grab samples

The consolidated core sample in the liner should then be capped at both ends, and labeled at the top of
the tube. The sample in the core catcher should be discarded unless the sample is small and there is no
opportunity to get another. If the tube is very long, it can be cut into shorter lengths for easier handling. All
cut surfaces should be capped. All sections should be labeled at the tops, clearly designating their position
in the total core (i.e., top, top-I, top-2, bottom, or some such scheme). For most ocean dumping work, it
probably will not be necessary to cut the cores into shorter lengths. Care should be taken while cutting core
liner tubes that plastic chips are not allowed to fall into the sample. When possible, the samples should be
stored in a vertical position with tops up.
If a liner is not used, a rod with a piston on the end with a diameter slightly less than the inside
diameter of the pipe is usually used as an extruding tool. This is a poor method as corrosion flakes or metal
shavings from the piston and barrel can contaminate the sediment. Use of the core liners is preferred.
Different methods of forcing penetration and of sediment sample retention result in different designs
for coring devices. Various types of corers have been developed including the gravity corer, piston corer,
box corer, pile d riven corer, manual thrust corer, and vibratory corer. Combinations of some of these types
are available (Hopkins 1964; Myers et al. 1969). Four of these coring devices are described briefly in the
following sections.
l. Gravity Corer
The coring tube of the gravity corer is weighted at its upper end by lead or steel weights and allowed to
fall freely to penetrate the sea bed. A vane is sometimes attached to the top of the corer, just above the
weight, to maintain the corer's vertical alignment during the fall. The gravity corer is lowered with a boat
winch. It is rigged to be triggered to freefall 6-9 m (20-30 ft) to the sea floor (Figure 3). The depth of
penetration depends on the weight of the corer and the physical characteristics of the sediment. The gravity
corer is" top-heavy," and some may overturn if allowed to freefall over too great a distance.
Whenever possible, a gravity corer should be used since it is the simplest device with the fewest
moving parts to be damaged. Generally, only short samples, usually less than 3 m ( 10ft), will be taken. If a
longer sample is required, an alternative device may have to be used.
II. Piston Corer
On the piston corer, a triggering mechanism is employed which causes the corer to freefall over a
known distance. Correct adjustment of the length of wire on the trigger weight and in the corer enables the
core barrel to slide past a piston which remains stationary at the sediment surface. The piston creates a
partial vacuum, facilitating sample entry and reducing core compaction. The barrel may be forced into the
sediment by weight or by vibratory action.
As with the gravity corer, a cutting head, core liner. core catcher, weights and vane are usually used.
The cost of the piston corer is higher than the gravity corer but its efficiency may be beller.
III. Box Corer
Box corers are particularly useful where undisturbed cores of large cross-sections are required for
subsequent chronological examination or other studies of pore-water chemistry, sedimentary structures,
mechanical properties, etc. Initially the long square box-corer ("Kastenlot") was developed by Kogler
( 1963) as a gravity-type instrument but modified versions can be hand-operated by a diver. Other
modifications include removable side-walls, which permit immediate visual observations and varying
designs of core-catchers (Werner 1973 ).
The box-corer consists essentially ofa box of varying length, often square in cross-section and usually
has a core closing device consisting of two flaps. These flaps lie opposite each other in the open position and
have to rotate through 90° in the sediment in order to close the corer. Obviously such a mechanical
operation presents considerable difficulties most often related to the type and physical properties of the
sediment being cored. " Clean" uncontaminated samples can subsequently be obtained following shaving
6