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

To compensate for lateral or temporal divergence of converted-wave (PS) imaging as a function of azimuth, a two-tiered workflow for applying a non rigid matching (NRM) process is utilized. The resultant product is a final enhanced 3D PS volume with superior stack response and continuity from the optimum combination of two distinct azimuth sectors of a PS prestack time migration (PrSTM) dataset. The method allows for the crossline artifacts from the effect of azimuthal anisotropy on the convertedwave moveout to be almost completely removed. Furthermore, the severe acquisition footprint, from insufficient crossline aperture as a result of a suboptimum survey design, is effectively mitigated. Seismic data examples of 3D inlines, crosslines and time slices taken from a proprietary 3D/4C OBC survey from the Gulf of Mexico acquired and processed in 2006, demonstrate the compelling benefits of the NRM application and the robustness of the developed workflow. Finally, the output NRM displacement attributes (time-shift values) are found to possess a qualitative value that are not only powerful indicators of azimuthal anisotropy, but also through calibration, may yield valuable information on the magnitude of shear splitting and principal directions of polarization.

Palaeochannels are typical geomorphic features representing drainage streams or rivers, which were flowing either during the past time and now stand buried or shifted due to tectonic or geomorphological processes. They often contain considerable thicknesses of sediment with elements which have been dated from the Mesozoic through to the late Cainozoic. These sediments are known to host, or act as pathfinders, to economic mineralization, and are an important source of potable groundwater, particularly in remote areas. Located about 600km northwest of Alice Springs, the Tanami region of the Northern Territory is an emerging gold province in Australia. Although not genetically related, some of the gold deposits are located on the margins of palaeochannels. Outcrop in these areas is sparse and bedrock is generally covered by in situ and transported regolith materials, that at places may reach more than 300m. Interest from industry and extensive drilling activity in the region has provided us with a natural laboratory to apply a multi-disciplinary approach for studying regolith properties and processes. As part of the research activities of Co-operative Research Centre for Landscape Environments and Mineral Exploration (CRC-LEME), we have carried out ground transient electromagnetic (TEM) and down-hole EM measurements as well as hydrogeochemical sampling of the open drill holes at the Titania mineral prospect. Apart from identifying possible locations of mineralisation, these studies have clearly delineated the structure of the palaeochannel, the character of the fill and the properties of the groundwater. A detailed discussion on the results will be presented.

Major difficulties associated with airborne geophysical surveys are rapidly disappearing with the development of precise position systems. These techniques could be useful in observing the natural source EM inductive field from a moving airborne platform. Recent studies show that fluxgate magnetometers with high sensitivity along with real-time precise positioning techniques could be used in making such airborne measurements. The concept of airborne measurement of natural source EM induction is similar to the ground based geomagnetic depth sounding (GDS). The only difference is that instead of simultaneously observing the magnetic field with an array of ground stations, airborne system is flown over an area of interest and measures a range of high frequency signals at a pre-defined interval. The 3-component fluxgate magnetometer data thus collected could be processed using the GDS method to study the lateral conductivity variations within the subsurface. The depth from which the information is returned depends on the frequency (or periodicity) selected and subsurface conductivity situations. This method could therefore be successful applied to exploration in cratonic areas (e.g. Canadian Plateau) where there are lots of fresh rock exposures. However it is uncertain how successful this method could be applied in areas of thick regolith cover such as in Australian conditions. Numerical modeling could possibly test the likely success of this method under those conditions. In this paper I shall discuss how one can utilize the 3-D Finite Difference forward modelling approach to compute the airborne responses of shallow subsurface lateral conductivity anomalies under various surface/subsurface conditions.

A new seismic monitoring network installed in northern Western Australia in October 2005 seeks to record low-level seismicity in the region, with a view to understanding event frequency, magnitude, location, geological controls on seismicity and deep-crustal structure that may be contributing to crustal weakness. Data from a network of eight semi-permanent stations, located at localities between Shark Bay and the Dampier area, complements data from an additional 80 stations that were deployed for shorter periods as part of other research projects at the ANU between 2000 and 2006. This dense network of seismometers provides the opportunity for a detailed analysis of seismicity and shallow- and deep-crustal structure in the northwest of WA.

For events such as the M5.3 event in Shark Bay in February 2007, our network provided the detailed data required for accurate location and potential fault plane solutions. Potential fault plane orientations have significant implications for our understanding of the neotectonic evolution of this part of Australia, and mechanical contrasts in the crust that may be predisposing areas to failure. Despite the region hosting large earthquakes (Geraldton, 1885, M6.5; offshore WA, M6.2, 1920), including Australia’s largest known event (Meeberrie, M7.3, 1941), little is known about the frequency, magnitude or causes of seismicity in this region, which is far less known than the SW Seismic Zone.

This project is part of an Australian Research Council Linkage project hosted at The University of Western Australia and involving collaboration with The Australian National University and Geoscience Australia, sponsored by Woodside Energy.

There are three principal sources of risk in mining and exploration: the external market, process and mining risk and geological risk. Only the external risk is truly beyond the control of the owner/operator. Underlying all other risk is the quality of the understanding of the geological risk that underpins mining and processing decisions.

A number of case studies are discussed which highlight areas where Anglo and affiliated companies have employed geophysics to reduce the geological risk in exploration, delineation and production.

Geological risk is intrinsically 3-dimensional in nature. Despite the fact that the parameters that describe a geological entity can generally be measured with some precision, e.g. density, grade, chemical composition and hardness, lack of access in the third dimension (depth) combined with cost, means that we invariably deal with incomplete data sets.

Yet it is precisely the geological model that informs the type and size of mine and processing options. The chances are that the resulting mine/process options will be sub-optimal and in the worst cases completely wrong.

Acquiring the necessary information costs money. Not having the information inculcates risk. There must be an optimum position where the overall cost of the information and the reduced risk is at a minimum.

Traditional methods of exploration/delineation no longer suffice. A judicious mixture of traditional and geophysical methods offers an opportunity to gain an adequate understanding of the critical geological parameters and thereby make better decisions on mine/process design.

Spectrem Air Limited is jointly owned by Anglo American and De Beers. The Spectrem2000 system was upgraded to its current configuration in 2000, with significantly improved transmitter and acquisition capabilities. Since then there has been a steady progression of improvements to the acquisition and receiver components.

The Spectrem design philosophy was to build a broadband system with the following capabilities: high resolution mapping; deep penetration; ability to operate in conductive terrain; direct detection and safe operation.

To date over 1,000,000km of surveys have been flown in Africa, South America, North America and Europe. Due to Spectrem’s unique broadband configuration, it has been successful in both direct detection and mapping roles. Recent developments have been aimed at improving the ability to conduct AEM surveys in conductive areas. This includes building a low frequency EM bird and processing developments to obtain better discrimination for more conductive targets.

A number of case studies from various terrains are presented demonstrating the multi-faceted nature of the Spectrem system.

A combined interpretation of gravity, magnetic and seismic data has been used to investigate some key structural elements of the southern part of the Northland Basin which lies off the west coast of North Island, New Zealand. 2-D seismic reflection data reveal a thick sedimentary sequence, a number of significant structural highs and large Miocene volcanic complexes. The basement reflector is generally poorly imaged, especially below locations where volcanics are evident at shallower levels in the seismic sections. Gravity and magnetic maps clearly show the locations of the major volcanic centres which have strong anomalies (up to 50 mGal and 800 nT). In addition, a belt of positive magnetic anomalies (up to ~200 nT) trends N-S in the south and NW-SE in the north. This follows the strike of the main structural trends in the region and suggests the occurrence of a highly magnetic basement terrane. Preliminary 2.75D gravity and magnetic modelling shows that the Miocene volcanic bodies are broader than previously interpreted and extend to basement depths. Modelling also shows that the top of the basement is deeper (up to 9 km) than is evident in the seismic sections. The gravity and magnetic data therefore are invaluable in resolving some of the ambiguities in the seismic interpretation.

In this paper, we discuss the influence of the self-demagnetization effect on magnetic data and present an alternative means of quantitatively interpreting such data in highly magnetic environments. In particular, we present two important results based on simulation which one might consider in their interpretation of magnetic data when self-demagnetization is present. First, current methods for estimating total magnetization, which are typically applied to the problem of remanent magnetization, do not reliably recover this parameter when the anomalous source bodies have high magnetic susceptibilities. And second, a single value estimation of total magnetization does not provide adequate information to properly resolve subsurface geology through inversion. Numerical experiments demonstrate that directly inverting amplitude data, calculated from magnetic data yet weakly-dependent on magnetization direction, produces superior results when interpreting data generated in terrain with high magnetic susceptibilities.

A combined tuning/AVO model was used to predict the oil column height for the Tui, Amokura and Pateke fields in the offshore Taranaki Basin, New Zealand. The Palaeocene aged Kapuni F10 reservoir sands occur at a depth of approximately 3700 mSS. The closure height and areal extents of these fields were initially mapped by converting time structure maps to depth using average velocities derived from stacking velocities. However, the traps at Tui are broad low relief features, extending up to 4 km laterally but with only limited vertical relief. This geometry means that even very small velocity variations have a significant impact on the location and extent of closure.

Weak amplitude anomalies had been noted on the fields and a study was undertaken to review the tuning and AVO behaviour of the rocks in an attempt to understand the anomalies. Vp, Vs and density logs from the Tui-1 and Amokura-1 exploration wells showed that it was possible to map oil column heights using amplitudes extracted from the F10 sand seismic horizon. This modelling predicted an oil column height of 20 m at the crest of the Tui structure prior to the drilling of development well Tui-2H. In the same area a column height of about 10 m was predicted using the average velocity based depth maps. Drilling results showed a maximum column height of 21 m, confirming the validity of the modelling. This result led to the change in the direction of the next well Tui-3H. Four horizontal development wells have been successfully drilled with total estimated gross 2P reserves of 32 MMBO.

Gravity surveying has been available since the early 1900’s in the form of analogue gravity meters, barometric levelling and conventional optical line of sight surveying methods. The gravity method was more generally used for applications in the oil industry.

With the advent of GPS technology in the form of centimetre accuracy from GPS surveying methods, commercial gravity operations developed. Line of sight surveying methods were no longer required and gravity meters became digital, easier and more reliable to use.

Commercial gravity surveys are now undertaken on a routine basis from projects spanning hundreds of kilometers for mapping large sedimentary basins to very small localised engineering projects covering merely hundreds of meters.

This paper outlines the development of the GPS surveying technology and the digital gravity meter. The integration of the two technologies are discussed with respect to the development of the commercial gravity survey method. Several applications and case studies are described with specific examples given from various international gravity contractors.

A regional scale 3D geological map of the upper crustal sequence in the West Arnhem Land region, Northern Territory, was compiled from surface mapping, limited drilling information, and liberal amounts of geological inference. Modelling of the gravity and magnetic field response of this map was proposed as a means of evaluating the viability of this geological hypothesis. A relatively large number of mass density and magnetic property measurements were available to constrain the transformation of 3D geological maps into property models in preparation for potential field modelling. The presence of numerous magnetic dykes, sills, and stratigraphic horizons provided many challenges for producing geologically-realistic magnetic property models at a regional scale. Modelling of the gravity field at this scale was far more straight forward and successful. After completing various forward modelling experiments, a stochastic procedure will be used to derive a large number of geological maps by making small changes to the highly uncertain interpretive parts of the original 3D geological map. It is expected that a subset of these derived geological maps will have associated mass density models that can adequately reproduce the gravity field observations. The common characteristics of the geological models in this subset will be isolated using statistical techniques and used to refine our representation of the regional scale 3D geological features.

Seismic amplitudes and AVO have been used successfully in exploration worldwide. We present a general geologically-based methodology to characterize the expected amplitude and AVO behaviours of reservoirs and seal facies in a basin. We produce a rock physics model based on well data and consistent with the geological model for the basin. The result is summarized in a series of depth trend curves for rock properties and anticipated seismic responses, illustrated here with data from the West of Shetlands, UK, to predict amplitude and AVO responses throughout the basin. A by-product of the technique is the verification of the validity of the amplitude changes and flat-spots which can result from physical property changes across fluid interfaces. For example, seismic flat-spots cross-cutting dipping stratigraphy are commonly observed within the Triassic Mungaroo Formation of the Exmouth Plateau area of the North Carnarvon Basin. We show techniques for quantifying the consistency of flat-spots in 3D, assessing amplitude conformance with structure in map view and automatically determining fluid contact levels.

Shallow anomalous high speed events, evident on the shot gathers, are observed to be associated with gas chimneys in the Central Perth Basin. Shot gathers in the vicinity of gas chimneys display these events, which have apparent speeds of around 3000 m/s in the uppermost 300 ms, and appear to localised around the gas chimneys.

These high speed events have been investigated using existing seismic data from the region, with these high speed events modelled using wide aperture seismic (WAS) techniques.

Understanding the effect of these shallow high speed layers is important for the following reasons -

1. they may provide support for a petroleum prospect,

2. they may represent a possible geohazard during drilling,

3. these high speed layers can have a profound effect on the seismic imaging of the underlying strata.

Dissolved salt in aquifers is a potential threat to fresh water resources and the environment. Interpolated grid maps at aquifer depths, derived from borehole EC measurements on water samples, are combined with more detailed bulk electrical conductivities from airborne electromagnetics to provide detailed estimates of total dissolved salt load and subsurface porosity values. A resistive host matrix assumption implies that our calculated porosity and salt load values are maximum values. This mapping technique is a large area coverage remote senor method which has been tested in different areas of the salt-threatened Murray-Darling basin. This technique provides extensive information on the hydraulic properties of aquifers, important for quantitative hydrology.

Geophysical data from the southern Coromandel Volcanic Zone provide information critical to the investigation of epithermal mineral deposits and their regional structural setting. Regional gravity data show steep linear gradients that coincide with major faults bounding the Hauraki Rift to the west and the Waihi caldera to the east. Regional magnetic data are dominated by a large bi-polar anomaly coincident with the Waihi caldera, which may result from a sub-caldera intrusion, and otherwise by high-amplitude shorter wavelength magnetic anomalies associated with the volcanic rocks.

High-resolution aeromagnetic and radiometric data reveal distinct signatures associated with epithermal deposits. Extensive magnetically quiet areas clearly delineate the location and extent of hydrothermal alteration zones around the deposits, whereas more localised zones of high K/Th ratios in these magnetically quiet areas delineate areas of intense potassium metasomatism. Epithermal deposits exhibit gravity signatures with two contrasting modalities: i) small negative anomalies (e.g. ≤30 gu at Golden Cross and Scotia) and ii) small positive anomalies (e.g. 40-50 gu at Karangahake and Waihi-Favona). Near-surface, low-density clay-altered andesite and/or enhanced fracture volume can account for small negative anomalies. However, positive gravity signatures show that significant mass anomalies must occur at greater depths; these may be either dense intrusions and/or zones of concentrated sulphide mineralisation.

Reverse time pre-stack depth migration, which uses the two-way acoustic wave equation, is not a new concept. Conventionally the method has been very computationally intensive and, therefore, has been considered impractical for production 3D depth imaging projects. Here we describe an efficient algorithm that can be used on large scale 3D seismic data. To make it practical and efficient we employ explicit 2^nd order in time and high order in space domain finite differences. We also use threading and domain decomposition methods to split the image cube amongst multiple CPU’s, when necessary. Only a few finite differencing layers are communicated between related CPU’s, by message passing during domain decomposition. High order spatial finite differences handle numerical dispersion and allow larger time steps than those possible with the, more conventional, pseudo-spectral method. We will show an overview of the method along with 2D and 3D synthetic and real data examples.

The number of oil and gas wells drilled through and near salt structures continues to increase. These wells are some of the most expensive wells to drill and are prone to numerous drilling problems that drastically increase their associated costs. Better understanding of the geomechanical setting in proximity to salt results in more successful drilling and completion of these wells. One promising area of study for investigating the perturbation of the stress field in and around salt bodies is numerical modelling. However, in order to apply the results of the simulated stress field from a numerical model to a well drilled near a real salt body with confidence, it is necessary to compare the modelled results to data observations. Therefore, it is important to have independent observations of the stress state near the salt body. One technique for determining principal stress directions uses cross-dipole shear wave velocity anisotropy data. However, shear wave velocity anisotropy can be induced by structural mechanisms as well as stress-related mechanisms. In this study, we investigate a technique to identify structure-induced velocity anisotropy and to characterize possible stressinduced velocity anisotropy. The investigation uses cross-dipole sonic data from three deep water, sub-salt wells in the Gulf of Mexico. First, we determine the parameters necessary to ensure the quality of the fast azimuth data used in our analysis. We then characterize the quality controlled measured fast directions as either structure-induced or stress-induced by predicting the apparent structure-induced fast direction the dipole sonic tool should measure for known bedding planes. We find that this technique supplements the use of dispersion curve analysis for characterizing anisotropy mechanisms. We also find that this technique has the potential to provide information on the stresses that can be used to validate a numerical model of the salt-related stress perturbations.

The deep gold mines in the Witwatersrand Basin of South Africa are some of the deepest underground mines in the world. For over a century, numerous, often deadly, mining induced earthquakes have been observed. In this work, we develop and test a new technique for determining the virgin state of stress near the TauTona gold mine. This work was completed as part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. The technique we use to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. The final result is a well constrained stress state consistent with all the observed stress indicators. We find that the stress state is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal (denoted with a *). The maximum principal stress, Sv*, is deviated 0-20º from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0-20º from horizontal with an azimuth of 145º to 168º and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0-10º from horizontal with an azimuth of 235º to 258º and has a magnitude gradient of 13 to 15.5 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on pre-existing fault planes that are optimally oriented to the stress field. The stress concentrations caused by the mining activities can dramatically alter the range of fault orientations upon which fault slip could occur.

A wave propagation based method was investigated to improve the quality of seismic P-wave fracture prediction. The improvement is achieved through reducing the overburden influence superimposed onto a target layer. Rather than relying on evaluations of layer’s properties across an interface, our method predicts fractures using two “fracture functions” estimated from the top and bottom reflections of a target layer. The two functions are firstly served as a measure of overburden influence, and then incorporated in inversion to minimize the overburden effect that results in improved fracture prediction for a target layer.

Tests on physical model data suggest the viability of the method that operates on pre-stack data, and is applicable to a wide range of seismic surveys including 2D, 3D and VSP.