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Sidescan vs Forward Looking Multibeam Sonar

Which sonar is best for Search and Recovery operations? We discuss.                                                                

First of all, what is sonar and why do we need it?

Underwater it can be very difficult to see things with cameras due to dirty water or just poor lighting.  Even if the water is clear and there is sufficient lighting, a camera cannot see very far therefore using a camera to find something is not easy or efficient.

To overcome this, people use sonars as they can see in waters where a camera cannot see anything.  Imaging sonars come in 3 types; MechanicalMultibeam and Sidescan:                                                       

        In this article we will discuss the Multibeam and Sidescan sonars to explore the benefits of each.  We will not discuss Mechanical sonars, however, they also have a place but they don't really offer any advantages over either the Multibeam or the Sidescan.

In an ideal world, Search and Recovery teams will have both Multibeam and Sidescan but where there are budget constraints this guide should help decide which one to choose. 

Let's start by looking at Sidescan sonars:

These are often the go-to product for Search and Recovery (SAR) teams as they are portable, easy to deploy and allow for a search to be undertaken quickly.  The Tritech Starfish Sidescan sonar is the one most commonly selected by SAR teams with its portability and simple operation key, as well as its relatively low cost of ownership.

A Sidescan is operated by either towing it behind a boat or by pole mounting it over the side of a boat.  The Sidescan provides a sonar image of what is below it, both to the left and right side of where the Sidescan is positioned.

If the Sidescan is stationary, it is unable to build up a picture of the seabed as it is only imaging a fine line of the seabed on both sides of where it is positioned, therefore it needs to be moving forward to give the user an image of the seabed.

When moving forward, the Sidescan produces a map of the seabed both to the left and right side of where it is positioned (towed behind the boat or pole mounted) and it can therefore cover a large area search in a relatively quick time.

Sidescan Sonar Positives

Large area coverage – The Sidescan can cover a large area search due to it scanning both sides of where it is positioned.

Bird's-eye-view - Having scanned an area, the system generates a bird's-eye-view still image which represents a snapshot of the search area which can be easily and quickly reviewed.

Accurate location – Integrating a GPS allows the system to be configured so that you can identify the above water GPS position of targets of interest, allowing you to return to an area of interest for closer inspection.

Sidescan Sonar Negatives

Image resolution – To cover a large area quickly a lower frequency sonar may be required. The increased wavelength of the pulse results in an unavoidable drop in resolution of the scan area. In addition, the sonar will have to be maintained at a higher altitude above the seabed and the result of these two points is that when imaging small targets is much more difficult to pick up the detail.

Needle in a haystack – When scanning large areas it is possible that targets may appear very small or weak in comparison to the surrounding environment making it difficult to identify the target.

Stick to search plan – A Sidescan image is built-up as it is moved forward through the water which requires it to be moved steadily in one direction meaning you must stick to a structured search pattern and not deviate off to look at each and every target as it's identified.  This can be frustrating when targets of interest are seen but you need to stick to your search plan otherwise you will end up missing areas when you return to the search plan.

Next, looking at multibeam sonars:

Multibeam imaging sonars are intended to look forward and not to the side as Sidescan sonars do.  This means the Multibeam gives you an image of what is in front of it and you can point it to look in any direction, providing more flexibility than you have with a Sidescan sonar.

This allows the Multibeam to be used on remote vehicles as they search for targets of interest or it can be mounted to the side of a vessel like a Sidescan but it does not give the same coverage to the left and right that you get from a Sidescan sonar.

A Multibeam can be used to generate a Sidescan type image by utilising mosaicking software however the coverage to the left and right of the Multibeam is considerably less than that of a Sidescan and a structured search will therefore take longer.

Ideally, a Multibeam will be used to support a Sidescan search as each sonar has its place in a SAR operation.

The entry level Gemini 720im is the smallest multibeam sonar in the world and when it is used in conjunction with a Sidescan search, where the Sidescan gives the greater detail, the Gemini 720im can be used to navigate an ROV to the targets of interest. You can then rely on the camera to identify the target.

More advanced Multibeam sonars such as the Gemini 720ik and 1200ik have the ability to provide a higher level of resolution such as that you get from a Sidescan.  This provides the flexibility to carry out a structured area search but also uses the same sonar to navigate to a target of interest and investigate it.

 Multibeam Sonar Positives

Instant results – The Multibeam sonar does not require you to move to get an image of the seabed allowing you to remain stationary and scan an area very quickly.

Versatility – The Multibeam has the ability to scan in front of itself while stationary and also allows you to use this data to build up a sidescan type map of the seabed by carrying out a structured search as you would do with a Sidescan.

Multiple installations – The Multibeam has the benefit of being used for both structured searches by mounting it over the side of a boat and carrying out a traditional structured search pattern however while mounted on a boat you can also sweep areas and move in on targets of interest.  The Multibeam can then also be reconfigured onto an ROV and using the Multibeam, the ROV can be flown down to investigate targets of interest.   

Multibeam Sonar Negatives

Scan area – Although only looking out to the side, a Sidescan covers a far greater area quickly.  A Multibeam only looks forward through a wedge of 60Ëš or 120Ëš, depending on the Multibeam type.   

Interpretation – Multibeam sonar data can be more difficult to interpret than a map that is created by a Sidescan sonar, although a Multibeam can be integrated with other software to produce a sidescan map!

Deployment – A Sidescan can be very quickly deployed by simply jumping onto a boat and throwing the sonar into the water to be towed behind the boat.  A Multibeam needs to be mounted to the boat in some rigid way which is relatively easy but it does require a bit more planning.

Tritech sonars are available to buy or rent with both long and short term agreements available to ensure Search and Recovery teams have an opportunity to find the sonar that is right for them. Our technical support is available 24 hours a day. 


*This article was originally published in April 2020.