A new multibeam survey of Loch Eriboll — the only deepwater sea loch on the north coast of Scotland — is underway to help us better understand our submerged landscape by producing a detailed bathymetric map.
The present day sea level is just an arbitrary point on the landscape of Scotland. There is evidence all around the coast for former sea levels both higher and lower than at present, even in the geologically recent past. Therefore we can expect the variation in morphology we see onshore to occur offshore and processes that are active today may have affected areas now submerged when they were previously exposed.
However, we still have a poor understanding of the submerged landscape around the coast of Scotland. In most areas this is limited to a few spot soundings that provide us with only a broad scale image of the shape of the sea bed.
Multibeam surveying of Loch Eriboll will provide us with detailed sea bed morphology. This will improve our geological understanding of the area.
Onshore we can see evidence of glaciers in the area only 15 000 years ago and it is probable that the moraines, and other glacial materials they deposited, extend offshore and may even allow us to link features on one side of the loch with those on the other.
Some rockfalls along the loch side extend to the shoreline. To understand better this instability it would be helpful to map the full extent of the rockfall deposits. There may also be underwater instability with evidence of sediment sliding.
An accurate map will allow interpretation of various sedimentary processes active on the sea floor, recording how and where sediment is moving. Such information forms the basis for habitat mapping so that biological information can be placed in context.
Loch Eriboll has played an important role in naval warfare and it is believed much material, including several vessels, has ended up on the floor of the loch. Having an accurate map of such obstacles can be used to update the archaeological records and will provide targets for sports divers who can undertake their interest with improved safety.
It will also mean better marine spatial planning for future users including the economically important aquaculture as Loch Eriboll is the location of several fish and mussel farms.
It is hoped that the results of this survey will also contribute to public understanding of science as another aspect of geology to be shown in Britain's first Geopark; the North West Highlands of Scotland.
The full multibeam bathymetric survey of Loch Eriboll is performed using our new survey boat — the White Ribbon. This is a small power catamaran, equipped with all the latest multibeam survey technology. Its small size and draft allow survey operations to be undertaken in very shallow water depths.
A multibeam echosounder uses a fan or swathe of narrow acoustic beams to completely cover the sea floor. To enable accurate sea floor mapping we also have to position the survey vessel very accurately.
We use DGPS (differential global positional system) to locate it and then correct for motion and heading changes using onboard sensors. A tide gauge is used to reference all depths to a common level (Chart Datum) and sound velocity changes in the water column are indentified using various sound velocity probes. The swathe of high frequency beams can now be accurately positioned and a depth resolved to create a very detailed and complete image of the sea floor.