the gemax corer for soft sediments

GEMAX
The ultimate corer
for
soft sediments
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THE GEMAX CORER FOR SOFT SEDIMENTS
by
Boris Winterhalter*)
ABSTRACT
The GEMAX gravity corer is an improvement of the very successful Niemistöcorer originally designed by Dr. Lauri Niemistö in the late sixties. The
hydrodynamically balanced corer is quick and easy to use from any sized vessel
equipped with a light-weight winch with a lifting capability of a few hundred
kilograms and with a wire speed of around 1 m/s. The corer is ideally suited for
undisturbed sampling of soft muds for environmental monitoring purposes. The
extruder unit permits very precise slicing of the sediment core for detailed analysis
either directly on the wet sample using various probe technologies or for later
analysis with more conventional methods.
The photo on the front page shows the the designer,
Dr. Lauri Niemistö and the original GEMINI corer in
its cocked setup ready to be lowered down for easy
and controlled sampling of unconsolidated muddy
sediments on the continental shelf of the Weddell Sea
*)
Boris Winterhalter, senior marine geologist
Geological Survey of Finland
02150 Espoo, Finland
off Antarctica. Picture was taken in 1996 on board
the Finnish r/v Aranda.
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INTRODUCTION
Dr. Lauri Niemistö designed in the late sixties a highly successful gravity corer for
environmental sampling of muds (Niemistö, 1974). The ease and reliability of handling the corer
and the effective procedure for sub-sampling of the sediment core made the Niemistö-corer
quickly the Baltic Sea “standard” for marine sampling of recently deposited sediments. The need
for larger sub-samples led to further development work and the GEMINI-corer evolved (cover
photo). The GEMINI-corer, true to its name consists of two independent core barrels for more
sample material in thinner slices. The special extruder unit for sub-sampling (slicing) the
sediment core was an integral part of the system.
In both the Niemistö-corer and the GEMINI, in very watery sediments, material from the “fluffy”
top surface had a tendency to flow down between the inside wall of the plastic core liner and the
sediment core. In more sticky sediments (e.g. glacial clay) the disturbed contact surface between
sediment and core barrel was also a source of minor contamination of sub-samples during
slicing. Although the amount of mixing can in many cases be accepted and is anyhow a reality
in bioturbated sediments, the mixing can be a critical source of error when trying to determine
the exact depth of e.g. an environmentally significant change in sediment character (chemical
pollution).
The development in analytical equipment has
led to increased requirements for preventing
contamination of the sub-samples during
slicing. This led to the construction of the
ultimate corer, the GEMAX. The main change
is the increase of the internal diameter form 80
mm to 90 mm. The slicer unit has seen the
major change in the design. The larger corebarrel diameter of GEMAX together with the
new slicer head with the peeler facility, limits
sample contamination to a minimum without
loss in sample volume. This is a very
important feature when collecting sediment
samples for environmental monitoring and
assessment of anthropogenic pollution.
Figure 1. The GEMAX corer with large diameter core
barrels in ready-to-go configuration.
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Figure 2. The GEMAX twin barrel corer
for environmental sampling of soft seabed
sediments. The figure shows the main
components of the corer unit.
CONSTRUCTION
The GEMAX corer consists of two
identical and independently functioning
stainless steel core barrels attached
symmetrically to the main body and
release mechanism (Figure 2). The plastic
(methylmeta-acrylate or polycarbonate)
core liner with the stainless steel core
cutter are attached to the corer body with
a simple screw-locked bayonet fitting.
The aluminum fins attached to a pivoted PVC frame have a dual function. During lowering they
improve the hydrodynamics of the corer helping to steer it vertically down into the bottom. Upon
retrieval the fins help to close and lock the core catcher thus preventing premature extrusion of
the sediment material.
The rubber flanged plastic (polyamid) valves at the top of each barrel help to retain the sediment
sample during initial pullout from the sea bed. They have been designed to cause minimum
restriction to water flowing thru the barrels during lowering, thus ensuring good sediment
penetration.
The sediment core extruder consists of a wide, shallow PVC box which helps contain spilled out
sediment from spreading on deck. The central pylon (column) is set in an aluminum base which
itself is fixed inside the PVC box. The extruder piston, with o-ring seal at the end and screwed
to the extender rod, fits exactly into the core liner. The core liner clamped to the slide and moved
vertically under control of the threaded shaft is used to extrude the sediment through the top of
the liner. (Figure 3).
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Figure 3. The sediment extruder used for subsampling of the sediment core. Clockwise
rotation of the knob at the top of the threaded
shaft causes the clamp fixture to move down
along the pylon pressing the sediment out of
the core liner for sectioning.
Sub-sampling of the sediment core is
controlled with the help of the slicer unit
(Figure 4). The unit is fixed to the top of the
liner with the aid of three screw knobs. The
diameter of the hole thru the peeling slicer
base is smaller than the inner diameter of the
core liner and the core itself. When extruding
a 5 mm thick peel (side wall contamination) is
removed from the core and discarded thru the
spoil orifices.
.
Figure 4. The peeling slicer. A
simpler slicer with an extrusion
diameter equal to the core liner
diameter is also available
THANKS TO SOUND DESIGN AND THE USE OF PROPER RAW MATERIALS, THE
GEMAX SYSTEM WILL GIVE YEARS OF ALMOST MAINTENANCE FREE SERVICE
PROVIDED THE UNIT IS PROPERLY CLEANED AFTER EACH DEPLOYMENT.
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OPERATION
Figure 5. Detail of the
trigger unit with the string
loops secured in a cocked
position by the spring loaded
pins.
The winch wire is attached to the trigger unit (Figure 5), which consists of two individual spring
loaded cocking pins. The looped strings from the valves and the fins are locked by these pins
while the weight of the entire corer hangs from the wire. When the corer hits the sea bottom a
minor slack on the wire triggers off the release mechanism. The weight on the trigger unit forces
the cocking pins against their springs to release the strings attached to the fins and the valves.
It is important that prior to deployment, the core catcher and the check valves be properly
attached to the trigger device. This ensures verticality during lowering and completely
unobstructed flow of water through the corer and subsequent sediment penetration. It is
advantageous to use a swivel, as seen in the cover photo, between the corer and the lifting wire.
The penetration of corer into the seabed is a function of the amount of attached lead weight,
winch speed, and sediment compaction. In soft muddy sediments, typical of Baltic Sea anoxic
basins, a wire speed of 1 m/s and two standard weights have generally given optimum
penetration of ca. 50-60 cm. Upon end of penetration into the seabed, sufficient slight slack has
to be given the lifting wire to trigger the release mechanism. This allows the upper check valves
to close thus preventing sediment loss during the initial retrieval process. Once free of the seabed
the core catchers swing into place by hydrodynamic force exerted by the fins and finally locked
in place by dual permanent magnets.
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The core catcher is gently swung out while capping the core cutter (lower end of core liner) by
the palm of a hand. The plastic liner with the sediment inside is removed downwards from the
corer and transferred in a vertical position to the fixed piston on the extruder unit (Figure 3). The
slider unit and clamp, uncoupled from the threaded shaft, is brought up around the core cutter
and screwed tight. Also the threaded yolk is closed around the shaft.
Figure 5. The slicer unit with peeler mounted on top of the core liner. During the extraction and
sub-sampling the contaminated outer part of the core is removed. The spoil (black ooze) can be
seen flowing out thru the side orifices on the slicer unit. The sub-sampler with centimetre scaling
can be used to measure the amount of sediment to be sliced. Alternatively special slice holders
with a hole equal to the inner diameter of the extruded sample and with thin plastic sliding
covers can be used in the sub-sampling process (see Figure 6) .
The slicer unit is attached to the top of the plastic liner. The sediment extrusion process is started
by rotating the shaft in a clockwise movement. Each full turn of the shaft amounts to two
millimetres of extrusion. Thus two and a half turns extrudes 10 millimetres of sediment. In the
initial phase the thin plastic slide covering the opening in the slicer unit is kept slightly open to
allow removal of excess water above the sediment either to be wasted or sampled as “bottomnear-water”. When almost all water is removed from the top of the sediment, slicing can
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commence. The plastic cover is fully opened, the sub-sampler measuring ring (Figure 5) is
placed so as to allow sediment to enter. When the required amount of sediment is extruded, the
plastic slide is closed cutting the sediment. The sediment in the measuring ring is then gently slid
off the slicer into a plastic bag or sample container. The plastic slide is opened and the next
length of sediment is extruded and transferred into a sub-sample container. The thickness of the
sub-sampled slice can be controlled either with the measuring ring or just by counting the
revolutions of the shaft.
For alternative sub-sampling without the measuring ring, special sample holders of various
thickness with thin plastic slides above and below are available (Figure 6). The covers on these
holders fit tightly enough to keep the sample from contact with air for a short time, being
normally sufficient for in situ Ph, Eh and other probe measurements.
Figure 6. Special plastic sub-sample holder with top and bottom sliding covers can also be used
in sample handling. The sliding covers fit snugly to protect the sample from air contamination
for some time. The drawing depicts the holder used in the original Niemistö corer with a 5 cm
core barrel.
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After coring and sub-sampling the core liners are rinsed and inserted back into the GEMAX
corer (Figure 7), the unit is cocked and ready for the next sampling station.
Figure 7. When preparing the corer for deployment, the core liner with attached stainless steel
core cutter are inserted into the core barrel and locked with a twist. The wing screw is tightened
to prevent loss of cutter and liner.
Reference:
Niemistö, L., 1974: A Gravity Corer for Studies of Soft Sediments. Merentutkimuslait.
Julk/Havsforskningsinst. Skr. 238, 33-38.
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