How understanding past landscapes might inform present-day site investigations: a case study from Dogger Bank, southern central North Sea

The integration of geophysical and geotechnical datasets acquired during a site survey for the Dogger Bank wind farm has enabled a new litho- and seismo-stratigraphy to be established. Although previously believed to be a relatively simple “layer-cake”, the data reveal that the sedimentary sequence within the foundation zone includes a complex series of buried landscapes with implications for both foundation siting and design. The most significant is a Weichselian glacially derived landscape dominated by a large thrust-block moraine complex buried beneath a thin Holocene sequence. This glacial landscape profoundly affects the structure and physical properties of sediments within the foundation zone due to locally intense glaciotectonic deformation and the occurrence of sub-aerially desiccated horizons recording fluctuating palaeo-climatic conditions. Understanding these landscapes, coupled with the geophysical and geotechnical data, enables the development of a predictive “geo-model” that may be used to target areas of uncertainty, reducing the requirement for boreholes (over Cone Penetration Tests) at every potential foundation location

Dogger Bank : a geo challenge

The Dogger Bank zone is the largest of the Round 3 offshore wind zones extending over ~8660km2. It is located between 125km and 290km northeast of Yorkshire on the Dogger Bank, a topographic high point in the middle of the North Sea, with water depths of 18–63m. The sheer size of this zone is considerably greater than that of standard shallow-water oil and gas site investigations, leading to the generation of extremely large data volumes to manage, interpret and move between partners. This paper outlines the methodologies that Forewind, a consortium of energy companies, is utilising to prepare for the zone's development. This includes activities undertaken to date to develop a 3D model of the geology, stratigraphy and geotechnical conditions to assist in optimal selection of wind turbine foundations, both location and type. Survey programmes and preliminary results also are presented and discussed, illustrating experiences with ultra-high resolution multichannel sparker, shallow gas, glacial tectonics and fractured clays

Seismic inversion for site characterization: When, where and why should we use it?

The application of seismic inversion techniques to the foundation and drilling top hole zones has garnered significant interest in recent years. The shift towards more geologically complex and deeper water sites, combined with the global economic climate, has driven a requirement for more cost-effective site characterisation. More often used by the exploration industry, seismic inversion has been touted as a potentially valuable tool for quantifying the spatial and depth variability in sediment properties. In doing so, this approach can reduce the risk of encountering unforeseen ground conditions and the need for excessive over-design. Despite its potential, the inversion of high-resolution seismic data has yet to see widespread use, leaving unanswered questions regarding how and where this tool can best fit into the site characterization work flow. We test the potential usefulness of seismic inversion using a range of existing site investigation data sets. We apply several different inversion methods, including acoustic impedance and seismic quality factor inversion, as well as artificial neural network multi-attribute regression, to tackle end-member potential uses. First, explore early-phase potential uses, showing how seismic quality factor and acoustic impedance inversion can be used to capture the spatial variability in facies architecture and bulk sediment properties that could be used in appraisal and pre-FEED studies to optimize borehole and penetrometer (CPT) depths/locations and to ensure effective site-wide characterization. Second, we apply a combined acoustic impedance and artificial neural network workflow to link seismic properties with CPT profiles. These results demonstrate the potential late-phase use of seismic inversion for short-range interpolation/extrapolation of more complex geotechnical properties through the generation of synthetic CPT profiles useful for infrastructure design and micro-siting late in the development cycle. While not a comprehensive list of applications, together these examples illustrate how seismic inversion can be utilized throughout the development cycle. If the required objectives are clearly defined and an appropriate inversion workflow developed, seismic inversion can help to reduce uncertainty in site-wide characterization and drive efficiencies in layout and design studies throughout a project lifetime

Pockmarks and methanogenic carbonates above the giant Troll gas field in the Norwegian North Sea

Acoustic imaging has revealed more than 7000 pockmarks on the seafloor above the Troll East gas field in the Norwegian North Sea. We present the first comprehensive study conducted on one of the World's largest pockmark fields complementing the acoustic data with extensive sampling, geochemical and petrographical studies. Specifically, we aimed at detecting possible active seepage still present over this vast area. The pockmarks are present as isolated structures, on average ~ 35 m wide and up to 100 m in size. In addition, smaller satellite pockmarks surround some of the pockmarks. In contrast to the muddy surroundings, parts of the investigated pockmarks contain laterally extensive carbonate deposits or meter sized carbonate blocks. These blocks provide shelter to abundant colonies of benthic megafauna. The carbonate blocks are comprised of micritic Mg-calcite and calcite, micritic aragonite, and botryoidal aragonite. Framboidal pyrite is also commonly present. Carbon isotopic values of the carbonates are 13C-depleted (δ13C as low as − 59.7‰) and with δ18O up to 4.5‰, indicating a methanogenic origin, possibly linked to gas hydrate dissociation. Pore water extracted from shallow cores from the centre and the flanks of the pockmarks show similar Cl and SO4 profiles as the reference cores outside the pockmarks, ruling out active methane seepage. This conclusion is also supported by seafloor video observations that did not reveal any evidence of visual fluid seepage, and by the absence of microbial mats and by the fact that the carbonate blocks are exposed on the seafloor and party oxidized on the surface. We conclude that methane seepage formed this extensive gas field following to gas hydrate dissociation

State-of-the-art remote characterization of shallow marine sediments: the road to a fully integrated solution

Current methods for characterizing near-surface marine sediments rely on extensive coring/penetrometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismic facies architecture. Even these fundamental seismostratigraphic nterpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time-to-depth conversion of geophysical data and potential loss and/or compression of high-porosity and under-consolidated seafloor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. In contrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude-versus-angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well-developed interface between these geophysical and reservoir engineering fields via rock physics. In this paper, we collate and review the different published inversion methods for high-resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can be related in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field

State-of-the-art remote characterization of shallow marine sediments: the road to a fully integrated solution

Current methods for characterizing near-surface marine sediments rely on extensive coring/penetrometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismicfacies architecture. Even these fundamental seismostratigraphic nterpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time-to-depth conversion of geophysical data and potential loss and/or compression of high-porosity and under-consolidated seafloor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. Incontrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude-versus-angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well-developed interface between these geophysical and reservoir engineering fields via rock physics.In this paper, we collate and review the different published inversion methods for high-resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can berelated in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field.<br/

Fission-product transport analysis for the loss of decay-heat-removal accident sequence at Browns Ferry

Non