Multibeam sonar is a mapping technology used by research ships and some submarines to measure the shape of the seafloor. Instead of sending one narrow sound pulse straight down, it sends many sound beams in a wide fan beneath the vessel. This lets scientists image a broad strip, or swath, of the seabed during each pass.
Accurate seafloor maps matter for navigation, habitat studies, earthquake research, and locating underwater hazards.
Key Facts
- Depth from echo time: d = vt/2, where v is sound speed and t is the round-trip travel time.
- Typical seawater sound speed is about v = 1500 m/s, but it changes with temperature, salinity, and pressure.
- A multibeam sonar maps a swath by sending many narrow beams at different angles across the ship's path.
- Outer beams travel farther than center beams, so angle and travel time must both be used to calculate seafloor position.
- Swath width often increases with water depth, but data quality can decrease at very steep beam angles.
- Bathymetry is the measurement and mapping of underwater depth and seafloor shape.
Vocabulary
- Multibeam sonar
- A sonar system that sends many sound beams in a fan pattern to map a wide area of the seafloor.
- Transducer
- A device that converts electrical signals into sound pulses and converts returning echoes back into electrical signals.
- Swath
- The wide strip of seafloor measured by a multibeam sonar during one pass of a ship.
- Bathymetry
- The study and measurement of underwater depth and the shape of the seafloor.
- Sound speed profile
- A record of how the speed of sound changes with depth in seawater.
Common Mistakes to Avoid
- Using d = vt instead of d = vt/2 is wrong because the sonar time includes the trip from the ship to the seafloor and back.
- Assuming all beams measure the same depth is wrong because angled beams strike the seafloor away from the ship and must be corrected for geometry.
- Treating seawater sound speed as perfectly constant is wrong because temperature, salinity, and pressure can bend sound paths and shift the mapped position.
- Confusing image brightness with depth is wrong because echo strength depends on seafloor material and slope, while depth comes from travel time and angle.
Practice Questions
- 1 A multibeam sonar records an echo return time of 0.80 s from the center beam. If the sound speed is 1500 m/s, what is the water depth directly below the ship?
- 2 A ship maps a swath that is 1200 m wide on each pass. How many straight passes are needed to cover a 6.0 km wide survey area if there is no overlap?
- 3 Explain why a multibeam sonar can map the seafloor faster than a single-beam sonar, and describe one reason the outer beams may be less accurate.