Reading sonar honestly: why the sea produces contacts that are not there
Half the cases in this section rest on a sonar return. Sonar is an inference, not a photograph, and the ocean is very good at producing returns from nothing. A working guide for anyone assessing these reports.
Photo by Derwin Edwards on Pexels
A large share of USO reporting comes down to one thing: an operator saw a contact and could not account for it. To assess those reports at all, you need to know what sonar does and, more importantly, what it does wrong. This piece is the reference the rest of the section points at.
Sonar is inference
Active sonar emits a pulse and times the return. Passive sonar listens. Neither produces an image of an object; both produce a measurement from which an object is inferred. The inference depends on assumptions about how sound travelled through the water — and in the ocean those assumptions are frequently wrong.
The thermocline
The most consequential feature is the thermocline: a depth at which temperature drops sharply. Sound crossing it refracts, and a good deal reflects. A submarine below a strong layer is very hard to detect from above it, and vice versa — which is why submariners talk about "hiding under the layer".
For an investigator, the implication runs both ways. A contact that appears and vanishes abruptly may be crossing a layer rather than doing anything remarkable. And a contact that appears to move impossibly fast may be two different reflections of the same sound.
Biologics
The deep scattering layer is a band of small marine organisms dense enough to return sonar as though it were the seabed. It rises towards the surface at night and descends at dawn, every day, across most of the world’s oceans. Early sonar operators recorded it as a false bottom that moved.
Whales, dolphins and large fish schools all produce strong returns and all move in ways that can look purposeful, because they are purposeful. A pod changing direction sharply and simultaneously is a common source of contacts that read as coordinated craft.
Artefacts of the equipment
Multipath returns arrive by more than one route and place a single object in several positions. Reverberation from the seabed or the surface masks real targets and generates false ones. Wake and cavitation from the observing vessel produce their own returns. On side-scan imaging, variation in tow speed stretches and compresses the picture, and the result routinely looks more geometric than the seabed is.
What a good report contains
Depth of the contact and depth of the observing platform. Whether a layer was present and where. Sea state and bottom type. Which system, active or passive, and at what frequency. What the operator ruled out and why. Whether any other sensor held the same contact.
Where a report contains those things, it can be argued about productively. Where it does not, the honest answer is not "unexplained" — it is "not assessable", and the difference matters.
Join the discussion
Still open on this one:
- 01
Why are supporting conditions almost never published with USO sonar reports?
Partly classification, partly that the people retelling the reports do not know the conditions matter. The second is fixable.
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See also
Everything this piece touches
Sources
Open them and check for yourself. Where a record is public, it is linked.
What is sonar, and how does sound behave in the ocean?
NOAA Ocean Exploration
NOAA Ocean Exploration
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