Do I need a multibeam echo sounder, or will a 3D side scan do?

The five options, honestly

Single-beam echo sounder. One depth reading directly beneath the vehicle. Cheap - bathymetry systems start around €780 - and still the right answer for cross-sections, bridge clearance checks, and anywhere you need a defensible depth number rather than coverage. You will be driving a lot of lines.

Conventional side-scan sonar. A narrow fan beam thrown sideways, producing a high-resolution acoustic image of a wide strip of seabed. Excellent at finding things. It measures the range to each return but not the angle that return arrived from vertically, so the image is projected onto a flat plane. You infer height from shadows. The Cerulean Omniscan 450 SS is this category at from €2,275, with 150 m range per side and a 0.5° beam.

Multibeam echo sounder (MBES). Looks down, forms many depth soundings across a swath per ping, and is the standard instrument for hydrographic survey. The Cerulean Surveyor 240-16 is a small-vehicle example at from €4,375 - 240 kHz, a 16-element receive array, an 80° cross-track transmit beam and a 4° along-track beam, resolving angle of arrival to better than one degree and typically detecting 10 to 15 targets across track per ping out to around 50 m slant range.

3D or interferometric side scan. Side-scan transmit geometry with a multi-element receive array added, so vertical angle is resolved per return and each ping produces points in space rather than a line of intensities. You get the side-scan image and a point cloud from the same pass. The Cerulean Omniscan 3D 450 SS is this category at from €7,000 - 450 kHz, 16 receive channels, 150 m per side in 2D and 100 m per side for the 3D point cloud.

Combined bathymetric side-scan systems from manufacturers such as EdgeTech, Klein and Norbit occupy the professional tier above all of this. They are priced for survey contractors rather than small-vehicle operators, and we do not carry them. If your contract requires a formally compliant hydrographic deliverable at depth, that is the category you should be pricing, and you should stop reading here.

The comparison, with real numbers

Prices are SepcoTech euro figures for the equipment only. Specifications are from Cerulean Sonar’s published documentation.

What the look angle actually buys you

This is the specification that decides the job, and it is not on the front of any datasheet.

A multibeam’s swath is a triangle hanging beneath the vehicle. Its width is proportional to altitude. Cerulean’s own survey guidance for the Surveyor 240-16 makes this concrete: plan your transect spacing at roughly 0.7 times the water depth. In 10 m of water, start with 7 m line spacing. In 3 m of water, that is about 2 m per line.

Work out what that means for a river survey. A 200 m wide channel at 3 m depth needs on the order of a hundred lines. At the Surveyor’s tested speed envelope of 0.6 to 2 m/s, that is not an afternoon. Nothing is wrong with the sonar - this is geometry, and it applies to every downward-looking multibeam ever built, from a €4,375 unit to a €400,000 one.

A side-scan geometry does not have this problem. Looking 30 to 45 degrees down, coverage per line is limited by how far the sound travels before the return becomes useless, not by how far the bottom is beneath you. That is why Cerulean states the Omniscan 3D captures bathymetric data over a wide swath even in shallow water - and it is the single strongest reason to choose it over a multibeam for shallow work.

The corollary is equally important and rarely said out loud: in deeper water the advantage reverses. Once you have altitude to work with, the multibeam’s swath opens up, its geometry beneath the vehicle is better conditioned than an angled sensor’s, and it is the more sensible instrument.

So the honest rule is not “3D side scan is better”. It is: shallow and wide favours the side-scan geometry; deeper and chart-grade favours the multibeam.

The caveat nobody writes down

Angle resolution is not depth accuracy.

Both Cerulean instruments advertise angle-of-arrival resolution better than one degree. That figure describes how precisely the sonar can tell you the direction an echo came from. It says nothing about how accurate the resulting depth is once that echo has been turned into a point on a chart.

Your real vertical uncertainty is dominated by three things that have nothing to do with the sonar you bought:

• Sound-velocity error. You are converting time into distance using an assumed speed of sound. Get it wrong by a few metres per second and every point moves, with the error growing across the swath.
• Heading error. At 100 m slant range, one degree of heading error puts a target roughly 1.7 m out of position. A vehicle magnetometer sitting next to a thruster is not a survey heading source.
• Position quality. Uncorrected GNSS is a metre or two at best. Your point cloud cannot be better than the positions you painted it with.

A 3D side scan carries one additional limitation: it does not see well directly beneath itself. Because depth is derived from the angle a return arrives at, returns from near-nadir arrive at angles that are hard to separate, and the geometry degrades. The Cerulean Omniscan 3D compounds this by not supporting a vertical mounting orientation at all - point it straight down and it misreads its own orientation, putting starboard returns on the port side. A downward-looking multibeam has its best geometry exactly where a 3D side scan has its worst.

If your deliverable has to satisfy a formal hydrographic standard, that is a question about your whole survey system - sound velocity profiling, calibration, positioning and procedure - and not a question about which sonar is in the bracket.

The question everybody asks next: what else do I have to buy?

This catches people out more often than the sonar choice does, and it is worth stating before you build a budget.

None of these sensors produces georeferenced data on its own. They measure range and angle relative to themselves. Turning that into a point on a map requires an external position and heading source, and the sonar has no idea whether yours is any good.

For the Cerulean products specifically, SonarView needs position and heading over NMEA 0183, NMEA 2000 or mavlink2rest. If you are running a Blue Robotics BlueBoat or another ArduPilot vehicle, you have this already and the marginal cost is zero. If you are working from an ROV below the surface, you do not - GNSS does not reach - and you need acoustic positioning. A Cerulean Omnitrack USBL system starts at €7,000, which is to say roughly the price of the sonar again.

Budget the positioning before you commit to the sonar. A €7,000 sensor fed by a €200 heading source produces a point cloud that looks impressive and measures badly.

Deciding, in three lines

• Your deliverable is a depth chart, and you have altitude to work with → multibeam echo sounder. The Surveyor 240-16 from €4,375.
• You need to find things and are happy reading shadows → conventional side scan. The Omniscan 450 SS from €2,275.
• You need imagery and measurable geometry from one pass, and you work shallow → 3D side scan. The Omniscan 3D 450 SS from €7,000.

If two of those describe you, the honest answer is usually to buy the cheaper one first and find out which problem is actually costing you time.

Frequently asked questions

Tell us the job, not the product name

We distribute Cerulean Sonar and the Blue Robotics platforms these sensors mount to. Tell us the water depth, the vehicle and what has to come out the other end, and we will name the model and the part number in euro - and tell you what we ruled out and why. If the answer is the cheaper sonar, that is what we will quote.

tech@sepcotech.com · (+45) 6916 2400 · Contact page

Related: Omniscan 3D 450 SS · Surveyor 240-16 and bathymetry · The Omniscan imaging range