Finding old shipwrecks on the ocean floor is a bit like searching for treasure in the dark. But thanks to sonar, we can “see” beneath the surface even before we dive.

Sonar technology has developed enormously since the 1930s and 40s. The first sonars could only measure distance by listening to the echo, but they could not tell anything about the material or structure. In the 1980s, digital sonar came along, which could give a hint about the density of the object – much like dolphins do when they echolocate. In the early 2000s, chirp sonar came along, which sends a full spectrum of frequencies at once, which gives clearer images, longer range and less interference from the environment.

We mainly use sidescan sonar. It is perfect for searching large areas and can be mounted directly on the boat’s hull or towed behind the boat. It scans to the sides and shows the seabed in detail, and with the help of GPS we can mark interesting finds for future dives.

With the classic 2D sonar, we look straight down below the boat. Sidescan works a little differently: Imagine a flashlight shining out to both sides of the boat. The “light” reflects off of structures on the bottom, creating shadows on the screen. Long shadows mean objects stand out high, while short shadows show low objects. Dense, solid objects reflect sound strongly and appear bright, while softer materials produce weaker echoes and darker images.

To ensure that the sonar images are as clear as possible, we follow a few basic rules: first, it is good to know what you are looking for. Information about the shape and size is needed to be able to make the right settings. For example, if it is a large wreck you are looking for, you can search an area with a wider search field than if the object you are looking for is smaller. You therefore set a well-balanced range, so that the object fills the screen just enough when it appears.

Which frequency you choose to search with is also crucial: Low frequencies reach far and deeper but provide less detail, high frequencies provide sharper images but shorter range and do not reach as deep into the water.

The search speed is also important. If we go too fast, we miss details, if we go too slow, the image can become noisy. Marking finds is also crucial: we want to be able to return to them.

Finally, it is a major success factor to lay down systematic search patterns, straight lines and accurate GPS recording make the hunt for wrecks both effective and exciting.