Fathometri

Fathometri, or fathometry, is the measurement of the depth of water by emitting an acoustic signal and measuring the time it takes for the echo to return from the seabed or lakebed. The instrument used is commonly called a fathometer or depth sounder; modern systems are often referred to as echo sounders. The term relates to the unit fathom, a traditional measure of depth.

Principle and theory: A transducer mounted on a vessel or platform sends a brief downward pulse. The sound travels to the bottom, reflects, and returns to the transducer. Depth is calculated from the travel time and the speed of sound in water: depth ≈ (speed of sound in water × travel time) / 2. Because the speed of sound in water depends on temperature, salinity, and pressure, accurate depth measurements typically use a sound velocity profile to adjust calculations.

Technologies and methods: Single-beam echosounders provide a single depth value directly beneath the source, while multibeam and higher‑definition sonars map larger areas of the seabed to produce bathymetric charts. Shallow-water surveys use higher frequencies for better resolution, and deeper surveys use lower frequencies for range. Raw data are corrected for tide, vessel motion, and refraction, and are tied to a vertical datum for charting and analysis.

Applications and significance: Fathometry is essential in hydrography, navigation, offshore engineering, dredging, and environmental studies. It supports marine charting, seabed mapping, and depth monitoring, with measurements commonly reported in meters or fathoms and aligned to standardized vertical datums.