Thursday, 29 September 2011

INFOMAR mapping of the Shannon estuary

INFOMAR mapping of the river Shannon commenced onboard the Marine Institute’s research vessel Celtic Voyager on July 19th. Over the course of the two-week survey leg, the estuary mouth was mapped up to the 10m contour, with physical sampling taking place at key locations in order to determine the seabed type (e.g. rock, sand, shell hash etc). The new seabed coverage was merged with existing datasets offshore and upriver. In addition to the geological mapping, a detailed survey was carried out over the wreck of the S.S. Premier – a British cargo steamer that was lost in 1898. This 54m long wreck lies upriver from the survey area, in a general water depth of 21m.

An overview of the survey area at the mouth of the Shannon estuary showing the newly acquired bathymetric data. The solid-colour areas indicate the extent of previous survey work.

The survey area is particularly important for a number of reasons – not only does the area incorporate a busy shipping channel, it is a Special Area of Conservation (SAC) for resident bottlenose dolphins, and from a geological perspective it overlies an important subterranean structure called the Iapetus Suture Zone – a large seam running through Ireland’s ancient bedrock that marks the closing of an ocean pre-dating the Atlantic by many millions of years.

The survey comprised bathymetric mapping of the seafloor from Kerry Head to Loop Head, with the inner extent lying along the 10m contour. As the area is an SAC for resident dolphins, the acoustic surveying took place during daylight hours only, with seabed sampling operations taking place at night. The sampling was carried out with a mechanical instrument called a “Shipek Grab” – this is a spring-loaded device that, upon being lowered to the seafloor, snaps closed and scoops up a bucket of material. Along with a variety of sediment types, the grab also returned a host of interesting biological specimens – squat lobsters, brittle stars, shrimp, crabs and solitary corals – all of which were logged before being returned safely to the sea.
In addition to the seabed mapping and sampling work, two survey lines were run at night outside the SAC, using a towed “Sparker” system to record a sub-bottom profile of the seabed sediments and bedrock. This device uses an electrical arc to create an acoustic pulse that penetrates the seabed. Where the bedrock is buried beneath soft sediments, this pulse can return a detailed image of the sediment layers and provide information on their thickness.
A variety of interesting seabed features were mapped in detail, and are detailed in the following images. Note that the colour scale represents depth, from deeper (blue) to shallower (red):

Outcrop of folded bedrock off Inishaboy Point, Co Kerry.

Shaded relief bathymetry indicates a channel feature visible in rocky seabed lying north of Kerry Head.
Distinctive layers of limestone off Horse Island, Co Clare.
Folded and faulted bedrock off Loop Head. A 3D multibeam image of the S.S. Premier wreck showing the hull of the vessel lying in a general water depth of 21m.

Multibeam echosounder image of the S.S. Premier in plan view.

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