Sedimentary Coring and Drilling
Sedimentary coring and drilling for scientific purposes is generally confined to the seafloor and the top few metres of the substrate. An exception to this is the Integrated Ocean Drilling Program (IODP) in which drill holes completed for marine geoscience research on a regional scale are kilometres deep. The following methods and equipment are those normally used by Geoscience Australia during routine seabed sampling programs.
Coring and rotary drilling
Marine sediments vary widely in grain size, mineralogy, fabric and hardness and a wide range of coring and sampling devices are required to recover useful samples.
The gravity corer is designed for use in areas of soft, unconsolidated sediment. It is a general purpose tool that relies on its weight for penetration into the seafloor. Geoscience Australia's gravity corer consists of a one tonne weight atop a steel core barrel which houses an internal PVC core sleeve. The unit is lowered to a predetermined height above the seafloor using a wire rope before being allowed to freefall. The resulting core enters the internal sleeve and is held in place by a core catcher. The wire rope is then reeled in and the inner sleeve is removed from the core barrel and the core is processed. Gravity cores up to 12 metres in length can be collected with Geoscience Australia's system but in practice recoveries are typically less than six metres.
The piston corer is a variant of the gravity corer and is used on soft, unconsolidated sediments. Geoscience Australia's piston corer has a similar weight and length to the gravity corer and is lowered by wire rope in the same manner. However, it has a trigger device that hits the seafloor before the core barrel and releases the corer allowing it to freefall. As the barrel enters the sediment a special internal piston creates a vacuum and helps to draw the core into the barrel. This suction reduces compaction of the sample in the inner sleeve. Piston cores up to 12 metres in length can be collected with Geoscience Australia's system and typically recoveries are longer than the gravity corer.
Vibracoring is a technique for collecting core samples in harder, unconsolidated sediments. It works particularly well in sediments with relatively high sand contents. The vibracorer is mounted in a tower which is lowered to the seafloor before three phase electric power is applied. It involves driving a thin-walled core tube into the sediments using an electronically-powered head unit that causes the core barrel to vibrate. It achieves this by creating a standing wave along the barrel that liquifies the sediment outside the barrel but leaves the inner sediments relatively undisturbed. This method allows the barrel to penetrate the seafloor and also reduces sediment compaction (which is common in gravity coring). The core is retained in the outer core barrel and held in place with a core catcher. Vibrocores up to six metres in length can be collected with Geoscience Australia's system and in practice cores of this length are regularly recovered.
Geoscience Australia has a small underwater rotary corer designed to operate on the continental shelf (<200 metres water depth). It is used to sample consolidated or partially lithified sediments. Core length is limited to three metres. The rotary corer is lowered to the seafloor before three-phase electric power is applied for coring to commence. A steel barrel with liner is rotated with hydraulic motors and the substrate is cored using diamond or tungsten-tipped bits. Variable hydraulic pressure is used to push the barrel into the seafloor and used to recover the barrel and core. Fragments are flushed from the bits with seawater to prevent the corer from jamming in the hole. The rotary corer has two video cameras and lights for site selection and monitoring of coring. All of the systems are controlled by computer on board the vessel with real time feedback of electrical, hydraulic and video systems.
This corer is used for geochemistry nutrient studies by Geoscience Australia and is ideally suited to shallow estuarine or river environments with fine-grained (muddy) unconsolidated sediments. It is a smaller hand-held device made up of a length of clear plastic tubing attached to a corer head and extension poles. In the marine environment, the push corer is lowered overboard and manually pushed into the sediment before a valve at the top of the core, is closed creating a vacuum. The core and barrel is then lifted out of the surrounding sediment and back to the surface. The operation is the same on land or in shallow waters where a boat is not required. Cores are generally limited to less than two metres in length.
The Mackerreth corer is designed for coring of very soft, unconsolidated fine-grain sediments. It has been used by Geoscience Australia for coring freshwater lake (lacustrine) deposits and is often used for palaeoclimate and palaeomagnetic studies. It is powered by compressed air which applies suction to both attach the corer securely to the bottom and drive the corer head into the sediment. Cores of up to three metres are regularly recovered.
Livingstone corer/section corer
These small hand held corers are a complement for push corers and are useful for relatively short cores in shallow water depths where a boat is not required. Cores up to three metres are regularly recovered.
Grab sampling and dredging
A variety of dredges, sleds and sampling tools are used for collecting sediment, rock and ecological samples from the seafloor surface.
Smith McIntyre grab
The Smith McIntrye grab is used to sample seafloor sediments, macro benthic plants and animals in either shallow or deep ocean water. It is our most regularly used seafloor sampling device. It has two spring hinged jaws with triggers on either side of the grab frame. The jaws are 'locked' in place within the frame until two pads hit the seafloor allowing the spring-loaded jaws to snap shut and collect the sediment. The strong hinges aid penetration of dense sediment while the large jaws return a substantial volume of sediment. Access to the sediment is through flaps on the upper side. The Smith McIntyre grab is used to get a quick sample of the seafloor and is not designed to get an undistributed sample (although this can sometimes occur in rare situations).
These samplers come in a variety of sizes and can be used in either deep sea or shallow water environments. They return a rectangular or square section of sediment from 50 mm to 1 m deep, as well as the overlying bottom water. They are designed to recover a relatively undisturbed sample of the seafloor to preserve surface or sub-surface structures. Geoscience Australia uses box cores to collect samples for biological and geochemistry research. The box core relies on its own weight for penetration of the seafloor and has a single swing arm which closes after being triggered to retain the sample on retrieval. The box is then removed from the corer enabling unrestricted access to the sample surface and sides.
The Shipek grab is another quick seafloor sampling device that is smaller than the Smith McIntyre grab. Geoscience Australia uses Shipek grabs to collect samples for sedimentology and geochemsitry studies. The grab is comprised of two semi-circular scoops. The outer scoop is fixed and allows the inner scoop to collect the sediment. The Shipek grab is triggered by its own weight releasing a pin on contact with the seafloor which causes the spring-loaded recovery scoop to spin around and dig into the sediment. The sample is then covered by the outer scoop to isolate it from the surrounding water. This grab is ideal for sampling sands and shelly sediments but is not designed to return an undisturbed sample.
Van Veen Grabs
The Van Veen grab is used at Geoscience Australia for biological and sedimentology studies. They are one of the more frequently used seafloor sampling devices because of their simplicity and success in returning sediment. All Van Veen grabs comprise two opposing 'jaws' that are held open by a hook. The grab is activated on contact with the seafloor causing the jaws to dig into the sediment. It is not designed to recover an undisturbed sample and must be deployed slowly to prevent the hook releasing and triggering the grab in the water.
The Wildco-Ekman grab has a more specialised function, taking quantitative and qualitative samples of macroscopic benthic fauna. It can be used to determine the productivity of soft sediments, particularly those composed of finely divided muds, ooze, submerged marl and fine peaty materials. It is not recommended for rocky or sandy environments or where there is moderate benthic vegetation because small pebbles or vegetation stems can prevent proper jaw closure. Two thin, hinged overlapping lids open while the equipment is descending which close on retrieval and are held shut by water pressure. The grab is triggered by a messenger weight dropped down the rope.
Rock dredges are used to recover lithified samples from rock strata exposed at the seafloor. The dredge is lowered to the bottom by a wire rope and is towed along the seafloor. It is designed to dislodge rock fragments from the seafloor while being towed. The style of dredge used at Geoscience Australia consists of a rectangular metal collar framed by outward facing teeth connected to a heavy chain bag that collects large rock fragments. Two metal pipe dredges are attached to the back of the chain bag to collect unconsolidated material and smaller fragments.
These devices are designed to sample benthic flora and fauna and are towed along the seafloor. They have a secondary net to study plankton and can be fitted with video or stills cameras to monitor sampling in real time.