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Surveying

Ground Truthing/Seabed Sampling

We collect sediment samples from the seabed. Looking at the samples alongside the bathymetry and backscatter maps allows us to group seabed areas of similar nature, and create seabed habitat maps.

Grab samplers are used to take samples from the seabed surface. They ‘grab’ the top few centimetres of sediment using a container with a set of jaws (clamshell bucket).


Coring methods use an empty core tube, which penetrates the sea floor, and fills the tube with sediment. These methods retain the vertical structure of the sediment.

Grab Sampler

The grab sampler is the most popular method of ground truthing used by the INFOMAR project. A range of grabs have been used depending on which vessel it is operated from. Day, Shipek and Van Veen grabs are all used to recover sediment samples from the seafloor. These grab samples are used to provide a cross reference to the seabed type classifications that are made from the MBES backscatter datasets.

Shipek Grab
Shipek Grab
Shipek Grab Sample
Van Veen Grab
Van Veen Sample

Images of shipek grab; Van Veen grab with video camera attached that allows sampling team to describe the seabed morphology and biodiversity before sediment is recovered. From this it is possible to determine if the sample is representative of the immediate area from which the sediment is taken; Recovery using a Van Veen grab of a fine grained mud/silt sample from Galway Bay from RV Geo; Recovery using the Shipek grab of a coarse gravel with minor mud from the Irish Sea off Howth Head, Co. Dublin from RV Celtic Voyager.

 

Video of Van Veen grab sampler deployed from RV Geo in operation in Mulroy Bay, Co. Donegal. Click 'Play' to view.

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Box Corer

Another method of retrieving sediment from the seabed, the box core differs from grab samples in that it retains the vertical structure of the sediment. It comprises of a cylinder or box which has a weight attached to it within a frame. This is left to free fall through the water column and under the force of gravity is driven down through the seabed. A hinged grab then pivots to trap the sediment within the box which is then recovered onboard. Average penetration is 30 to 40 cm but is dependent on sediment type.

Images of a Box corer (left) and a successful recovery with internal structure of the coarse shelly sand is conserved in the core from the Celtic Sea off the south coast of Ireland (right).

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Vibrocorer

The Geo Resources 3000 + 6000 vibrocore is another method of taking samples from beneath the seabed. It can recover cores of 3 or 6 metres depending on the sediment type, with best penetration in fine grained sediments. As the name suggests, the vibrocore is made up a base which sits on the seabed, a motor that crates vibrations that allows a metal cylinder (into which the plastic liner is inserted) to go through the sediments collecting sediment in the cylinder as it goes. When the core is recovered from the seabed, the plastic liner is removed with the undisturbed sediments safe inside. These are later cut in half, photographed, scanned and analysed to determine the changes in sediment through the core that may reveal information about the depositional environment in the past. Many of these cores are now being used to investigate changes in past climate which in turn may help scientist to understand current and future changes to climate. Click here for information on the most recent INFOMAR vibrocore leg off the west coast.

Images (Top to Bottom) of the vibrocore on the deck of the RV Celtic Explorer, Removing the plastic core liner which contains the undisturbed sediment from the vibrocore after a successful deployment, Core sediment from west of the Porcupine Bank in the Atlantic Ocean.

View Seabed Sediment Core Extraction Animation by RealSim Movies

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Vessels | Acquisition | Tides | Referencing | Sound Velocity | Ground Truthing | Survey Progress

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Geological Survey of Ireland Geological Survey, Ireland
Beggars Bush, Haddington Road
Dublin 4
Marine Institute Marine Institute Headquarters,
Rinville, Oranmore
Co. Galway
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