SUGAR: SUwanee Suture and GA Rift basin experiment
The rocks beneath the coastal plain of Georgia were at the center of the most fundamental tectonic events to shape eastern North America: continental collision around 290 million years ago to form the super continent of Pangea; continental breakup leading to the formation of the Atlantic Ocean beginning around 230 million years ago; and one of the biggest magmatic events in Earth’s history around 200 million years ago, the Central Atlantic Magmatic Province. A record of these events and possible relationships between them is preserved by structures in the crust of southern Georgia, including a suture between two different types of continent, the largest failed rift basin along the east coast of North America and igneous rocks from the Central Atlantic Magmatic Province. We will collect seismic refraction data, which can be used to image structures in the crust to understand these tectonic events. During March 2014, over 1000 geophones will be deployed along a ~300-km-long profile across the suture and the basin, which will record sound waves generated by a series of controlled blasts spaced ~20 km apart. The speed that sound waves travel through rocks varies with rock type. We will use these data to create velocity models that reveal the distribution of igneous rocks, variations in the thickness of the crust and variations in crustal composition. Besides a better understanding of fundamental tectonic processes, other benefits of this program include training and education of students, and characterization of basins and igneous rocks that might be good targets for carbon sequestration.
If you want to image the Earth’s crust and upper mantle with seismic data, you need to record the arrival of seismic waves that have propagated down to, in our case, depths of up to ~30 km.
These deep-diving phases travel quickly through the denser, higher velocity rocks of the lower crust and upper mantle, and they arrive back at the surface ahead of shallower phases at long source-receiver offsets (see video below).
To record these lower-crustal and upper-mantle phases as “first arrivals”, where they are not obscured by the arrival of energy from shallow paths, we use long lines. Long lines mean lots of receivers
and lots of driving to deploy and recover these instruments. We could have used lots of sources instead, but the blasts we used to get seismic energy into the lower crust and upper mantle in this experiment take a lot of time and money to setup
. Receivers are much cheaper, so we used a lot of them. (For similar wide-angle/long-offset work at sea, airgun sources are cheaper than putting seismometers on the seafloor
, so we use many shots and a smaller number of receivers out there.)
This time-lapse video shows Team 13 of 14 recovering 89 of the 1200 total short-period seismograph stations from where our line crossed Fort Benning, near the northwestern end of the line.
|Weather map during deployment. |
When the time came to install our 1200 small seismographs across Georgia at the flagged positions
, the rains came….
A lot of rain.
During our first deployment day, we received 1-2 inches of rain, and another wave of rain clouds came through on Day 2 (check out map). Roads that used to be easily passable became mudholes or were flooded with water. All-wheel-drive vehicles and drill rigs alike got stuck, and a few station locations could only be reached on foot. Our hard-working field crew labored in the rain
digging holes and deploying seismometers.
Vehicles, equipment and people were covered in the famous Georgia red clay (and other muds and sands of Georgia and northernmost Florida). Adding insult to injury, problems with the programming of some of the instruments meant that we actually had to pick up and redeploy many of them. It was a mudbath.
Nonetheless, our field crew managed to deploy 1200 seismometers across Georgia by Tuesday at sundown. It was an impressive show of endurance, and an inspiring display of positivity given the number of people that were still smiling and upbeat at the end of it all.
A couple of days later, after our seismic shots, it was already time to pick up the instruments, and the weather changed completely. The sun shined on SW Georgia, and we picked up almost every last seismometer in just one day under blue skies….
Donna Shillington, LDEO
Shooting a land refraction experiment is more difficult in almost every way than collecting a comparable dataset at sea. Far more difficult. But I can't think of anything at sea that compares to the experience of setting off a series of shots at night. On the first night of shooting, Steve, Nathan, Meghan and I detonated shots L1-05, 06, 07 and 08, while Galen, Donna and Natalie shot 14, 13, 11, and 10, and Tina, Adrian, James and Semir shot L1-04. I recorded the video clips linked below at our shots (05-08).
To someone who hasn't seen a seismic source shot before, there really isn't a good way to describe what a good shot feels like, except as something you haven't felt before. We had a number of students watching L1-05 being shot, since this location is quite close to Americus. The video of L1-05 is completely lacking in drama, which is a good thing; but that shot gave us all a great ride. The 100 pounders 06 and 07 were also surprisingly good.
We made gathers for most of the shots today. The dataset is fantastic, and 05, 06 and 07 produced super record sections. L1-08 committed most if its energy to the air, but it shook the ground nicely and I've got a feeling those data are going to be great too.
As a follow follow up to Chastity's post, I thought a few random pictures from the road would be entertaining. I have been part of group 5 and as such responsible for the part of the line that spans from Hahira in the south to just north of Adel.
|South-central part of the seismic line. The yellow line is team 5's section. |
We have been in a relatively rural part of Georgia and as a result have not encountered many locals save a few who have stopped to ask if we are ok. However, we have seen quite a few interesting things that are quite out of the ordinary (to me at least).
|Friendly Muscovy duck.|
|Rocks in a stream bed with associated pink spongy material (?)|
|Linguoid (current) ripples on a washed out road.|
We have also seen quite a few old abandoned farm houses in various stages of aging...
|At least 10-15 dogs were standing guard at this house, including about 8 puppies. |
|Caroline making some new friends.|
All said we have dug 122 holes in team 5's stretch. We have also helped deploy instruments in other sections as well and while doing so have seen others hard at work.
|Meghan and Nate getting it done!|
Along the way the cars have taken quite a beating and have actually held up pretty well. Although there have been a few instances where people got stuck, I think that the people with the toughest job will be the guys that have to detail the cars upon their return...
|A more appropriate vehicle (?)|
And lastly here's a couple more random pictures that I thought were interesting.
|The large disparity in fuel grade gas prices.|
|A ~perfectly leveled geophone (it's harder than you'd think).|
Hopefully this random selection of pictures was entertaining. Up next we will post about last night's "shots." In the meantime, I can say that they were all successful with varying degrees of excitement. The most important thing is that all of our hard work is being realized as the instruments are recording refractions from buried geology that will help us unravel some of the mystery that surrounds events that happened in this area long ago.
James Gibson, LDEO
Our first day of deployment did not go as planned. Inclement weather slightly dampened our spirits, but it’s amazing how omni-dry pants and waterproof boots can combat that! Our second day was still wet, but the rain was not nearly as torrential. The good news was that the heavy rain had softened many of the sites so it was not as difficult to dig. In fact, many of the deployment teams were able to bury over half of their geophones in that second day.
Deployment, like surveying, has its own interesting encounters - from curious neighbors to farm animals and reptiles.
Curious neighbors have both shared their enthusiasm for the project as well as offered their services due to our flashing hazard lights when we pulled over to deploy.
Cows also seem to be very interested in our activities a
s many times they have curiously walked over and stared intently in our direction while we work, all the while “mooing” as if alarming the others.
We also have seen many calves and kids suckling, which yielded many “awwwws.”
The scariest encounter is most likely that of snakes.
They are sneaky, as they hide in the brush!
Deployment is tough work. By the end of the day, we are tired from digging and navigating on soggy and muddy dirt roads. After returning back to home base (the gym), we get together for dinner and enjoy a few laughs and tears (happy ones, of course!). It is this camaraderie that makes the SUGAR project all the more better because of the camaraderie.
Georgia Institute of Technology
Many of the SUGAR field team arrived in Americus, GA on Wednesday to start helping with the massive charge of deploying 1200 seismic instruments along the SUGAR seismic line. The seismic line spans 200 miles from northwest Georgia to just past the Georgia-Florida border; a 4+ hour car drive from end to end! Everyone gathered early Thursday morning on the idyllic Georgia Southwestern State campus to meet with the chief scientists and learn about the proper techniques for identifying installation sites for the seismographs (just the first step in installing the instruments). With neon orange safety jackets, numerous maps, GPS devices, packets of official permitting documents, and heads full of safety precautions the field team split into seven two-person pairs each equipped with their own squeaky clean rental car (though they didn’t stay clean for very long!).
|The fleet of SUGAR rental cars looking clean and shiny before being driven|
into the field where they undoubtedly got a little mud on their tires.
Each pair of field assistants was given a segment of the seismic line to drive and flag locations for instrument installation deemed safe both from the seismograph (i.e. dry, firm soil) and the install team (i.e. a safe distance from the road). Given the shear distance of the seismic line, teams found themselves amid diverse backdrops from rolling farmland with overly friendly cows to buzzing residential neighborhoods to sandy stretches flanked by towering groves of Ponderosa Pine trees.
|Antonio placing a flag and using a GPS device to note the location where a|
seismograph will be installed amid the sandy surroundings of a Ponderosa Pine farm.
Every team was able to flag all their sites within just two days leaving us the luxury of a sunny Saturday morning free for exploring more of our beautiful Georgia surroundings. Next up is the actual task of installing the 1200 seismographs which will involve twice the people, six more (temporarily clean) vehicles, and of course countless exciting adventures from the field. Happy (almost) St. Patrick’s Day from Americus!
|A picturesque county road near Jasper, FL along which instruments will be deployed.|
-- Natalie Accardo, LDEO
The source of sound waves for the SUGAR experiment will be a series of controlled blasts along the profile. For each of these, we drill a 60-100 ft deep hole, place emulsion explosives with boosters and caps at the base of the hole, and fill in the rest of the hole with dirt and gravel. Each seismic source location requires a substantial amount of work by drillers and the UTEP seismic source team. Below, Adrian Gutierrez shows a day in the life of the source team with pictures (Donna Shillington, 13 March 2014)
Adrian Gutierrez, 13 March 14
|7:30 am: Leave Georgia Southwestern State University, where we are staying, and head to the site|
|8:20 am: Arrive at site |
|8:30 am: Start drilling and take geological samples every 5 ft.|
|9:00 am: Dyno Nobel truck arrives; load emulsion into cut PVC pipe sections that serve as a holders for emulsion. |
|Boxes with seismographs and other equipment|
Fortunately, we have a lot of space! Our field headquarters is located in a historic gym on the campus of Georgia Southwestern State University
in Americus, GA. Faculty and staff at GSW have been extraordinarily generous with their time and expertise. They are allowing us to use the Florrie Chappel gym as our base of operations, and they have helped us enormously with Georgia geology and logistics coordination, handling our huge shipment of equipment and supplies, housing on the campus (many of us are staying in one of the dorms!), setting up the gym with internet access, power, and tables, and much, much more. Today, they moved all of the boxes with our seismic equipment from the shipping warehouse to our field headquarters in the gym. I can sense that all of our seismic instruments are itching to be deployed....
|Pallets waiting outside the Florrie Chappell gym|
11 March 2014
|A geophone in grass, awaiting sound waves.|
To prepare for our big seismic experiment, we have collected a couple of miniature seismic datasets. The shallow geology varies substantially along our profile and is very important for planning the depth of drilling and size of our seismic sources. In particular, we need to determine the depth to a limestone layer in a few places. The same seismic method that we will use to understand the deep geological structures beneath Georgia can also be used at a smaller scale to examine layering in the upper ~100 ft (~30 m) beneath the surface. We recorded the data on 24 geophones attached to a 230-ft-long (70-m-long) cable. The source was a modified shot gun that looks like a pogo stick. We drilled small holes in the ground, loaded the gun and stuck it in the hole. To limit the kickback, we weighted the gun down with a metal plate topped by two heavy jugs filled with sand. Hit the plate with a mallet and – BANG – a seismic source! Not a bad way to spend a sunny Sunday!
Check out Dan firing the seismic source...
9 March 14
We have just arrived in sunny Americus, GA from the cold north to ramp up for the SUGAR project. The peaceful, pastoral landscapes of southern Georgia mask geological structures created by a series of dramatic events that were central to the formation of the North American continent. During SUGAR, we will use sound waves to image these geological structures. Less than 2 weeks from now, we’ll deploy 1200 small seismographs along a 200-mile-long line that extends from north of Columbus to south of Valdosta with the help of a cadre of students from across Georgia and beyond. These instruments will record sound waves generated by a series of controlled blasts in deep drill holes.
|Spanish moss lined trees along our transect south of Valdosta|
Collecting these data will involve a week of intense work by >30 people. However, just laying the groundwork for this effort has already required a long list of (sometimes novel) tasks. When we conceived of this project, we drew a couple of straight lines on a map that would enable us to capture the geological features that we wish to study: the South Georgia Basin, the Suwanne Suture, and frozen magmas from the huge Central Atlantic Magmatic Province. In reality, we must create this line by knitting together a patchwork of roads. During a couple of planning trips, we bumped along on dirt roads, cruised county lanes, and zoomed down state highways mapping out the best route.
Our seismometers will line county and state roads across southwestern Georgia, and both seismometers and seismic sources will cross private properties. Identifying private landowners to request permission has transformed us into detectives. In most cases, the name and address of the owner are easily found on the tax assessor's website for each county. But actually getting in touch with people is not so easy! We mailed letters. We put flyers directly into people’s mailboxes. We searched for phone numbers online and left messages (sometimes multiple messages…). We found websites and email addresses for companies, and sometimes wrote to people about our project through website forms (including those for a bank, a dentist's office and a website selling organic beef!). Happily, once we made contact, individuals and companies have been very welcoming and graciously granted us permission – southern hospitality in action! A litany of other preparations have already been completed or are currently underway. Drilling of the holes for seismic sources has just begun, and the seismometers will arrive very soon. We are definitely ready for the transition from preparing to doing....
|Dan and Steve scouting our route.|
5 March 2014