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Read the Rainbow: Seafloor Mapping Glossary

When the Nautilus team is seafloor mapping, the screens of Nautilus Live are awash in colors as our sonar systems reveal high resolution imagery of seafloor landscapes. Learn to read the rainbow with this seafloor mapping glossary of common terms you'll hear used by our mappers, navigators and science team! 

Compiled with Seafloor Mapping Intern  & Mapping Coordinator .
Seamount image above processed with software. 

Multibeam Sonar Terms

Mounted on the hull of Nautilus, the multibeam sonar transducer emits two powerful pulses of sound (or “pings”) that travel to the seafloor. As the pulses bounce off the seafloor and travel back up to the receiving array, the distance to the seafloor can be determined from the travel time and the speed of sound through water. Unlike early mapping systems that emitted a single beam of sound to the seafloor and only returned one sounding (e.g. one depth measurement per ping), the multibeam sonar emits two fans of beams, resulting in a wider "swath width" with up to 864 soundings per ping! Learn more about using

During a mapping expedition, the acquisition screen of the Seafloor Information System is the featured video stream on Nautilus Live, used by both the mapping team aboard E/V Nautilus and Scientists Ashore guiding the expedition. 

Bathymetry- Bathymetry is the topography of the sea floor. Mapping tools are used to determine the shape of the seafloor. Undersea mountains, canyons, basins, or flat plains are examples of seafloor features that are the bathymetric features of the area.

Backscatter- (acoustic return intensity) Backscatter is what we use to determine the type of material or sediment on the seafloor. Backscatter is displayed in gray scale to show intensity and the lighter the return normally indicates a harder seafloor. A light area could be a rocky area, whereas darker shades indicate soft sediment like silt or mud.

Center beam and across-track profile- The depth immediately below the hull of the ship, where the center point of the multibeam is located. The profile shows the 432 soundings taken per “ping” across the beam of the vessel.

Perspective view of bathymetry colored by depth- The main screen shows a bird’s eye view of our mapping progress. The rainbow of colors correlate with relative depth. A red area means it is the shallowest area we are mapping in the area and a purple color means it is the deepest.

Ship motion data- In the bottom left of the screen you can see a tri-color graph. This graph is tracking the ship’s motion, including heave (up and down motion), pitch (front and back alternate up and down motion), and roll (side-to-side motion). The multibeam uses this information to compensate for the ship’s movement when receiving measurements from the seafloor.

Sensor data display- This sidebar includes data streams from supporting sensors, including position, sound velocity, depth, and more for monitoring performance of the mapping system.

Water column mapping- Whenever the multibeam is collecting data about the seafloor, it is also collecting data about what is in the water column. This is of particular interest to us when we are looking for methane seeps. After the bathymetry data is processed we examine the water column data to see if we can find any vertical signatures that could be a seep. The purple or red horizontal line marks the seafloor, while vertical plumes may indicate possible seafloor seeps. 

Sub-bottom Sonar Terms

The sub-bottom sonar system mounted on the hull of E/V Nautilus provides data on the geological structure and sediment types below the surface of the seafloor. Data from this system is used to identify geological features like paleoshorelines or hydrothermal vent systems. 

In sub-bottom sonar data, higher intensity returns from sonar "chirps" appear darker, indicating harder sediments below the surface of the seafloor. 

Pulse and Power- Both the pulse and the power are increased with increased depth. When we are mapping in deeper waters we need to increase both in order to send a stronger signal and give the chirp more energy to reach the seafloor.

Range- The range is set depending on depth and how often the bathymetry is changing. In shallow waters, the range can be 50 meters while it can be 200 meters in a deeper area with steep drop-offs.

Phase- Current Depth being viewed. Software typically displays a 200-meter range so an example of a phase would be 300-500 meters.

Chirp- We often refer to the subbottom signal as sending down a “chirp” or a “ping” to the seafloor.

Mapping Technology Terms

Deploying a UCTD takes a team to launch and recover the baseball bat-shaped instrument. Right to left, Seafloor Mapping Interns  and are assisted by Video Engineers and

UCTD- This is another instrument to measure the sound velocity of the water column to adjust the multibeam sonar. The Underway CTD collects conductivity, temperature, and depth information while the ship is underway and moving at speeds of 8-10 knots. This information helps our mapping team adjust the sonar instruments to suit current seawater conditions and ensure more accurate mapping data is collected.

XBT- Often when we are mapping we go out on the back deck to launch an XBT, an expendable bathythermograph. An XBT is launched when we move into an area where the physical oceanography of the water is different from the previous area. These are done approximately every 4 hours. An XBT is used to measure the sound-speed profile of the water by collecting depth and temperature information, and later adding in salinity information as well. This is sent to the multibeam to adjust for changes in the spped as sound from the surface to the seafloor.

Sound speed profile: A sound speed profile shows the speed of sound moving through water at different layers in the ocean. Sound speed is affected by temperature, depth, and salinity of the water. We launch XBTs and UCTDs to determine sound speed in seawater.

Hypack- Hypack is the navigation software we use to plan our lines and plan out the course of a survey. This is the same software that the bridge uses to direct ship movement.

Fledermaus- This is a suite of software that we use to analyze the backscatter intensity and the water column data of the sites that we are studying.

Qimera- This is a software we use to process the bathymetry from the multibeam sonar. We compile all the lines during a survey and clean up the data and remove any outliers which may be in the data due to bad weather or returns from the water column.