Australian North West Shelf Research Articles

Emplacement and landscape controls on ancient submarine volcanism: 3D seismic geomorphological analysis of Cretaceous volcanic activity in the Browse Basin, Australian North West Shelf

Ancient submarine volcanic systems preserved within sedimentary basins provide opportunities to investigate the time and spatial distributions of intrusive and extrusive rocks, though existing work has largely focused on the geometrical features of submarine volcanic products, with limited interrogation of their interactions with sedimentary processes. This study investigates a buried submarine volcanic system within the Browse Basin, Australian North West Shelf. Through the integration of three-dimensional seismic reflection survey with borehole data, our study reveals intricate geometric features of a submarine volcano and lava channels, and their relationship to an associated intrusive network of Early Cretaceous age that has been preserved beneath ∼3.5 km of sedimentary overburden. In planform, the volcano has a diameter >4.5 km with a preserved height reaching 650 m. Meandering lava flow channels extend ∼20 km southward of the volcanic edifice, forming lava lobes with pressure ridges at the flow termini. Abundant sheet intrusions, identified within underlying Jurassic and older strata, indicate a magma transport from the north. The localised accumulation of sills led to variations in paleo-seafloor topography, influencing the direction of lava flows and post-volcanic sedimentation patterns. Our findings have a broad range of implications encompassing submarine volcanism and their impacts on basin dynamics.

Early Pleistocene Orbital‐Scale Variability in Australian Northwest Shelf Sediments

AbstractPaleoclimate proxy records from regions sensitive to humidity/aridity extremes provide crucial insights into the natural forcing mechanisms underlying long‐term climate variability in broader regions. One such area is Northwest Australia, where the Australian monsoon impacts its northernmost fringes, which are bordered by the Great Sandy Desert inland. Marine sediments from the Australian Northwest Shelf record fluvial run‐off and eolian dust input during the wet and dry seasons, respectively. The location is therefore ideal for investigating long‐term variability in the Australian monsoon and Northwest Australian dust flux over orbital timescales. However, there are few continuous, high‐resolution paleoclimate records from this region spanning the early Pleistocene, when strong ice‐climate feedbacks of the late Pleistocene did not yet dominate global climate. Here, we present geochemical and environmental magnetic proxy records that reveal %CaCO3 and dust‐flux variability between 2.9 and 1.6 million years (Myr) ago from International Ocean Discovery Program Expedition 356 Site U1464 on the Australian Northwest Shelf. We establish a new, orbitally‐tuned chronology for Site U1464, primarily based on ∼400 thousand year (kyr) eccentricity cyclicity in %CaCO3, and observe strong obliquity variability (41 and 54 kyr periodicities) but almost no precession signal in the dust‐flux records. We propose that the 41 kyr cycle in Northwest Australian dust flux could be a linear response to East Asian winter monsoon intensity and/or the summer inter‐tropical insolation gradient (SITIG), whereas the 54 kyr cyclicity might be a non‐linear response to obliquity amplitude modulation via the SITIG effect on the cross‐equatorial atmospheric circulations.

The spatio-temporal variation of exhumation and its impacts on the resource distribution in the Northern Carnarvon Basin, Australia

Deformation and rock fracture during exhumation can enhance rock porosity and permeability, thereby improving fluid migration through reservoirs and destroying seal integrity in caprocks. Therefore, analysing the spatio-temporal distribution of exhumation in connection to fluid migration models can be crucial for energy resource exploration and storage. Geological and geophysical evidence suggests that parts of the Northern Carnarvon Basin (NCB) were exhumed. However, the spatio-temporal distribution of this exhumation and its impacts on the region's resource distribution is yet to be established. We analysed porosity, sonic logs and paleotemperature data from 210 boreholes using compaction and vitrinite reflectance analysis techniques to constrain the spatio-temporal distribution of exhumation and its correlation to the region's resource distribution. We discovered that multiple exhumation phases occurred since the Late Triassic. The highest exhumation occurred in the Mid Jurassic, where approximately 1000 m of sediment thickness was eroded from the western and southwestern parts of the Exmouth Plateau. Exhumation at this time spatially correlates with magmatic intrusions in the NCB, and paleotemperature profiles indicate that sediments in the exhumed regions were partly heated by increased paleo-basal heat flow, suggesting that magmatic bodies through underplating and crustal intrusions were the primary causes of this exhumation. Another significant exhumation phase occurred in the Early Cretaceous, removing ∼800 m of sediments from the Exmouth, Barrow and Dampier Sub-basins. Depth-dependent extension events and magmatic intrusions possibly caused exhumation at this time. Significant exhumation ceased in the Early Cretaceous, and our study has established that the exhumation episodes in the NCB were part of large-scale regional events that affected the Australian Northwest Shelf. Combining results from different estimation techniques increased the spatio-temporal coverage of the analysis and improved the precision and accuracy of exhumation results.

Diagenetic dolomite in planktonic foraminifera on the Australian Northwest Shelf

ABSTRACTPlanktonic foraminiferal shells are widely used to investigate past oceanographic and climatic variations via their trace elements and stable isotopes. However, these geochemical methods may be compromised by the presence of diagenetic high‐Mg calcite. In this study, dolomite crystals are observed in planktonic foraminifera from International Ocean Discovery Program Site U1464 sediments on the Australian Northwest shelf, a shallow marine environment (<300 m). This study investigates the formation of the dolomite using new scanning electron microscopy, X‐ray diffraction, stable isotope (carbon and oxygen), Mg/Ca and rare earth element data, combined with shipboard geochemistry and available literature. This study finds that foraminiferal dolomite formation was probably associated with early diagenesis, microbial and ageing processes. Two potential diagenetic processes (seafloor diagenesis and post‐depositional diagenesis) are proposed, and both indicate that the favourable redox condition of dolomite formation is low oxygen, which may activate methanogenic catalysis to facilitate dolomite formation. It is postulated that high‐Mg calcite may be the precursor of foraminiferal dolomite in this case, and that dissolution–precipitation may be the ageing process of dolomite formation. In contrast to deeper, more normal pelagic environments, this study shows that the marine environment can facilitate dolomite precipitation by higher fluxes of organic matter to the seafloor.

Trends in catch rates of sawfish on the Australian North West Shelf

Northwestern Australia is thought to have some of the world’s last remaining viable sawfish populations, although little quantitative data exists on their status or trends. This study examined 17 years of logbook bycatch records (n = 815) for green sawfish Pristis zijsron and narrow sawfish Anoxypristis cuspidata from a trawl fishery operating on the Australian North West Shelf. Incidental sawfish captures by the fishery are rare, occurring approximately once every 75 trawls (~199 trawl hours). To standardize catch rates and account for excess zeros in the data, we employed generalized additive models for location, scale, and shape (GAMLSSs) using a zero-inflated Poisson distribution. For green sawfish, catch rates approximately doubled over the study period, while an oscillating trend was observed for narrow sawfish catch rates. Reported captures occurred throughout the management boundaries of the fishery, which operates in mid-shelf waters from 48 to 121 m. A weak seasonal signal in catch rates was detected for both species, with the highest capture numbers occurring during autumn-winter, consistent with an expected inshore migration for parturition during spring-summer. Logbook trends were partly corroborated by independently verified data collected in a subset of years, which also showed an increasing proportion of green sawfish in the catch. Our findings emphasize the importance of sawfish populations in northwestern Australia within the context of global conservation efforts for this taxon.

Ocean Mixing in a Shelf Sea Driven by Energetic Internal Waves

AbstractInternal waves drive ocean mixing and enhance the transport of heat, momentum and other tracers in shelf seas. We collected observations of mixing over a 30‐day period from three cross‐shore moorings placed on the 330, 200 and 150 m isobaths on the offshore side of a pelagic ridge on the Australian North West Shelf. The region is forced by energetic surface and internal tides, exhibits non‐linear internal waves, experiences flow‐topography interactions, and is subject to episodic intense wind events. This complex forcing drove energetic diapycnal mixing at all sites. We identified five dominant internal wave forcing categories: mode‐1 waves at low‐frequency (time scales from double the buoyancy period to 4 hr), mode‐1 waves at high‐frequency (HF) (time scales between the buoyancy period and double the buoyancy period), mode‐2 waves, internal bores, and internal hydraulic jumps. Overall, just 15% of mixing events accounted for 90% of the total observed heat flux over the record. Mixing during internal wave events accounted for as much as 50% of the total heat flux in some locations. Of the internal wave categories, HF mode‐1 waves were the most significant contributors to the total heat flux at all sites (∼20%). On the other hand, internal bores made significant contributions to mixing only at the 200 and 150 m moorings; they made no contribution to mixing at the 330 m mooring. At the shallowest mooring, the different internal wave categories all made similar contributions to the total flux, indicating an increasingly complicated relationship between the evolving internal wavefield and the mixing.

Differentiating reefal ridges from relict coastal ridges: Lessons from the seismic geomorphologic study of buried Miocene buildups (North West Shelf, Australia)

AbstractLinear buildups formed in tropical carbonate environments are often interpreted as bioconstructed reefs. Nevertheless, coastal processes can also form extensive sedimentary ridges exhibiting buildup morphologies. This study investigates two Miocene ridges developed along the Australian North West Shelf using 3D seismic and well data.Ridge 1is ca. 30 m thick and >60 km long, and it is made of foraminiferal pack‐grainstones. It protects a lagoon with pinnacle morphologies.Ridge 2is ca. 150 m thick and >80 km long. It is composed of quartz sand forming lobes. Both ridges have a continuous curvilinear front and are in a mid‐shelf setting. They mimic the modern Australian coastline. It is then proposed thatRidge 1is either: (1) a barrier reef developed on a drowned shoreline, or (2) stacked carbonate aeolianites and beachrocks acting as a barrier.Ridge 2is interpreted as stacked deltaic sands. This study demonstrates that lithified and buried coastal features of carbonate and siliciclastic nature can form extensive ridges exhibiting buildup morphologies. It is proposed that ridges formed by stacked coastal features are overall continuous with a curvilinear front, while reefal ridges are more discontinuous and exhibit deeper and more stable passes.

Petrophysical properties of igneous rocks in continental rift basins – a case study from the Mesozoic sequence of the Browse Basin, North West Shelf of Australia

Capturing the ubiquitous distribution and properties of igneous rocks is necessary to succeed in hydrocarbon exploration, field development and underground gas storage when dealing with magma-rich rift margins. In recent years, numerous researches have revealed detailed morphologies of intrusive and extrusive rock units embedded within sedimentary basins owing to advances in 3D seismic data. Outcrop studies have also provided deeper understandings of their occurrence and geometries. However, there has been comparatively little focus on the rock physics of igneous bodies and their relationships to seismic expression. To bridge this gap between the outcrop and the seismic information, we undertook a well-log-based petrophysical study of igneous rocks using subsurface dataset from the Browse Basin located in the Australian North West Shelf. In this contribution, we describe a classification of volcanic facies (e.g. lava flows, volcaniclastics, intrusive sills) based on well log motifs and textures apparent in borehole image data. Statistics of the petrophysical properties of each volcanic facies are also analysed in order to examine their correlation to seismic patterns. Our study implies the importance of detailed characterisation of igneous petrophysical properties of igneous rock units where concentrated within a sedimentary basin, which will help reconcile interpretations of seismic data.

A Directional Decomposition Method to Estimate the Reflection and Transmission of Nonlinear Internal Waves Over a Slope

AbstractMulti‐directional propagating nonlinear internal waves (NLIWs) create complex spatial patterns, often making it difficult to quantitatively estimate the reflection and dissipation processes of NLIW energy over a slope. To identify the onshore‐ and offshore‐going wave signals in a regional model, we apply a directional Fourier filter (DFF) method to clarify wave dynamics over varying slopes. First, a series of two‐dimensional analytical solutions of either solitons or NLIW packets are utilized. Next, two‐dimensional numerical experiments indicate that the rate of reflection (dissipation) of energy for the shoaling NLIWs is much lower (higher) than that for the shoaling internal tides over a slope, regardless of varying slope criticality and height. Finally, we apply the DFF method in a three‐dimensional non‐hydrostatic regional model (MITgcm) to directionally decompose the onshore‐ and offshore‐going internal waves (IWs) on the Australian North West Shelf. The model results show that mode‐1 incoming internal tides gradually steepen into NLIWs during the shoaling processes over the slope, and then the reflecting IWs are propagate offshore in the main form of linear IW beams. In addition, the reflectivity of IWs around the Imperieuse Reef is 60% and the offshore‐reflecting IWs quickly dissipate accompanied by an e‐folding length scale of ∼22 km.

Observation of strong turbulent mixing in the Australian North West Shelf

The North West Shelf (NWS) of Australia is a region with strong barotropic tides up to 10 m, which provide a source for intense turbulent mixing. In the study, we used turbulent mixing measurements obtained on the NWS from October to November 2017 to investigate the mixing characteristics during the austral spring season. In the surface mixed layer, the averaged dissipation rate reached 1 × 10−5 W/kg. The dissipation rate decreased to 1 × 10−8 W/kg in the thermocline and increased to 7 × 10−8 W/kg near the bottom. The averaged diffusivity in the surface mixed layer reached 7 × 10−1 m2/s, decreased to 1 × 10−4 m2/s in the thermocline, and increased to 7 × 10−3 m2/s near the seafloor. These values were significantly larger than those in the open ocean. The study proposed a parameterization method that scaled the diffusivity in a function of the Richardson number, κρ=κ0+κm(Ri)−1.5, where κ0 and κm were derived from the observation data, to take into account the strong shear instability conditions in our observation. Applications of this method for other shelf regions need further validation.

3-D seismic chronostratigraphy of reefs and drifts in the Browse Basin, NW Australia

Abstract In its subsurface, the Browse Basin on the Australian Northwest Shelf accommodates one of the largest Miocene–Pliocene carbonate platform provinces on Earth. This study presents a comprehensive three-dimensional (3-D) documentation and investigation of the Neogene Browse Basin carbonates based on the analysis of 15 3-D seismic reflection volumes covering ca. 33,000 km2 and 22 industry boreholes. Eight basin-wide seismic marker horizons tied to Sr isotope and biostratigraphic data provide a robust 3-D chronostratigraphic framework that (1) newly documents the occurrence of Miocene to recent carbonate systems in vast inboard basin areas; (2) presents the 3-D distribution and dynamic development of carbonate platforms through time constrained by absolute ages; and (3) supports the detailed 3-D interpretation of Miocene to recent depositional processes and key stratigraphic controls. Around 18.6 Ma, the extent of all Browse Basin carbonate platforms was ca. 8600 km2, rising to >10,000 km2 between 15.6 Ma and 12 Ma. By ca. 9.7 Ma, the total platform extent had decreased to ca. 5700 km2, waning to <2800 km2 in the Pliocene. The observed reef demise is less abrupt than previously thought and comprises two steps, with the first in the late Miocene on the outer shelf and the second in the Pliocene on the middle and inner shelf. The Miocene outer-shelf platform demise coincided with (1) strong subsidence that outpaced sedimentation, (2) an increase in NW–SE-oriented tidal current activity, (3) the development of NNE–SSW-oriented bottom currents, and (4) the onset of drift sedimentation. The Pliocene demise of middle-shelf reefs coincided with (1) an increase of clastic sediment input from land and (2) stalling of the Indonesian Throughflow around 3.7 Ma. The basin-wide, 3-D seismic chronostratigraphic analysis presented supports a re-evaluation of the key controls for Miocene and Pliocene reef growth and decay on the NW Shelf and highlights the complexity of interacting global, regional, and local processes and peculiarities in carbonate platform development.

Origin of seafloor pockmarks overlying submarine landslides: Insights from semi-automated mapping of 3D seismic horizons (North West Shelf, Australia)

Pockmarks are geohazards that can impact offshore developments, and an understanding of their formation assists in determination of their physical characteristics and in the prediction of their future behaviour. This study documents how the presence of submarine landslides - either on the seafloor or buried - can favour pockmark formation through the characterisation of two pockmark fields from the Australian North West Shelf. Analysis was carried out through an innovative workflow combining full-volume interpretation of exploration 3D seismic data and semi-automated mapping of seismic horizons. The Gorgon Pockmark Field (GPF) extends over ∼125 km2 in water depths ranging from 210 to 480 m. It contains ∼500 pockmarks with diameters of ∼150–200 m. The pockmark field overlies a submarine landslide and was likely created by the expulsion of fluids and/or liquefied sediment during or after the formation of the landslide. The Carnarvon Pockmark Field (CPF), located 100 km southwest of the GPF, covers around ∼150 km2 in water depth ranging from 680 to 930 m. It contains ∼250 pockmarks, each having a diameter of ∼250–300 m. The CPF developed on a buried paleo-landslide. While the seafloor pockmarks and the paleo-landslide are physically disconnected by a sedimentary drape, seafloor pockmarks are abundant above the paleo-landslide and present a characteristic morphology, then suggesting that fluids circulating through the paleo-landslide induced the formation of the seafloor pockmarks. Study of the Gorgon and Carnarvon pockmark fields presents new evidence that submarine landslides - either on the seafloor or buried - can control the development of pockmarks over huge areas of the seafloor.

A Seasonal Harmonic Model for Internal Tide Amplitude Prediction

AbstractWe present an empirical model of the seasonal variability of the internal tide using seasonal harmonics to modulate the amplitude of the fundamental tidal constituents. Internal tide data, from both long‐term, in‐situ moorings and a mesoscale‐resolving and internal tide‐resolving ocean model, are used to demonstrate the performance of the seasonal harmonic model for the Indo‐Australian Basin region. The seasonal model describes up to 15% more of the observed (baroclinic) sea surface height variance than a fixed‐amplitude harmonic model at the mooring sites. The ocean model results demonstrate that the study region, which includes the Australian North West Shelf (NWS), Timor Sea, and southern Indonesian Islands, is dominated by standing wave interference patterns due to the presence of multiple generation sites. The seasonal harmonic model reveals that temporal shifts in the standing wave patterns coincide with seasonal variations in density stratification. This shift is particularly evident within distances of 2–3 internal wave lengths from strong generation sites. The fraction of the variance of the internal tide signal explained by seasonal modulations is largest in standing wave node regions, contributing to differences in predictive skill of the seasonal harmonic model at two moorings separated by only 38 km. Output of the harmonic model also demonstrates that the seasonally evolving internal tide propagating southward from Lombok Strait had a small amplitude in October when shear from the Indonesian Throughflow was strongest.

Quantifying exhumation using compaction and vitrinite reflectance in the Southern Bonaparte Basin, North West Shelf, Australia

Rock fracture during exhumation can enhance hydrocarbon fluid migration through reservoir rocks but can also breach seal rocks and cause fluids to escape. Therefore, understanding the spatial and temporal distribution of exhumation within sedimentary basins can help optimize oil and gas systems targeting and help identify potential subsurface locations for CO2 storage. Structural, tectonic and stratigraphic studies suggest that the hydrocarbon-rich Bonaparte Basin of the Australian North West Shelf has undergone multiple episodes of exhumation since the Palaeozoic. However, the distribution of this exhumation and its correlation with the distribution of petroleum systems in the basin is not well established. In this work, we combine compaction and vitrinite reflectance analysis to estimate exhumation on selected boreholes, then model its temporal and spatial distribution within the southern Bonaparte Basin. The compaction and vitrinite reflectance analysis use porosity and paleotemperature data of sediments, respectively, to estimate the maximum burial depth of sediments and subsequently calculate exhumation. Our results indicate that the southern Bonaparte Basin went through an average of ∼1.9 km total exhumation from the Permian to the present with a maximum exhumation of ∼2.7 km in the Londonderry High. Vitrinite reflectance data suggest that sediments in the Ashmore Platform were subjected to heat from magmatic events. From the Permian to the Late Triassic, exhumation was confined within the Ashmore Platform (∼0.8 km) and Vulcan Sub-basin. Then from the Late Triassic to the present, exhumation shifted eastwards towards the Londonderry High and Petrel Sub-basin. The results show that the latest Cretaceous and Cenozoic tectonic events produced around 0.8 km of exhumation in the Londonderry High and 0.3 km in the Ashmore Platform. This exhumation likely impeded the formation of Mesozoic petroleum systems in the central Bonaparte Basin while preserving the older Paleozoic system whose hydrocarbons migrated towards traps in around the Londonderry High. Our results indicate porosity values between 20% and 25% for the Vulcan Sub-basin and the Ashmore Platform with little exhumation making them potential areas for CCS.

Early Miocene carbonate ramp development in a warm ocean, North West Shelf, Australia

AbstractAlthough carbonate ramps are widely described from the geological record, there is still a debate on the relative influence of water temperature, trophic conditions and type of carbonate factories on their development. The ca 2400 km long Australian North West Shelf is among the largest Cenozoic carbonate provinces worldwide, and records a transition from an early Miocene ramp to a middle Miocene rimmed platform. This change is observable on publicly available seismic data, giving the opportunity to investigate environmental influences on platform evolution. This study combines macroscopic and petrographic descriptions of early Miocene strata cropping out in the Cape Range Anticline (North West Cape, southern end of the North West Shelf) and of time‐equivalent well cuttings from the adjacent, offshore Exmouth Sub‐basin. Particular emphasis is placed on the identification of larger benthic foraminifera at a broad generic level, because differing taxa have a limited range of habitable conditions that serve as environmental proxies. The results show that early Miocene strata are dominantly composed of larger benthic foraminifera with minor coralline algae in the proximal platform, grading to micropackstones in the more distal platform. A ramp margin is inferred from the lithological data on the basis of the lack of framework builders and the presence of open oceanic indicators. Facies shallow upward through individual outcrops, with a proximal to distal trend towards the north‐west. These trends along outcrops are consistent with the seismic interpretations. Identification of taxa with warm, oligotrophic water affinity suggests that the ramp was formed in an oligotrophic and warm ocean, despite the absence of coral reefs. Changes of carbonate facies with depth do not seem to be associated with changes in ramp morphology, and the latter may have been controlled by physical oceanic parameters, such as offshore currents and waves.

Reservoir Characterization, Scenario-Based Models Optimize Development Planning

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202273, “Reservoir Characterization and Scenario-Based Modeling To Optimize Development Planning of the Jurassic Plover Formation in the Ichthys Field, North West Shelf of Australia,” by Kazuyuki Yamamoto, SPE, Shuji Yamamoto, and Toby Jones, INPEX, et al., prepared for the 2020 SPE Asia Pacific Oil and Gas Conference and Exhibition, originally scheduled to be held in Perth, Australia, 20–22 October. The paper has not been peer reviewed. The Plover Formation is one of two reservoirs in the Ichthys field of the Australian North West Shelf. The objective of this study is to build multiple scenario-based models to optimize development planning in preparation for the upcoming production phase. The authors have integrated data and interpretations of thin sections, cores, well logs, and seismic data to create multiple geological concepts for the field and to identify key geological uncertainties. Introduction The Ichthys liquefied natural gas project is one of the world’s largest and involves the development of a gas-condensate field in the Browse Basin. The field is approximately 220 km offshore Western Australia and covers an area of approximately 800 km2 with an average water depth of approximately 250 m. The field is currently under preparation for the development of the Plover Formation. The authors conducted an integrated subsurface evaluation and built reservoir models with newly reprocessed 3D seismic data to optimize Plover development planning. Considering the geological uncertainty given the limited production data gathered before the production phase, the multiple deterministic approach was selected as the best option to optimize development planning. In this approach, it is important to capture all geological scenarios that may occur in the field and to build reservoir models in which the scenario concepts are explicitly integrated. A multidisciplinary team was organized to conduct this study. Geology of the Plover Formation The Plover Formation consists of sandstones, shales, igneous rocks, and a minor amount of coal. The depositional environment is fluvial to shallow marine. The average thickness of the formation is approximately 360 m. Based on the stratigraphic correlation anchored by mainly palynological biostratigraphic data, the formation has been subdivided into five stratigraphic members. The formation is overlain by the Ichthys Formation, which is composed mainly of argillaceous sandstone and shale deposited in a tidally influenced, lower-delta-to-shelf environment. Although the nine existing exploration wells broadly cover the entire field, the spacing between the wells is still large (8–10 km). Further-more, core coverage is low, especially in the reservoir sandstone intervals. Igneous rock, predominantly basaltic in composition and likely extrusive in origin, occurs more in the eastern part of the field. Igneous activity has complicated the distribution and connectivity of the reservoir sandstones in the Ichthys field. Key Geological Uncertainties Rock Type (RT). The results of petrographic analysis of rock samples from the Ichthys field and other surrounding fields indicate that the sandstones of the Plover Formation can be separated into multiple RTs: RT1 and RT2, with RT1 subdivided into RT1a and RT1b. Very little data exist from RT2 in the fluvial/distributary channel sandstones in the Ichthys field in comparison with other fields; this is considered to be the result of limited core. The assumption that RT2 exists in the field is critical to prepare for the possibility that a future well may be drilled that might have some nontrivial quantity of RT2.

Directional decomposition of internal tides propagating from multiple generation sites

Internal tides generated from multiple sites create complicated interference patterns, often making it difficult to identify the origin, direction of propagation, and dissipation of individual waves. To identify these constituent waves, we apply a directional Fourier filter (DFF) method to analyse multimodal, broad-wavenumber, internal wave fields found over varying topography. We apply the method to data derived from both a numerical model and satellite remote sensing observations. Using a series of two-dimensional (i.e. x–z) simulations, we first demonstrate the capability of the DFF method to separate incoming and reflected internal tides emanating from a supercritical slope. The results show the DFF method requires model sampling rates of Δt∕T<0.15 and horizontal cell sizes of Δx∕λ<0.2, where T is the wave period and λ is the wavelength. Remote internal tides can significantly affect the generation and propagation of local internal tides in regional ocean simulations. Using a gridded satellite altimetry product at three globally diverse regions, we demonstrate how the DFF method can be used to specify the internal tide boundary conditions at the open boundaries of regional ocean models. Lastly, we apply the DFF method to a three-dimensional hydrostatic numerical simulation of the southern Australian North West Shelf to estimate the energy decay characteristics of internal tides. The energy decay length scale, for offshore-propagating waves from the continental slope, in the model was estimated to be two mode-1 wavelengths (i.e. 240 km), in close agreement with an estimate derived from a satellite-derived sea surface height anomaly.

Observations of Breaking Internal Tides on the Australian North West Shelf Edge

A comprehensive observational data set was used to examine shoreward propagating semidiurnal internal tides as they shoal, break and run-up as turbulent boluses across the edge of the Australian North West Shelf (NWS), offshore Dampier, during late winter 2013. The measured waveforms and wavefields supported the grouping of events into two distinct categories: (1) pre-; and, (2) post- wave breaking. It was found that the transition from (1) to (2) was marked by the rise of nonlinear steepening (α) and reduction in dispersion (β), both coefficients that parameterize nonlinear wave effects on the Korteweg-de Vries (KdV) equation. We introduced a criterion for wave breaking from the dimensionless parameter (δ) that relates these two terms: wave breaking occurs when δ &amp;lt; 1. In the first group, dispersive effects were dominant to spread energy out of the semidiurnal wave to a dispersive wave packet of short-period internal solitary waves (ISWs). In the second, dispersion was considered small compared to the cumulative effect of nonlinear steepening. Here, the semidiurnal wave built sufficient energy at its rear face to generate wave breaking, which has been known to produce multiple turbulent boluses. Similar observations have not been described for this region during winter months and highlight that the nonlinear internal wave field is an important feature on the NWS throughout the year. Additionally, measurements obtained through autonomous ocean glider profiles revealed some of the post-breaking characteristics that included intensive vertical mixing and transport of dense water and suspended material onshore of the shelf break.

Novel collaborative P&amp;amp;A campaign using inflatable packers and bismuth alloy plugs

Chevron Australia, a leading O&amp;amp;G operator on the Australian North West Shelf, executed a plug and abandonment (P&amp;amp;A) campaign where 34 wells (19 offshore and 15 onshore) incorporated a novel bismuth alloy barrier system to the traditional cement plug. The challenge of isolating shallow gaseous zones that cause sustained casing pressure and free gas flow to surface behind the production casing was overcome by this new innovative use of collaborative technologies to provide an optimised P&amp;amp;A solution. The offshore/onshore P&amp;amp;A campaign was completed successfully with significant cost savings, eliminating the lengthy process of section milling more than 100ft of casing on each well, providing a long-term bismuth alloy barrier in the well and eliminating the potential need to reenter the well later due to a leaking cement plug.

The Reversal Indian Ocean Waters

AbstractThe Reversal Indian Ocean Waters (RIOWs), carried by flows in a direction opposite to the Indonesian Throughflow (ITF), is investigated in the Lagrangian framework using the simulative results of a high‐resolution western Pacific and northern Indian Oceans Model (PIOM). The results suggest that the RIOWs have two sources: They are the Australian Northwest Shelf Inflow linking with the Leeuwin UnderCurrent (LUC) along with the previously identified South Java Coastal Undercurrent (SJCU). After entering the Indonesian seas, the RIOWs mix with other water masses and tend to make waters in the Indonesian seas cooler, saltier, and denser. Ultimately, about half of the RIOWs return to the Indian Ocean by joining the ITF, and the other half enter the Pacific Ocean and contribute to the North Equatorial CounterCurrent (NECC) and the North Intermediate CounterCurrent (NICC).