Samplers and Corers
The Woods Hole Science Center operates a variety of samplers and corers to collect a representative and minimally-disturbed sample of sea-floor sediment. The Sea-floor Mapping Group studies generally use a modified Van Veen grab sampler and a gravity corer in their ground-truth efforts.
Van Veen Grab Sampler
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| Van Veen sediment grab sampler | Close up of Van Veen sediment grab sampler |
This light-weight sampler has a clam-shell type scoop to sample the top 20 cm. of the sea floor within a sampling area of 0.1 square meter. The jaws of the sampler are opened prior to deployment. During descent to the bottom, the design allows sea water to flow through the sampler to prevent premature closure of the jaws in rough seas and also, to maintain a vertical pathway. When contact is made with the sea floor, the stabilizing plates prevent over-penetration, so when the tow-cable is made taught causing the jaws to close, thereby capturing a sample, it equally samples all depths with a minimum of disturbance. The tension on the cable prevents the jaws from opening, preventing any sediment loss during retrieval.
Gravity Corer
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| Deployment of the gravity corer |
The gravity corer is basically a weighted tube mounted within a frame that descends by gravity from the research vessel to the sea floor, where it penetrates the sediment to a given depth, filling the tube with sediment in the process. The hydraulically-damped gravity corer has a slow rate of penetration that is controlled by a water-filled piston. Disturbance of the water-sediment interface is minimal and maximum penetration in mud is about 70 cm. A core-catcher on the bottom of the tube moves into place when retrieval begins, trapping the sediment sample in the corer. A winch, onboard the ship, slowly brings the corer back to the surface, where the sediment core can either be stored for later analysis back onshore, or in some cases be sampled or extruded onboard for immediate analyses.
Video cameras can be attached to either the corer or the grab sampler to provide a view of sediments prior to sampling and to confirm that disturbance with collection was minimal.
System Operation
The WHSC studies generally use the Van Veen sampler as part of the SEABOSS systems (please navigate to the description from the page to the right). Power supplies and winches are required onboard the research vessel from which the sampler is deployed.
The gravity corer requires a power winch as well, to deploy and retrieve the corer. Also, sufficient cable is required to reach from the ship to the sea floor.It can be difficult to retrieve, especially in heavy seas.
System Usage
The Van Veen sampler mounted on the SEABOSS can be used in water depths up to 200m. The Mini SEABOSS is designed for use in more shallow areas, including lakes, rivers, and estuaries. Samples obtained with the Van Veen grab, can mix to some degree. The Woods Hole Science Center has used it extensively to collect samples during projects offshore Massachusetts and New York. The samples undergo size, texture, and composition analyses in the labs at the Woods Hole Science Center. That information yields bottom type and characteristic information that improves interpretation of sidescan-sonar and seismic data.
Click here for information regarding WHSC analytical labs
Gravity-cores maintain the stratigraphic relation of the sediment sample. So, in addition to size, texture, and compositional analyses, dating of the sediments can carried out.
