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

There is a popular assumption that damage to roads due to high soil salinity is one of the greatest economic effects of dryland salinity. There is little scientific evidence to back up these claims. The effect of salt on road materials is not documented or understood. This study started as an attempt to develop a risk management tool for roads in saline areas using geophysical techniques. EM38 and EM31 surveys were carried out effectively and safely on major and minor rural roads. They relate to changes in salt content in the road basecourse and underlying subgrade. The lack of scientific understanding of the effects of salt on road material has resulted in a focus on understanding how the road is affected. Mineralogical and geochemical changes are indicated that may result in volumetric and strength changes to the road pavement but more work is needed.

Various rules-of-thumb (e.g. Fresnel Radius, Rayleigh Limit) are commonly used to predict seismic resolution, based on image wavelength. However, seismic resolution ultimately depends on more fundamental parameters including survey design, source bandwidth, geology and data processing. A more instructive analysis is possible via numerical modelling of the acquisition process. Here we demonstrate the improved insight available with this approach, with examples taken from the coal and petroleum sectors.

We use viscoelastic finite-difference modelling to simulate 2D multi-component acquisition sequences. The ability to allow for anelastic attenuation is important as it permits a more realistic comparison of the resolution achievable on P-wave and PS-wave imagery.

Analysis of a typical coal target suggests that barren-zones of width 5-10 metres can be resolved. The interplay of wavelength and attenuation is such that the PS-wave image is likely to exhibit comparable, or slightly reduced, lateral resolution where statics are not a problem. Resolution can be downgraded significantly if statics are more severe, and in practice this is likely to have greater impact on the PS image.

A second example examines detection of lens-like features at petroleum depth. The resolving ability on the P-wave imagery is broadly consistent with analytical predictions (100m laterally and 40 m vertically). In terms of resolution, the PS-images are perhaps less competitive than at the coal scale, because the longer path lengths yield greater relative attenuation. Realistic numerical modelling, simulating the full acquisition and processing sequence, leads to a more pragmatic understanding of seismic resolution issues. It is a valuable tool for survey planning and image interpretation.

In this paper, we present a reprocessing case study that applied the latest processing technologies to improve the seismic imaging inside the granite basement reservoir. The highlight of this effort is the application of the latest Controlled Beam Migration (CBM) technology, and a stack sweep method for updating velocity inside the basement.

Seismic prospecting and evaluation of the rock mechanical properties require knowledge of shear-wave (S-wave) velocity. However, frequently the S-wave logs are not available, especially in past years. Prediction of Swave velocity is still attracting researchers’ attention because of its importance in petroleum exploration and engineering application.

A new rock physics model of clay-sand mixture is developed based on Self-Consistent Approximations (SCA) and Gassmann model. The model assumes that total pore space consists of two parts: (1) pores from sand grains and (2) pores from clays. The elastic moduli of clay-fluid mixture and sand-pore mixture are obtained by SCA. The clay-fluid mixture first fills up the total porosity, then replace the matrix (sand grains), taking the form of structure clay. The model can explain the distinctive changing relations between clay content and compressional velocity of the sand-clay mixture observed by Marion.

The model can be applied to predict compressional and shear wave velocities of the rock using conventional logs such as acoustic velocity, density and gamma ray logs. Parameters of the sand clay mixture model are chosen according to the fitness between predicted and measured compressional wave velocity. After that, we use the improved model to predict shear wave velocity, and the computational results indicate that the predicted shear wave velocities agree well with the measured velocities. The reliability of the shear wave velocity estimation can be validated by the predicted and measured compressional wave velocities fitness.

We conducted high-resolution shallow seismic exploration and ground penetrating radar (GPR) exploration to assess the earthquake hazard along the speculated north-south striking Chaujou Fault that is along the mountain foot at the western edge of the Central Range in the southern tip of Taiwan. Shallow seismic and GPR survey lines are approximately eastwest oriented, perpendicular to the speculated fault. All these survey lines run across the speculated fault. A fault plane of an 50° east-dipping angle is interpreted using shallow seismic profiles and one GPR profile. The interpreted fault locations are consistent with the scarp on the earth’s surface with an error of only a few meters, indicating that the scarp marks the Chaujou Fault’s surface location. The GPR profile even shows fault planes within a few meters depth, indicating fault rupturing within the past centuries. As Taiwan has humid climate with high erosion and deposition rate, these evidences imply that the Chaujou Fault really exists and is an active fault, that it has been displaced within the past centuries, and that it is a potential earthquakeinducing mechanism.

The Beenyup wastewater treatment plant is situated in close proximity to residential properties approximately 20 km north of the central business district of Perth, Western Australia. It is proposed that treated wastewater from the plant will be injected into the sub-surface in preference to releasing it to the ocean. In order for this to occur over a long period of time a good understanding of the connectivity of the sub-surface aquifers in the vicinity of the injection site is required.

High resolution surface seismic together with vertical seismic profiling data were collected at the site using a high-power impact source. The resulting sub-surface images correlate well with geophysical well-logs and clearly show the injection zone and confining seals.

For contributing to precise depth delineation in the Ichthys giant gas field offshore northwest Australia, we investigated shallow velocity heterogeneity effects using forward modelling and seismic data review. We discovered that their removal through applications of high-resolution tomographic velocity modelling and prestack depth migration (PSDM) enabled correct representation of the target reservoir structure. Analysis of synthetic seismic pre-stack gathers generated from forward modelling demonstrated that small velocity anomalies, such as channels, in shallow overburdens could give rise to apparent seismic RMS velocity artefacts at deeper target levels. Delineation of “true” velocity anomalies and implementation of PSDM using precise shallow velocity model were required for solving these problems.

In accordance with the phenomenon predicted by forward modelling, examination of coinciding patterns of timethickness, amplitude and deep prestack time migration (PSTM) velocity allowed determination of the shallow heterogeneous layers that caused target velocity undulation.

We employed two iterations of tomographic velocity model updating for PSDM velocity model building. First, the shallow heterogeneous velocity patterns were successfully identified by utilising the dense residual moveout picking and the layer-based 3D high-resolution finite-offset tomography. Subsequent grid-based global tomography with constraints was used for updating the entire velocity field and delivered stable velocity pattern at the deep. This two-step approach successfully eliminated deep velocity artefacts.

The current Australian gravity datum, Isogal84, is defined by the Australian Fundamental Gravity Network (AFGN). The AFGN consists of about 950 stations at over 250 locations throughout Australia with the first stations in the network being established in the early 1950s. Prior to Isogal84, the datum was based on relative ties to overseas sites. The Isogal84 datum is based on 5 absolute gravity sites within Australia that were established in 1979 using a Soviet absolute gravimeter.

Absolute gravity measurements conducted at 60 AFGN sites by Geoscience Australia using a portable absolute gravimeter have shown that the Isogal84 datum is 78 microgals (1 microgal = 1x10^-8 m/s²) higher than the absolute measurements. A new gravity datum, the Australian Absolute Gravity Datum 2007 (AAGD07), has been defined based on these absolute gravity measurements and the AFGN and the Australian National Gravity Database (ANGD) have been adjusted to this new datum.

Concurrent with implementing AAGD07, the formulae used for reducing gravity data in the ANGD have been reviewed and updated. These changes include using the 1980 International Gravity Formula, global horizontal and vertical datums, and a spherical cap Bouguer correction that accounts for the Earth’s curvature. These new formulae provide more accurate anomalies, particularly in longer wavelengths which will be beneficial to regional studies.

In August 2006 pilot high resolution 2D seismic surveys were carried out on Lake Lefroy near Kambalda in WA to provide a “proof-of-concept” for the use of surface reflection surveys for guiding exploration for nickel sulphide and gold in this area.

The pilot survey demonstrated the basalt ultramafic boundary is usually a good reflector and mapped a number known and previously unknown faults and shears. A correlation was also noted between zones of reduced reflection and intrusive porphyries.

Since their introduction by Nigel Anstey and Tury Taner in the1970’s, attributes have become an integral tool in the interpreter’s arsenal. At present, as emphasised by Taner, no direct relationships have been established between all attributes and physical and geological characteristics of the subsurface. Their discriminatory properties, however, allow very useful classifications to be performed. This paper deals with various attribute related issues. Firstly, we consider the theoretical and physical aspects concerning instantaneous attributes, particularly instantaneous phase. This attribute is of central importance since it describes the location of events in the seismic trace and leads to the computation of other instantaneous quantities. Secondly, we deal with the issue of information content. It has often been implied that attributes convey no more information than that present in the original seismic trace from which they are derived. This, however, is akin to claiming that David contains no more information than the raw marble from which Michelangelo freed him. A seismic attribute section provides that much more information. The attribute in time attempts to enhance resolution, whereas the attribute property in the spatial dimension emphasises continuity. These important and interesting issues will be dealt with theoretically and by example.

Finally, we present and illustrate by means of a real data examples, a novel, hybrid attribute which has been constructed to provide high resolution information. We must point out that, as is always the case, our attribute is dependent on the phase of the source wavelet.

The Barrow Group strata (Macedon, Pyrenees and Barrow sands) of the Exmouth Sub-Basin host significant accumulations of gas and liquid hydrocarbons. There is currently production from the Macedon sands at Enfield and ongoing development drilling at Stybarrow. Active appraisal and exploration is underway, including the mutli-field Pyrenees Development. In the course of assessing these discoveries, BHP Billiton and its joint venture partners have undertaken a hydrodynamic study in order to better understand the sealing mechanisms, the position of freewater-levels (FWL’s), and the likelihood of compartmentalisation within the discoveries.

Whilst the region is faulted with a predominant southwest-northeast grain, the potentiometric gradient is surprisingly flat indicating that the individual sands are hydraulically well connected. Other than the Macedon Gas Field, there is no pressure data that indicate intraformational seals have been breached. Thus, top and bottom seal capacity is likely not limiting pool size. Rather, structural spill points and fault seal capacity appear the significant factors in determining pool geometry, with the underlying aquifer being regionally connected around fault tips.

On the field-scale, the flat hydraulic gradient allows for the calculated FWL’s to have a high confidence. Pressure data from the hydrocarbon phases indicate that in some cases, fault zones effectively compartmentalize a field into multiple pools. The Macedon Gas Field, on the eastern edge of the play fairway, marks a change in the trapping character with intraformational and fault 2 seals having been breached resulting in a single continuous gas pool despite internal structural complexity.

Deepwater Taranaki is investigated for its petroleum potential using using all available seismic data tied to shallow water wells. It contains up to 10 km of sediment. An early rift sequence is overlain by a large Late Cretaceous delta which culminates with Rakopi Formation coal measures. This sequence marks the breakup unconformity following the start of Tasman Sea spreading. A passive margin succession follows as the New Zealand mini-continent gradually subsided, with sediments becoming gradually finer grained until carbonates dominate during the Oligocene. Initiation of the present plate boundary about the start of the Miocene caused uplift and renewed clastic deposition in the form of spectacular channel and turbidite complexes.

The present reconnaissance seismic grid indicates at least six subtle structures that are large enough to contain a billion barrels of oil or several TCF of gas, suggesting that the first drilling targets may be Late Cretaceous fluvial and marine sands draped across gentle basement structures. Cretaceous structures are commonly overlain by Miocene channel and turbidite sands that are also draped across underlying highs. The similar, but much smaller structures of Tui, Amokura and Pateke, below the Taranaki shelf, are currently being developed by AWE. Future drilling will take discoveries closer to the shelf edge and ultimately the larger prizes will be sought in deeper water.

A GEOTEK multi-sensor core logger (MSCL), which was originally developed to log soft-sediment cores, has been adapted to allow simultaneous measurement of a range of petrophysical parameters on diamond drill core. The system can measure density, P-wave velocity, electrical conductivity and magnetic susceptibility of either whole or split core. It also acquires high resolution digital colour imagery of the core. System operation, sensor modification, sensor calibration, data accuracy and repeatability are described in this paper. The GEOTEK system is currently being used to acquire detailed petrophysical data on archival drill core from metalliferous mines for correlation with metallurgical parameters (AMIRA Project P843) but it has significant potential for use in other applications, e.g. environmental.

The management of health and safety in the petroleum industry has seen many significant changes since the introduction of the safety case regime after the Piper Alpha tragedy on 6 July 1998. Environmental legislation in Australia has also undergone significant changes since the introduction of the Petroleum (Submerged Lands) Act 1967, Management of Environment Regulations 1999, and the Environment Protection Biodiversity Conservation Act 1999.

Seismic vessels are not considered to be petroleum facilities and hence the safety case principles do not apply. Marine seismic acquisition is not overseen by the National Offshore Petroleum Safety Authority (NOPSA), with the responsibility for the management of health and safety on seismic vessels in Commonwealth Waters being under the jurisdiction of the Australian Maritime Safety Authority (AMSA). The environmental aspects of marine seismic acquisition are managed by the State Designated Authorities and the Department of Environment and Water Resources (DEWR) in Canberra.

The effective management of health, safety and environment in seismic operations cannot be achieved by the regulatory authorities and the operators of petroleum titles alone. The integration and commitment of the seismic acquisition contractors to achieving the HSE goals of the regulatory authorities, the operator, and their own organisation is the key to achieving successful HSE outcomes.

The responsibilities for HSE management are shared between the regulatory authorities, operators and contractors, but the accountability rests solely with the operators.

New insights into biological functions of roots casts doubt on many entrenched abiotic theories on soil formation-to the extent that understanding processes in the Rhizosphere is now increasingly cited as the new frontier of regolith science. Critical field observations and information drawn from a number of disciplines suggests that many pedogenic processes are causally related to niche-building activities of higher plants. This paper examines some edaphic features and associated formative effects of competing plant communities in semiarid settings of south-western Australia and presents evidence that bioengineering by higher plants and their associates influences the landforms of their habitat as well as the trajectory of their own evolution. Examples given relate to patterns of chemical variation visible in radiometric imagery of uplands, and what appear to be biotic responses to stripping and head ward incision.

The spatially constrained inversion (SCI) produces quasi-3D modelling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. Information migrate horizontally through spatial constraints and allow resolution of layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. It also allows creation of subsets suitable for parallel computing. In this study the SCI was applied to airborne time domain EM soundings (SkyTEM data from the Island of Lolland, Denmark), but it can be implemented also with all other data types. The field case study proves that the SCI produces laterally consistent results that respect the 3D geological variations of the sedimentary settings. The resolution of near surface tills and water bearing sand and gravel layers is greatly enhanced and so is the boarder of the saline groundwater intrusion beneath the island. The SCI also suppresses the elongated artifacts commonly seen in profile oriented data sets.

The use of airborne electromagnetic (AEM) methods for measuring water depth and estimating sediment thickness has been demonstrated using commercial AEM equipment that is not optimized for marine surveying. A new prototype helicopter time domain AEM system, SeaTEM(0), is under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, NSW, in shallow water up to ~ 30 m in depth. The SeaTEM(0) system was untested and the Broken Bay survey identified instrumentation problems that will be addressed in future surveys. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Waters and Pittwater which join in Broken Bay give rise to paleo-valleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ~ 200 m. Sediment thickness and water depth is predicted from stitched 1D inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ~ ±1 m with known water depths in areas less than 20 m deep. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleochannels to depth limits of ~ 80 m below sea level in 30 m water depth.

Underwater seismic refraction (USR) with advanced interpretation approaches makes important contributions to shallow marine exploration and geotechnical investigations in Australia’s coastal areas.

We present a series of recent case studies to demonstrate the application of continuous and static USR methods to river crossing and port infrastructure projects around Australia.

In Sydney, static USR and borehole seismic imaging assisted the development of improved geotechnical models that reduced construction risk for a tunnel crossing of the Lane Cove River. In Victoria, combining conventional boomer seismic reflection and continuous USR improved the definition of a submerged, buried basalt flow and dredging assessment for navigation channel upgrades at Geelong Ports. Sand quality assessment with continuous USR and widely spaced borehole information led to improved commercial decisions on available sand resources for the reclamation phase of development at the Port of Brisbane.

Buried reefs and indurated layers occur in Australian coastal sediments with the characteristics of laterally limited, high velocity, cap layers within lower velocity materials. If these features are not recognised then significant error in depth determination to deeper refractors can occur. Application of advanced seismic refraction inversion using wavefront eikonal tomography and continuous USR data obtained along the route of a proposed offshore pipeline near Fremantle allowed these layers and the underlying bedrock refractor to be accurately imaged. Static USR and the same interpretation approach was used to image the drowned granitic regolith beneath sediments and indurated layers in the northern area of Western Australia at a proposed new berthing site where deep piling was required. This accurately mapped the regolith and identified preferred piling sites that minimised pile lengths.

USR can be expected to find increased application to shallow marine exploration and geotechnical investigations in Australia’s coastal areas as economic growth continues.

Resistivity and induced polarisation data are very useful for defining lithological boundaries, shear zones (often with conductive and chargeable graphite), and sulphide alteration zones. The useful depth for interpretation depends upon the magnitude of the measured voltages and the effective current penetration, which in turn are determined most often by surface conditions at the electrode locations and subsurface electrical structure. Downhole resistivity and IP measurements are relatively quick to acquire, and can add significantly to the depth information of the surrounding surface survey. Bottomof-hole to surface surveying can be performed without any specialised downhole IP equipment. Examples over shear zones in southern Mali demonstrate the added information from inverting surface data with data from bottom-of-hole current injection with surface receiver electrodes.