In November 2023 a hydrographic survey was carried out of an area in the Bothnian Bay where we want to harvest mineral-rich manganese nodules. The final report has now arrived.
Why has it taken so long? Because it involves an incredibly large quantity of data. We have not yet had time to work through the details ourselves.
But already I can say, having worked on similar projects for over 30 years, that the data we have received is very interesting. To be able to map the seabed with such accuracy is little short of incredible. We can see, for example, that the seabed consists of large flat areas covered only by silt and clay.
The data from the survey consists of a film-like stream, roughly 520 hours long.
I would like to highlight the instruments used, because they have been absolutely decisive.
1. Multibeam
This gives a 3D image of larger areas of the seabed. The technique works by sending over 500 sound pulses down towards the bottom, spread like a fan. The instrument measures the topography of the seabed, that is, the underwater landscape.
The image shows how a multibeam survey is carried out.
An area of the seabed measured with a multibeam. The red areas are shallower and the blue deeper.
2. Side-Scan Sonar
It looks like a small torpedo and is towed behind the vessel on a long cable that in our waters needs to be several hundred metres long. The side-scanner gives a 2D image of the seabed seen straight from above, at high resolution.
The image shows a small part of the seabed in the surveyed area, about 400 x 300 m. To the right is a ridge rising slightly above the surrounding bottom. The darker parts are shadows that arise where the sound pulses do not strike anything behind the object. As if you shine a torch at an object in a dark room.
3. Sub-Bottom Profiler (SBP)
The data from this is perhaps the most interesting for us as a company, if one looks only at the economic possibilities. The SBP sends a low-frequency sound pulse straight down into the seabed. It can penetrate about 25 metres of clay, sand, gravel and other material before the signal dies out. It is therefore possible to see the sequence of layers in the seabed with great accuracy. This also appears to work for measuring the thickness of what we are interested in, the manganese nodule bed where the minerals are.
When we can later verify the SBP data with the help of seabed sampling, we will probably be able to make quantity assessments by studying the SBP data alone. Something that would make the process faster and smoother.
The image above shows an extract from the sub-bottom profile data with clear layer sequences down in the seabed. The image is compressed because there are 100 metres between each vertical line while the thickness of the dark section is only about 2 to 3 metres. The seabed is therefore considerably flatter than this image suggests.
4. Magnetometer
The instrument is usually towed behind the vessel, but during our survey it was mounted together with the side-scanner. The magnetometer measures deviations in the surrounding magnetic field. If, for example, a steel wreck lies on the bottom, the magnetometer will give a strong reading.
The instrument can also help to find old mines from the First and Second World Wars that may have sunk into the seabed, and that other instruments do not detect.
For me the final results from the survey in the Bothnian Bay feel both exciting and positive. Step by step it is proving that we are on the right track.
The data from the survey will form part of our coming application documents for the extraction permit. That what we deliver holds high quality is absolutely decisive. That is why it feels reassuring that we have the hydrographic survey company Clinton and our environmental consultants from Sweco to help us.
Now our journey continues. I hope you follow it here on the blog.
