If you're using a StarFish towed sidescan sonar for your underwater surveys, this guide offers practical advice to help you optimise your imagery and maximise the potential of your system.
ONE | Watch Your Speed
When towing the StarFish, keep in mind that the sonar 'pings' at a steady rate. If you speed up, images on the display may look compressed, as the sonar will be covering more distance before each ping reaches the seabed and returns. Higher towing speeds also increase drag on the transducer head and cable, making the StarFish sit higher in the water, affecting the ping's potential reach. For best results, aim for a towing speed of 1 to 3 knots.
TWO | Depth Matters
The acoustic shadows captured on sonar data play a key role in helping operators identify underwater objects. It is important to keep in mind that the depth the sonar sits in the water can influence the appearance of acoustic shadows, towing too close to the seabed will make shadows appear very elongated (like shadows cast from objects at sunset), while towing too high will give a 'birds-eye-view' with minimal shadows, making it difficult gauge the height of submerged objects.
THREE | Get in Position
It is important to make sure the sonar is positioned below the boat's keel to reduce the chance of echoes bouncing back from the hull, which can show up as 'ghost' images on the display. Be sure to deploy the sonar head carefully to ensure it isn't sat at an angle in the water, and if you’re adding any attachments (such as weights), make sure they’re symmetrically balanced as this will help the transducer head to tow smoothly without tilting.
FOUR | Master Smooth Sailing
When surveying an area, think about how you'll steer your boat. Try to break the area into a grid with long, straight paths and 180° turns at the ends. Keep in mind that when the boat turns, the acoustic beams on the inside of the turn will overlap, while those on the outside will cover more seabed, this can make the images look distorted and hard to read in those spots. Make sure to give enough space between your boat and other vessels or any hazards in the water, and try to avoid crossing through the aerated water left in the wake of other boats, as this can affect your sonar readings.
FIVE | Mind The Waves
Since your StarFish operates several meters below the surface, it’s generally shielded from waves and surface chop. However, it can still be affected by movements on the towing cable. In rough conditions, it’s a good idea to use longer lengths of cable to help reduce the angle between the cable and the sonar transducer head. Just keep in mind that your StarFish won’t perform as well in stormy weather, so it's best to check the forecast before heading out.