Coring
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		Description The corer consists of a corer head, a PVC sampling tube, a stainless wire rope attached to the corer head, a stainless piston wire rope, a weight (10-20 kg) with static ropes for lowering and lifting, a piston (three pieces with a central bolt and two sealing rings), wire clamps, a core catcher and a jack (Fig. 1). The length of the static ropes for the weight, the length of the piston wire rope, and the wire rope attached to the corer head are determined by the water depth.

When the piston corer is used from ice, a hole (with diameter 3-5cm larger than that of the corer head) has to be drilled. Cores up to 3-4 m length can normally be lowered and raised vertically through the hole in the ice. To retrieve long (>3-4 m) and heavy cores, however, it is commonly necessary to cut an oblong trench in the ice by means of a chain saw and/or an ice auger. However, step ladder placed by the hole allows one person to climb sufficiently high to guide the top of the corer.

Holes on the PVC core barrel for the corer head and core catcher are made by a hand/power drill through premade holes on a jig.

The piston wire rope and the main wire rope are fed through the steel pipe and the corer head from above and attached. Dish-washing liquid or vacuum grease is put on the piston corer to reduce friction between the piston corer and the PVC sampling tube. The piston and piston wire are fed into the PVC sampling tube by means of a plastic rod until the piston gets in a position close to the core catcher.

The main wire rope is attached to the corer head before the PVC sampling tube is fastened to the head. For extra security, two hose clamps may be fastened just above and below the screws at the corer head.

The PVC sampling tube has then to be filled with water through the outflow hole at the corer head in order to prevent the piston corer from sliding upwards in the sampling tube during lowering of the sampler into the water. The water depth at the coring site must be measured precisely by a measuring tape and the depth transformed to the main wire rope to ensure to get the soft top sediments. An alternative way to measure the water depth is to attach a light plate to the piston corer (before the piston is placed on the wire) and send it down to rest lightly on the sediment surface to prevent any significant disturbance. An electrical tape is put on the wire rope at the ice surface as an exact measure of water depth. But by far the best way to control the coring operation is to use echo-sounding equipment for exact measuring and monitoring of the process.

The sampler is lowered by hand down to the sediment surface by holding the main wire rope attached to the corer head (note: not the piston wire). When the lower end of the PVC sampling tube is on, or just above, the sediment surface (to get the sediment/water interface), the piston wire is attached with a wire clamp onto a bolt/carabiner at the jack lifting arm, preventing the piston form entering the sediments.

Two long static ropes are attached to the weight. When coring in deep water (100-150m) wires and ropes tend to twist together. The two static ropes may therefore be replaced with one.

The oblong slit piece on the weight is removed before attaching the weight onto both wire ropes through the oblong slit on the weight. Then the slit piece is attached to the weight and the weight can be lowered to the corer head. Before coring begins, the length of the PVC sampling tube is marked by electrical tape on the main wire rope attached to the corer head to see where to stop penetration into the sediments. By manually raising (commonly not more ten 1.5m) and dropping the weight with the two static ropes (alternatively on static rope) through pulleys on each side of the jack, the PVC sampling tube is driven into the sediments. Because the closely fitting piston and the sediment surface during penetration, the sediments are sucked evenly and undisturbed into the sampling tube. When not using a piston, the friction of the sediment inside the core tube normally prevents more than a few decimetres of sediments from entering the sampling tube.

When the PVC sampling tube has penetrated the sediments, or the weight starts bouncing when reaching when reaching impenetrable sediments (bedrock, stone, diamicton etc.), the weight is pulled up along the wire ropes to the surface and laid on the ice/raft. Then, both wires are attached to the wire clamps on the lifting arm of the jack to pull the corer out of the sediments. When the sampling tube is out of the sediments, the sampler can normally be pulled through the water column by hand. Before pulling out long cores (>3-4m) containing mostly minerogenic material, the oblong fissure in the ice allows gradual lowering of the top of the sampler. Ropes fastened to the corer head and around the sampling tube aid in retrieval.

When coring from a raft, anchoring is necessary to hold station while the coring operation is carried out. On a raft the sampler and core tube can be fastened underneath before going to the lake shore. Thereafter, the sediment catcher and the corer head are removed before the sampling tube is corked and labelled. Cores must be transported carefully and stored unfrozen in order to prevent the sediments form being turbated.

Advantages and Disadvantages of the corer

The main advantages of the corer are:

1. Light-weight (about 100 kg), easily transportable on a snowmobile sled and quite simple to manufacture at a cost of about dollar: 8000.
2. Easily handled by 3 persons
3. The sampler can core up to 6m of sediments in water depths to at least 60 m.
4. Little of no disturbance of the sampled sediments.
5. Operable both from ice and rafts.

The main disadvantages are:
1. Some problems with transport 6m PVC tubes.
2. Only one sample can be obtained from the same hole.
3. The length of the core is commonly limited by the length of the PVC sampling tube (usually supplied in lengths of 6m). If necessary, two lengths of core tube could be fitted together with an external collar. The limitations with such long cores are (1) the difficulty in handling a long tube, (2) the stiffness of sediments encountered at greater depths and the friction of the sediments on both the outside and inside of the core tube, and (3) vibration as the tube is struck by the weight and much of the energy is lost as the tube wobbles from side to side.

The coring equipment

Corer head, PVC sampling tubes, jig, piston, weight (10-20 kg), plastic rods for pushing the piston into the sampling tube, hex screws and hex keys for sediment catcher, two long static ropes (as long as maximum water depth), short ropes (5-10 m), carabiners, wire clamps, piston wire rope (stainless) (as long as maximum water depth), wire rope to attach the corer head (stainless) (as long as the maximum water depth), spools for the wire ropes, hose clamps, sediment core catcher (optional), jack, wood jack stand, pulleys, screws for the corer head and sediment core catchers, chain saw, ice auger, ice chisel, ice axe, snow shovels, plastic bucket, funnel, plastic caps to cork the core barrels, kit of tools (screwdrivers, hammer, adjustable spanner, pliers, saw, hand drill, knife, rasp, file), dish-washing liquid/vacuum grease, measuring tape, tape (water proof), electrical tape to flag the wire ropes, paper towels, gloves.

Reference:

Nesje, A. 1992: A piston Corer for Lacustrine and Marine sediments. Artic and Alpine Research, Vol. 24, No. 3, 1992, pp. 257 - 259.