ASEG Extended Abstracts - ASEG2012 - 22nd Geophysical Conference, 2012

There are a number of magnetic inversion methods that have been developed to map the structure and depth of sedimentary basins, assuming that sediments are nonmagnetic and underlain by magnetic basement. Gridding/mapping the basement depth estimates from such methods has two significant problems: 1) the magnetic results are dominated by signals coming from the top edges of basement faults such that the depth information from the down thrown sides of the faults is not captured and 2) within the centres of large basins there is often little variation in the magnetic anomaly to provide depth estimates such that when gridding the depth data, grid interpolation has to rely on a sparse distribution of estimated depths.

In this study we convert the magnetic data into pseudogravity which is then inverted to produce a 3D basin model with a constant susceptibility basement. This overcomes the interpolation problem since the 3D model now uses the complete pseudo-gravity field as well as the magnetic depth estimates to constrain the depth model. The method is applied to the Abu Gharadig basin, Western Desert, Egypt and generates results which match well controls on basement depth. The advantages of this method over 3D gravity inversion are that the pseudogravity response is not affected by structure within the sediments and is not compensated isostatically as the gravity response of basins often is affected.

As part of the Australian Government’s Onshore Energy Security Program, Geoscience Australia has acquired magnetotelluric data along 12 deep crustal seismic reflection transects in Australia.

The magnetotelluric projects, which total more than 640 stations over 3700 km in distance, have been undertaken by Geoscience Australia in collaboration with state and territory geoscience agencies. Broadband and long period MT data have been acquired for investigating geological structures from the shallow surface to upper mantle.

These data, along with the deep seismic reflection, magnetic, gravity and geological data form the basis for multi-disciplinary investigations of crustal architecture, and energy and mineral potential, providing precompetitive information to industry and researchers.

Thiel Surface Impedance Method (TSIM) is a surface electromagnetic device that uses VLF radio waves at a single frequency to measure apparent resistivity changes in the subsurface. The technique allows the interpreter to accurately delineate known geological features and identify previously unrecognised discontinuities.

The aim of the TSIM survey at Coppabella Mine was to determine areas of heavy intrusion in the coal. The survey was conducted in a grid pattern with 16 lines and a total line length of 3,600 m.

TSIM has been successfully used at Coppabella Mine to identify areas of low apparent resistivity occurring where intrusion is expected.

Integrated interpretation through physical properties of rocks provides better approach towards understanding of geological set-up. High resolution heliborne time domain electromagnetic, magnetic and radiometric dataset acquired over Palaeo Proterozoic Bijawar Basin has been interpreted to enhance the geological understanding. The process involved knowledge driven analysis through correlation with geological units that outcrop and data driven analysis for extrapolation of information. Radiometric data in conjunction with spectral remote sensing data was used for generation of surface geology map. Spatial distribution of rock magnetization properties defines the subsurface extension of litho-structural elements and magnetic basement configuration. Electromagnetic data aided demarcation of resistive basement topography and various conductive layers. Radio-elemental distribution refines the unconformable contact shared by Bijawar Group with overlying and underlying group of rocks and also outlined the formational boundaries of arenaceous-argillaceouscarbonate-volcano-sedimentary sequence. Subsurface extent of basic volcanics is much more than its surface manifestation and appears to extend well below the overlying Vindhyan Supergroup of rocks. The unconformable contact of Bijawar Group with underlying basement and overlying Vindhyan Supergroup is marked by distinct change in conductivity parameter and has been outlined in depth to resistive basement topography. Fault system and its manifestation over different litho-units indicate multiple tectonic episodes.

Geological interpretation from the Kaladgi Basin is based on integration of heliborne time domain electromagnetic (HELITEM), magnetic and gamma-ray spectrometry data with available geological mapping, ground exploration data, various published papers and spectral remote sensing data such as Landsat 7ETM+ and ASTER GDEM imagery.

Electromagnetic (EM) data, magnetic, radiometric, Landsat TM and Aster datasets were used for delineation of surface and subsurface extent of geological units and structural elements and for better understanding of the overall structural pattern of the Kaladgi Basin, including distribution of its local depocentres, the resistive basement surface and the magnetic basement topography. The interpretation of the available data details the localisation of intrusive bodies, basaltic lava flows, mafic dykes, basement faults and major intrabasinal fault systems.

Three dimensional grids (voxel models) of EM data were used for visualization in three dimensions.

Interferometric Synthetic Aperture Radar (InSAR) is a technique for mapping subtle surface deformations over a two-dimensional areas with high spatial resolution. The objective of this study is to evaluate the capability of InSAR analysis using both up-going and down-going orbits data for monitoring the long-range ground deformation caused by the environmental disaster. Differential InSAR (DInSAR) analysis and InSAR time series analysis were performed around disaster areas of the Kirishima Mountains in Japan (Fig. 1). The data used in this study were images from the Advanced Land Observation Satellite/Phased Array Synthetic Aperture Radar (ALOS/PALSAR) observed from 2007 to 2011. We performed DInSAR analysis and InSAR time-series analysis with a commercial software and attempted to precisely estimate vertical and horizontal displacements by using the vector composition method from the observation data of both orbits. The results show that InSAR analysis is effective for the disaster monitoring of volcanic eruptions. Uplift and subsidence were detected around the Kirishima Mountains before the last eruption on January 26, 2011. This result suggests that long-range InSAR analysis has a capability to detect the symptoms of volcanic eruptions.

This paper presents a processing workflow to attenuate the strong tube waves from VSP surveys acquired in Perth, Western Australia. An array of 24 hydrophones spaced at 10 metre intervals is attached to a cable run down the borehole. The hydrophones were unclamped, so they record significant amount of the tube waves which entirely mask the upgoing waves. F-K filtering was used for wavefield decomposition of flattened downgoing waves. Linear moveout correction was applied to flatten the tube waves and removed unusual aliased energy due to shot depth variations and hydrophone positioning errors through the F-K filtering. The flattened data were transformed to tau-pi domain using a very narrow window slowness to produce a model of tube waves. An adaptive wavefield subtraction algorithm was used to subtract the tube wave model using a matching filter from the input data. The result was processed through a F-K filter and moderate tau-pi filter to remove the residual undesired energy. After attenuating the multiple energy and true amplitude recovery, pre stack depth migration, a CDP-VSP mapping and stacking were applied to produce CDP equivalent depth and time images.

India has developed an ambitious research program after successfully unearthing gas hydrate layer at site 10 in Krishna Godavari (KG) Offshore, during first National Gas Hydrate Program (NGHP-1). After studying the result from NGHP-1, second NGHP has been firmed up with an objective to identify the Gas Hydrate prospect for test drilling using seismic data and well analysis in KG offshore areas.

First time a 3D seismic data volume has been studied with objective to identify Gas Hydrate prone areas in Indian water.

To achieve this objective seismic data has been calibrated with well log to identify the Top and bottom of Gas Hydrate Stability Zone. The extent of Gas hydrate is inferred on seismic section by large amplitude (opposite polarity of with respect to Sea bottom) bottom simulating reflector (BSR), which is following the sea bottom and also at places cutting across dipping srata. In this study, the BSR mapped and then RMS amplitude extracted within window of 100ms above of it. RMS amplitude distribution suggests the variance in Gas hydrate concentration spatially primarily guided by the porosity variation in shape of channel and scroll bar. Further RMS amplitude extraction 20 ms below BSR suggest the presence of free gas areas associated with Gas hydrate in the down dip towards the regional lows.

The study concludes that there are three to four areas where positive acoustic impedance within GHSZ is available, probably indicative of high Gas Hydrate concentration due to better reservoir condition and may be tested.

The impact of small-scale and ‘branched’ faults in the assessment of fault-trap integrity often remains unknown or unresolved when risking/evaluating a lead or prospect in regards to its petroleum prospectivity. The following study considered 40 leads in the eastern Gippsland Basin and assessed the overall impact of branch lines on fault trap integrity.

Faults and surfaces were interpreted within a 30X50 km area using a 3-D seismic volume, depth-converted using purpose-programmed software and, a fault seal analysis carried out on the top three ranked leads. The fault seal analysis was underpinned by logs from 13 wells located in and adjacent to the leads.

It was found that the likelihood of fault reactivation increased for 32% of the branch-line cases considered, reduced for 26% of cases while the change being minimal for the remaining 42%. Overall, modelling of the volume of shale attribute indicated that it is unlikely that sand-onshale windows can be maintained along fault-strike lengths that range up to 20 km.

The study conclusively demonstrates that the primary factor affecting fault-trap integrity in the Halibut Subgroup is the high proportion of across-fault sand-onsand windows; by comparison, the contribution of fault tips, ‘branched’ faults and presence of shale smear is secondary. One implication is that the fault-trap integrity for leads located in the eastern Gippsland Basin is more likely to be enhanced closer to the depocentre where the shale content is anticipated to be higher.

Aeromagnetic surveys collected over bodies of high susceptibility record fields that have been significantly affected by self-demagnetisation. These effects complicate interpretation as the amplitude of the magnetic response scales non-linearly with the susceptibility. Analytic modelling is only carried out for compact ellipsoidal bodies and most approaches break down in the high susceptibility limit. It therefore of interest to develop tools for understanding and modelling of self-demagnetisation effects to support geophysical exploration. We present a modelling scheme based on multiple prismatic sub-volume bodies in a 3D grid for investigating and testing self-demagnetisation effects. This scheme, and another recent iterative scheme based on multiple spherical voxels, is tested to demonstrate self-demagnetisation effects on simple bodies. The schemes are verified against the known analytic results for a spherical body and the results for a cubic body. The prismatic scheme is then used to model a dipping magnetite sheet for comparison against real data where self-demagnetisation effects are thought to be important. Finally, we demonstrate the main advantage of the correction scheme by modelling multiple magnetic bodies in a 3D grid. These correction schemes are aimed at improving current magnetic modelling techniques and providing solutions for dealing with complex and highly susceptible bodies.

Large-scale high-resolution 3D inverse models have successfully been built for the Wallaby Plateau and Capricorn region of Western Australia using the National Computational Infrastructure supercomputer facility. This work highlights the benefit of using increased computing power to achieve higher grades of detail in 3D inversion modelling.

Geophysical modelling often requires High-Performance Computing (HPC) when applying complex calculations to large datasets. To meet HPC requirements, access to parallel grid computing services via the internet, termed cloud computing, has been established for geophysical inverse modelling using UBC-GIF codes GRAV3D and MAG3D. Portals and workflows have been developed to simplify the inversion modelling process and to allow more efficient access to inversion modelling tools.

A mechanism which allows inversion modelling jobs to be submitted to cloud computing resources using a web browser (a geophysical portal) is being developed and tested at the National Computational Infrastructure (NCI) housed at the Australian National University (ANU). This work is the result of a collaborative project between Geoscience Australia, CSIRO and NCI.

This work was done in the frame of a greater project aimed at assessing possible impacts of climate change on the groundwater resources in Timor-Leste.

With the intention to contribute to the understanding of the area’s aquifer characteristics and groundwater quality, two lines of time domain EM soundings were acquired in different coastal locations. The acquired data where then inverted and interpreted with in a contextual framework.

The geophysical component within the broader project was brought in to assist with coastal aquifer characterisation and to better understand the aquifers’ architecture. The method employed has proven to be an effective way of sounding the ground in Timor-Leste. Some of the results we have produced have contributed to fill-in gaps between well logs and water chemistry and have helped identify the potential presence of a seawater wedge in proximity of some of the city’s pumping wells.

Uncertainty in the tomographic inversion of near-surface seismic refraction data can be separated into aleatory variability, which describes the misfit errors and epistemic uncertainty, which describes the suite of acceptable models. Common implementations of refraction tomography usually focus on reducing aleatory variability and frequently disregard epistemic uncertainty.

In this study, the tomograms generated with three models of the seismic velocities in both the weathering and in the sub-weathering, using the generalized reciprocal method (GRM), are consistent with the traveltime data. However, only one tomogram is consistent with the optimum XY value and the attributes derived from the head wave amplitudes and seismic velocities. This study demonstrates that epistemic uncertainty can be explicitly addressed with the GRM, because the most probable tomogram can be selected objectively from a number of acceptable alternatives.

The GRM-based tomogram successfully detects, defines and differentiates narrow regions with low seismic velocities which represent shear zones and a massive sulphide ore body. None of these zones is detected with the tomogram generated with the default starting model using smooth vertical velocity gradients.

It is concluded that minimizing epistemic uncertainty through the use of the most appropriate starting model is more important than minimizing aleatory variability.

New ways of energy production through the use of coal seam gas plays and geothermal hot dry rock and hot sedimentary aquifer systems pose challenges in identifying and monitoring fluid in the subsurface. We propose the use of the magnetotelluric (MT) method to image static and dynamic fluid distributions in the subsurface exhausting the contrast in electrical conductivity between resistive host rock and conductive fluid-filled, porous rock. Base line MT measurements provide reference transfer functions and inverse models to characterise the electrical conductivity distribute on which is linked with bore hole and other geophysical data to obtain knowledge about fluid distribution at depth. The reference models are used to accurately forward model fluid injection or extraction temporally and spatially. This work shows results from fluid injections at a hot dry rock system at Paralana, South Australia, and its applicability to other geothermal and coal seam gas systems.

Superparamagnetic effects in soil and rock cause responses in a variety of EM survey types that are routinely used in mineral exploration surveys. The identification of these effects can be difficult and the response may be very similar to that from targeted base metal orebodies. Identification of these effects in TDEM datasets is critical for prioritising ground follow-up of targets and failing to correctly identify SPM anomalies frequently leads to inaccurate inversions, misdirected field work, and unnecessary, deep drill holes. As signalto-noise ratios in acquisition systems increase, SPM effects will become an increasingly common issue in both airborne and ground EM surveys.

This paper presents examples of SPM responses in data collected with the latest airborne, ground, and drill hole EM systems used in minerals exploration. Ground follow-up results confirm the SPM responses.

Techniques that have been found to be effective to firstly avoid, and secondly to identify SPM anomalies in survey data are presented. These techniques are based around minimising the primary energising magnetic field at the source of the SPM, and by distancing the sensor so that it is separated from the source. The main technique for recognising SPM effects directly in data remains the fairly predictable power law decay rate. Secondary techniques such as association of anomalies with alluvial sediments, topographical features (e.g. drainage, hills), and palaeochannel patterns also provide indications of SPM. Associations of anomalies with low transmitter loop height in airborne systems is also indicative of SPM, however systems that can better discriminate SPM need to be developed.

Over the past twenty-five years Geoscientists have acquired more than 550,000 square kilometers of 3D seismic data [APPEA statistics] over continental and offshore Australia in the pursuit of mineral and petroleum deposits. Whether the target is hydrocarbons of any phase (solid, liquid or gas) or minerals, the information extracted from the 3D seismic data when integrated with other geological and geophysical data helps form models of the subsurface. These models are the foundation upon which decisions are made, directing the design and the development of many aspects of the mine site. The success of these activities often depends upon the accuracy of these models.

Many advances in acquisition, processing and interpretation methods have been implemented since the first 3D seismic surveys were acquired in Australia during the 1980s. As a consequence of these advances, the geoscientist today is faced with dramatic increases in the volumes of high quality data available for analysis. However, the time available for the thorough examination, analysis, extraction and integration of the information from these large often multi-volume datasets is always limited and is becoming more problematic. Typically, the geoscientist will spend most of their available time extracting information from small portions of these datasets with a disproportionately amount of time spent thinking about the significance of the results.

Fortunately, Geoscientists are not the only, or the first Scientists, to face challenges associated with the analysis of large amounts of data. Specifically, ideas developed during the course of the thirteen (13) year Human Genome Project (HGP) have been adapted to help interpret seismic data by automatically segmenting and identifying all surfaces within a 3D volume of data; which is then stored into a visual database. Using this technology enables the Geoscientist the ability to analyse large amounts of data in a unbiased manner and thereby incorporate much more data into their models. The application of this patented technology is discussed and demonstrated on several 3D seismic datasets collected over several Queensland Bowen Basin Coal Mine sites.

Six high resolution 3D seismic surveys comprising approximately 31 km² were recorded for an underground coal mine in the Permian aged Bowen Basin Central Queensland. The colliery had previously interpreted the data with view to identifying hazards at the mined seam level.

The seismic data and an extensive set of borehole data were made available for a Masters Project at the Queensland University of technology with the focus being to study the geology of the interburden.

This paper discusses the background to the seismic surveys and the work flow used for the interburden mapping. Conclusions and recommendations are drawn

For decades, Mongolia remained largely underexplored because it is a landlocked nation sandwiched between Russia and China, and interest for their mineral deposits was minimal. As the center of global economic growth shifted to Asia in the last decade, many foreign investors became interested in Mongolia as the country has many known rich mineral deposits. What was then an isolated nation has become a positive strategic position as China could purchase many of the minerals Mongolia can produce to fuel its robust economic growth.

As a result, geological teams and geophysical surveying companies were quickly formed to explore and evaluate the country’s mineral resource potential. Two recent world-class deposits were announced for development, i.e., Oyu Tolgoi (copper and gold) and Tavan Tolgoi (coal). Some geophysical case studies are presented to highlight the opportunities and challenges required to better understand the complex geologic and stratigraphic structures that gave Mongolia its diverse and rich mineral deposits.