Mapping the ocean floor: swath bathymetry

As well as data collected at stations by the instruments we deploy, we also take measurements underway as the ship is steaming. This is done by instruments mounted on the hull of the ship, and include water temperature, salinity, and sound speed, but perhaps most importantly, seabed depth.

The instrument, called a multi-beam echo sounder, uses a fan of beams of sound to map the ocean floor beneath the ship. This works under similar principles to sonar: the instrument measures the time it takes for the beams to reflect off the seafloor and return to the receiver on the ship, and this time is then converted to distance, giving seabed depth. This maps a swath of the ocean floor, thus is known as swath bathymetry data (with bathymetry meaning the topography of the seafloor). To enable an accurate conversion of time to distance, it is important to have an accurate velocity model of the ocean water column. This is measured as often as possible using a sound velocity probe attached to the CTD.

Multibeam swath acquisition (credit: Fisheries and Oceans Canada)

After acquisition, the swath data must be 'cleaned', to remove false reflections that we know are not the true seafloor. After processing, we then have a usable map of the ocean floor, revealing canyons, seamounts, vast plains and even underwater rivers.

A potential underwater river system, formed by streams of denser ocean water.

This map is then used to help aid the placement of our instruments: we may want to deploy a CTD over a trough, for example, or try and find a flatter area that will have accumulated enough sediment for heat probe measurements. It is also interesting to see the changes in the shape of the seafloor from the Costa Rica Rift outwards. Close to the rift depths are shallower and topography rougher, full of ridges and troughs formed by the basalt lavas at the rift, with no sediment cover. Older oceanic crust further from the rift is starting to develop into flatter abyssal plains, with thicker sediment cover.

Sediment ponds between ridges in the Ecuador Fracture Zone

Swath bathymetry data is also used for non-scientific purposes, perhaps to map out harbours or sunken ships. The shallower the water, the greater the resolution of the resulting map. Our map will also eventually be added to a global database, which allows anyone to view all swath data currently collected around the world.