Multi-Frequency Sonar: Fine Tuning Your Pipe Inspection System

Do you remember back to the days when the “rabbit ears” antennas were used to get a TV channel?   You had to adjust the knob to get a good signal — otherwise you were watching “snow” and then you had to keep adjusting the knob when the wind blew or it was raining to get just the right location and picture?   Even worse, it was before the remote control age, so you had to keep getting up and down to adjust it.  We’ve come a long way since then for TV — and for pipes, too.  Getting a good signal is imperative, but not all conditions are the same.   Having the capability to adjust to existing conditions and not having a “one size fits all” sensor is fundamental,  especially when dealing with a changing  environment such as a live sewer.  It is even more specific when trying to collect data under the flow using SONAR.

In a previous article about profiling sonar, we talked about SONAR and its usefulness in estimating the distribution and amount of sediment within a pipeline. We also talked about how sonar can also be used to obtain measurements of internal dimensions, storage capacity, as well as locate major blockages and identify pipe collapses. In each of these examples, sonar is used to obtain measurements from within the pipe. Sonar has many other capabilities that make it an indispensable tool for every pipeline inspection toolbox.

Knowing the differences between single frequency and multi-frequency sonar units is key to understanding how to best use them. Most pipeline inspection systems use single frequency sonar units;  these are suitable for obtaining measurements from pipelines under normal conditions. Multi-frequency sonar units are less common but have a wide range of benefits that include operation in very large pipes, pipes with high turbulence, and pipes with dense, clumped content that can obscure single frequency units.

The benefits of multi-frequency sonar are possible because different frequencies interact with the pipe in different ways. For example, higher frequency sonar can measure with great resolution but does not have much penetrating power, which poses a problem with large pipes, turbulence, and solids. On the other hand, lower frequency sonar has great penetrating power, enabling it to handle the problems mentioned with high frequency sonar; however, the results have much less resolution. Multi-frequency sonar enables you to have the best of both those worlds. With multi-frequency sonar, the frequencies can be adjusted to provide useful information in extra large pipes, excessive flows, turbid waters, or high amounts of suspended solids.

Multi-frequency sonar can also be used to discriminate between the different kinds of materials that might be found in a pipe. This is possible because all materials conduct sound in different ways. For example, it is commonly known that water is much better at conducting sound than air  It’s important, when evaluating this capability, to understand both that the ability of a material to conduct sound  varies with the frequency of sound and to remember that we know what that value is.  That means that it’s possible to use a multi-frequency sonar unit to scan the objects in the pipe at multiple frequencies and understand what might be causing a blockage. As an example, multi-frequency scans have been used to interpret pipe profiles that were misdiagnosed as pipe collapse when they were only scanned with a single frequency sonar inspection.  Scanning the same pipeline segment at multiple frequencies clearly showed the source of misdiagnosis as grease deposits rather than collapse of the host pipe.

Choosing Between Single Frequency and Multi-Frequency Sonar Inspection

So how do you select between single frequency and multi-frequency sonar inspection?  When the object of the inspection is to obtain accurate sediment volumes in traditional pipe sizes with reasonable amounts of flow, low content of suspended solids, not too much clumping of dense materials (flocculent), a standard single-frequency pipe inspection sonar will suffice. If the pipe to be inspected is large, has very high flows, lots of turbulence, heavy suspended solids, or heavy flocculent, a multi-frequency sonar has a higher probability of providing good data. Additionally, multi-frequency eliminates the guesswork from in-pipe estimation of sediment density and material classification.

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