VISION AND SCIENTIFIC GOALS

The research infrastructure "Geosystem 3D Seismic Imaging (G3)" aims to investigate complex and dynamic geosystems of gas hydrates, geofluids and geohazards in marine environments from the shelf to the deep sea. It consists of a mobile high-resolution three-dimensional (3D) seismic imaging system for mapping and visualization of subsurface structures in unprecedented detail. Such high-resolution subsurface structure studies are crucial for future potential CO2 storage and European Large Scale Infrastructure (ESFRI) ocean observatory site investigations (EMSO: European Multidisciplinary Seafloor Observations).

Understanding complex and dynamic marine geosystems hinges on detailed knowledge of the subsurface. The proliferation of 3D seismic technology is one of the most exciting developments in the Earth Sciences over the past century. During the 80ies and early 90ies, 3D seismic technology has revolutionized the geophysical industry with far-reaching effect on the exploration and development of oil and gas fields worldwide (Nostvold, 1994). Only during the last decade, academic and other research institutions have begun to exploit the enormous volume of geological information contained in industrial 3D seismic surveys. A wealth of fascinating (and surprising) discoveries have since been made and at the same time stimulated novel research into geological processes and products for many decades to come (Cartwright and Huuse, 2005; Cartwright, 2007).

However, due to the enormous costs related to the acquisition of 3D seismic industry data, academic institutions have not been able to acquire 3D seismic data on their own objectives in their preferred target areas. Until very recently, academia was therefore limited to areas of hydrocarbon exploration. Industry 3D seismic technology plays a strategic role throughout the whole value chain of identifying, appraising and developing oil and gas fields. Moreover, these conventional 3D seismic surveys are designed to image very deep-seated structures potentially hosting hydrocarbon reservoirs. Thus, their main frequency bandwidth often ranges up to only 40 Hz, and their need for covering large areas requires a large inline spacing of receiver arrays. Therefore, the temporal and spatial resolution of these data sets is limited.

Figure 1

Figure 1. (a): Schematic diagram of P-Cable 3D seismic system: a seismic cable is towed perpendicular to the vessel's steaming direction. Many single-channel seismic streamers are attached to this perpendicular wire, which is held in place by two doors. (b) and (c): Comparison between P-Cable 3D seismic data (bin size 10m) and high-resolution 3D seismic exploration data (bin size 12.5 m, interpolated) (Gay et al., 2007) of very similar fluid-escape structures. Horizontal and vertical scale are identical. Note the significantly higher resolution of the P-Cable data. Particularly, the spatial resolution shows much more details of the fluidescape structures than the conventional 3D exploration data, which looks rather blocky and doesn't allow visualizing the fine-scale nature of these features.

The research infrastructure "Geosystem 3D Seismic Imaging (G3)" will be a national facility for the acquisition of high-resolution 3 D seismic data. It will be headed by two of the leading marine geology and geophysics departments for high-resolution seismic (University of Tromsø and University of Bergen). The technology is based on the P-Cable 3D seismic system (Figure 1a) (Norwegian patent No. 317652, Planke and Berndt, 2003; Planke et al., 2009) that will be operational on R/V Jan Mayen, R/V Håkon Mosby, R/V G.O.Sars, and the new icebreaking research vessel under detailed planning in Norway and the planned European research vessel Aurora Borealis. The P-Cable 3D seismic system is container-based and thus grants a high mobility and flexibility on small and large research vessels. The high-resolution 3D seismic system offers forefront marine technology developed by a consortium of Norwegian Universities and companies in cooperation with European partners (National Oceanographic Centre Southampton and IFMGEOMAR). It allows the acquisition of high-resolution seismic data with 16-24 digital streamers on a short spread with high-frequency airgun sources. The spatial resolution of such a system is at least one order of magnitude higher than conventional 3D seismic, whereas the temporal resolution is improved 3-5 times. The increases in resolution facilitate a much better target identification and achieve a much more accurate imaging of for example shallow subsurface structures and fluid flow systems. The advantages and benefits of the P-Cable seismic system with respect to data quality are illustrated in Fig. 1b and 1c. It shows a comparison of high-resolution 3D seismic exploration data from the Lower Congo basin (Gay et al., 2007) with P-Cable data from a comparable target. Both images have the same horizontal and vertical scale and the P-Cable data shows superior quality in both, horizontal and vertical resolution.

Today's and future research targets address geosphere-biosphere interactions, environment and energy including CO2 storage sites that clearly underline the need for such new high-resolution marine technologies. For example, 3D imaging of gas-hydrate reservoirs is of major importance as no such data exists from Arctic regions where these ice-like and methane-trapping structures may occur in large quantities. The areas may rapidly release methane and may cause significant changes in the carbon cycle and greenhouse gas concentrations. Given the ongoing global warming, we need to better understand the distribution of gas hydrates in sub-seafloor sediments. Their response to ocean temperature changes and, when and how potential greenhouse gas (methane) releases may occur appears to be of major environmental concern.

Hundreds if not thousands of shallow gas accumulations occur in the Barents Sea and adjacent Arctic regions. These accumulations are often uncharted or poorly imaged using conventional 2D or sometimes 3D seismic industry data. We have no clear indication what is trapping the gas at shallow depths. It may be some kind of cap rock, fault seal or even gas hydrates. However, the enormous amount of these gas accumulations makes them a major geohazard. They constitute a risk for safe drilling operations in this frontier basin and they may pose a threat to global climate if the seal that is trapping them is breached. We need to understand the accumulation mechanisms of these reservoirs, their detailed structure and trapping mechanisms, and to assess seal integrity and conditions for leakage from the shallow reservoir. The P-Cable 3D seismic system is ideally suited to provide answers for many of these critical and societal relevant issues.

Leakage systems with focused fluid flow are important elements in the development of sedimentary basins (Berndt, 2005). However, the processes responsible for the formation of these so-called sealbypass systems (Cartwright et al., 2007) and their implications for large-scale fluid migration in the sedimentary basin are not well understood. These seal-bypass systems are often recognized as vertical pipes or chimneys and they constitute markers for fluid release from overpressured formations in the subsurface. Their occurrence amongst other plays an important role for petroleum systems or seabed ecosystems. However, these vertical structures are at the limit of spatial resolution. Even inline spacing of receiver groups of conventional 3D seismic technology may not be able to capture the true nature of the focused fluid flow structures, and as a consequence spatial aliasing will impede the interpretation. The P-Cable 3D seismic data with its spatial resolution an order of magnitude higher than that of conventional 3D seismic data will allow imaging these fluidescape structures in unparalleled detail and shed new light on the processes that create them.

The University of Tromsø has developed a prototype of the P-Cable (p= perpendicular) technology in close cooperation with Volcanic Basin Petroleum Research (VBPR, Oslo), National Oceanographic Centre Southampton (NOCS, UK), and IFM-GEOMAR (Germany) to acquire detailed three-dimensional images of target areas in the sub-seabed in a cost-efficient way (Figure 1a). It presently consists of 8 seismic streamers towed perpendicular to the research vessel's steaming direction.

Building on the acquired knowledge and existing technology, we propose to purchase and integrate a fully operational and mobile P-Cable 3D seismic system that can be widely used by academia and industry. High-resolution 3D seismic images to be collected with 16-24 digital streamers offer forefront marine technology. Conventional 3D seismic technology relies on very long streamers (several up to 10 km long streamers is common), large air-gun sources, large seismic vessels, and thus very costly operations without reach of universities. In contrast, the proposed P-cable system will be light-weight and fast to deploy from smaller vessels such as RV Jan Mayen or RV GO Sars. Only a small array of airgun sources is required as the system is designed for shallow to intermediate sub-seabed imaging targets (< 1500m). The P-Cable system is particularly useful for the acquisition of small seismic cubes, i.e. 10-50 km2. The rapid deployment and recovery time makes it possible to acquire several 3D cubes during one research cruise. Such a system will add to bring geo-marine research institutions in Norway to the forefront in polar continental margin research and marine-science technology.

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