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

AEM systems are essential tools for a wide range of mineral exploration and geological or environmental mapping applications. The product of peak dipole moment and the Liu waveform factor provides a quantitative estimate of the effective signal strength of a TEM system at a specific base frequency, and can be used to “compare” systems. Given also the noise levels of an AEM system and its frequency or time sampling, it is easy to predict the capabilities and resolution of the system.

The most challenging development required of AEM is the development of systems operating at 5 Hz or less to penetrate conductive cover and assist in the discrimination of very conductive copper/nickel sulphide deposits. Altimeter errors provide the main limitations in depth resolution of shallow environmental targets. 2D and 3D imaging and inversion strategies are not yet reliable or fast enough for routine application.

Earthquake a phenomenon,thought to be out of control of human is no longer an uncontrolled menace. This paper deals with the cause and effects of earthquakes and its subsequent negation. The shakes and tremors were thought to have been caused by plate movements and related occurrences.

We have presented an alternate hypothesis for these phenomena. Our hypothesis is competitive enough to convincingly deal with the anomalies which are present in the plate tectonic and subordinate theories.

Success with heavy oil production depends as much on knowing the geology of reservoir as it does on understanding the fluid properties of the reservoir. Heavy oils are defined as having high densities and extremely high viscosities. Due to their viscoelastic behaviour the traditional rock physics based on Gassmann theory becomes inapplicable in principle. In this paper, we use effective-medium approach known as coherent potential approximation or CPA as an alternative fluid substitution scheme for rocks saturated with viscoelastic fluids. Such rocks are modelled as solids with elliptical fluid inclusions when fluid concentration is small, and as suspensions of solid particles in the fluid when the solid concentration is small. This approach is consistent with the concepts of percolation and critical porosity, and allows one to model both sandstones and unconsolidated sand.

To test the approach against the known solutions, first, we apply CPA to a rock with fluid pore fill and compare the obtained estimates with Gassmann results. Second, we compare CPA predictions for solid-solid mixtures with numerical simulations. Good match between the results confirms the usefulness of the proposed fluid substitution scheme. We extend the CPA scheme to predict the effective frequency- and temperaturedependent properties of heavy oil rock. We also use viscoelastic extension of Hashin-Shtrikman (H-S) bounds to obtain alternative estimates. The proposed fluid substitution scheme provides realistic estimates of the properties of a rock saturated with heavy oil which lie between H-S bounds.

Fracture-induced anisotropy is of moderate to large magnitude in sand reservoirs and can be examined in order to further advance our knowledge of fractures in the reservoirs. Production and increasing recovery rate of the existing oil and gas fields have shown to attract arrested attention and new emphasis. Fractured reservoirs contain an enormous amount of oil and natural gas. Fractures control in situ permeability; fluid storage and mobility; and rock strength; and thus the reservoir itself. Therefore, a clear insight into the subsurface fracture network will definitely promote optimization of well planning and production. Rock properties in a fractured reservoir indicate the amount of hydrocarbons and the convenience or inconvenience of production. Ignoring open fractures may lead to fluid flux, loss of circulation and other unsuccessful exploration and production efforts.

Hyperspectral drill core scanning undertaken using the CSIRO HyLogger^™ between 2002 and 2005 focused on accumulating spectral data from a series of signature holes from across the State of South Australia. One component of the software used to process the data provides a summary for each hole indicating the amount of each detected mineral as a percentage of the scanned hole. By converting this percentage into the number of metres of detected mineral present in the drill hole this information can be presented in a GIS.

Four mineral suites are coming to the fore in alteration mineral mapping using HyLogger^™: white micas, chlorites, carbonates and iron oxides. Each of these suites can relate to Eh-pH conditions in a mineralising system. For white mica, the transition from muscovite through phengite, as measured by the progressive change in wavelength position of the ~2200 nm absorption feature to longer wavelengths, corresponds with increasing replacement of aluminium in the crystal structure by iron or magnesium. Empirical studies show a correlation between concentration of economic metals and the presence of phengite that may also reflect local fluid pressure conditions. The ratio of Fe/Mg in chlorite has also been shown to vary in proximity to mineralisation. Calcium, iron and magnesium carbonates are a third component. Spectral studies have distinguished between hematite, Fe²⁺ goethite and Fe³⁺ goethite. HyLogger detects in wavelengths appropriate to these suites and software interprets relative abundances. Plotting the number of downhole metres of core containing these assemblages in their relative geographic locations permits interpretation of regional patterns of alteration. With some 600 holes (including 300 on open file) and 61,000 metres of core scanned across South Australia, regional patterns are starting to appear.

The Airborne Geophysical survey of the PNG Highlands and the Papuan Peninsula involves the collection of geophysical data using helicopter-borne magnetic and gamma-ray spectrometric instruments over 2 areas of Papua New Guinea (PNG), where little or no prior geophysical coverage is available. The availability of these data in both digital and paper-map formats is expected to encourage and facilitate exploration in PNG by international mining companies.

All of Area 1, comprising the PNG Highlands portion of the survey, is anticipated to be completed by the end of the 2007 field season in late October. Flight operations will recommence in March 2008 in the Papuan Peninsula. Preliminary interpretation of the magnetic and radiometric data collected to date is providing a much higher degree of resolution and accuracy than has been available to date; with more detail in some areas, and new features being identified in others.

As part of an appraisal program by InterOil of the Bwata gas resource and prior to undertaking further 2D seismic surveying, a 3D geology model of the project was rapidly built using the 3D GeoModeller software.

The software implements a methodology developed in the BRGM to jointly interpolate geological contact data and dips of geology formations. The method uses the chronostratigraphic order of geological formations, and their rock-relationships. The model is calculated using an implicit 3D potential function as the interpolator for each component part of the geological history. The order and relationships recorded in the stratigraphic column are used to automatically resolve the intersections between component parts, and produce volume reconstructions. The methodology allows the geologist to focus on geological issues and consider alternative interpretations.

The 3D structural geology model was built using a single 2D seismic line, well data from the Bwata-1 and Triceratops-1 wells, surface geological data and airborne gravity data. Eight cross sections across the Bwata Anticline were created from surface geology, seismic and well data in 2D Move. These sections were imported into 3D GeoModeller. A 3D model was then created and the forward gravity response computed. Density variations from general background of 2.24 t/m³ are provided by the Cretaceous Ieru Formation at 2.40 ± 0.05 t/m³ and the Puri and Mendi Limestones at 2.70 ± 0.05 t/m³. The computed response was compared to observed data derived from an airborne gravity survey. On the basis of such comparisons several iterations of geologic revision were proposed to improve the fit between the computed and observed data.

The outcome of this study was the prediction of the geological setting and the extent and thickness of the limestone beds. The model incorporated c. 30 degree dipping thrusts and a steeper backthrust and introduced two NE/SW near vertical faults which exhibit a sinistral strike slip and east-side-up displacement. A substantial increase in the size of the field was interpreted at its western end. Using the 3D model will enable InterOil to design a follow-up 2D seismic survey with greater confidence that the survey will meet program objectives.

The objective of this paper is to seek a generalized understanding for the influence of incorporating the gradient of the model’s physical properties (e.g., conductivity, velocity) in the forward modelling numerical algorithm. In order to take a step towards that, we examine an example from Geophysics for solving 3-D Maxwell’s equations using finite difference (FD) methods. The 3-D FD methods to obtain discrete solutions of Maxwell’s equations include the staggeredgrid and balance methods. The balance method 3-D algorithm exploits the conductivity gradient in order to make the FD formulation a seven-point scheme and the resulting matrix a banded septa block diagonal but not symmetric. The staggered grid algorithm is free of conductivity gradient and results in a symmetric 13-diagonal banded matrix. The objective now is to examine and understand better the influence of the conductivity gradient incorporated in the FD equations on the accuracy of the electromagnetic (EM) modelling for two 3-D benchmark models. We use three various discretizations (fine, mildly coarse, and coarse) for each model. The modelling results of each discretization have been computed separately by the balance method and staggered grid method. We have found that the staggered grid method produces accurate results for all the three discretizations investigated. However, the balance method encountered some inaccuracies for the mildly coarse and coarse discretizations. This appears to be due to the presence of the conductivity gradient in the 3-D modelling algorithm. The model studies also suggest that the thicknesses of the horizontal and vertical discretizations at the conductivity boundaries should be about 1/25 and 1/100 skin depth to maintain accurate modelling results when the conductivity derivatives exist in the 3-D modelling algorithm.

We have developed a new inversion algorithm to successively determine the depth (z), polarization angle, and the electric dipole moment of a buried structure from the self-potential (SP) data measured along profile. By utilizing the entire values of the SP profile, the problem of depth determination is formulated into the problem of solving a non-linear equation of the form f (z) = 0. Using the estimated depth and by applying the leastsquares method, the polarization angle is then determined. Finally, having known the depth and polarization angle, the electric dipole moment is determined in a least-squares sense. The proposed SP inverse algorithm has been derived for semi-infinite vertical cylinder, infinitely long horizontal cylinder, and sphere anomalous bodies. The method is tested on synthetic examples with and without random errors, and applied to a field example from Germany for mineral exploration. The estimated depths and other SP model parameters are found in good agreement with the known actual values. The results obtained will be presented and discussed in the conference.

The Tanami 3D crustal-scale model integrates geological mapping with seismic interpretation and gravity modelling in order to enhance our understanding of the 3D architecture of the region.

The Tanami Region is a multiply deformed Palaeoproterozoic terrain consisting of the predominantly turbiditic Tanami Group, the volcanically derived sedimentary and extrusive rocks of the Ware Group and the Mount Charles Formation. Thought to be underlain by Archaean basement, the region was intruded by numerous granitoid bodies during the period 1820-1790 Ma and hosts the Callie world-class and several smaller gold deposits.

The 2005 Tanami seismic survey consisted of four regional traverses. Interpreted sections show the presence of a series of crustal-penetrating structures, that appear to be fundamental to the evolution of the Tanami Region. A number of these interpreted structures appear to link the mid-crust to ‘thin-skinned’ structures within the uppermost crust. The seismic also shows the presence of a southeast-dipping suture and associated pop-up structure, separating the Tanami crust from the Arunta crust. Known ore deposits are all located in close proximity to these interpreted through-going thrust faults and the associated ramp anticlines. Gravity modelling of the southeast-trending backbone seismic traverse (05GAT1) supports the seismic interpretation.

The current 3D model was constructed using 3D GeoModeller incorporating seismic data, modelled gravity, interpreted magnetic and gravity data and geological mapping. The 3D model forms the basis of the Tanami X3D web viewing tool, which allows the model and associated data to be viewed and manipulated over the web.

Converted-wave refraction statics is an algorithm that incorporates both compressional (P-) and shear (S-) wave refraction events to correct S-wave static errors in multicomponent seismic data. Converted refraction (PPS) events are picked on an inline component shot record in the same way as first-breaks are picked on a normal vertical component record. These picks are then analysed using the reciprocal method to create a near-surface model from which S-wave receiver statics are derived.

The derived PPS refraction statics have a similar shortwavelength character to the S-statics obtained via statistical analysis of converted-wave reflections. Based on standard P-wave practice, we believe that an optimal production approach will include converted-refraction analysis, followed by converted-wave residual statics.

Although the thrust of this work has been towards derivation of S-wave statics, an interesting auxiliary output is also available. Based on theoretical modelling, the S-to-P time-depth ratios can be tuned to provide P-to-S velocity ratios (and hence dynamic Poisson’s ratios) for the near-surface. This has interesting implications for lithological and rock-strength analysis in the mining, environmental and engineering contexts.

Tar or bitumen deposits in the Canning Basin, Western Australia, indicate possible paths of petroleum migration. Neutron and X-ray tomography is applied to rocks containing biodegraded tar in order to investigate the distribution of petroleum within the pore system of the samples. Neutron tomography allows the imaging of hydrogen-rich components, such as tar and bitumen, within a sample, whereas X-ray tomography assists density analysis of the grains and pore space.

The results indicate that the methodology discriminates excellently between tar and the other rock matrix and open void components. The images permit analysis of the connectivity of pore paths and enable conclusions to be drawn about petroleum migration.

In the case history presented, the images demonstrate the injection of petroleum into a dilated fault zone, and the interaction of the petroleum with the fault gouge material. The images also demonstrate the nature of petroleum movement into sandstone beds, which were in communication with this fault zone.

This paper presents a novel approach to identifying/tracking interlocking Palaeocene sand and shale-filled channels (based on forward modelling, AVO and inversion-of-inversion techniques) in an appraised subtle Palaeocene deep-marine turbidite, Central North Sea. The Brenda Field has been an excellent example of where rock physics can act as the bridge between petrophysics and seismic in the world of interpretation.

Geophysical, particularly electrical, methods have the potential to provide detailed spatio-temporal information on the distribution of salinity in soils and groundwater that characterise the floodplains of the Murray River in southeastern Australia, thereby assisting our understanding of floodplain processes. This knowledge can help manage the ecology of these settings, particularly in a time of severe drought when floodplain salinisation, vegetation dieback or health decline is a growing problem.

In this paper we examine the relative merits of high resolution helicopter EM technologies for elucidating floodplain processes. Specifically we consider the relative performance of the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems for defining variations in near surface conductivity and sediment salt load across the Bookpurnong Floodplain in the Riverland of South Australia. Results from coincident surveys are reviewed as are strategies for the inversion of these data. Data are examined against available borehole information including sediment chloride content and groundwater conductivity.

FTG Gravity data acquired on airborne and marine platforms measure 5 independent Tensor components that collectively describe a total gravity field. The components capture unique signature patterns related to specific attributes of target geology that when collectively interpreted enable detailed imagery of the target itself in terms of geometry, composition and depth of burial.

The horizontal tensor components Txx, Tyy, Txy, Txz & Tyz are commonly used to identify and map lineaments associated with structural and/or stratigraphic changes or target geometry in a survey area. The vertical tensor component, Tzz, is used to estimate depth and predict compositional information related to target geology. However, these components have traditionally been interpreted separately from one another and often run the risk of missing out on key information.

This paper describes application of a semi-automated approach that combines the individual components into singular representations to best extract the signature pattern common to all components as revealed by the underlying geology. The examples presented are taken from an Air-FTG^® survey onshore Brazil to image the structural framework and identify target geology ahead of a seismic programme, and a Marine-FTG^® survey offshore Norway to resolve salt body geometries imaging areas of overhang development.

The resultant interpretation enables the end-user to fasttrack the exploration initiative by quickly evaluating target geology for detailed follow-up.

Completion of a 1:100 000 geological mapping program over the Koonenberry region of north-western New South Wales has allowed the Geological Survey of New South Wales to develop techniques for potential field modelling of serial, sheet-by-sheet cross-sections, with the ultimate aim of constructing a 3D interpretative tectonic model for this Delamerian margin. Modelling has been aided by comparison of magnetic and gravity structural trends, revealed by edge analysis of upwardly continued fields (“worming”), made practical by improved data density and quality resulting from the Exploration NSW initiative. Worms aid modelling in two senses: indirectly, through qualitative assessment of tectonic styles; and directly, by fixing the position and trend of otherwise poorly imaged deep sources to constrain inversion. We exploit the different rate of decrease during upward continuation of anomalies due to dipole (magnetic) and monopole (gravity) sources, to distinguish structural differences between relatively shallow (<2–5 km) and deeper rocks. This has been particularly useful for examining the fold-and-thrust tectonics of the region. Comparison of synthetic worms derived from the model anomalies with worms derived from the gridded data allows a powerful test of the validity of the model.

Resulting profiles have been combined in GOCAD to produce a 3D model of the belt. Definition of surfaces linking the profiles is guided by an inverted image of the potential field worms. The final 3D model reveals a series of features related to repeated late Neoproterozoic to Cambrian rifting, amalgamation of the Delamerian and Thomson orogens, and subsequent transpression.

For the past decades nuclear magnetic resonance (NMR) technology has gained acceptance as petrophysical tool for evaluating reservoir quality. Comprehensive formation evaluation requires determination of irreducible fluids, movable fluids, and permeability. The presence of clay, their occurrences and distributions, however, in some reservoir rocks tends to introduce complexity in any formation evaluation activities. This can also cause problem for NMR log interpretation. In the presence of clays the most commonly used T2 cut off values, a constant value throughout a formation, seem to eventually yield inaccurate permeability estimates. Therefore, NMR measurements should be integrated with other measurements from conventional cores for a comprehensive formation evaluation, in which T2 cut off may vary for reservoir with different reservoir qualities. This paper presents results of a study that focuses on NMR measurements on Tirrawarra shaly sands taken from 3 wells situated in Cooper Basin, South Australia. The study suggests that the T2 cut off values for the samples vary significantly in order for NMR-derived irreducible water to match core-derived irreducible water. This is also true for NMR-derived permeability estimates when compared to measured permeability values. Comparisons between estimates produced using the normally used ‘constant T2 cut off’ and the suggested ‘varied T2 cut off’, as well as their effect on formation evaluation, are also discussed. In general, the results highlight the need to study T2 cut off values more directly for specific reservoir rocks before their practical uses in the field.

This paper aims to estimate elastic parameters such as Pwave and S-wave velocity, Thomsen’s anisotropy parameters (, ε density, and layer thickness of a layered medium using pre-stack seismic data and some prior information.

Three different algorithms have been developed in an object oriented environment: Travel time inversion, waveform inversion, and joint traveltime and reflectivity inversion. These algorithms can use compressional or joint compressional and converted shear waves data respectively. We have implemented exact parametric travel time equations in a VTI medium for a stack of either anisotropic or isotropic homogeneous layers. Reflection and transmission coefficients for the whole ray path have been calculated. A convolution model is used to calculate synthetic seismograms.

To minimize an objective function a non-linear conjugate gradient (CG) algorithm has been implemented. All of the partial derivatives have been calculated analytically which results in a fast and robust differentiation of traveltimes and reflection and transmission coefficients.

A numerical example of a stack of 10 layers has been tested for different elastic parameters and thicknesses. Compressional and shear waves velocities and thicknesses have been varied 25 percent from true values while  and ε have been varied 100 percent from their true values. Joint traveltime inversion of P  P & P  Sv has successfully estimated the true values of the parameters. Waveform inversion is highly non-linear; hence only anisotropic parameters and density while varying up to 100 percent are estimated. Waveform inversion results in a full estimation of the true values of  and ε and a fairly good estimate of density.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In this study, subcritical crack growth in rock was investigated in distilled water (pH=6) and aqueous solution of sodium hydroxide (NaOHaq, pH=12). Especially, comparing the results in water to those in air, the effect of water was investigated. Additionally, the effect of pH in aqueous environment was also investigated. Rock sample were andesite and granite. The relation between the crack velocity and the stress intensity factor was determined by Double-Torsion method. All experiments were conducted with controlling temperature. It was shown that the crack velocities in water were higher than those in air. These results are in accordance with the results by many researchers who reported that the crack velocity increased in water. Comparing the results in NaOHaq to those in water, however, it was shown that the crack velocity at the same stress intensity factor didn’t change even though pH of surrounding environment changed. This result doesn’t agree with conventional concept that the hydroxyl ion accelerates subcritical crack growth in silicate materials. It is concluded that water accelerates the crack velocity and pH has little effect within the environmental conditions in this study.