Side Scan Sonar Explained: Mapping Wrecks, Pipelines, and Seabed Obstructions

Side scan sonar is one of those bits of technology that sounds very specialist at first, but once you understand what it actually does, it is surprisingly intuitive and incredibly useful in real world projects.

If you are responsible for subsea infrastructure, marine archaeology, or coastal development, it can show you what is really lying on the seabed before you commit to any works. That is often the difference between a smooth project and a very expensive problem.

What is Side Scan Sonar?

Side scan sonar is a type of underwater imaging system that uses sound to create detailed, photograph like pictures of the seabed and anything sitting on it. Instead of simply telling you how deep the water is, it reveals shapes, textures, and obstructions, from shipwrecks and boulders to pipelines and cables.

Rather than pointing straight down like a traditional depth sounder, side scan sonar sends sound pulses out to both sides of the survey track. Those pulses bounce off the seabed and objects on it, and the returning echoes are converted into an image. The result looks a bit like a black and white aerial photo, only you are looking at the seafloor instead of the land.

For civil engineers, marine archaeologists, utility project managers, and environmental consultants, that visual element is a big step up from a set of contour lines. You are not just working from numbers on a chart, you are looking at clear imagery that helps you make safer, more informed decisions before a diver ever gets in the water.

Beyond Bathymetry, Why Imagery Matters

Standard bathymetry gives you the depth and shape of the seabed. That is vital for navigation, dredging and hydraulic modelling, but it does not always tell you what individual objects are or how they are lying.

Side scan sonar fills that gap.

Because it produces acoustic imagery, you can pick out:

• Isolated boulders and rocky outcrops
• Lost anchors, containers, or vehicles
• Pipelines and cables sitting proud of the seabed
• The outline and condition of wrecks and debris fields

Imagine you are planning to lay a new cable across a busy estuary. A bathymetric survey will tell you where the seabed slopes, how deep it is, and where the channels are. Side scan sonar will then show you whether there is a wreck, rock, or piece of historic pipeline sitting directly in your proposed route. That combination is where the real value is.

How Side Scanning Sonar Works, The Technical Process

The technology behind side scanning sonar is clever, but the core idea is straightforward once you strip it back.

The Towfish and Transducer Setup

Most commercial side scan sonar systems are made up of three main parts, a towfish, a tow cable, and a topside processing unit.

The towfish is a streamlined device towed behind or beside the vessel, kept at a consistent height above the seabed. Inside it are the transducers, which emit fan shaped sound pulses out across the seabed to both port and starboard.

As the vessel moves forward, this fan sweeps the seabed line by line. Every time the sound pulse hits the seabed or an object, some of that energy returns to the towfish. Those returning echoes are recorded and turned into an image strip. Line by line, those strips build up into a continuous picture of the seabed.

Controlling the towfish is important. Too high and your resolution drops. Too low and you risk hitting the bottom or losing coverage. Surveyors constantly balance vessel speed, tow cable length, and water depth to keep things at the right level. It is a bit like flying a kite underwater, only with rather more maths involved.

Understanding Backscatter and Acoustic Shadows

When the sound pulse reaches the seabed, different materials reflect it differently. Hard surfaces such as rock, concrete, or metal tend to reflect more energy, so they show up as brighter areas in the image. Softer sediments like mud or sand absorb more sound and appear darker.

A simple way to think about it is to imagine walking into a dark room with a torch. Wherever the beam hits, you see the surface reflect the light back. Anything that sticks up blocks the light and throws a shadow. Side scan sonar works in a very similar way, except the torch beam is a sound pulse and the shadows are acoustic rather than visual.

Those shadows are where things get really interesting.

Shadow Analysis, Measuring Object Height

When an object rises above the seabed, it creates two important features in side scan data, a bright highlight where the sonar beam hits the object, and an acoustic shadow behind it where the sound cannot reach.

By measuring the length of that shadow and knowing the geometry of the sonar system and the height of the towfish above the seabed, surveyors can calculate how high that object stands proud of the bottom.

This technique, often called shadow analysis, allows you to estimate the height of:

• Wreck superstructures and masts
• Pipelines spanning above the seabed
• Debris, rock pinnacles, or other protrusions that could pose a clearance hazard

This is where professional side scan work really moves beyond basic “fish finder” style displays. The shadows are not just visual, they are measurable, and those measurements can directly inform engineering and safety decisions.

Key Applications for Marine Infrastructure and Archaeology

Side scan sonar is used wherever there is a need to see and understand what is on the seabed, not just how deep it is.

Identifying Underwater Obstructions and Hazards

For ports, harbour authorities, river managers, and offshore contractors, underwater obstruction detection is a key part of safe operations.

Side scan sonar is ideal for searching for:

• Lost containers, anchors, or moorings
• Submerged vehicles and large debris
• Natural hazards such as boulders, outcrops, and scour holes

Because the imagery is so clear, it becomes much easier to identify potential hazards that might snag a cable, damage a vessel, or interfere with construction works. You can then plan diver or ROV inspections in a targeted way, rather than searching blind.

Pipeline and Cable Route Surveys

Pipelines and subsea cables are often critical assets. If they fail, the consequences can be serious, both financially and environmentally.

Side scan sonar plays a major role in both planning new routes and inspecting existing ones. It helps answer questions such as:

• Is the pipeline fully supported, or is it spanning between high points, leaving sections hanging?
• Has scour developed around the pipe, exposing it and making it more vulnerable?
• Are there rock berms, dumped materials, or debris that could interfere with a new cable route?

In a typical project, side scan imagery will be combined with bathymetric data and sometimes sub bottom profiling. That way, engineers can see not only how the seabed looks on the surface, but also how it is shaped and what lies beneath. It is a much more complete picture for design and maintenance.

Marine Archaeology and Shipwreck Mapping

If you work in marine archaeology, especially in UK waters where visibility can be limited to almost nothing, side scan sonar can feel like a bit of a lifeline.

It allows archaeologists to:

• Detect previously unknown wrecks or submerged sites
• Map the outline of a wreck and associated debris fields
• Monitor changes over time without disturbing the site

Because it is non intrusive, side scan sonar is ideal for sensitive heritage sites. You can document and study them while keeping physical impact to an absolute minimum. Once a site has been mapped acoustically, divers or ROVs can be deployed much more efficiently, heading straight to areas of interest rather than searching a wide area.

Side Scan Sonar vs Multibeam, Which Do You Need?

Side scan sonar and multibeam echo sounders are often mentioned together, and for good reason. They do different things, but they work brilliantly as a team.

Here is a simple comparison:

In most commercial projects, the question is not “which one should we use”, but “how do we use both effectively”.

Multibeam gives you a detailed digital terrain model of the seabed, which is perfect for dredging, navigation, and hydraulic modelling. Side scan sonar then adds the rich imagery that shows what is sitting on that terrain, from wrecks and rock outcrops to pipelines and debris. Together, they give a much stronger basis for decision making.

If you would like to explore the depth mapping side in more detail, it is worth looking at a dedicated bathymetric survey guide, then comparing it to how side scan imagery is used within professional hydrographic surveys. It also ties neatly into the broader hydrographer’s role in infrastructure projects.

Processing the Data, From Sound to Sonar Mosaic

Out on the water, collecting data can feel like the main event. In reality, the real value emerges when that data is processed and turned into clear, reliable information.

Once the survey is complete, hydrographers will:

• Clean the raw data, removing noise and correcting for signal strength
• Apply motion corrections for heave, pitch, and roll, so the image is properly stabilised
• Correct for slant range, so distances across the swath are accurately represented

Individual lines of data are then georeferenced and stitched together to form a sonar mosaic. Think of it as a patchwork quilt of image strips that covers the entire survey area in one continuous picture.

These mosaics can be supplied in various formats, for example:

• GIS ready raster files, such as GeoTIFF
• AutoCAD backgrounds for engineering drawings
• High resolution PDFs for reports, stakeholder engagement, or consent applications

A good mosaic is easy to read, easy to share, and easy to integrate with other spatial data. That is exactly what you want when multiple teams, from engineers and archaeologists to regulators and clients, all need to understand the same site.

Why Professional Expertise is Essential for Sonar Interpretation

On a finished plot, side scan sonar imagery can look deceptively simple. Clear shadows, bright targets, neat mosaics. Behind that, though, there is a lot of judgment and experience at work.

Several factors can complicate both data collection and interpretation:

• Vessel speed and heading need to be controlled carefully to maintain coverage and image quality
• Vessel motion, including heave, pitch, and roll, can distort the imagery if it is not measured and corrected properly
• Different seabed types can produce similar responses, making it tricky to distinguish, for example, between compacted sand and rock without supporting data

Experienced hydrographers know how to manage these challenges. They understand the behaviour of the equipment, the impact of sea state and currents, and the subtle differences between genuine targets and artefacts in the data.

From a commercial point of view, that expertise is not just a technical nicety, it is a core part of risk management. Mis reading sonar imagery could mean missing an obstruction that later causes damage, or mis classifying a feature and spending time and money investigating something that is not actually an issue.

By working with a professional team, you are not just buying access to a side scan sonar system. You are accessing a complete service that includes:

• Thoughtful survey design and line planning
• Controlled data acquisition with the right equipment for the site
• Careful processing and quality control
• Clear, practical interpretation of what the imagery shows and what it means for your project

How Side Scan Sonar Helps You De‑Risk Subsea Works

When you add it all up, side scan sonar offers a very practical benefit, it gives you a clear view of the seabed and its features before you commit to work.

It helps you to:

• Spot wrecks, debris, and other obstructions that could affect construction, maintenance, or navigation
• Assess the condition and support of pipelines and cables, and identify areas of scour or exposure
• Support marine archaeology and environmental work in low visibility waters, without intrusive methods
• Combine imagery with bathymetry and other data types as part of a wider, professional hydrographic survey

For decision makers, that means fewer surprises, better planning, and a stronger basis for design and consent. Instead of working from partial information and hoping for the best, you are looking at a clear, well interpreted picture of what is actually there.

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This post was written by Paul Jackson