscience Australia is filling this gap by acquiring new airborne magnetic data as part of the Australian Government’s new energy initiative. Intermediate wavelengths are important, for example, for better definition of sedimentary basins for petroleum prospectivity evaluation, for interpreted depths to bottom of magnetic sources in attempts to define the Curie point isotherm, and for regional removal in modelling. The AWAGS2 project (Australia-wide Airborne Geophysical Survey) is collecting both gamma-ray spectrometric and total-field magnetic data across Australia with long north-south lines spaced 75 km apart and 80 m terrain clearance. Accurate spatial crustal magnetic data depend upon accurate removal of time variations of Earth’s magnetic field. These are recorded at pairs of ground sites, concurrently with the airborne acquisition, and supplemented by geomagnetic observatory and other data. Routine airborne magnetic surveys rarely use more than one base magnetometer and the long lines of AWAGS2 present a challenge. It cannot be assumed that time variations are spatially uniform (eg., induction in the oceans and crust creates spatial non-uniformities). Removal of time variations depends upon interpolation of data recorded at ground sites and knowledge of induction effects. Removal of the latter is aided by using highresolution vector data recorded at the base sites. Using the AWAGS2 corrected traverses will improve the accuracy of intermediate wavelengths in the Australian Digital Magnetic Anomaly Map

Technical Area: Geophysics in Government Surveys

The individual surveys that comprise the national gamma-ray spectrometric radioelement database are not all registered to the same datum. Older survey results are presented in units of counts/sec, which depend on factors such as survey flying height and detector volume. Even recent surveys can have a significant mismatch along common borders due to limitations in spectrometer calibration and data processing procedures, as well as environmental effects that result in temporal changes in the gamma-radiation fluence rate at the earth’s surface. To solve these problems, Geoscience Australia is currently flying (under contract) an Australia-wide airborne geophysical (magnetic and radiometric) tie-line survey (AWAGS2) that will be used to bring all of the surveys in the national database to a common datum. The AWAGS2 survey is being funded under the Australian Government’s Onshore Energy Security Program and is due for completion early in 2008. The data are being acquired and processed according to international standards, and the final estimates of radioelement concentrations along the AWAGS2 tie-lines will be consistent with the International Atomic Energy Agency’s (IAEA) radioelement datum. The national database is being levelled by estimating survey correction factors that, once applied, minimize both the differences in radioelement estimates between surveys (where these surveys overlap) and the differences between the surveys and the AWAGS2 traverses. This effectively levels the surveys to the IAEA datum, and significantly enhances the value of a database that is essential for informed decision-making about Australia’s onshore energy resources, mineral exploration and environmental protection.

Technical Area: Minerals – Global Datasets

INTRODUCTION: Integrated studies play a vital role in exploration projects. This study demonstrates the importance of collaboration of geologists and geophysicists in better locating drilling points.

ABSTRACT: Within exploration studies for oil in the Central Iran, the first seismic survey led to Zavareh 1 exploratory well (3900 meters deep), which turned to be a dry well. In this research, we have tried to conduct an integrated geological and geophysical study for identifying major geological structures in order to better locate exploratory well. For this purpose, firstly, the geology of the Zavareh–Ardestan area was carefully studied and the relevant hydrocarbon horizon was indicated. Then, by using gravity, magnetic and seismic methods the important geological structures including anticlines A, B and C were mapped. By reprocessing and interpreting a number of seismic lines and specially the one intersecting Zavareh 1 exploratory well, we concluded that the drilled well had not reached the target horizon. Based on this study, we suggested the Ghom Formation as the oil producing horizon and anticline C as a suitable target for future exploratory drilling. In addition, we recommended National Iranian Oil Company to conduct denser gravity, magnetic and 3D seismic surveys in order to more precisely determine the new position of the well.

Technical Area: Interpretation and case histories

Seismic data with appropriate processing tools, often come to the rescue of interpretation, minimizing the ambiguities involved in analysing complex geological situations. Stratigraphic features associated with structure, play critical role in hydrocarbon entrapment within the Middle- Late Cretaceous reservoirs in the Al-Khafji offshore area. A total of 61 square kilometres high resolution 3D OBC seismic data were acquired in the North Khafji area, with objectives to explore strati-structural traps and their associated reservoir settings. New petroleum plays are intended to be explored after integrating new 3D seismic vintage with older Al-Khafji seismic data interpretation.

Present study is aimed at comparing seismic datasets that are processed by different companies. Authors examine and identify the relative merits of data attributes that are suitable for structural and strati-structural traps interpretation. The initial interpretation done during 1997, reports a possible carbonate build-up, correlatable to Gudair formation, which is equivalent to a horizon associated with Aruma-Wasia unconformity. Comparative study made between Khafji and north of Khafji field could not conclusively establish the extension of this carbonate build-up in the North-Khafji area, because of ambiguous nature of structural and stratigraphic anomalies. At places, there is inconsistency among scales of these datasets. However, based on the interpretation of the newly processed seismic data, seismic structure and attributes have been integrated and reinterpreted for scalable anomalies. Present study strongly suggests an exploration potential in the North-Khafji area and stipulates pursuing further geological studies, detailing these seismic anomalies and converting them into commercial petroleum plays in the offshore basin.