By Lindsay McKenna - NOAA Office of Ocean Exploration and Research
June 6, 2015
NOAA Ship Okeanos Explorer collects baseline ocean mapping data using sonars, which stands for SOund Navigation and Ranging. Sonar systems use transducers to transmit an acoustic pulse of sound into the water. When the pulse of sound comes in contact with the seafloor, some of the sound reflects off the seafloor and returns to the ship as an “echo.” The time elapsed between the sound transmit and receive is used to calculate depth of the seafloor. Some sonar systems have one transducer that both emits and receives sound pulses; others have separate transmit and receive transducers.
We have three sonars on the Okeanos Explorer. Our “workhorse” sonar is the Kongsberg EM302 30 kHz multibeam sonar. The EM302 offers a large swatch width, increased data density, and higher resolution. On the Okeanos Explorer, we use the EM302 to collect seafloor bathymetry, seafloor backscatter, and water column backscatter. Backscatter represents the strength of the acoustic signal reflected from some target, whether that’s the seafloor or bubbles in the water column.
The EM302 has separate transmit and receive transducers that are mounted to the hull of the ship as modular linear arrays in a Mills Cross configuration. This system produces a swath of sound and can generate up to 864 soundings per ping. The maximum observed swath of the EM302 is approximately eight kilometers and diminishes in depths greater than 5,000 meters. Beam focusing can be applied automatically during both transmit and receive.
The EM302 is designed to map the seafloor in water depths of 10 to 7,000 meters, and our system was able to detect the seafloor at 8,000 meters over the Puerto Rico Trench during a recent Océano Profundo cruise.
The Knudsen Chirp 3260 3.5 kHz subbottom profiler sonar is used to collect shallow seismic reflection profiles, which provide details about the geology below the seafloor. This system can survey the seafloor at depths up to 10,000 meters.
The third sonar on Okeanos Explorer is an 18 kHz Simrad EK60 split-beam sonar, which provides both single-point reflection seafloor bathymetry and calibrated water-column backscatter. Calibrated backscatter can be used in bubble detection and to map biomass in the water-column.