20 January, 1998
Greetings from the Palmer! We've had a very busy day here on the Nathaniel B. Palmer. One of the things that we did was take cores of the bottom sediments. We have four different ways to get bottom sediments from here -- 3 methods of coring and a grab sampler!
The grab sampler does just what it sounds like -- it grabs samples of the sediments, but it doesn't really keep them in any kind of order. It brings up about one large bucketful of sediments at a time. Why do you supposed that we would want to see a bucketful of sediments rather than sending down a nice core every time?
Two of the corers always work together. One is called a piston core, and the other is called a trigger core. The trigger core is smaller, and when it hits the bottom it triggers the larger piston core to drop. The trigger core ends up getting a sample of the upper layer of sediments (a couple of feet) and the piston core gets a little deeper (6-7 feet). All of the samples that we are bringing up contain mud that was under the ice sheet, mud that wasn't under the ice sheet, and/or bits of sand and debris.
After we bring those cores back up to the ship, they are capped at both ends and saved in a walk-in cooler. These cores will be shipped back to the U.S. to be analyzed later. They look like long PVC pipes with red caps on the end! We also have to make sure that they are labeled correctly so that people will know later exactly where the core was taken and which direction is up or down.
The other type of corer is called a kasten core. It's larger, like the piston core, but it doesn't need a trigger. In addition, the box that the sediments come in is long and square rather than long and round. This corer drops free-fall to the ocean bottom and gently drops down through the sediments. Once it is pulled to the surface, the sediments from this core are analyzed immediately. On one side of the square are hinged doors that we can open to look at the layers of sediments.
The other new instrument that we began using today uses sound waves to determine the large scale vertical profile of the sediments. We call it shooting seismics. Behind the ship, we tow a huge "air gun" and a long rope (called a streamer). The "air gun" shoots out a large bubble of air which creates a compressional wave. When this happens, a loud "pop" is created and those waves travel down to the ocean floor. Some waves bounce off the surface of the sediments, but others bounce off specific layers of sediments beneath the surface. The streamer has things called transducers (we call them phones) in it that listen for the return of the sound waves and record how long they take to get there. In the computer room, we have a computer monitor that records this information and we have a printer that makes a printout of what the seafloor (and the layers beneath) look like. The scientists that I am with am specifically looking at the top 250 meters of sediments with this instrument (although it penetrates much deeper). Other scientists may use much larger guns to penetrate all the way down to the earth's mantle!
Looking at yesterday's questions -- Why do you suppose we walk around and check our equipment every 15 minutes instead of once each hour? That way, if something goes wrong we don't loose too much data. We actually have a "log" that we write in every 15 minutes. We record our speed and direction, our latitude and longitude, and the depth of the water. In addition, we record the water temperature and write down exactly what equipment we are using (cores, seismic, etc.). We also keep track of exactly when things like cores and seismic go in the water and when they come back out. That way, when the scientists are analyzing the data after they get home they know exactly where we were when the data was collected. We have a few extra jobs each hour -- like plotting our latitude/longitude on a map and saving the data on the computers to some sort of disk. There are many precautions taken so that the data will be able to be used after we get back home.
If you have any questions, don't hesitate to write. In addition, I'll be sending out another sent of pictures in a couple of days. Is there anything in parcticular that you would like to see a picture of? Thanks to all of you for keep in touch! Until tomorrow . . .
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