Solutions in Depth

What are the differences in the sonars that you use?

When first discussing profilers with the uninitiated, there is sometimes some confusion as to how they differ from an imaging scanning sonar or a sidescan sonar.

First of all, let’s look at the difference between a scanning sonar and a sidescan sonar. With a scanning sonar, a small transducer emitting thousands of ultra sonic audio pulses per second is rotated about an axis using a small motor to drive it. The transducer rotates around an axis much as a lighthouse lamp rotates. In contrast, a sidescan sonar has a fixed transducer which also emits thousands of pulses per second but its movement is a linear one generated by towing the transducer through the water rather then an angular movement achieved with a motor.

Next we must differentiate between an imaging sonar and a profiling sonar. Imaging sonars are typically used to provide an image of the seabed or water column much as a radar does on land. They provide a multicolored (‘chromatic’) display which shows stronger echo returns as brighter colors than points with weaker echos. E.g. you may get a bright yellow image from a strong return off of the side of a steel shipwreck and dark blue image off of the weak return from a smooth sand bed.

A profiling sonar on the other hand provides a digitized version of the echo returns. The sonar’s processor looks at the return signal for each pulse and decides where along that pulse’s return time lays the strongest return. Rather than providing an analog range of colors for each pulse it provides a single dot or x,y point at the point of strongest return.

Another differentiating factor between imaging and profiling sonars is the shape of the transducers beam pattern. An imaging sonar is typically a fan beam whereas the profiler emits a beam pattern like a spot or pencil beam.

The imaging sonar fan beam (typically around half power +/-15 deg from horizontal) is configured to ensure that all targets above and below horizontal are detected while retaining an angular resolution (about 1.7 deg). The profiler on the other hand is a very specific tool for performing engineering measurements. Its spot or pencil beam (half power about 1.7deg from both horizontal and angular rotation) means that it is able to provide high resolution measurements both in terms of the horizontal plane and of angular displacement around its motor axis.

OK, so what does all this mean to you? When we are inspecting a pipeline, our profiler takes very specific measurements of the internal dimensions of the pipe at that location. Not an average over the fan beam as it would be if we used and imaging sonar. The profiler then provides us with either a series of digitized “dots” in a screen shot (as in figure below) or it can save those thousands of digitized points as an ASCII file which can then be imported into CAD software for post processing.

If combined with the recorded penetration depth we now have the necessary x,y,z points required to build a 3D model of the pipe (x & Y from the profiler and z from the umbilical counter). Once input to CAD, such parameters as volume of sedimentation or percentage restriction in pipe can be calculated.

If you would like a further, more detailed description of sonar theory, Imagenex provide an excellent summary on their website www.imagenex.com or you can click this link to download it directly  Basic Sonar Theory.  N.B.  The sonar diagrams images used in this FAQ answer are used with kind permission from Imagenex.