Seabed Sampling & Vibrocores
Our Geophysical survey services include a range of techniques to recover sample material from the seabed and underlying sub-strata for testing and analytical purposes.
These include both cores and surface grabs, using a range of equipment and methodologies appropriate to the site conditions.
Vibrocoring, a technique for collecting core samples of the seabed sub-strata sediments, consists of a vibrating mechanism attached to a metallic core which is driven into the sediment by the force of gravity, enhanced by vibration energy.
The vibrocore apparatus is generally a lightweight rig and as such does not rely on overall mass as an additional means of penetration.
The portability of this equipment facilitates rapid deployment at an alternative core location should the original location provide a poor return.
Vibrocores are most effective in unconsolidated, waterlogged sediments and soils, with sandy and silty sediments of mixed grain size easiest to core. Vibrocores are less effective for relatively dry soils such as clay, compacted sand or any consolidated materials. Given that the seabed sub-strata composition at site is unknown, core retrieval depths can vary.
Once the core tube has fully penetrated the sediment and the vibration source is turned off, the core is withdrawn without losing the sample. On retrieval of the cores, they are sub-sampled into sections – Top, Middle & Bottom, and are then photographed and logged before being despatched to an accredited laboratory.
In addition to core samples, surface grabs can be recovered where the area of study is more focused on the seabed, possibly for benthic analysis or for particle size distribution.
A Day Grab, with a sampling area of 0.1m² can be deployed via a stainless steel wire bridle, which is attached to the grab’s bucket arms. As the unit is lowered and the bottom of the sampler frame makes contact with the seabed, the catch plates release the buckets which drive into the surface sediment.
On retrieval, as the bridle comes under tension, the buckets close automatically, securing the sediment sample which is then brought to the surface, generally by means of a winch system. The grab sits in a purpose built table, enabling plastic trays to be placed underneath to hold the samples when the grab is opened.
A smaller version of the Day Grab, a Van Veen Grab with a sampling area of 0.045m², can also be used where smaller quantities of material are required or where conditions are such that it is not possible to deploy a larger system. The Van Veen grab buckets also have stainless steel flaps which are hinged, allowing access into the top of the buckets for sub-sampling.
The sample material is despatched to a UKAS accredited laboratory for analysis against an agreed set of criteria, e.g. Marine Scotland Revised Action Level 1 and on receipt of the laboratory findings a review of the data is undertaken and a factual report generated detailing the results and findings.
Sampling works are usually undertaken in anticipation of a dredging campaign, whereby the client is required to obtain a license to facilitate the removal of material from a site with disposal either in a recognised landfill site or seaward disposal area.
