Vibracoring: Reconstructing Earth’s past
from sediments
Narrator: Geologists rely on information from
deep beneath the Earth’s surface to reconstruct
the past. As sediments accumulate over time,
they create records geologists use to understand
Earth history and to predict future processes
and trends.
The most common way to get this information
is to drill a hole in the Earth where sediments
have been deposited over time. The sediments
drilled are recovered in the form of a core
that will contain an intact record of the
past. [Map with explanation: ‘Cores from
the Sabine National Wildlife Refuge study
area shown’. Two cores are shown, each with
a Description and Photo.] Core samples are
used to assess the geologic history of an
area, such as its geomorphology; coastal,
marine, and terrestrial processes; and changes
in environmental quality.
The type of drilling tool used for cores depends
on how deep and how hard the sediments are.
Vibracoring is one of the tools used in shallow
coastal areas where sediments consist of soft
sand and mud.
Hi, and welcome to the U.S. Geological Survey
Coastal and Marine Geology Podcast. I’m
Matthew Cimitile. I’m here with two marine
geologists, Kyle Kelso and Nancy Dewitt. They
are part of the St. Petersburg Coastal and
Marine Science Center’s seafloor mapping
group. They have taken cores in coastal areas
all around the U.S. Together this team of
scientists analyzes geologic information to
reconstruct Earth history and to predict future
trends. We’ll be talking to them about what
it takes to collect a sediment core using
a vibracoring rig from a boat platform. Nancy,
can you tell me what we’re seeing here?
ND: That’s the RV/GK Gilbert, that we use
for seafloor mapping purposes. What you are
going to see today is taking sediment and
vibracores off of the boat.
KK: It has a nice big deck to hold the tripod
and also to handle all of the equipment that
is necessary to perform our fieldwork.
ND: We have a captain on the top of the fly
bridge, and we are positioning over the place
that we’re going to take a sediment core.
In order to do that you have to have a stationary
boat so we have a two-point anchor in this
situation. Sometimes, in rougher seas, you
need a three-point anchoring situation.
KK: We are hooking up the pipe to the vibracore
head. It connects just by a little clamp there,
and then we raise it up and getting ready
to insert the entire rig into the water.
ND: Rich is raising it up with the crane,
and then we lock the barrel in place so it
stays vertical.
KK: The rig will sit on the seafloor bottom
and it‘ll vibrate and penetrate the sediment
until the head reaches the bottom.
ND: In this situation, this rig is setup for
20-foot barrels. Sometimes you can take it
down to 10 feet. On a manual vibracoring setup,
you can use 40-foot barrels to get a continuous
record. The goal is to get a continuous record
as long as possible. Normally, this rig is
underwater but since we are only in about
6 or 7 feet, you get to actually see the electronic
vibracore head. The black part right there
actually shakes the barrel. It liquefies the
sediment around the barrel, which allows the
barrel to enter the seafloor.
KK: It’s pretty shallow water here but we
are able to collect in water depths in excess
of 200 feet. We are guiding it now back on
to the deck of the boat. You can see the weight
of the coring rig pulling the boat to the
side and sometimes when the core gets stuck
in the sediment the angle of the deck is about
30 degrees in some instances. You have to
make sure you wear closed-toed shoes.
ND: That’s a two-ton crane that Rich is
operating there, allows us to handle the weight.
KK: And the sediment in those barrels do weigh
a lot, I’ll tell you that much. Tripod is
now in place; rinsing it down.
ND: You end up marking the outside of the
barrel. It is very important which end is
up because we are taking a section of geologic
history and we are trying to piece history
back together. So it is chronologically important
to know which end is up.
KK: Right now, Chandra was tapping to see
where the sediment surface was and she is
releasing the water above the sediment surface
in order to be able to cut the core without
any danger, because those cores get really
heavy, especially when they are saturated
like that.
ND: She is using a pipe cutter. Plumbers use
those things a lot. It is just a cleaner cut,
a straight cut, and it is faster than a hacksaw.
KK: So here we are cutting again right at
the sediment surface. That initial cut was
just an estimation because we didn’t really
know where the sediment surface was. Getting
rid of as much void in the core as possible.
And you will see us stuffing, in this case
paper towels, in order to keep the sediment
from moving around while it is in the core.
ND: That is a very, very, very heavy sediment
core right there. You would be surprised.
It looks like a toothpick but it is very heavy.
It takes two people to move it.
KK: That was Chandra marking the top and naming
the core and marking the top and bottom.
Narrator: Thanks, Kyle and Nancy, for your
time and for explaining the process of vibracoring.
Now, once the core is brought back to the
lab, it will be cut into meter sections and
split lengthwise into two halves. One core
will be kept for archiving and one core will
be kept for sampling. The archiving core would
be photographed and stored in plastic bags
for future reference. The sediment core could
be analyzed for grain-size analysis, and age
through radiometric dating.
Learn more about USGS science online at usgs.gov.
The Coastal and Marine Geology podcast is
produced in St. Petersburg, Florida, and is
a product of the U.S. Geological Survey, Department
of the Interior.