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SurveyingGround Truthing/Seabed SamplingTo verify the results of remotely sensed sonar data, it is important to collect physical sediment samples from the seabed. These may be from the surface where grabs are used or may penetrate through the seabed and retain the vertical structure of the sediment by using various coring methods. The samples are then analysed for to determine their effects on and interactions with the sonar data but also to determine to environmental conditions under which they were deposited. 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.
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. 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). 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. Vessels | Acquisition | Tides | Referencing | Sound Velocity | Ground Truthing | Survey Progress |
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Geological Survey of Ireland Beggars Bush, Haddington Road Dublin 4 |
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Marine Institute Headquarters,
Rinville, Oranmore Co. Galway |
 
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