Seafloor mapping: Tools of the trade

By: Lloyd Godson

One of the most common instruments used in oceanographic research on the RV Investigator is a CTD rosette system. This instrument, made up of a set of small probes attached to a water bottle rosette system, is deployed off the starboard side of the ship using a crane and lowered vertically through the water column with a winch. Each deployment of the CTD is called a cast.

The CTDs electronic probes measure Conductivity (salinity), Temperature, and Depth (pressure) at set time intervals. The water sampler collects discrete samples for later analysis in the vessel’s lab. Data from a CTD cast is then used to create water column profiles to show differences between various marine locations or depths. Although you can’t see it from the surface, oceans are not one continuous type of water from surface to seafloor. Rather, there are chemical and physical fluctuations of parameters that change due to geographical location, seafloor features, weather patterns, depth and current systems.

So how can CTDs assist with seafloor mapping? The speed of sound in water (sound velocity) is affected by water pressure, temperature, and salinity. A CTD calculates the sound velocity based on the data it collects. Mappers use the sound velocity to calibrate the multibeam mapping sonar to provide accurate calculations of water depth. CTDs are also useful in detecting anomalies (data that deviates from the normal or expected state) or used in conjunction with other data sets to help characterise seafloor habitats. Since no other oceanographic research is taking place during this hydrographic (seafloor mapping) survey, we are deploying a much smaller device off the port side of the ship at regular intervals which measures sound velocity directly, rather than calculating it from CTD data.