# Will Ping Sonar measure sediment depth?

We are looking for a way to measure the depth of sediment that we find at the bottom of the storage water tanks that we survey. For example, the depth of sediment can be said to be between 5 and 25mm, however we would like to take an accurate measurement.

Would Ping Sonar be able to recognise the distance from the BlueROV2 to the sediment, and then the depth of the sediment to the bottom of the tank?

Thanks

A single beam sonar will give you a return of the top surface of the sediment. If you need sediment thickness I would look at a dual frequency sonar or a sub-bottom profiler.

Hi @Slee,

The sediment layer depth could potentially show up on the profile, but it would depend on the relative density of the three layers involved (water, sediment, tank bottom), as well as the structure of the sediment (if it has lots of pockets then sound will transfer poorly).

Density and structure of sediment isn’t something I have experience with, so I can’t confidently say how likely it is to work or not work. I wouldn’t discount it as an option without trying it, but also wouldn’t assume it would work without trying it / knowing more about the materials involved.

Our Ping Sonar technical details state the range resolution is 0.5% of the range, so when the sonar is profiling a 2m range the resolution is 1cm. I’m unsure whether that’s sufficiently accurate for your requirements.

If you know where the bottom is relative to the vehicle (e.g. from an initial full scan early on) you can set a offset the scan start and use a shorter length (the current firmware has a minimum 1000mm scan length (so 5mm resolution), although we should be able to provide one with a lower length limit if relevant).

It’s worth noting that the further the sonar is from the tank bottom, the more spread out the return signal will be, and the more impact there’ll be from slight tilting of the vehicle.

Doing some rough maths, and assuming the sonar is perfectly facing the tank bottom, the 30 degree beam width means the sound from the edges of the beam travels ~3.5% further than the sound in the middle, so the response from the sediment surface would be spread out over time (which lowers the precision of the data). If 3.5% of the range is longer than the sediment depth then even if the acoustic properties of the transitions are suitable for detection, the return signal from the sediment surface may be indistinguishable from the tank bottom signal. Making a conservative assumption that response at the edges is just as strong as the middle (in an attempt to have some reduced response after the sediment surface),
3.5% <= 5mm → max range <= ~140mm

Presumably it’s not feasible to keep the vehicle within 140mm of the tank bottom, particularly if you’re trying to measure sediment depths (without stirring it up). That’s also below the 0.5m minimum range (the region where the sonar is generally still vibrating from its transmission), so likely wouldn’t yield any usable data anyway.

It may be possible to recess the sonar to reduce the beam width, but using the sensor “as-is” wouldn’t be suitable for the measurements you’re trying to take

@FairweatherIT’s suggestions are also worth considering