Marine sediment samples are obtained by either by “grabbing” material from near the sediment-water interface (i.e. the sea floor) where the sediment is mostly unconsolidated and quite soft, or by inserting a hollow box or cylinder, called a corer. The corer will usually be constructed of some combination of metal (steel or aluminium for strength) and an inert material (plastic or polycarbonate “liner” to prevent chemical reaction between the corer and the sediments). The deeper you penetrate into the sediment, the more compacted and harder it becomes and so the more rigid the corer must be and the more energy must be used to insert it. Coring almost always disturbs unconsolidated sediment to some extent but care can be taken to minimise or at least account for this disturbance. A range of different coring techniques are available, the selection of which depends on:
- Water depth (a general rule of thumb is the deeper the water, the more elaborate the coring equipment, and the bigger the boat required)
- Desired Core length (the deeper the penetration, the more elaborate the equipment)
- Type and amount of material (the larger the diameter of the core, the heavier it will be to extract
A number of practical solutions to these concerns are available, and there are a multitude of named corers with specific attributes. Most are variations on a basic theme of how the corer is inserted in the sediment. The major types are listed below:
- Hammer or push core – core barrel pushed or hammered into the sediment
- Gravity Coring – weight of corer free dropped into water assists penetration
- Vibracoring – high-frequency vibration of the core barrel liquefies sediment to aid penetration.
- Piston Coring – Core retrieved in short sections involving multiple re-entry of the same hole; piston (one-way valve) reduces contamination and plugging. Insertion methods range from hand pushing, to hydraulic rams, or vibration.
- Rotary Coring – core barrel is rotated and a drill bit grinds away the sediment around the core barrel allowing penetration
- Auger Coring – device with a helical flighting which when rotated about its axis “screws” into the sediment.
An additional issue of importance is whether or not the sea floor (sediment-water interface, or sometimes called the mud line) is well defined, or diffuse. In the latter case there will be a transition from almost 100 percent seawater (with some suspended particulate matter) to “sediment” which, depending on its composition, may range between 50 and 90 percent or more, water in the pore spaces between particles, over a depth of a few centimetres to metres into the sediment. In this case it may be desirable to preserve the structure of the transition zone, and this can be achieved by using a range of freeze corers.
All coring systems comprise at least four parts:
- The corer itself (with core barrel and liner)
- A mechanism to assist insertion of the corer into the sediment (ranging from pushing or hammering, to complex pieces of powered equipment to achieve the same result)
- A mechanism to pull the core barrel out of the sediment once full (usually involving a pulley system and/or power assisted winching)
- A device to prevent the sediment falling out of the corer during retrieval (called a core catcher).