Coping With Cores
|Drilling unit at night.
Credit: Aaron Gronstal
Every hundred thousand years or so, a large asteroid or comet hits the Earth. The explosive force of the impact sends tons of material into the air and punches a big hole in the crust. We don’t see a lot of these old craters, however, because Earth is a dynamic environment — wind, water, and tectonic plate shuffling constantly changes the appearance of the surface. Over time, many impact craters become altered or buried deep underground.
One such buried crater is underneath Chesapeake Bay in Virginia. Carved out 35 million years ago, the crater now can only be accessed by intensive drilling. In November, Aaron Gronstal, a microbiology student at the Open University in the United Kingdom, visited a drilling project at the Chesapeake crater. His goal: to discover what sort of microbes may now live deep underground in the ancient impact environment. This portion of his journal is part 3 of a 4-part series.
Crater Diary, Part 3
Coping With Cores and Contamination
November 17, 2005
The storm passed, leaving behind clear skies and brisk air this morning. Our arrival at the drill site was greeted with the news that the night shift had drilled past the 5,000-foot mark. Upon reaching this depth there is a noticeable difference in the texture of the samples coming up. The packed material is a bit more crumbly to the touch, making my task of collecting samples for the biology experiments much easier.
The cores being drawn up now are larger than before, each bringing up about 20 feet or so of material, and progress downward into the crater is going well. There is still hope that we’ll reach a mile before Thanksgiving.
The noise from the drilling is continuous, a loud incessant drone that large machinery tends to make. It isn’t so loud that we need to yell to be heard, but it does leave your ears humming a bit when you lay down to sleep after a day at the site. You get used to it quickly. I can always tell when someone is entering or leaving the trailer – when they open the door there’s a definite change in the drill’s volume.
November 18, 2005
|Strapping on latex gloves to sample the graphitic schist.
Credit: Aaron Gronstal
Progress toward the basement of the crater has been slow today, but at least the small cores are interesting. The material is very crumbly and graphite-like. The geologists say that it is metamorphic rock composed mostly of quartz and mica, and they are referring to the material as a ‘graphitic schist’. To my eyes, the cores look like big pieces of pencil lead. This created a messy situation for the geologists, whose hands were quickly coated in the shiny black powder. Luckily, I had plenty of latex gloves.
The maximum length a core can be depends on the size of the barrel they send down to collect it, but the size of a core also depends on the rock consistency and strength. Usually they have to pull up a core when the drill barrel starts getting clogged. As rock and sediment is forced into the drill barrel, it can get wedged in and prevent the drill from going down any further. This is happening now as we’re digging through the graphitic schist.
Due to the consistency of the material, the cores keep breaking into small pieces and are difficult to bring up from the drill hole. Their crumbly, porous nature could also make uncontaminated samples for the biology experiments difficult to come by.
A great deal of the biological sampling is monitoring the contamination level of the core. A good sample is a solid core that doesn’t have any cracks. Because there’s a lot of mud and water circulating through the drill, seepage in through any cracks in a core would contaminate the sample. We put things like tiny florescent beads and halon gas in with the drilling mud and then check to see if it appears on the inside of the core. In the trailer where we have the biology lab set up, the core is then cracked open and samples are taken from the (hopefully uncontaminated) interior.
|Gathering biological samples from core rock.
Credit: Aaron Gronstal
The sample is also no good if it’s too solid. When the team was drilling through the huge block of granite, no biology samples could be taken because there was no way to scrape out small samples of material from the granite into vials. Granite also doesn’t have much in the way of small spaces and holes in which bacteria can live.
This evening there was a stunning sunset over the bay. Everyone brought out their cameras, and for a short while, standing in the puddles of core mud, under the pounding drone of the drilling rig, everyone caught their breath as they stared off toward the western horizon. All that was missing were a few well-placed candles and a bottle of Merlot…
The night ended early for the scientists, since the drill bit had to be changed. This process takes many hours because the drillers have to bring up the entire length of pipe that reaches down into the hole. So we spent the last portion of the shift cleaning, reading books, and simply holding down the fort until the night shift arrived to take over. It also means that we are currently stuck at around 5,105 feet.
But at least the funding situation seems to have been sorted out, so the drilling will continue through the next couple of weeks. The promise of reaching the crater floor is still alive, and the mile deep mark is definitely within reach.
November 19, 2005
The day started with a farewell breakfast for two of the geologists, who are now headed back to their home in Texas. It was my first experience with a regional breakfast delicacy known as ‘scrapple.’ When asked what the thick patty of fried, spongy, gray material was, the waitress shot me a look that said, "You don’t want to know." As it turns out, she was right. Scrapple is leftover pig odds and ends, mixed with cornmeal and spices and then fried. The taste wasn’t so bad — it just tasted ‘fried.’ It was the consistency that made it hard to swallow.
When we returned to the drill site after breakfast, the reports from the night shift crew were dim. One unlucky driller had spent six hours perched atop the drilling rig as the bit was being changed. Unfortunately for him, it was the coldest night of the project thus far. But at least now he can lay claim to some good bragging rights.
Only one small core had been brought up since changing the drill bit. Members of the night shift, left with nothing to do, spent some of their shift sleeping on the floor of the trailer.
|Clusters of core sample vials.
Credit: Aaron Gronstal
Our noon-to-midnight shift was slow as well. Numerous cores were brought to the surface but most of them were in the range of three to five feet, making progress toward the crater floor slow.
More of the staff left later in the day — two geologists and a physicist — to enjoy Thanksgiving turkeys with their respective families and friends. Their replacements toured the site, and will be starting work with the night shift at midnight. The turnaround of scientists coming and going has been fairly rapid, making it difficult to keep track of names, but at least there’s always a new face to meet.
Towards the end of the night larger cores were being brought up, and the geologists’ eyes are beginning to gleam with hope. Memories of the 20-foot cores of days past are starting to bubble to the surface.
November 20, 2005
The drill remained in the graphitic schist for most of the day, and hours of drilling only brought a few feet of rock to the surface. Added to the schist are a few veins of hard quartz that have further slowed the drilling process. By the end of the day, however, we finally moved out of the schist, and at a depth of around 5,200 feet the cores started reaching lengths greater than five feet. Everyone is hopeful that we’ll be able to dig more quickly now.
The weather remains nice, so much time has been spent with a cup of coffee in hand watching the deer in the fields, or dodging wild turkey with the pickup when making a food run. Thanksgiving is only days away, and it seems the turkeys in this part of the country are still searching for dinner parties to join.