Geomorphometric characterization of pockmarks by using a GIS-based semi-automated toolbox

Pockmarks are seabed depressions developed by fluid flow processes that can be found in vast numbers in many marine and lacustrine environments. Manual mapping of these features based on geophysical data is, however, extremely time-consuming and subjective. Here, we present results from a semi-automated mapping toolbox developed to allow more efficient and objective mapping of pockmarks. This ArcGIS-based toolbox recognizes, spatially delineates, and morphometrically describes pockmarks. Since it was first developed, the toolbox has helped to map and characterize several thousands of pockmarks on the UK continental shelf, especially within the central North Sea. This paper presents the latest developments in the functionality of the toolbox and its adaptability for application to other geographic areas (Barents Sea, Norway, and Malin Deep, Ireland) with varied pockmark and seabed morphologies, and in different geological settings. The morphometric characterization of vast numbers of pockmarks allows an unprecedented statistical analysis of their morphology. The outputs from the toolbox provide an objective, quantitative baseline for combining this information with the geological and oceanographical knowledge of individual areas, which can provide further insights into the processes responsible for their development and their influence on local seabed conditions and habitat

Cartografia morfo-estrutural da área da falha Marquês de Pombal : utilização de imagens de sonar lateral de alta resolução (TOBI) e de perfis sísmicos de reflexão para cartogarfia de áreas submarinas

O estudo mono-estrutural detalhado da área da Falha Marquês de Pombal (>3300 km2 agora apresentado, baseou-se na interpretação de: linhas sísmica mu1ti-canal, imagens de sonar lateral de alta resolução (TOBI) e da análise de dados batimétricos. Resultaram deste estudo: i) a quantificação parcial da deformação ao longo Falha Marquês de Pombal, ii) modelo digital de terreno da área estudada, iii) mapas de declive e outros produtos de analise espacial iv) mapas de pormenor dos movimentos de massa existentes na área v) mapa mono-estrutural da área da Falha Marquês de Pombal, à escala 1:250.000. Este trabalho mostra que a morfologia da área de estudo está intimamente associada à sua compartimentação estrutural e às diferenças na evolução tectónica do Miocénico Médio até ao Presente. Dobras e falhas inversas afectando unidades do Quaternário assinalam actividade tectónica actual. Canais e escorregamentos submarinos são frequentes em toda a área. Na vertente associada a Falha Marquês de Pombal foi identificado um grande escorregamento complexo (com uma área total de ~110 km2). Abstract: Morpho-Structural Mapping of the Marquês de Pombal Fault Area (off SW Portugal). This thesis is a morpho-structural study of the Marques de Pombal Fault (MPF) and surrounding area (>3300 km2), based on the study and interpretation of multi channel seismic reflection profiles, high-resolution side-scan sonar image (TOBI) and bathymetry data. The outcomes of the study are: i) the partial quantification of the deformation along the MPF, ii) digital terrain models of the study area, iii) slope maps and results of other surface analysis, iv) maps detailing the mass movements of the study area, v) a morpho-structural map of the MPF at a scale of 1:250 000. The analysis of the data shows that the morphological differentiations of the study area are intimately associated with its structural compartmentalization and the tectonic evolution from the Middle Miocene to present time. Folding and reverse faulting of the Quaternary units along this structure indicate present-day tectonic activity. Gully-incised slope failures and submarine landslides are common features. One major complex submarine landslide, disrupting a total area of ~.110 km2, was identified at the Marques de Pombal thrust front

Structural controls on shallow fluid flow and associated pockmark fields in the East Breaks area, northern Gulf of Mexico

Three-dimensional (3D) seismic data, combined with semi-automated mapping in ArcGIS, were used to analyse the morphology and distribution of 720 pockmarks and 62 mud volcanoes in the northern Gulf of Mexico. The relationship amongst salt bodies, faults and the distribution of pockmarks and mud volcanoes stresses the significance of these structures in focusing fluid flow on continental margins. The pockmarks were classified according to their structural setting and depth of source, and later correlated with seep data from the Bureau of Ocean Energy Management (BOEM). Key findings include: a) half of the pockmarks are located within faults rooted on the top of salt diapirs, whilst 96% of the pockmarks are associated with salt diapirs – emphasising the importance of salt and crestal faults in focusing fluid flow to the sea floor; b) diffusion flow through the salt minibasins is clear due to the presence of soft amplitude anomalies (indicating fluids) and pockmarks located far from salt or faults; c) oil and gas are actively leaking to the sea floor; d) a higher density of fluid flow features are found in areas with steeper minibasin dips and greater catchment areas. While no clear correlation is evident between the morphological attributes and depth of source, the shallow plumbing system is dominated by pockmarks, whereas mud volcanoes are sourced from the deeper parts of the salt minibasins. In summary, this study uses a novel approach to analyse the plumbing system in a salt-rich basin based on the recognition of surface fluid flow features. The importance of characterising the fluid flow features and associated structures to reduce risk and uncertainty is stressed in terms of both shallow gas hazards and hydrocarbon leakage from deeper reservoirs

Morphology of small-scale submarine mass movement events across the northwest United Kingdom

A review of multibeam echo sounder (MBES) survey data from five locations around the United Kingdom northwest coast has led to the identification of a total of 14 separate subaqueous mass movement scars and deposits within the fjords (sea lochs) and coastal inlets of mainland Scotland, and the channels between the islands of the Inner Hebrides. In these areas, Quaternary sediment deposition was dominated by glacial and glaciomarine processes. Analysis of the morphometric parameters of each submarine mass movement has revealed that they fall into four distinct groups of subaqueous landslides; Singular Slumps, Singular Translational, Multiple Single-Type, and Complex (translational & rotational) failures. The Singular Slump Group includes discrete, individual subaqueous slumps that exhibit no evidence of modification through the merging of several scars. The Singular Translational Group comprise a single slide that displays characteristics associated with a single translational (planar) failure with no merging of multiple events. The Multiple Single-Type Group incorporates scars and deposits that displayed morphometric features consistent with the amalgamation of several failure events of the same type (e.g. debris flows or slumps). Finally, the Complex (translational & rotational) Group comprises landslides that exhibited complex styles of failures, including both translational and rotational mechanisms controlling the same slide. The submarine mass movements that comprise this dataset are then discussed in relation to global fjordic and glaciomarine nearshore settings, and slope failure trigger mechanisms associated with these environments are described with tentative links to individual submarine landslides from the database, where appropriate. It is acknowledged that additional MBES data are needed not only to expand this database, but also in order to create a more statistically robust study. However, this initial study provides the basis for a much wider investigation of subaqueous mass movements and correlations between their morphometric parameters

An integrated geological and GIS-based method to assess caprock risk in mature basins proposed for carbon capture and storage

Subsurface injection of carbon dioxide (CO2) is a technique to enhance oil recovery and so the economic value of depleting fields. It complements carbon capture and storage, which is a key technology to mitigate greenhouse gas emissions. In this work, an integrated method developed by the British Geological Survey and Cardiff University uses high-resolution 3D seismic and borehole data from the Jæren High to analyse potential seal breaches and fluid flow paths in a frontier area of the North Sea, ultimately assessing the risk of a possible carbon capture and storage site. We integrate the spatial analysis of subsurface fluid flow features with borehole and geochemical data to model the burial and thermal history of potential storage sites, estimating the timing of fluid expulsion. On seismic data, fluid pipes connect reservoir intervals of different ages. Spatial analysis reveals clustering of fluid flow features above strata grounded onto deep reservoirs intervals. Our integrated method shows that gas matured from Dinantian coal and migrated up-dip during the Triassic-Jurassic into the lower sandstone reservoir of the Rotliegend Group. The containing seal rock was breached once sufficiently large volumes of gas generated high overpressures in the reservoir. Some of these fluid flow features may still be active conduits, as indicated by bright amplitude anomalies within the pipes. This study shows how integrated analyses may enhance our understanding of fluid-flow pathways, de-risking prospective sites for carbon capture and storage. The method proposed in this work is particularly important to assess the suitability of area with trapped gas pockets and understand tertiary migration in areas proposed for geological storage of CO

Land below sea: a new generation of seabed geology mapping

Spurred on by the increasing availability of high-resolution bathymetry data, as well as growing awareness in the importance of the seabed environment, Dayton Dove and colleagues discuss an initiative to renew mapping of the seabed geology around the U

Rule-based semi-automated tools for mapping seabed morphology from bathymetry data

Seabed morphology maps and data are critical for knowledge-building and best practice management of marine environments. To facilitate objective and repeatable production of these maps, we have developed a number of semi-automated, rule-based GIS tools (Geoscience Australia’s Semi-automated Morphological Mapping Tools - GA-SaMMT) to operationalise the mapping of a common set of bathymetric high and bathymetric low seabed Morphological Features. The tools have a graphical user interface and were developed using Python scripts under the widely-adopted proprietary ArcGIS Pro platform. The utility of these tools was tested across nine case study areas that represent a diverse range of complex bathymetric and physiographic settings. Overall, the mapping results are found to be more consistent than manual mapping and allow for capture of greater detail across a range of spatial scales. The mapping results demonstrate a number of advantages of GA-SaMMT, including: 1) requirement of only a bathymetry grid as sole data input; 2) flexibility to apply domain knowledge to user-defined tool parameters, or to instead use the default parameter settings; 3) repeatability and consistency in the mapping outputs when using a consistent set of tool parameters (user defined or default); 4) high-degree of objectivity; and 5) efficiency in mapping a large number (thousands) of seabed morphology features in a single dataset. In addition, GA-SaMMT can comprehensively quantify the characteristics of individual seabed bathymetric high and low features, respectively generating 34 and 46 metrics for each type of feature. Our results indicate that attribute metrics are invaluable in the interpretation and modelling of mapped Morphology Features and provide insights into their formative processes and habitat potential for marine communities

Origin of high density seabed pockmark fields and their use in inferring bottom currents

Some of the highest density pockmark fields in the world have been observed on the northwest Australian continental shelf (>700/km2) where they occur in muddy, organic-rich sediment around carbonate banks and paleochannels. Here we developed a semi-automated method to map and quantify the form and density of these pockmark fields (~220,000 pockmarks) and characterise their geochemical, sedimentological and biological properties to provide insight into their formative processes. These data indicate that pockmarks formed due to the release of gas derived from the breakdown of near-surface organic material, with gas accumulation aided by the sealing properties of the sediments. Sources of organic matter include adjacent carbonate banks and buried paleochannels. Polychaetes biodiversity appears to be affected negatively by the conditions surrounding dense pockmark fields since higher biodiversity is associated with low density fields. While regional bi-directionality of pockmark scours corresponds to modelled tidal flow, localised scattering around banks suggests turbulence. This multi-scale information therefore suggests that pockmark scours can act as proxy for bottom currents, which could help to inform modelling of benthic biodiversity pattern

Geochemistry and related studies of Clyde Estuary sediments

Geochemical and related studies have been made of near-surface sediments from the River Clyde estuary and adjoining areas, extending from Glasgow to the N, and W as far as the Holy Loch on the W coast of Scotland, UK. Multibeam echosounder, sidescan sonar and shallow seismic data, taken with core information, indicate that a shallow layer of modern sediment, often less than a metre thick, rests on earlier glacial and post-glacial sediments. The offshore Quaternary history can be aligned with onshore sequences, with the recognition of buried drumlins, settlement of muds from quieter water, probably behind an ice dam, and later tidal delta deposits. The geochemistry of contaminants within the cores also indicates shallow contaminated sediments, often resting on pristine pre-industrial deposits at depths less than 1 m. The distribution of different contaminants with depth in the sediment, such as Pb (and Pb isotopes), organics and radionuclides, allow chronologies of contamination from different sources to be suggested. Dating was also attempted using microfossils, radiocarbon and 210Pb, but with limited success. Some of the spatial distribution of contaminants in the surface sediments can be related to grain-size variations. Contaminants are highest, both in absolute terms and in enrichment relative to the natural background, in the urban and inner estuary and in the Holy Loch, reflecting the concentration of industrial activity

Submarine mass movement processes on the North Sea Fan as interpreted from the 3D seismic data

This research has been focused on the characterisation and analysis of the deposits of large-scale mass movement events that shaped the North Sea Fan since the Mid-Pleistocene. Located at the mouth of the cross-shelf trough Norwegian Channel, the North Sea Fan is one of the largest through-mouth fans in the glaciated european margin with an area of approximately 142,000 km2. Submarine mass movement processed have occurred intermittenrly throughout the Quarternary history of the North Sea Fan, related to recurrent climate-related episodes of growth and retreat of the ice sheets. These processes can transport large amounts of sediment from the upper shelf up to the abyssal basins, playing an important role on the evolution of continental margins and can also reporesnet major geological hazards. This thesis uses mainly 3D seismic data to investigate the external geometry and internal structure of large-scale mass movement deposits. The high spatial resolution provided by the 3D seismic data has allowed a detailed geomorpholocial analysis of these deposits, This study involved the interpretation of the seismic data and the detailed pickling of key reflectors followed by tge extraction of both horizon and window-based seismic attributes. Digital elevation models of the key reflectors and their seismic attribute maps were then transferred to a geographical information system (GIS) where they were interactively interpreted using spatial analysis tools and the full visualisation potential of the software. The outcomes of this study highlight the importance of detailed horizon pickling and interactice interpretation followed by spatial analysis and visualisation in GIS environment. The identification of acoustic patterns within deposits that are normally described from 2D seismic as chaotic or acoustically transparent emphasizes the potential of detailed analysis of 3D seismic data. It gives an example of how this type of data can provide new insights into the mechanisms and processes associated with mass movements. In particular, amplitude and RMS amplitude maps provide remarkable detailed information of internal deformation structures whereas slope, shaded-relief and thickness maps allowed detailed characterisation of the external geometry. Various types of kinematic indicators can be recognized within the mass movement deposits through combined seismic analysis and detaield morphological mapping