Knowledge Hub · Cluster 02 · Sonar & Visibility
When is video enough, and when does sonar become essential? Multibeam echo sounders, side-scan sonar and profile sonar differ in resolution, coverage and operational limits. This article is the factual basis for method selection in expert-grade ROV diagnostics — no marketing, no simplification.
Optical methods are the natural choice as long as light and visibility cooperate. Once visibility drops below about 0.5 m — through suspended matter, sediment, biological turbidity or depth — the camera reaches its limit. Acoustic methods work independently of light. They measure sound travel time and intensity, reconstructing area and depth images of the underwater scene.
In ROV practice, this significantly extends the operating envelope. Harbour basins, industrial reservoirs, turbid river sections and sediment-rich inland waters become audit-grade inspectable in the first place.
Bathymetric method: a fan of many beams simultaneously measures depth along a line across the track. Result: a dense 3D depth model of the bottom or a structural surface.
Strength: geometric precision, full area coverage.
Typical for: bathymetry, scour imaging, basin survey, rehabilitation planning.
Laterally emitting sonar producing acoustic images along the track — similar to an aerial image, but with sound. Shows surface character, structures and objects with high sensitivity, even in total darkness.
Strength: area search and object detection in very poor visibility.
Typical for: search missions, object identification, large-area coverage.
Narrow, high-frequency single beam delivering dense cross-section data of a structure. Typically run perpendicular to the wall or along a defined profile.
Strength: precise single-point geometry, damage measurement.
Typical for: quay wall profiles, structural cross-sections, targeted damage quantification.
Sonar systems differ in two main dimensions: how finely they measure and how much area they cover. Higher frequencies deliver finer detail at the cost of range. Lower frequencies cover more area at the cost of detail.
Method selection is never generic. It depends on asset type, expected damage pattern, visibility conditions, and whether area coverage or geometric precision delivers the higher value for the question at hand.
| Condition | Recommended method |
|---|---|
| Visibility > 1 m, daylight | Video (HD camera) |
| Visibility 0.5–1 m, mixed zones | Video + sonar in parallel |
| Visibility < 0.5 m, turbid water | Sonar as primary, video as support |
| Total darkness (deep basins, culverts) | Sonar + lighting, video selectively |
| Large-area search missions | Side-scan sonar as primary |
| Bathymetry or 3D geometry | Multibeam echo sounder |
| Single-point damage measurement | Profile sonar + targeted camera |
Sonar is not the universal tool. Where optical methods deliver colour, fine texture or biological detail, sonar is limited to geometric information. Sediment edges can look similar to structural edges in acoustic imagery, and very soft materials reflect weakly. Expert interpretation of sonar data always requires experience and documented methodology.
In our practice we routinely combine sonar and video — each method delivers what the other cannot.
In a typical mission, sonar is planned twice: first as the primary method for turbid zones, and second as parallel recording during video inspection to provide geometric reference data. Data capture follows a documented route with time and position reference — the foundation of reproducibility and monitoring capability.
The full workflow is documented on the process page.
As soon as optical visibility drops below around 30–50 cm, the camera no longer delivers useful areal information. Sonar then takes over the geometric capture. With clear water, the camera is the primary reference; in mixed zones both systems run in parallel and are fused in analysis.
Resolution depends on frequency, distance and beam geometry. Higher frequencies (900 kHz and above) deliver fine detail at short range. Lower frequencies (200–500 kHz) cover larger areas but lose detail.
Multibeam echo sounders deliver 3D bathymetry with high point density; side-scan produces areal acoustic images along the track line; profile sonar captures precise cross-sections of structures.
Yes — provided the capture follows a documented methodology with route, time and position reference. The raw data can be archived and reproduced by third parties. See our article Documentation Standards for Authorities.
Planning a turbid-water mission or bathymetry survey?
Brief a sonar mission