Seabed canyons: slope instability problems, or just interesting features?

Abstract

Abstract This paper describes and illustrates a number of submarine canyons from various deepwater areas. It is based upon swathe bathymetry and geophysical field data acquired for geohazard site surveys and upon re-processed short offset 3D data. The morphology of these canyons is discussed, together with their potential for slope instability. These factors represent potential constraints to the continuing exploration and production activities in some deepwater areas. Introduction Submarine canyons have long been recognised as distinct channel features cutting into the majority of continental slope areas so far investigated1,2. As technology advances and mapping techniques improve, these canyons are being resolved in increasingly clearer detail. Particularly useful are deep towed long range sonar for covering large regional extents, whilst methods such as swathe bathymetry and short offset processed 3D seismic data can provide very detailed grids of seabed data. These latter two methods are providing unprecedented images over areas of typically a thousand square kilometres or more, at grid sizes of 25m by 25m and even to 12.5m by 12.5m. The association of these deep canyon systems with recent or present flow activity, such as turbidity currents, has long been recognised. Much has been published following indirect evidence concerning episodic events such as the 1929 Grand Banks and 1971 Nice/Var canyon related submarine cable breakages. As exploration and production continues in deep water, upper continental slope areas, these canyons are being encountered with increasing frequency. Thus, concerns are raised as to their stability combined with the present level of activity within the canyon systems3. Current research work is recognising the relatively recent nature of a number of events4. This is particularly important when, for example, sub-sea facilities are constrained by reservoir characteristics to being positioned close to, or within, such canyon systems. The new techniques mentioned above are progressively revealing much finer detail and will in time allow the frequency of minor flows to be better estimated. Improvements in visualisation and presentation of these results also allow a much more "user friendly" display, which should improve interpretation of actual processes involved. Three different areas are presented below to show how recent data sets are not only providing interesting views of submarine canyons, but can also add to the understanding of present day activity in these canyons. Also, the risk of slope instability for exploration and production operations in their vicinity may be better quantified. Offshore Nova Scotia This data set is from 3D seismic and covers an area of approximately 1500 sq km with a grid size of 25m by 25m. Water depths range from typically 200m on the shelf to 2475m in the deepest area to the southeast. It shows a complex tributary drainage pattern, (Figure 1). This is dominated by a single large-scale canyon, which has cut back into the continental shelf edge and then passes down the upper slope, continuing beyond the extent of the data set.