How can we find out what lies underneath the ocean surface? We can use a multibeam echosounder or sonar, which is a device that uses sound to map the seabed by determining the bathymetry (i.e. depth of ocean) and the nature of the seabed.
This device is typically attached to the keel of the boat and therefore sits permanently underwater, and can collect data whilst the ship is travelling. It has a sonar transmitter and receiver which are called transducers because they convert energy into another form (electrical signal). The sonar sends out fan-shaped sound waves from its transmitter which travel down to the seafloor where they are bounced back (reflected) towards the surface and detected by the receiver – this interval is called the two-way travel time, and from this we can calculate the water depth. The more time it takes for the sonar pulse to return, the further away the object is (i.e., the deeper the seafloor). These measurements are used to build up a picture of the topography of the seafloor.
The frequency of the sound wave is chosen based on how far the wave has to travel. Higher frequency waves, which approach the surface of the object at more of an angle, are more likely to bounce off the object instead of passing through it, allowing for better resolution images. More returned sound waves mean better images. Therefore, for high resolution pictures, a high frequency sound wave is desirable; but if the seafloor is so deep that the high frequencies cannot reach, then lower frequency waves may be the better choice. If the wave attenuates (fades) before it reaches an object, no echo is returned to create an image. When the resolution is right you can detect many things on the seafloor such as sunken ships, underwater volcanoes, coral reefs or even individual coral colonies.
During the iMirabilis2 expedition, existing multibeam data will be used to plan the routes (transects) that the remotely operated vehicle (ROV) will take to film video. However, since only 20% of the ocean is mapped, more multibeam data will be collected during the ship’s transit to the working area, and in specific target areas.
The research vessel used for the iMirabilis2 expedition – RV Sarmiento de Gamboa – has an acoustic “gondola” equipped with multibeam (shallow and deep waters), single beam (hydrographic) and parametric echosounder transducers installed on the lower surface, which provides very high-resolution seafloor mapping and penetration into sub-seafloor sediments respectively. This “gondola” is an airplane-like structure mounted on the hull of the ship. This structure is separated from the hull, thus avoiding the bubbles produced by the bow of the ship that could affect the acoustic transducers installed on the lower surface. The ship also has two drop keels installed in the middle part of the ship. They can be lowered 3 metres below the keel to separate the transducers from the hull, avoiding the acoustic noise produced by water flux and from hull itself. This helps to keep the acoustic data as “clean” and free from background noise as possible, meaning better quality acoustic imagery.
The animation below, created by the University of Washington’s School of Oceanography, shows how multibeam sonar technology works.