Sediment Corers

Sediment Sampling

A variety of sediment samplers are for recovering different types of sediment mainly based on their stiffness. Soft sediments are usually collected using short cores with wide barrels that do not disturb the sediment-water interface. Corers with deeper penetration usually encounter sediment with increasing stiffness and therefore need greater weight or other features, such as vibration or rotation to enable deeper penetration.

NIU percussion corer (Designed by DOER-Marine and S. Vogel and built by DOER-Marine)

Figure 1: DOER engineering CAD of the NIU Percussion Corer

The percussion corer (Fig. 1) is designed to be lowered to the lake or sea floor on the smart cable of the multipurpose winch, and then to hammer a core barrel up to 5m-long into stiff over-consolidated sediment such as subglacial till. This is a significant technology development, intended to further Antarctic geoscience by significantly extending sedimentological sampling studies beyond the range currently accessible using piston coring and recovering sediment that rotary drilling cannot.

As the percussion corer is deployed, a 2000lb mass is released within its casing by unbolting an extension section from inside the casing (Figs 2 and 3; refer to pink section in Fig. 1). Once deployed on the sediment surface, the hydraulic motor is commanded to drive a piston that raises the mass to its maximum height within its casing, and then is tripped to be released in a free-fall, to then strike a plate on the top of the core barrel. This process is automatically repeated every 20-30 seconds until commanded to stop.

Figure 2. Upper stages of the percussion corer including the power and hydraulic motor stage and the telemetry stage. Below these is the drop-weight stage (the weight can be seen through the holes at the bottom of the stage in image on the left.

Figure 3: Unbolting the extension section to release the drop-weight before deploying the percussion corer. On the right, the corer is at its full extent with the 5m core barrel being the lowest stage.

A linear position sensor is used to measure the penetration distance with each strike. Coring is stopped when there is either a lack of further penetration or the 5m-barrel is fully buried in the bottom sediment. To avoid large pullout strains beyond the capacity of the cable, which is 10,000lb, the hydraulic system is designed to help extract the core barrel. The hydraulics can be commanded to force pressurized-water down between the core liner and core barrel; the water exiting via jets through holes in the core cutter head (Fig. 4). The water is then forced up the outside of the core barrel to decrease friction between it and the sediment.

Figure 4: Specifics of the core barrel of the percussion corer. Clockwise the images are: water jet holes at the base of the drop-weight stage that attaches to the top of the core barrel; top flange of the core barrel with water jet holes; linking the base of the drop-weight stage to the core barrel (two images); core cutting head; looking up the core cutting head at the core catcher; the core liner exposed without the core barrel attached to the core cutter with custom sealing ring with water jet holes.

Also for added safety if the hydraulic flushing process fails to extract the barrel, weak-link bolts that fail in tension will break away at the top of the barrel so that the rest of the corer assembly can be recovered and only the barrel is lost.

Multicorer (Designed and built by Uwitec)

The multicorer is a lightly modified off-the-shelf system designed by the Austrian Uwitech Company (Fig. 5) that is well-tried in many lakes around the world. It is designed to take three replicate cores at the one time after self-triggering on striking the bottom sediment during descent. It recovers undisturbed top-most sediment, the sediment-water interface and the bottom water in contact with the sediment in each of the tree core liners. Thus it is also an efffloor sediment.ective bottom water sampler. It also comes with a custom sediment slicer with which you can extrude sediment in discrete intervals for sequential sampling through the top-most lake sediment.

Figure 5: Uwitech multicorer that takes three replicate cores at once preserving the top-most sediment, the sediment-water interface and the water column. Two bottom right images show a sample being recovered.

 

Piston corer (Designed and built by University of California Santa Cruz) Figure 6: Preparing and deploying the UCSC piston corer. Bottom right image is from the ROV of the protruding bent core barrel with its lower section being stuff in stiff till.

The piston corer is modeled on the previous CalTech corer (Fig. 6) and has its own light-weight winch and cable.