ASEG Extended Abstracts - ASEG2010 - 21st Geophysical Conference, 2010

The exponential increase of seismic velocities with effective stress has usually been explained by the presence of pores with a broad distribution of aspect ratios. More recently, a stress-related closure of soft pores with a narrow distribution of compliances (e.g., grain contacts) has been suggested to be sufficient to explain such exponential stress dependency.

This theoretical interpretation has been verified here using laboratory measurements on dry sandstones. On the basis of these experimental data, linear dependency of elastic compressibility on soft porosity and exponential decay of soft porosity and elastic compressibility with effective stress up to 60 MPa is confirmed.

Soft porosity, estimated from the fitting coefficients of elastic compressibilities, is on the same order of magnitude but slightly lower than obtained from strain measurements. The results confirm applicability of previously proposed stress sensitivity models and provide justification for using this approach to model stress dependency of elastic properties for isotropic and anisotropic rocks.

A new scanning magnetic microscope to image magnetic fields with micrometric resolution has been constructed based on a giant magnetoresistance (GMR) sensor. Operating in zerofield, the GMR sensor is sensitive to the components of the magnetic field arising from the remanent magnetisation parallel to the plane of the cut rock surface. The field component perpendicular to the surface therefore has to be calculated using FFT relationships. The spatial resolution of the system is 20 μm, and its peak-to-peak noise during operation is 250 nT. Its high spatial resolution and a minimum sensor-to-sample distance of only 30 μm compensate for its rather modest field sensitivity.

Once all three components of the field are known it is possible to determine the magnetic moments of individual fine particles. The individual moments of fine particles can be combined vectorially and the total magnetic moment per volume (i.e. the magnetic intensity) can be estimated and compared with macroscopic measurements, either on standard samples or through ground-or aero-magnetic surveys.

This procedure has been performed on a hemo-ilmenite - magnetite norite sample from the 930 Ma Bjerkreim-Sokndal (BKS) layered intrusion of south Norway. The BKS is a 7 kmthick norite to quartz mangerite layered intrusion, part of the early Neoproterozoic Rogaland Anorthosite Province. The sample with a strong lattice preferred orientation (LPO) of orthopyroxene and moderately strong LPO of hemo-ilmenite, has an NRM of 53-60 A/m inclined at ~65^° to the polished surface. Highly variable magnetisation in the BKS, caused by progressive magmatic crystallization giving rise to differing combinations of opaque minerals, highlights some of the possibilities to be considered in evaluating crustal magnetic anomalies.

The Elk/Antelope gas field is hosted in Miocene reefal and deepwater carbonates in Papua New Guinea. The carbonates exhibit multiple diagenetic overprints and complex internal seismic reflector heterogeneity.

The objective of this study is to identify the main cause(s) of seismic reflectors within the Elk/Antelope gas field and to ascertain if diagenetic boundaries are resolvable in seismic data.

Wireline logs in the Antelope-1 and Antelope-2 wells were corrected and P-Impedance logs were generated. Wireline logs were correlated with thin section petrological studies to characterize lithological facies and diagenetic overprint. A well to 2D seismic tie for the Antelope-1 well was generated and forward modelling of synthetic seismic was conducted at the Antelope-2 well.

We find that the dominant control on seismic reflection events within the reservoir are porosity variations. Lithological variations due to diagenetic overprint exert a secondary influence. Within the reservoir at the Antelope-1 well, chronostratigraphic boundaries are associated with the highest amplitude seismic reflection events. At the Antelope-2 well we model two prominent reflectors below the top carbonate event and we interpret these events to be related to diagenetically controlled porosity variation.

Stress dependency of the TI elastic tensor of shales is important for seismic interpretation, fluid identification, 4D monitoring, etc. Using Sayers-Kachanov formalism, we develop a new model for transversely isotropic (TI) media which predicts stress dependency of all five elastic coefficients. The four fitting parameters (namely, tangential compliance, the ratio of normal to tangential compliance of a single crack, characteristic pressure and crack orientation anisotropy parameter) are calculated for stress dependencies of elastic properties of about 20 shales obtained from our laboratory measurements and also from a literature survey. The ratio of normal to tangential compliance of a single crack is equal to 0.1-0.2 for the majority of the shales and shown to be close to unity just for a few shales. Thus, it appears that the socalled scalar crack approximation (that implies that shear and normal compliances are equal) is not generally applicable to shales and the full form of Sayers-Kachanov formulae is used for further analysis. The four fitting parameters show moderate correlations with the depth from which the shale was extracted. With increasing depth, the tangential compliance broadly decreases exponentially and the ratio of normal to tangential compliance generally increases linearly. The crack orientation anisotropy parameter exponentially increases with the depth for most of the shales indicating that cracks may be more aligned in the bedding plane. The characteristic pressure shows no simple correlation with depth. The suggested model allows prediction of the stress dependency of all five elastic coefficients if only three of them are known. This can be useful, for instance, for the reconstruction of all five elastic coefficients of shale from log data.

Uranium deposits are known to occur in soda-metasomatised rocks and metasediments at several places in the world constituting 18% of the world’s uranium production. Uraniferous metasomatites/albitites are identified at several locations along the 320 km long albitite zone in the environs of North and South Delhi Fold Belts of Rajasthan, India. A multi disciplinary geoscientific approach followed over the last few decades, led to the discovery of a uranium deposit along the albitite line. Fracture controlled uranium mineralisation is associated with weak to moderate albitisation and pyroxenisation of metasediments, as well as with metallic sulphides and carbonaceous phyllites. Mineralisation is mainly in the form of uraninite associated with copper, molybdenum and sulphides. The low resistivity of the fractures associated with metallic minerals produces a significant contrast with the host rock, which can be located with electromagnetic methods. Highresolution heliborne geophysical surveys were conducted to identify deposits in the albitite zone. This paper demonstrates the results of heliborne and ground follow-up geophysical surveys to prioritise targets for uranium exploration.

Analysis and integrated interpretation of the heliborne high-resolution multiparameter magnetic, transient electromagnetic (VTEM) and gamma ray spectrometric surveys conducted in Archaean–Proterozoic metallogenic province of North Delhi Fold Belt, Rajasthan has delineated several targets for uranium exploration. A ground based Time Domain Electromagnetic survey employing Geonics make EM37 system were conducted over one of the target area. Interpreted results from ground data correlate well with the spatial locations of the EM conductors delineated from heliborne surveys. Modelling of both heliborne and ground based EM data revealed the presence of subsurface conducting bodies with comparable model parameters. Plate models generated from both the data sets provided useful input to ongoing exploration programme.

Lowering receiver noise levels of EM systems has the same benefits as increasing the transmitter signal, because detectability, discrimination and resolvability of target responses improves as the signal to noise ratio increases. Unfortunately the peak dipole moment (where peak dipole moment is considered to be the product of the peak current and the effective transmitter loop area) of TEM systems is often still perceived as being the most significant factor contributing to the signal to noise ratio. The importance of also taking into account the excitation waveform pulse width and shape when determining target response is well published. However, the quantitative effects of reducing receiver noise levels have not received much coverage in literature. Modeling experiments using real VTEM system noise samples from 2006 and 2009 indicates that a four times reduction in system noise amplitude can improve detectability of targets up to 100 m deeper and also enables target conductance and geometry to be modeled as accurately for targets up to 150 m deeper. These improvements are better than can be modeled with a simulated increase in peak dipole moment giving the same signal to noise ratio. This is most likely due to the fact that the real character of noise should be taken into account and not only the standard deviation.

The Geological Survey of Namibia hosts a world leading airborne geophysical data coverage library that has amassed from years of large scale regional airborne magnetic and radiometric survey acquisition with detailed high resolution survey follow up to promote the exploration and mineral wealth of the country. In 2005 the Geological Survey of Namibia acquired a strategically planned Airborne Electromagnetic Survey in Northeast Namibia. Following completion of this initial phase one reconnaissance survey (532 line km), two areas were selected for detailed high resolution follow up, namely Elandspan and Eiseb. In total just over 5,000 line km of AEM was acquired.

Initial qualitative interpretation indicated that the Tempest AEM system could detect conductors beneath the Kalahari sediments. Recently a more detailed interpretation has been undertaken with many 2D and 3D mapping products generated. Conductivity Depth Images along with early, mid and late time EM products were created illustrating the surficial effects of a river system and a number of deep basement conductors. The 2D and 3D AEM products led to an integrated geological interpretation of the area and significantly added value to an aeromagnetic interpretation of the area. The CDI’s generated allowed the thickness of the Kalahari sands to be quantified, far shallower than anticipated, increasing the exploration prospectivity of the region and a geological map of the area, based on these results, has been created in an area with very little data coverage and no outcrop.

Gravity anomalies from geological features in the upper crust are masked by large amplitude long wavelength gravity variations from isostatic roots. An isostatic residual (IR) gravity anomaly grid of onshore Australia has been produced by Geoscience Australia which has these long wavelength features removed. This gravity map reveals more clearly the density distributions of geological interest within the upper crust.

The depth to mantle model and subsequent isostatic corrections were produced using a modified version of the USGS program AIRYROOT provided by Intrepid Geophysics. Geoscience Australia’s 2009 Bathymetry and Topography Grid was used to calculate the depth to crustal bottom following the Airy-Heiskanen crustal-root model. The isostatic corrections were then applied to the complete Bouguer anomalies to produce the Isostatic Residual Gravity Anomaly Grid of Australia.

CSEM has been applied as a risk mitigation tool in the Krishna-Godavari Basin, East Coast India. A workflow for application of CSEM in a portfolio ranking process is described, ranging from feasibility assessment to interpretation and post-drilling analysis.

Data interpretation was based on relative response analysis, unconstrained inversion and 3D modelling. A major challenge, encountered in the data interpretation was the lack of suitable calibration areas for the interpretation model. Nevertheless, several robust resistivity anomalies were identified, which were then prioritised for drilling.

The drilling of a CSEM positive prospect yielded commercial hydrocarbons. This additional calibration information, obtained from the well, was then fed back into the integration work-flow and robustness analysis for the previously lower-ranked resistors. Even using this more complex model, the presence of the resistors was confirmed. This motivated Reliance to review the seismic to upgrade the CSEM leads to prospects.

Recognising the imprint of invading fluids on primary geology using geophysical and hyperspectral airborne imagery can help locating sites where there may have existed suitable physicochemical contrasts able to precipitate Au. This is tested in the St Ives area of Western Australia, for regionally metamorphosed ultramafic rocks. Airborne magnetic TMI data are used to map areas enriched in magnetite, which could have developed after serpentinisation (hydration reactions). However, magnetic susceptibility often changes along strike of mapped ultramafic units. The hyperspectral data provide mineralogical links between this geophysical response and the mapped geology. That is, areas of higher magnetic response are often associated with relatively ferrous-poor silicate Mg-OH-bearing minerals. The ultramafic units are characterised by high contents of talcamphibole. Cross-cutting zones of potential Kmetasomatism can be assessed using the hyperspectral chlorite (and biotite) information and gamma radiometrics.

The northwestern margin of Australia is a prolific hydrocarbon province, with large areas affected by volcanism and igneous intrusions, such as the outer Exmouth and Browse basins as well as parts of the Canning Basin. Volcanic features can easily be mistaken for potential hydrocarbon traps. Some have subsequently been drilled with disappointing and costly results. Proper delineation of volcanic features is therefore crucial in exploration efforts especially with current NW shelf exploration progressing in ever deeper water closer to the continent-ocean boundary. Most igneous rocks drilled on the northwest shelf consist of doleritic intrusions, basalt flows and volcaniclastics. Normally doleritic intrusions show a sharp contrast on seismic data due to their high acoustic impedance resulting in bright amplitudes on the seismic stack as seen on spectacular sill complexes on the Exmouth plateau as well as in the Canning basin. The high impedance contrast of volcanics also contributes to seismic imaging problems below the volcanic flows or intrusions, this becomes more evident when basalt flows thicken as evidenced on the North Atlantic margin. Similar potential thickening of basalt and volcaniclastic sequences is encountered on the NW edge of the Browse basin. This has important consequences for prospectivity in this region. Another common imaging problem is interbed multiple generation due to the high impedance of the volcanics. Other problems arise when volcanics have been exposed to significant weathering and subsequent burial resulting in lower impedance values, making it more difficult to distinguish them from clastic rocks. Large volcanic features such as igneous complexes and volcanoes, can mimic antiformal structures on seismic data, but can be easily de-risked by integrating gravity and magnetic data, especially by using data filtering techniques. More subtle volcanic features can be more difficult to distinguish and could be mistaken for reefs or potential hydrocarbon traps. Submerged volcanoes might also be prospective due to reefs growing on top of them as evidenced by examples from western India.

Automated shape recognition technology has been developed for application to porphyry exploration, through a joint research initiative between Barrick Gold and the Centre of Exploration Targeting at the University of Western Australia, commencing in 2009. The result is the Development of the Porphyry Texture Filter for application to magnetic datasets.

Many mineralised porphyries display concentric zonation in their magnetic character as a by-product of extensive hydrothermal alteration systems and secondary magnetite development/destruction. This characteristic magnetic signature can be exploited by image processing techniques enabling the enhancement, identification and quantification of features. Features must agree with a user-defined set of criteria for size, shape and magnetic contrast.

Development of the technique was carried out on the world class Reko Diq porphyry system resulting in successful identification of all major known mineralised porphyry centres and additional targets within the camp. User control over filter parameters has resulted in the successful application of the filter on projects in a range of geological and erosional environments.

The ability to rapidly characterise porphyry-like signatures using mathematical principles and geometries results in an unbiased geophysical target layer. When integrated with other geoscientific data, the filter has consistently supported target generation activities.

Examples of the Porphyry Texture Filter application and results from Reko Diq, Grasberg and active exploration projects are shown.

In 2008 Geoscience Australia acquired regional seismic reflection, wide-angle refraction/reflection, gravity, and magnetotelluric (MT) data along a 253km east-west transect across northern Eyre Peninsula, Gawler Craton in South Australia as part of the Australian Government’s Onshore Energy Security Program (OESP). These datasets provide complementary information on the crustal architecture and evolution of this part of the Gawler Craton.

Twenty three Earth Data recorders were deployed along the reflection transect as a fixed, wide-angle recording array, with 5 and 10 km interval spacing, using threecomponent seismometers. The high quality data of up to 60-70 km offsets were collected using vibroseis sources. MT data were acquired at 40 broadband and 12 long period sites along the reflection transect.

This paper presents acquisition parameters and processing steps for these datasets, to highlight their advantages and limitations, and improve the quality of the geological interpretation over the Gawler Craton.

Petrophysical properties of carbonate reservoirs, which are the majority of all hydrocarbon reservoirs worldwide, are much less predictable than the properties of siliciclastic reservoirs. This is mainly due to chemical interactions of carbonate rocks with percolating fluids that cause dissolution, ion exchange, and recrystallization in geological time. These processes result in dramatic changes in density, porosity, and permeability leading to inhomogeneity of carbonate rocks on micro-and meso-scales. Quantification of the elastic variability of carbonate grains on the microscale is the first step to constrain models and to obtain more realistic predictions of practically important rock properties of carbonate reservoirs. In this study, we present elastic moduli of an oolite sample from the Dampier Formation - Pleistocene of Southern Carnarvon Basin, Western Australia, obtained by nanoindentation. Young’s moduli of this highly heterogeneous sample are measured at 49 points regularly distributed in a 70x70micrometer rectangular grid on the sample’s surface. The frequency diagram shows bimodal distribution of the Young’s moduli oriented around values of 56 GPa and 144 GPa that correspond to low and high density components of the grain respectively. These two solid phases are apparent in the high resolution scanning electron microscope images. We used the obtained moduli of this composite grain for numerical modelling using finite element approach of elastic properties of the carbonate sample from micro-Computed Tomography images. The results compare favourably with the elastic moduli obtained from acoustic velocities measured by ultrasonic technique.

Interpretation of geoscientific data is a difficult and sometimes an impossible task. The interpretation process involves the interplay between what are obviously or objectively noticeable phenomena in observations and what interpreters bring (i.e. subjectivity) to the interpretation in regards to presuppositions and expectations. As a first step towards understanding subjectivity and human biases in interpretation, our study focuses on quantitative profiling of the behaviour of interpreters. Whilst the data is being observed, we use an eye tracker that captures eye gaze information associated with the visualization of the data. The specific aims of this study include the analysis of: (1) target spotting accuracy and efficiency between interpreters with different levels of experience and different geoscience expertise; (2) the impact of commonly used enhancement tools for data interpretation. A preliminary experiment was conducted using an aeromagnetic dataset from Ontario, Canada, and the 1^st order vertical derivative (1VD) of the corresponding data, to characterise the observation patterns in free viewing and target spotting accuracy for specific geological features (faults, granitoid intrusions, kimberlite pipes). The results showed: some distinct observation patterns between experienced and inexperienced interpreters; and the impact of the 1VD enhanced data in interpretation. The study outcomes may impact on a wide range of geoscientific activities including: risk management in decision making for the mining industry; education and training of geoscientists; and the development of enhancement tools for geoscientific data.

A full width image of the radiometrics of Australia is offered as a standard source for radiometric images of subset areas. High pass filtering each of the component grids has allowed colour transforms to be obtained for the whole continent that approximate the transforms that would be required to equalise each zoomed subset.

Standard practice for radiometric mapping has been to cut subset areas from the separate radiometric grids and use a proprietary image processing software to obtain an equalised colour stretch of the area.

The Australia wide radiometric image already has the image processing completed, so the subset can be taken of the image itself. Consequently, specialist software is not necessary to obtain sufficiently equalised images of the radiometrics of any area within Australia.

The image is available as a TIFF file on GADDS, from which subsets can be downloaded and studied in ordinary office software.

We show that it is practical to invert entire airborne electromagnetic (AEM) surveys to 3D conductivity models with hundreds of thousands of cells within a day on a workstation. We have exploited the fact that the area of the footprint of an AEM system is significantly smaller than the area of an AEM survey to develop a robust 3D inversion method which uses a moving footprint. Our implementation is based on the 3D integral equation method for computing AEM data and sensitivities, and the re-weighted regularized conjugate gradient method is used to minimize the objective functional. Even for terranes which are arguably as close to 1D as geologically possible, we demonstrate that results from our 3D inversion are a significant improvement over those models obtained from layered earth inversion. We demonstrate this with 3D inversion of RESOLVE frequency-domain AEM data acquired for salinity mapping over the Bookpurnong Irrigation District in South Australia.

ZTEM is a helicopter-borne AFMAG system that measures the magnetic-field response in the frequency range 25-600 Hz of naturally occurring currents in the subsurface. The resolution of this system is analyzed by forward modeling and inverting synthetic ZTEM data using a 2D algorithm for a range of conductivity scenarios.

ZTEM data acquired at the Forrestania test site are compared with overlapping VTEM data. Conductivitydepth sections derived from both data sets show broad agreement, but indicate better spatial resolution for the VTEM data. The response due to bedrock conductor IR2 is strong for the VTEM system and subtle on the ZTEM profiles, which appear to be dominated by responses to larger, elongated structures. Products derived from the ZTEM data, including apparent conductivity, phase and Karous-Hjelt filtered grids appear to map geologic structure, complementing the information gathered from the VTEM data.

Petroleum exploration of the Permo-Carboniferous Pedirka Basin (central Australia) has been limited due to the high costs of exploring in this region. Modern high resolution magnetics can be used as an effective tool to map major structures that may influence migration and trapping of hydrocarbons, but careful processing is needed because geologically sourced variations in TMI are comparatively small in the study area. Aeromagnetic data at 400m line spacing was collected in eleven datasets from government sources. The minimum curvature and bidirectional line gridding algorithms were applied on two datasets which were subsequently evaluated in the Fourier domain for undesired artefacts. The radially averaged power spectra highlighted power spikes related to the grid Nyquist frequency in the minimum curvature algorithm for grid cell sizes of 1/5, 1/4 and 1/3 the line spacing. These artefacts could be reduced with a value of 99.99999% for the % pass tolerance parameter. Bi-directional line gridding produced very good gridded results and exploited the high resolution along-line data. The separate grids were stitched together and were reduced to pole prior to the application of a vertical derivative filter. Textures of the resulting grid revealed five apparent depth-to-magneticsource zones. The deepest zone represents sediment thicknesses of the order of 1.5km – an area which includes the Eringa and Madigan troughs. Also visible in the vertical derivative filtered grid are stratigraphic features such as onlap margins, embayments and channel flows – all visible to moderate depths.