Surveys for Deep Ocean Observatories

Plans to install a variety of long-term sensors on the deep seafloor connected to land by submarine cables will require very high-resolution AUV and/or ROV surveys to achieve full effectiveness. These surveys should have sufficient accuracy so that they can detect crustal deformation when repeated at intervals over time.

There is worldwide scientific interest in establishing new and much more extensive ocean observatories on and near the seafloor. These monitoring stations provide long-term measurements of geological, biological and oceanographic conditions at fixed locations – a dimension that is not available from spot measurements obtained from research vessels. When connected to land by submarine cables, these observatories also provide the opportunity to acquire voluminous data, utilize sensors that might take considerable electrical power, and provide the high-bandwidth data in real time to researchers on land.

Seafloor maps with a new level of precision are required for both planning and placing these seafloor observatories. The first challenge is to install a sensor so that it has minimal impact on its surroundings. Most instruments will be designed for a lifetime of at least 25 years, so they need to be positioned for adequate long-term stability with minimal environmental impact.

Perhaps a greater challenge is that a wide variety of sensors will be installed at nodes in the seabed cabling network. Most of the instruments will be passive monitoring observatories, but some proposed instruments may be active; for example, making acoustic measurements or even measuring hydrothermal circulation by pumping water into the seabed. These active experiments must be installed such that they do not interfere with the passive instruments, or affect the environment, for 25 years.

The regions around complex groupings of seafloor observatories need to be mapped to decimetre precision and instruments will need to be placed on the seabed in precise locations defined by the high-resolution maps. If done today, these maps would require using AUVs or ROVs flying at low altitude above the seabed to have the necessary resolution and accuracy.

Obviously, many observatories are planned in geologically active areas such as crustal spreading centres, near active faults, submarine volcanoes and hydrothermal vents, as well as convergent margins where the oceanic plate is being subducted. In such areas, a very significant goal of the observatories will be to precisely measure the deform–ation of the plate and associated changes in other physical characteristics that may accompany this tectonic activity. Thus, during the lifetime of an observatory on an actively deforming seafloor, we will eventually want to have repeated very high-resolution maps of the instrumented active portions of the plate. Using present AUV technology coupled with precise acoustic positioning it is possible to measure bathymetry and seafloor features to an absolute accuracy of 10 centimetres. Such surveys will open new dimensions in our understanding of the evolution of ocean basins and surrounding continental margins.

Very high-resolution seabed mapping around ocean observatories will greatly enhance the usefulness of the data acquired at these sites, as well as the long-term effectiveness of the instruments on the deep seafloor.