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 together with re-processed short offset 3D data. The morphology of these canyons is discussed, together with their potential for slope instability. These factors represent potential geohazards to the continuing exploration and production activities in some deepwater areas.
Submarine canyons have long been recognised as distinct channel features cutting into the majority of continental slope areas so far investigated1,2As 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 recent advent of autonomous underwater vehicles, (AUVs), means that detail and resolution will be improved still further due to their ability to acquire swathe bathymetry data closer to the seabed.
The stability of submarine canyon systems may, in simplistic terms, initially be related to seabed slope angles, particularly of the canyon sides. The potential for instability then depends on the likelihood of processes modifying these slopes and external triggering mechanisms causing them to fail. Modifying processes are typically currents along and downslope, which may cause erosion and sediment input or deposition. Triggering mechanisms include, for example, seismicity, soils strength variations due to pore pressure changes, disturbance due to halokinesis, shallow gas and hydrate sublimation and cyclical loading.
The association of deepwater canyon systems with recent or present flow activity, such as turbidity currents, has long been recognised. Much has also been published following indirect evidence concerning episodic events such as the 1929 Grand Banks and the 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 intrinsic stability combined with the present levels of bottom current and sediment activity within the canyon systems3. Research work has also been 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.