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Core
Sampling
Core
sampling is used to take deep samples of sediment and mud from the
sea floor. Core samples can be taken deep below the surface of the
ocean. On this trip we will be taking cores from depths of 3,000
meters but it is possible to sample at 10,000 meter depths (almost
as deep as the Mariana Trench). The location for core sampling can
be predetermined with a great deal of accuracy. Jason can pick the
site visually and mark it with a transponder. The navigator can
put the ship on the site using the transponder net set at the beginning
of the study.
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The
crane and front (bottom) of the core pipe.
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The
ship's sonar can also be used to determine the characteristics of the
sediment below the ship for sampling. The scientists try to sample areas
free of large rocks. The ship has two types of sonar. One uses sound frequency
at 12 kilohertz which maps the topography of the sea floor. Generally,
the steeper the slope, the rockier the terrain, so the scientists look
for more level areas for core sampling. The second type of sonar uses
a lower sound frequency of 3.5 kHz. These sound waves can penetrate the
layers of sediment. Hard areas reflect the sound waves and indicate areas
that would be difficult to core. Two methods are used to obtain core samples:
gravity coring and piston coring. Both methods will be used on this trip
using a 3,500 pound core weight on the end of a wire. Under the weight
are sections of steel core pipe in 10- foot sections. Up to 4 additional
600-pound weights can be added, one at a time, for a maximum of 5,900
pounds.
Gravity
Coring
The
gravity method generally uses only one 10-foot section of pipe. The gravity
method is restricted in depth because, although the pipe can go deeper,
the sediments will only travel a limited distance up the inside of the
pipe.
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The
recovered core length is limited by the density of the mud, the penetration
characteristics of the mud and the wall friction in the pipe. The
coring apparatus is lowered to the sea floor using a winch. The speed
is dependent on the abilities of the winch. Under normal conditions
the decent speed is about 60 meters/minute. Inside the steel pipe
is a plastic 4-inch core liner. |
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the mud sample is brought on the ship, the core liner is removed from
the steel pipe and cut with a circular saw lengthwise on both sides.
The mud sample in the core liner is then cut with a long length of
monofilament fishing line and the sample is opened to give a cross
section. |
Closeup
of the valve that keeps the sediment from falling out on the way
back to the surface. |
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Piston
Coring
Piston
coring is more complex than gravity coring but cores can be taken
from a greater depth. The depth of the core is limited by the length
of pipe that can be set up on the deck of the ship. Ten foot sections
of pipe are linked together along the side of the ship before they
are lowered into the water using the winch.
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On
this ship the maximum depth of core samples is 70 feet. The French are
currently using up to 30,000 pounds of weight to take cores up to 160
feet in length.
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