Header image caption: High-resolution seafloor mapping revealed unusual pancake-like features of a seamount in the Moonless Mountains chain in the Eastern Pacific.

To plan efficient and safe operations, Nautilus (sea exploration ship) often creates its own seafloor maps—particularly when exploring little-known regions of the ocean. To facilitate this operation, the ship incorporates various equipment that provides high-quality seafloor maps at depths to 7,000 meters (23,000 feet). 

Whether focused on a canyon, seamount, or shipwreck, creating a map allows the crew to identify potential targets, cutting down exploration time and boosting mission efficiency. Before ROVs are deployed, the team must first map the area to understand the characteristics of the region and identify potential benthic habitats, seeps, and other environments and resources worthy of exploration. In addition to informing dive objectives, Nautilus transit routes cover unmapped areas of the ocean and contribute to the Seabed 2030 initiative, an international collaborative project to combine all bathymetric data in order to create a comprehensive map of the ocean floor. 

Multibeam Echosounder

Mounted on the hull of the vessel is a Kongsberg EM302 multibeam echosounder capable of accurately producing state-of-the-art maps covering large areas of the seafloor. The echosounder maps the seafloor at depths between 50 and 7,000 meters (300 to 23,000 feet) while cruising at ship speeds up to 12 knots (14 mph). 

The transmit array emits acoustic pulses that ensonify the seafloor with a wide fan-shaped swath of sound, while a second transducer receives the return signal echoes. Each pulse sends many beams of sound in a fan shape toward the seafloor. When these pulses strike the seafloor and return to the transducer/receiver combination mounted on the hull of the ship, the system computes a “sounding” associated with each returning pulse via the time it took to travel down and up through the water column. 

Because the ship is moving between the transmit and receive functions, a motion sensor connected to the system allows the echosounder to “steer” the sound pulses to correct for the ship’s rolling and swaying motions. This allows the ship to collect an even distribution of data from the seafloor. Received soundings are combined with the ship’s Global Navigation Satellite System (GNSS) information, to produce a grid or “digital elevation model” of ocean bathymetry—essentially a topographic map of the seafloor. Images such as those from Google Earth and other satellites offer very little modern depth observations and only provide general highs and lows of deep-sea topography. 

The multibeam echosounder acoustically “sees” different scales and resolutions at different depths. When Nautilus is mapping, the multibeam sonar fan covers a different width (scale) on the seafloor depending on the depth, however the number of measurements across the swath of the fan remains the same. In shallow water, the soundings are closer together delivering many details of the seafloor in a small area (higher resolution data). In deeper water, fewer details are available (lower resolution) but the multibeam fan of soundings covers a much wider area. 

In addition to the depth, the signal strength that the sonar receives back from the seafloor (“backscatter”) will be different depending on the type of seafloor that reflects the ping. By making corrections to this signal to account for the changes as it went through the seawater from the ship and back, the processing can extract information to indicate variations in the seafloor type. Reflections from rocky seafloor will generally provide a stronger signal than a muddy area. Backscatter measurements are then combined in another grid called a backscatter mosaic, which can be combined with the bathymetry grid to provide a better understanding of the shape and seafloor type. 

The multibeam echosounder can also detect phenomena in the water column, such as plumes of bubbles emanating from the seafloor that indicate gas seeps. To date, the Nautilus has documented thousands of methane seeps along the Cascadia Margin off the Oregon and Washington coast.

Exploring Sub-surface Faults 

Revealing structures below the seabed is just as important as discovering the seascape and habitats above. To complement the multibeam mapping work, the team uses a Knudsen 3260 sub-bottom profiler and echosounder. Mounted inside the hull of Nautilus, the echosounder operates at low frequencies to penetrate and reflect off of the layers of sediment, revealing a cross-section of the seafloor structure.

The dual-frequency profiler operates at 3.5Khz or 15Khz (two discrete channels with separate transducers) and is capable of full ocean depth soundings. An acoustic pulse is directed through the water column to the seafloor and then captured by the system as it bounces back from each layer. Scientists use this data to identify subsurface geological structures such as faults, ancient channels, and buried levees.

In early 2023, Ocean Exploration Trust installed a Kongsberg Simrad EC150-3C 150 kHz transducer on E/V Nautilus. Mounted within the ship's hull, the EC150-3C is the first of its kind to combine an acoustic Doppler current profiler (ADCP) and an EK80 split-beam fisheries sonar into one instrument. The ADCP, which measures the speed and direction of currents at various depths underneath the ship supports safe remotely-operated vehicle (ROV) operations and provides data for improving oceanographic current models. The integrated split-beam echosounder maps and characterize features found within the water column, such as biology, scattering layers, and potentially bubble plumes. The EC150 will equip E/V Nautilus with the capability to better serve as an operations hub for multi-vehicle operations, increase OET’s capacity to explore and map the water column, and to collaborate with partners from the Ocean Exploration Cooperative Institute to advance combined robotics and new technologies to increase and advance the pace of ocean exploration. 

For more information: 

Ocean Exploration Trust & Nautilus Live

Kongsberg Simrad EC150-C

Seabed 2030

Qimera