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

Due to the nature of the Fourier transform geophysical data must be prepared before the transform is calculated. This preparation usually takes the form of the removal of any trend from the data combined with the padding of the data to 2^N points at the data edges. However, no data preparation procedure is perfect, and the result is that problems (in the form of edge effects) appear in the filtered data. When high-pass filters (such as derivatives or downward continuation) are used then these edge effects become particularly apparent.

This paper suggests three methods for the stable downward continuation of geophysical data (two of which may be combined). The first method is applied to an integrated horizontal or vertical derivative of the data rather than to the data itself. Since the derivatives can be calculated in the space domain where FFT edge effects are not present, this reduces the enhancement of the data at frequencies near the Nyquist, resulting in smaller edge effect problems. The second method measures the FFT-induced noise by comparing data that has been downward continued using both the space and frequency domain methods. The data are then compensated accordingly, and the compensated data may be downward continued to arbitrary distances that are not possible using space domain operators. The final method treats downward continuation as an inverse problem, which allows the control of both FFT-induced noise and other noise that is intrinsic to the dataset.

Textural analysis is a powerful tool, but it is rarely applied to geophysical potential field data because the results are often noisy and ambiguous. New texture filters (based on grey-level co-occurrence matrices (GLCMs)) have been specifically designed for gravity and magnetic data, and are useful for the detection of subtle monopolar and dipolar geophysical anomalies. The method uses two GLCM kernels simultaneously: in the first kernel the GLCM vectors are oriented along the contour lines of the feature to be located, and in the second the vectors are oriented in the ‘uphill’ direction ie orthogonal to those of the first kernel. The ratio of the GLCM texture amplitudes from the two kernels then yields a maximum when the kernel is centred over an example of the feature within the data. The method has general application and is not limited to potential fields. It works with any type of target, as long as an example is available. Applications to both synthetic data and real data are demonstrated.

The Shuttle Radar Topography Mission (SRTM) has generated a homogeneous near-global digital elevation model (DEM) of the Earth using single pass radar interferometry. The crew of Space Shuttle Endeavour (STS-99) operated the modified dual antenna synthetic aperture radar systems for 11 days in February 2000. SRTM acquired both C-band and X-Band synthetic aperture radar data, collecting 3-D data using a 60-metre mast extending from the shuttle payload bay, containing additional C-band and X-band receiver antennas.

SRTM DEM data have a horizontal resolution of 1 arc second (30 m at the equator) and vertical resolution of 10 m (C-band radar). SRTM-1 (1 arc second) data are only available for US territories with subsampled SRTM-3 (3 arc second) data provided for the rest of the world. The USGS are responsible for archiving the data with 3 arc second data being made available on a continent by continent basis. So far North and South America and Eurasia have been completed and the rest of the data processing is expected to be complete by July 2004. Comparison of SRTM DEM data with older GTOPO and altimetric DEMs shows a significant improvement in resolution, similar to that achieved by ASTER. The SRTM DEM data provide a useful new resource, especially in areas where limited topographic data are available. The data are free and in a simple format.

Separation or layer filtering of regional and residual magnetic fields is an important component of magnetic interpretation. Separation filtering depends fundamentally on the concept of random distributions of sources within discrete layers and assumes that there is no statistical difference in response along each ideal layer and no correlation between the distribution in each layer. Separation filtering becomes very difficult when there is considerable overlap in the spectra of individual depth ensembles. The degree of separation achieved depends on the spectral b/B ratio, the ratio of the amplitudes of the shallow and deep ensembles. A high b/B ratio is needed to deconvolve the effects of shallow sources with minimum contamination by deeper sources.

It is well known that derivatives of potential fields enhance the field component associated with shallow features and de-emphasise the field from deeper sources. Fractional vertical derivatives provide an objective, flexible approach to shallow layer separation filtering as the order of the fractional derivative can be selected to match the data and optimise enhancement of the shallow field component. The method avoids the uncertainties in selecting spectral matched filter parameters. Different order fractional derivatives can be combined to produce RGB images and this can be a significant aid to the interpretation of the data.

The application of fractional derivative separation filtering is illustrated using high-resolution aeromagnetic data covering the Ghanzi-Chobe fold belt in Botswana. Total magnetic intensity data are dominated by crystalline basement anomalies. Progressively increasing the order of fractional vertical derivatives provides rejection of deeper basement anomalies and provides improved resolution of subtle supracrustal anomalies than the conventional vertical gradient.

Aeromagnetic surveys play an important role in the detection and analysis of terrestrial impact structures as large semi-regional aeromagnetic surveys are widely available.

Impact craters can be divided into two groups based on morphostructure, namely simple and complex. Simple craters are relatively small bowl-shaped depressions with an upraised and fractured rim whereas complex craters are larger with a central uplift zone.

Magnetic signatures of terrestrial impact craters vary greatly, reflecting the target rocks, the impact-related magnetisation and effects of crater fill and post-impact sediments. In basement rocks, the common signature is a magnetic low, ranging in amplitude from a few nT up to a few hundred nT. The central peak or ring uplift of crushed basement may produce strong magnetic highs. The magnetic signature may be due to shock demagnetisation, shock remagnetisation, and thermal and chemical remanent magnetisation effects. Impact craters in sedimentary targets are usually subdued and amplitudes of a few nT up to 10 nT are common.

Enhancement of magnetic signatures of impact structures using filtering techniques is an important part of detection and analysis. Derivatives and shaded relief techniques, along with separation filtering, are probably the most used methods. Algorithms for fractional order derivatives and circular shaded relief have dramatically improved filter results. The fractional derivative order can be varied to optimise separation of the impact magnetic signature. Circular shaded relief treats all directions equally unlike the fade-out for features sub-parallel to the shading direction evident in conventional shading.

The fractional order derivative and circular shaded relief algorithms are illustrated from impact structures in Australia and Canada in both basement and sedimentary cover rocks.

eXploration and Mining Markup Language (XMML) is a GML-based catalogue of feature-types used in mineral exploration. XMML includes boreholes, observations, assay data, geophysical data, geological features (rock-unit, fault, etc) and supporting concepts (geological timescale). The GML basis ensures conformance with ISO standards, national Spatial Data Infrastructures, and compatibility with languages developed in other communities, within and outside the geosciences. GML languages are compatible with interfaces such as Web Feature Services (WFS), which provides a GML "view" of data sourced from GIS and DBMS.

GML was developed under the sponsorship of mainly Australian mining and service companies and agencies, who determined the requirements and initial priorities. However, further development of XMML is proceeding in collaboration also with the academic community, international geological surveys. XMML will be standardised through IUGS.

XMML provides an open, platform-neutral, standards-conformant means for transfer of exploration data between applications and between organisations. Using XMML and WFS provides for publication of data by data custodians at both fine and coarse-grained level. XMML and WFS/T provide a transactional interface. The Government Geologists Information Management Advisory Committee (GGIPAC), which has partly sponsored XMML, is considering that XMML be a technology for statutory reporting in the near future.

The farming communities around Jamestown in the Northern Agricultural District of South Australia have experienced periodic crop losses due to salinity and waterlogging. Detailed soil maps and localised studies had not provided a comprehensive understanding of the causes of salinity, and a larger-scale approach was required.

The area was flown with airborne electromagnetics (AEM - TEMPEST), magnetics and radiometrics in mid-2002 over three parallel N-S valleys with different expressions of salinity: Caltowie Valley suffers from surface scalding, infrastructure damage and very shallow, saline watertables. Jamestown/Bundaleer Valley has regions of crop failure across the valley floor, seasonal waterlogging and periodic shallow, saline watertables. In Belalie Valley isolated soil degradation and scalding occurs only on north-facing slopes, with minimal salinity issues on the valley floor.

Topography shows an increase in valley size from Caltowie to Jamestown to Belalie. Surface drainage is minimal (rainfall 400-500mm/a), but seasonal floods over thick clay soils result in almost yearly waterlogging, particularly at bottlenecks in the topography: in Caltowie and Jamestown Valleys.

Airborne magnetics reveal an extensive sub-surface drainage system, defining preferential groundwater pathways beneath broad, flat plains. This has important differences from the surface drainage, helping explain the observed patterns of salinity.

The AEM confirms these pathways adding the element of depth; explaining why and where salinity is, and is not, expressed at the surface.

Radiometrics highlight surface distribution of materials through and between valleys, particularly the association with salt-bearing waters.

Down-hole and groundwater chemistry confirms the relationships shown by the airborne geophysics, defining the scale of groundwater flow systems responsible for the salinity concerns of the region.

The Junee-Narromine Volcanic Belt is the westernmost and most poorly outcropping belt of Ordovician rocks that represent the dispersed remnants of the Ordovician Macquarie Arc. The northern part is under deep cover of Surat Basin.

In areas of poor or nonexistent outcrop such as these, most of the knowledge of the geometry and components of the geology comes from the interpretation and modelling of gravity and aeromagnetic data - interpolation between outcrops and extrapolation into areas of deep cover.

Most of the shape of the Junee-Narromine Volcanic Belt is a reflection of meridional bounding faults of the Tullamore Trend. Individual complexes are elongated N-S or NNW and deformation is generally low, except in the high strain zones. From north to south, Ordovician complexes with approximately meridional Tullamore trends become rotated into the Gilmore Trend and then become aligned along splay faults as thrust sheets (Gidginbung Volcanics) before being truncated by the Gilmore Fault.

Electrical and electromagnetic (EM) geophysical data from a line or grid are usually converted into two- or three-dimensional resistivity models of the Earth. In regions of relatively uniform geology, variability of measurements are assumed to be random and Gaussian, such that the Earth models are found from a maximum likelihood approach. In this paper, we show that such variability is not Gaussian, and has a scale-length dependence, expressed as a fractal dimension. We argue that the fractal dimension of electrical and EM data is causally determined from Earth heterogeneity, which provides a link with hydraulic conductivity in porous and fractured media that also has a fractal dimension.

This paper presents initial analyses of data from two sites within South Australia. NanoTEM time domain electromagnetic (TEM) data were collected at Tunkillia in the Gawler Craton with a target of identifying palaeochannels for gold exploration. Analysis of survey data revealed a good correlation between regions of low resistivity and high fractal dimension. On the other hand, a river-borne NanoTEM survey of the River Murray sediments showed much lower correlation. We conclude that analysis of the variability of EM data may provide useful additional constraints on subsurface properties.

Self Organizing Maps (SOM) are a tool for the visualization and interpretation of complex, multivariate data with many potential applications in geophysics.

We demonstrate the ability of the SOM method to analyse a data set with 50% missing data. This robustness of a SOM enables it to also be used to interpolate missing data and see through noise to underlying relationships between the data variables.

A study involving geophysical log data for the characterization and prediction of limologies with anomalous levels of radioactivity in the Bowen Basins is used to illustrate the calibration of a SOM. In this case the SOM was used to identify the radioactive sandstones. This was then used successfully as a classification template for data from another mine site, >30km to norm-east.

The concept of using gravity gradient measurements for gaining knowledge of the subterranean zone is not new. Instruments have been built and field measurements have been conducted over the past 100 years. Improvements in both sensor performance and survey procedures have resulted in the present capability of making high accuracy (sub-Eotvos) measurements in real-world areas of interest. A summary of recent field activity using Lockheed Martin gradiometer systems will be presented in this paper. Recent improvements have resulted in a land-based gravity gradiometer system capable for use in a broad range of land applications, including void and tunnel detection, void characterization, and time-lapse reservoir monitoring.

The Slave Craton in Canada, is a small Archaean fragment, bounded on the east by the Thelon magmatic arc (2.0-1.9 Ga) on the western edge of the Archaean Rae Province and on the west by the Great Bear magmatic arc of the Wopmay Orogen (1.88-1.84 Ga). The northern and northeastern part is overlapped by Late Proterozoic and Phanerozoic supracrustal rocks. On the south, the Slave Craton is separated by the Great Slave Lake Shear Zone from the Early Proterozoic Chinchaga and Buffalo Head terranes.

We use robust geochemical methods based on mantle-derived xenoliths, heavy mineral concentrates from over 25 kimberlites, and representative diamond populations and their inclusions to construct sections that delineate the composition, structure and thermal state of the lithospheric mantle across the Slave Craton. This analysis reveals a distinct two-layered lithosphere beneath the craton: a shallow ultradepleted, olivine-rich layer and a deeper less depleted layer, interpreted as an Archaean plume head.

We have mapped variations in the gravity/topography relationships across the Slave Province in terms of the effective elastic thickness (Te). The results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te < 25 km), probably related to the intrusion of the Mackenzie Plume (ca 1270 Ma). The strongest lithosphere is found in the eastern part of the craton (Te > 56 km). A N-S zone of low Te along the middle of the craton may map the deep extension of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of low Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd/Pb isotope lines which define a major crustal boundary at depth, and is a major locus of kimberlite intrusion.

Joint research on geophysical exploration for coal between the New Energy and Industrial Technology Development Organisation (NEDO) in Japan and the Queensland Department of Natural Resources, Mines and Energy covers three main components: (1) verification and evaluation of a previously developed coal exploration and assessment system;, (2) a Coal Potentiality Evaluation System;, and (3) the regional geophysical and geological framework.

A test site at Coppabella Mine was used to create a geological model using the Kinematic Modelling System. Input into the model included drilling, geophysical logging, vertical seismic profiling, 2D and 3D seismic and gravity. The model is being assessed against the results of mining. Airborne magnetic and radiometric data are being progressively collected over the Bowen Basin. Data collected and interpreted to date have enhanced our understanding of the basin, in particular, the tectonic and structural history. The Coal Potentiality Evaluation System consists of three main parts: a series of databases; a GIS; and the coal potentiality system which is an expert system. The GIS and Coal Potentiality Evaluation System interact through a mediator. The databases provide geological, environmental, mining and economic data. The Coal Potentiality Evaluation System was used to help define a project area in the northern Bowen Basin for further testing of the geophysical methods. Three fully cored boreholes support 10 kilometres of 2D seismic.

Collaboration at an international and a local scale has been highly effective. It has provided ongoing development of geophysical exploration techniques, has provided an impetus for the re-evaluation of the geology of the Bowen Basin and has provided an avenue for the exchange of geophysical technology and ideas.

A comparison of recently acquired seismic data in the Bowen Basin has highlighted inherent differences in signal content between dynamite and Vibroseis sources. This work was undertaken to assist selection of the source for proposed 2D and 3D seismic surveys in the Bowen Basin where recent drilling successes have revived interest in the hydrocarbon potential of the Permian sequence. Although comparisons between dynamite and Vibroseis sources have been made previously, the topic remains relevant because of developments in acquisition technology, the use of seismic source modelling and the area-dependency of seismic data quality.

The motivation for acquisition of further 2D and 3D seismic data in the Surat-Bowen Basin are petroleum discoveries in the Permian Tinowon Sandstone along the western flank of the Taroom Trough at Myall Creek, Churchie, Overston and Waggamba. Variable reservoir quality and extent make identification of Tinowon sands difficult and good quality, high resolution data are essential for exploration and development mapping of the prospective reservoir units. For this study, a pair of overlapping 2D seismic lines was selected, one recorded with a Vibroseis source in 1996 and the other with dynamite in 2000. Care was taken to select two modern lines with acquisition parameters that would minimise the attenuation of frequencies in the higher end of the signal spectrum, thus providing input data sets that would result in a valid Vibroseis-dynamite comparison.

Data processing was closely monitored to determine the effects of specific algorithms on signal content. Dynamite proved to have the greatest resolution in the Tinowon zone of interest, that is, around 1400 ms. Indeed, inspection of the signal spectra and particularly the bandpass filter panels on raw field records clearly showed that dynamite data contained higher frequencies and exhibited increased resolution relative to the Vibroseis data in the study area. Bandpass filtering is an effective means of evaluating seismic source characteristics.

Predictably, results showed that resolution of the dynamite and Vibroseis data converged with depth due to the natural frequency filter effect of the earth. Synthetic data produced from wavelets matching the Vibroseis and dynamite data demonstrated that the improved resolution expected from the dynamite source would improve imaging of the Permian section. Based on the results of this trial and other modelling, Origin Energy selected a dynamite source for their Myall Creek 3D survey. Mosaic Oil selected dynamite for both their 2003 2D and their 2004 3D seismic survey.

In this paper we present a methodology for performing lithology and fluid prediction in lightly explored basins. We use the deep water area of the Orange basin, offshore South Africa to illustrate the methodology. In the Orange basin, there are numerous wells drilled on the shelf, however, there have been no wells drilled in the deep water.

Firstly, petrophysical analysis of the shelf wells is performed to determine end member properties of the sands and shales. From the analysis, rock property trends such as Vp versus Vs and Vp versus density are determined. In addition, the uncertainties associated with the trends are also calculated. A critical step in extrapolating from the shelf to the deep water is to use seismic derived interval velocities to improve the estimate of Vp as a function of depth. Using seismic interval velocities and well data, we derive expressions for Vp of the sands and the shales that are functions both of depth and seismic interval velocity.

The rock property trends are used to perform stochastic AVO modelling for single interfaces. The AVO modelling gives an estimate of the average response of interfaces such as shale-to-brine sand and shale-to-gas sand interfaces, as well as a measure of the uncertainty of the estimate. Therefore, a range of AVO responses is provided. Lastly, the AVO modelling is compared with AVO anomalies observed on the seismic data.

A significant portion of South Africa’s current gold resources come from the Ventersdorp Contact Reef (VCR). The VCR is a tabular orebody, generally less than 1.2m thick. Slopes, terraces and faults impact on VCR geometry. Gold is generally concentrated in palaeochannels, corresponding to terrace elevations. Conversely, slopes are generally associated with lower gold grades. To be able to site support pillars, such that they correspond with lower gold grade slope areas, has significant financial benefit. The geophysical challenge is, thus, to model the reef geometry prior to mining in order to facilitate optimal ore extraction.

Borehole radar has proven to be applicable for mapping topography on the VCR. In this paper a case study is presented where radargrams and 3D visualization from three boreholes show continuity of geometrical features on the VCR target horizon. The improved confidence in the geological model has an immediate impact on resource estimation and mine planning with immediate financial benefits.

In order to extend seismic processing techniques to anisotropic media, it is required that we have a measure of the different anisotropy parameters.

The purpose of this study is to estimate the Thomsen’s parameters, µ and ´, for transversely isotropic (TI) media using the shifted hyperbola NMO equation by Castle.

A non-linear inversion technique has been developed to invert the Castle’s Shifted Hyperbola NMO Equation for obtaining the shift parameter.

This method will be used for the estimation of these parameters over the Blackfoot field in western Canada.

As oil fields produce liquids (oil or gas/condensate), they undergo pressure change, with the pressure reducing as a function of production rates. Equally, the high production of water from a reservoir can cause major reduction in pressures of gas fields resulting in an increase in the gas cap size and a potential loss of gas through spillage to the surrounding formations.

The injection of water or CO2 into reservoirs or underground storage areas causes an increase in local pressure which in turn, has the potential to cause local increases in permeability in the reservoir or at the storage site. Time-lapse three dimensional (3-D) seismic methods are used to monitor the fluid movement during both fluid extraction in producing fields and injection of water or CO2 for EOR or storage purposes.

Little is understood about the seismic effects caused by variations in both pore pressure or rock matrix stress as a result of these operations. During 2003, a pressure/volume/temperature (PVT) chamber was built at Curtin University to simulate such effects, and to establish the seismic reflection response under these variations.

This paper discusses the development of this facility and the applications of this unique PVT chamber.