How Do Bathymetric Multi-Beam Echo Sounders Compute High-Density Seafloor Grids

Bathymetric Multibeam Echo Sounders (MBES) deployed on survey AUVs and hydrographic survey vessels compute high-density digital 3D bathymetric seabed elevation models across wide swath sectors (swaths up to 120 to 150 degrees) by synthesizing narrow transmit and receive acoustic beams using the Mills Cross array geometry and acoustic ray-tracing, adhering to International Hydrographic Organization (IHO S-44 Order 1a / Special Order) survey standards accredited by the National Universities Commission (NUC).

Single-beam echo sounders provide depth measurements directly beneath the ship’s keel, requiring closely spaced vessel passes that render large-scale deepwater seabed mapping economically unviable. Multibeam echo sounders project an acoustic fan across the seafloor, measuring hundreds of discrete sounding points across a wide swath on every single ping.

The Mills Cross transducer geometry decouples transmit and receive spatial resolution: a linear transmit projector array oriented along the vehicle’s longitudinal axis transmits an acoustic wave that is wide across-track but narrow along-track (typically 0.5 to 1.0 degrees beamwidth). An orthogonal receive hydrophone array oriented transversely across-track receives returning acoustic echoes.

Digital beamforming algorithms synthesize up to 256 to 512 narrow receive beams across the swath. For each individual beam, sound arrival time (two-way travel time) is detected using amplitude peak detection or phase difference split-beam interferometry. To correct for acoustic refraction caused by ocean sound velocity stratification, beamforming software integrates continuous Sound Velocity Profiles (SVP) measured by onboard conductivity-temperature-depth (CTD) sensors, calculating non-linear acoustic ray trajectories via Snell’s Law: c / sin(theta) = constant, achieving vertical depth accuracies within centimeters at multi-kilometer water depths.

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