Once the ice has melted in April-May, Metsähallitus will launch seabed soundings off Korsnäs as part of the environmental impact assessment for the planned offshore wind farm. The soundings will take a few weeks to complete.
“The soundings will provide a wide range of information on seabed conditions. They will help to establish what kind of foundations will be needed for wind power plants and which routes are suitable for the electricity transmission cable”, explains Antti Lindfors, expert at Luode Oy, the company responsible for the soundings.
“Wind turbines need firm foundations, and this is why we are looking for a site with solid seabed conditions for them. Soundings are also conducted to locate suitable routes for the electricity transmission cable, making it possible to place it at a sufficient depth to prevent any interference with the anchoring of boats or ships, for example.“
“The aim is to avoid underwater slopes, rocks and rock fields. Sounding helps to assess dredging needs and survey cable routes. We are looking for sites that need to be modified as little as possible. This will minimise any impacts on the environment and fisheries. For example, the soundings will help to avoid the spawning areas of fish. One line of sounding was added to the plans specifically at the request of fishermen”, says Lindfors.
Later, the plan is to take sediment samples from the area as well as to examine benthic animals, including shellfish, crustaceans and bristle worms. The impacts of the project on underwater noise will also be investigated.
Archaeological sites may also be found
Seabed soundings are also important for recording any archaeological findings, such as wrecks or other old structures. Should any new archaeological sites be discovered, they would be reported to the Finnish Heritage Agency, which maintains a register of ancient relics and looks after underwater cultural heritage.
“In connection with the soundings, we can lower a camera to the sea bottom and take pictures of the site. We can also use echo sounding technology to obtain more information on it from different directions”, says Antti Lindfors.
Discovering seabed structures
The human ear can hear frequency ranges of between 20 Hz and 20 kHz. Fishermen’s echo sounders use frequencies ranging between 50 kHz and 500 kHz, which are clearly higher than the range of the human ear. The most sensitive sonar scans reach frequencies of over 1000 kHz and provide very accurate images. This means that echo sounding does not produce noise that could be picked up by the human ear.
The sonar sensor converts the electrical impulse produced by the device into a sound wave. When the sound wave hits an object in the water, part of the sound bounces back. The device’s receiver measures the volume of the reflected sound and the time that elapsed since the pulse was sent out and, using this data, calculates the distance to the object.
When surveying the seabed, broader frequency ranges than usual are used, as well as side scan sonar and 3D echo sounding technologies. The best understanding of seabed structures can be reached by combining different methods.
“The image produced by modern sonars is amazingly accurate. We can achieve an accuracy of a few centimetres both in the vertical and horizontal direction”, says Lindfors.
Spring offers optimal conditions for sounding
Spring is a suitable time for sounding as high pressure and calm weather occur more frequently. There are also fewer pleasure boats around.
The equipment and cables are towed behind the sounding vessels, and warning lights are used to alert other seafarers. The vessels stand out at sea because of their cranes and other deck structures.
For further information, please contact:
Wind Power Project Development Manager Ville Koskimäki, Metsähallitus, ville.koskimaki@metsa.fi, +358 536 9582