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.