Every sonar system, underwater survey, and subsea operation depends on knowing how fast sound travels through the water — and that number is never constant.
The speed of sound in seawater isn't fixed — it changes with depth, driven primarily by temperature, and to a lesser extent by salinity and pressure. This depth-dependent relationship is called the sound speed profile, or SSP, and it's a foundational input for nearly every acoustic system used underwater.
Because temperature, salinity, and pressure all shift with time, weather, and location, the SSP is constantly changing — sometimes significantly within the same day, in the same spot.
Illustrative sound speed profiles based on field measurements taken one day apart. The shift between the two casts shows how quickly the water column can change — even without a storm or a change in season.
SSP isn't a niche measurement — it's an input used throughout most work done on and under the water.
Multibeam and other sonar systems measure the round-trip travel time of an echo, then convert that time into range and position using the sound speed — get the sound speed wrong, and every conversion from arrival time to position is wrong with it.
Sound speed gradients affect streamer depth selection and can shift the apparent position of hydrophones and reflected signals during survey.
Acoustic positioning references used to hold vessels and ROVs on station, and to guide subsea infrastructure work, depend on accurate sound velocity.
Because sound speed is closely tied to temperature, SSP measurements are a valuable secondary source of information for ocean circulation and climate studies.
Using the wrong sound speed doesn't just introduce a small margin of error — it can shift where a system thinks something actually is.
An incorrect sound speed assumption causes blurring or an apparent shift in target position — the same effect seen in medical ultrasound imaging when the wrong sound speed is used to form the image.
When SSP is wrong, adjacent survey lines stop agreeing — the same patch of seafloor reads a different depth depending on which line covered it, creating false ridges and steps that have to be manually cleaned up or re-surveyed.
Sound bends as it passes through layers of different sound speed — similar to how light bends underwater. If the assumed sound speed profile is wrong, seismic systems misjudge that bend, producing subsurface images that are subtly misaligned and costly to reprocess.
Acoustic positioning references used to keep vessels and ROVs on station lose accuracy as the water column changes and the sound velocity data they rely on becomes outdated.
Because sound speed varies both spatially and temporally, it needs to be measured often — not just once per survey. But each conventional tool has a limitation: stop-and-cast systems like CTD and SVP are accurate but disruptive and slow, underway Moving Vessel Profilers require significant capital and deck space, and expendable probes like XBTs are single-use with recurring costs and limited resolution.
Providing accurate, frequent, underway SSP data without the cost and disruption of existing methods is exactly the problem Atlantic Echo is designed to solve.