ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

Acoustic attenuation has been proved to be an indicator of stress changes in solid structures. Acoustic coda, as a superposition of incoherent scattered waves, reflects small-scale random heterogeneities in solids. Acoustic coda attenuation, as a combination of intrinsic attenuation and scattering attenuation, contains information on stress changes as a result of changes in the physical state of small-scale heterogeneous structures. Based on the ultrasonic measurements of a rock sample with intra-grain pores and fractures under different pore-pressure induced effective stresses, we compute the stress-associated coda attenuation quality factors QPC and QSC as a function of frequencies. Based on the digital heterogeneous cores of the sample, the experimental results are validated and corrected with numerical results by the finite-difference simulation of Biot’s poroelastic equations and the Monte Carlo simulation of multiple scatterings, respectively. The quality factors characterize its scale dependence of scattering attenuation on stress variations in rocks. We compare them with the intrinsic attenuation quality factors QP and QS calculated by the spectral ratio method and BISQ model, respectively, from ultrasonic measurements. Comparisons demonstrate that the scattering attenuation is much stronger, particularly when ultrasonic wavelengths are comparable to the scale of pores and grains. The intrinsic and coda attenuations versus increasing effective stresses present quite different nonlinear features, where QPC and QSC show a greater sensitivity to pore pressure than QP and QS.

Distribution and growth history of isolated carbonate build-ups (ICBs) is controlled by complex interplay between various tectonic, eustatic, and oceanographic parameters. Quaternary ICBs in the Timor Sea (NW Australia) are located in tropical waters, and at present they form clusters of ~150 build-ups, developing 2 to 85 km from the edge of a wide continental shelf. The tectonic evolution of the Timor Sea lead to regional changes in the oceanography and flexural deformation of the NW Bonaparte Basin, which in turn had a major impact on the evolution of ICBs. Flexure-induced fault activity produced structural topography for the growth of ICBs over ‘highs’, while oceanic current through Timor Trough provided warm and nutrient-rich water. Our results demonstrate that, despite potentially good conditions for carbonate production, ICBs did not form until the Mid Pleistocene (ca. 0.582-0.8 Ma BP). This age corresponds to the onset of repeated, high-amplitude (+120 m) sea level fluctuations with rapid deglacial rises and slow falls. Thus, we infer that the NW Australia ICBs formed due to: (1) structural shaping of the margin; (2) oceanographic changes, and; most importantly, (3) onset of repeated short-term transgressions reactivating the carbonate production along isolated highs. The distribution and growth of ICBs could be useful to understand the evolution of ancient ICBs that formed along very wide shelves and epeiric seas.

Full Waveform Inversion (FWI) has recently emerged as one of the most exciting new techniques in the seismic industry, with the potential to deliver incredibly detailed velocity models. We applied FWI to 2D lines from the Exmouth basin, Western Australia. Results showed that FWI can produce excellent high resolution velocity models even if the starting velocity model is far from perfect providing that the input data is rich in low frequencies

We used laser Doppler interferometer for measuring the displacement on the sample surface. These measurements allow us to clearly separate different wave types, whose picked travel times are used for estimation of VTI anisotropy parameters. One of the observations in this study is the very strong amplitude of critically refracted SP wave at the measurement surface. We confirmed the characteristics of this wave by numerical modelling. We used this wave to improve the estimates of the anisotropy. The observed strong amplitude of this wave can have strong implications for the interpretation of ultrasonic measurements.

Rift basins are typically developed on heterogeneous continental crust. Inherited basement fabrics exert a fundamental control rift basin geometry, and on the geometry of individual faults. Many rift basins are also the result of multiple rift episodes and early formed structures will exert further control on the way in which faults evolve in subsequent rift events.

Inherited fabrics and fault reactivation are often invoked to explain rift orientation and segmentation, often with little independent evidence for their existence. However analogue models of orthogonal and oblique rifts show that predictable fault patterns result from the partitioning of stress between pre-existing structures and superimposed extension directions.

The Northern Carnarvon Basin provides an ideal laboratory in which to test these models. High resolution 3D seismic data allows detailed imaging of fault patterns developed during separate Lower-Middle Jurassic and Lower Cretaceous rift events. Fault patterns clearly reveal the influence of older structures, most likely related to Carboniferous and Permian rifting, enabling contemporaneous stress patterns to be revealed.

We use three-dimensional (3D) seismic reflection data to determine the structural evolution of thin-skinned listric fault growth, at the extensional top of a gravitationally driven delta system, in the central Ceduna Sub-Basin. We present analysis of a strike and dip-linked extensional fault system, which is decoupled at the base of a marine mud interval of late Albian age. The fault system is oriented NW-SE with strike-linkage of fault segments and dip-linkage through the Santonian interval which connects a Cenomanian-Santonian period of kilometre scale fault growth and post-Santonian normal faulting. Understanding the growth of listric faulting requires quantifying heave and throw, which involves simplistic depth conversion of fault plane time measurements to establish a fault plane model to translate throw into fault plane displacement. Our analysis constrains fault growth into six evolutionary stages: [1] early Cenomanian nucleation and isolated radial propagation of fault segments; [2] substantial segment linkage established by the latest Cenomanian; [3] late Santonian cessation of a majority of fault growth; [4] heavy erosion at continental breakup of Australia and Antarctica (c. 83 Ma); [5] early Campanian independent nucleation of the post-Santonian fault system; and [6] fault assemblages fully linked by the Cenozoic, with continued accumulation of displacement. The structural evolution of this fault system is compatible with the ‘isolated fault model’. In particular, we emphasise the importance of dip-linkage in this fault system, which controls the present day geometry of the fault array.

We introduce a new practical least-squares reverse-time migration (LSRTM) scheme and derive a steepest descent method for optimal imaging by adapting reverse-time migration (RTM) and demigration (RTDM) as the migration and modelling operators to maximize the cross-correlation between simulated and acquired seismic data. Through real data experiments, we demonstrate that the proposed LSRTM provides high quality images with balanced amplitudes, improved focusing and enhanced resolution. The method is also capable of removing the free surface ghosts generated in towed streamer acquisition and filling holes in the imaged structures due to imperfect acquisition.

Together with impedance perturbation technique, the proposed method is a useful tool for both seismic imaging and inversion.

The double absorbing boundary (DAB) is a new high-order absorbing boundary condition for the scalar acoustic wave equation. It suppresses scattered waves at the edge of a boundary layer in computational domain boundary by using destructive interference analogous to a noise-cancelling headphone. This method has advantages in that it addresses some of the shortfalls in existing boundary conditions, such as the need for tuning in Perfectly Matched Layers or complex formulations at corners such as in high-order absorbing boundary conditions. We extend the original formulation of the DAB to three dimensions and higher-order stencils. Through numerical simulation we test the performance of the DAB by comparison with a reflecting boundary. We find that the DABC is a broadband attenuator with a power attenuation of 20-30dB using only six boundary cells. Increasing the order of the method improves accuracy for wavelengths less than 10 cells, whereas increasing the layer width does not improve accuracy. The method shows promise as a robust and computationally efficient boundary condition for seismic applications.

A framework for the analysis of stratigraphic facies as emergent phases of self organization will be presented. An example will be given of turbidite deposition that is governed by a system of partial differential equations. It will be shown how the boundary conditions and coefficients of the PDEs parameterize a phase space that is divided into distinct phases, or what is more commonly called facies. A method of renormalization of the texture of geologic outcrops, seismic data, and well logs will be presented that gives the scale dependance of the PDE coefficients and boundary conditions. This specification of the running coupling coefficients or S-matrix of the physics gives the form of the PDE as well as the coefficients and boundary conditions. Practically this gives a unique fingerprint, or “attribute” (technically a metric) of the geologic facies. The mathematical framework is based on the Mallat Scattering Transformation - an iterative wavelet transformation.

Deformation of the Cretaceous Ceduna Delta system is dominated by gravitationally driven listric extensional faults. They were initiated as strongly listric faults during deposition of the Cenomanian White Pointer deltaic sequence, coincident with the final stages of rifting and break up between Australia and Antarctica. The faults were progressively reactivated during deposition of the post break up Santonian to Maastrichtian Hammerhead deltaic sequence, propagating upwards as relatively planar sequences associated with narrow zones of downward converging secondary faults.

Individual faults segments maintain a characteristic curved geometry in map view which link together to form relatively long continuous NW-SE trending faults which rotate to a NNW orientation in the west of the study area (towards the break of slope at the edge of the delta top). Previously unrecognised N-S trending faults that are confined to the lower part of the sequence control some of the segmentation of the NW-SE trending faults.

Understanding the evolution of these fault systems will help to better define the risks associated with Cretaceous plays in this highly prospective frontier petroleum province.

As high-quality 3D seismic data has become widely available, stratigraphic interpretation has significantly improved our ability to predict the subsurface distribution of lithologies. Stratigraphic interpretation of seismic data involves the integration of stratigraphy and geomorphology, with integrated section and plan view images yielding robust interpretations of stratigraphic architecture and associated lithology. Key aspects of successful application of seismic stratigraphic analysis are: 1) integrating section and plan views in an iterative workflow, 2) understanding and recognizing geologically-meaningful patterns both in section and plan view, and 3) having efficient and creative workflows to quickly analyze geophysical data.

Seismically-derived geologic interpretations can have significant impact on exploration and production in the following ways:

Geology: 1) prediction of lithology, 2) prediction of compartmentalization , 3) development of depositional analogs, 4) Enhanced understanding of geologic processes.

Geophysics: 1) provides depositional context for geophysical analyses (e.g., DHI analysis, reservoir properties from seismic), and 2) quality control for geophysical processing.

Numerous examples from a variety of different depositional settings will be shown and key workflows will be illustrated.

Seismic attenuation and anisotropy in the overburden can significantly affect seismic image quality, including amplitudes of the target horizons. Therefore, understanding magnitudes, causes and spatial distribution of attenuation and anisotropy is important for seismic imaging and reservoir characterization. Thin layering can cause both scattering attenuation and anisotropy. These phenomena can only be significant, if there is a strong contrast in elastic properties between the layers. We present a case study from North-West Shelf of Australia, where presence of shallow stiff carbonate layers can be responsible for deterioration of seismic data quality through both attenuation and anisotropy.

Complex seafloor bathymetry can create significant challenges for subsurface imaging and geologic interpretation of seismic exploration and monitoring data. Steep seafloor canyons that cut through continental shelf areas can produce very strong seismic wavefield distortions. Neglecting such wavefield complexity can result in inaccurate velocity models, significant imaging errors, misleading amplitudes and uncertain geologic interpretations. In this paper we investigate the kinematic and dynamic effects of seismic “prism waves” generated by seafloor canyons. Prism waves are waves that undergo multiple primary reflections at scattering interfaces before propagating to the recording sensor array. We demonstrate that strong prism waves can be generated for realistic seafloor canyon geometries, and show how their adverse effects can contaminate the seismic imaging process.

The early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which infilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. A study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and in places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which provides effective sealing capacity.

New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale and reveal the evolution of the early Cretaceous deltas.

Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the offshore central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and defines constraints on the possible location of valid plays.

We present models of random resistor networks to relate electrical resistivity to fracture permeability in the upper crust. In this approach, the upper crust is modelled as a network of resistors that are randomly assigned to be either electrically and hydraulically conductive or resistive based on a network-wide probability of connection. In the models presented here, the conductive resistors are assigned resistance values based on a constant fracture diameter of 1 mm and a fluid resistivity of 0.1 Ωm, with variable fault length distributions and probabilities of connection. We have found that the permeability is very sensitive to both of these parameters, increasing to 8.33 × 10⁸ times the matrix permeability in the fully connected case. The resistivity is less sensitive, increasing by a factor of 1000.

As in several other industries, progress in exploration seismology relies on the collection and processing of massive data volumes that grow exponentially in size as the survey area and desired resolution increase. This exponential growth-in combination with the increased complexity of the next generation of iterative wave equation-based inversion algorithms-puts strain on our acquisition systems and computational back ends, impeding progress in our field. During this talk, I will review how recent randomized algorithms from Compressive Sensing and Machine Learning can be used to overcome some of these challenges by fundamentally rethinking how we sample and process seismic data. The key idea here is to reduce acquisition and computational costs by deliberately working on small randomized subsets of the data at a desired accuracy. I will illustrate these concepts using a variety of compelling examples on realistic synthetics and field data.

The Perth Basin in southwestern Australia has an extended history involving multiple regional unconformity-forming events from the Permian to Cretaceous. The central and southern Perth Basin is the closest basin to the relict triple junction of eastern Gondwana and comprises a complete Permian to Recent stratigraphy, thus recording the full history of the breakup events. We use sonic transit time analysis to quantify the magnitudes of net exhumation and the minimum differences in net exhumation across different time intervals (here called ‘interval exhumation’) for four stratigraphic periods from 37 wells. We were able to quantify the minimum interval exhumation of the Permian-Triassic, Triassic-Jurassic, Early Cretaceous breakup and post-Early Cretaceous events. The Permian-Triassic and Triassic-Jurassic events recorded spatially varied exhumation, up to 1000 m, across sub-basins. These localized variations are caused primarily by reverse (re-) activation of NW- and N-striking faults in the Permian-Triassic and Triassic-Jurassic events, respectively. The Valanginian breakup unconformity (-133 Ma) records approximately 400 m of basin-wide interval exhumation during the breakup of Gondwana, which implies a change to relatively uniform exhumation on a regional scale. Using published uplift rates for volcanic and non-volcanic passive margins, estimates of the time required for 400 m of exhumation vary from 6 to 20 Ma, respectively. A volcanic margin is far more likely given that post-breakup sedimentation commenced 2-7 Ma after breakup. Lastly, post-breakup interval exhumation ranges from 0 to 800 m. The highest values are in the hangingwall blocks of faults. Up to 200 m may be locally caused by reverse fault re-activation due to the present-day compressional stress state of Australia. The remainder is attributed to regional exhumation caused by dynamic topography in the last 50 Ma.

Recognition of mixed processes on coastal-deltaic systems end members (relative power of W- wave, T -tide and F-fluvial processes) is important for both exploration and reservoir characterization. Mixed-influence systems impart asymmetry and heterogeneity that impact prediction of subsurface lithology (facies), static modelling of various connectivity scenarios, and ultimately exploration to development well planning. Numerous detailed studies of these mixed-influence systems from modern analogs, outcrop and core, and log data requires calibration with high resolution seismic visualisation.

Although typical stacking of genetic units (5-25m parasequence-scale) is at or below the resolution limits of most 3D seismic data, focused seismic stratigraphic workflows can image detailed geomorphic plan-forms, which reflect features at the limits of detection (<10m).

A range of seismic stratigraphic workflows are illustrated (single and multiple datums, horizon slicing, flattening, optical stacking, channel/feature chasing, and attribute calculations) with a variety of example seismic datasets. These workflows can produce detailed images of complex facies juxtapositions at or near the detection limit. Specifically, we show examples of varying degrees of wave, fluvial and tidal influence, recognized by characteristic plan-form features at element to complex scales including (but not limited) to the following:

1. High to low reflectivity, continuous elongate arcuate, divergent to subparallel reflections (either convex or concave in a basinward direction), indicative of wave-dominated (W), to wave-dominated, but tide-influenced (Wt) strand-plains and associated down-drift chenier-plains (Tw).
2. High reflectivity, continuous and sinuous channel-form reflection features adjacent to sets of recurved-lineations (convex-basinward), interpreted as the trace of tide -influenced estuarine channels (Tf) or distributary channels (F, Ft).
3. Transparent seismic reflections with internal channel-forms, and dendritic or reticulate planforms, indicative of tide-dominated shorelines including tidal flats and associated tidal creeks (T, Tw, Twf).
4. High to low reflectivity, continuous or discontinuous, low- to high-sinuosity channel-form reflections, either isolated or amalgamated, indicative of fluvial-dominated channel belts, and associated abandoned meander loops (F, Ft), associated with a background of transparent to highly reflective continuous to discontinuous reflections, representing the alluvial or coast-deltaic floodplain.

This approach can assist prediction of reservoir connectivity in wave -dominated systems, with the recognition of internal baffles and local barriers associated with shale -prone parts of the depositional system, both within and between parasequences.

In the past years X-ray Computed Tomography (CT) became more and more common in geo-scientific applications and is used from the u-scale (microfossils) up to the dm-scale (cores or soil columns). Hence a variety of different systems was adapted to these applications.

In the present paper we investigate CT results from an Opalinus Clay core (diameter ~100 mm) considering the 3D distribution of cracks. Two CT systems are compared both, with specific ad- and disadvantages: the large and flexible phoenix v|tome|x L300 high energy CT scanner and the high throughput speed|scan CT 64 helix CT system (both GE Measurement & Control).

The results are compared regarding the contrast resolution, spatial resolution, and scanning speed. The fast medical scanners provided a quick overview whereas the microfocus tube provided a more detailed view on cracks.