Explosive science

Detonating explosives is serious business. It requires high level of technical prowess and a healthy dose of focus, as explosives are rarely forgiving. In the old days people in the explosion business were often missing a finger or two (or worse), and there was a reason for that...

The experiment Dan, Steve and I are carrying out here, along the east coast of the US, is a classic example of "active source" seismology in which seismic waves are artificially generated (hence the name "active source") and recorded by numerous (700+ in our case) devices (the "Texans" RT125 digitizers + sensors)  deployed in a nearby region (often along a 2D line but not only). As the seismic waves propagate in the subsurface, they travel through the layers at the speed of the rocks that make up the structures in the crust and upper mantle, and they slow down or speed up, depending on those properties. To the eyes of a seismologist the arrival time and shape of the waves, as they come to the surface and are detected by the seismometers, are diagnostic of the geometry and speed of the layers they traveled through. Using this information seismologists can model the wave speed structure of the subsurface (whether a basin, or the whole crust and mantle down to a depth of few tens of km). In the case of our project this information is used to interpret the tectonic evolution of the passive margin and of the opening of the Atlantic Ocean.

The Incorporated Research Institutions for Seismology (IRIS) provides scientists who conduct research that requires imaging of the crust and upper mantle of any region of our planet using this technique with a National Seismic Source Facility (NSSF). The facility is hosted at UTEP and is run by Steve Harder with the assistance of Galen Kaip and of several students. The source facility team is often invisible in active source experiments, as they come to the field before the rest of the science team to drill the shot holes and load them with the explosives, and once the science team arrives and the deployment of the instruments starts, they do their job (firing the shots) in the darkness of the night, when the rest of the world is sleeping and the land is quiet (anthropogenic noise is the archenemy of the active source seismologist). So, despite their fundamental role in the experiment (they generate the waves we use to illuminate and "see" the subsurface), they are rarely in the spotlight of the scientific efforts, and have therefore earned the nickname of "Inglorious Blasters".

The source team (a.k.a. the Inglorious Blasters). From left to right: Steve Harder, Ashley Nauer, Galen Kaip, Afshin Gholamy, Felix Ziwu.

The ENAM land experiment fired 11 shots along the two lines, 6 along Line 2 (southern line) and 5 along Line 1 (northern line).

The shots are usually fired in one night, by two teams who move independently along the line and are assigned different firing times within the recording interval. Team 1 can fire at :00, :10, :20 min etc to the hour, while Team 2 can fire at :05, :15, :25 min etc to the hour, so that there is never the risk of the two teams firing at the same time. Things are so well coordinated that the two teams rarely communicate throughout the night.

To see what happens when shots are fired, I join Team 2 on shooting night along one of the two ENAM lines.

Shooting team 2 ready for action!

The plan for shooting the line is to start firing at 00:00AM. The first shot will be fired at the western end of the line by Team 1. We will drive to the opposite end of the line and will fire our shot immediately afterward at 00:05AM (if everything goes well). If not, our next opportunity will be 00:15AM. Once we fire our shot, we will pack our gear and move to the next one (about 1hr away) and fire again within our window. And so one until all shots along the line are fired. It's gonna be a long night, but not a chance of getting sleepy!

Before heading to the first shotpoint, we pick up Ken Taylor, State Geologist of North Carolina and two other colleagues who will observe the operations. Ken has been instrumental in the shot point permitting process in North Carolina.

If you expect a shot point to look special on the ground you are definitely under the wrong impression. This is pretty much what a shotpoint looks like as we drive up to it at night.

Not even a sign with ENAM SHOT POINT 21???

 If you look carefully however, you will find wires coming out of the ground....

Detonators connected to the charges at depth.

The orange wires are the detonators attached to the two tubes filled with 200 lb of emulsion (liquid explosive) each that were loaded in the hole at a depth of about 15-20 m (see "Shotpoint Operations along the ENAM profiles" post). Most of the job of firing the shots consists of "communicating" with the charges at depth via these cables, to make sure all is OK  to detonate. We follow four steps:

1) We check the detonators to test that they are working (for leakage of current in the detonator if the wires are broken or stripped)

2) We tag the detonators to the shot hole (so that the shooting system knows that they belong to this hole)

Galen tags the detonators to the shotpoint

3) We then connect the detonators to the Seismic Interface Unit (SIU) and make sure the microchips in the detonators are working

Galen connects the detonators to the Seismic Unit to check the microchips

4) We finally arm the the detonators in the SIU and then we arm them again 5 seconds before detonation through the shot box.

The SIU (Seismic Interface Unit). The "shotbox" for the seismologists

These multiple steps ensure that nobody can set off the explosions accidentally by connecting the cables to an electric charge.

Once everything is checked and we are ready to fire, we stand back at a safe distance and a few huddle around the shotbox. The shot is armed about 5 seconds before fire and fired on GPS time. One minute before firing the countdown begins...

As the water table is very shallow, the shot generates a tall geyser. The walls of the borehole hold, and the shot leaves a deep hole. We can hear the low sound of water gurgling in the hole, filling the space left by the explosion at depth.

Hole immediately after the explosion.

The site is cordoned off, for safety. During the next hours it will likely collapse and leave a crater.  The remediation of the shot sites is one of the many activities tasked to the source team.

Shot point closed off for safety after firing.

Tomorrow the source team will come and fill up the crater with sand and gravel and bring the site back to stable and safe conditions. For now we pack up our gear and move to the next shot point. We work along the line all night, setting up, firing and moving on until all charges are fired. It's dawn when we make it back to the hotel. By then the crews are getting ready to head out and start retrieving the seismometers that have recorded the shots, and soon we'll get to see some data. But for the source team it's time to go to bed. G'night.

ENAM Instrument Support “Wait” Day

Eric and I are from the IRIS/PASSCAL Instrument Center (PIC) located on the campus of New Mexico Tech in Socorro, New Mexico. IRIS/PASSCAL is funded through the National Science Foundation (NSF) and the PIC is basically a lending library of seismological equipment for NSF funded researchers. In addition to providing equipment for research experiments, we also provide user training as well as field support for the equipment, which brings Eric and I to North Carolina.

Supporting the equipment on a large experiment like ENAM is a bit of “hurry-up and wait” for us. We program all the recorders, RT125A “Texan” dataloggers, with the gains, sample rates, and times to start and stop recording data and help the deployment teams get their equipment loaded in their vehicles and sent off in the mornings, that’s the hurry-up part. But then there is the wait until the equipment is picked back up and returned, after the shots, when we offload the data (another hurry-up part). 

Early morning, deployment day. The ENAM instrument center is located at an old VOA radio station so there are large antennas all around the site

“Bix” Magnani, one of the PI’s, on the left, and Eric Makarewicz on the right, helping Jonathan Ward, of Deployment Team 6, get their boxes of equipment.

Loading up a vehicle under another antenna.

The equipment is packed and ready to go…

… and Team 6, Kameron Ortiz and Jonathan Ward, is ready to go too!

 During the wait part though, if possible, we try to explore the local area a little bit.  When Bix and Dan Lizarralde, one of the other PI’s, did not need us for the rest of the day Eric and I headed out on our adventure.

Passing more antennas as we head out.

 

Eric had not seen the Atlantic Ocean yet, so we headed for Cape Hatteras.  The trees and greenery of North Carolina is a huge switch from our normal brown, red and sandy New Mexico scenery. We enjoyed seeing large swaths of daylilies and sunflowers along the road, but we were really struck by the open expanse of water when we crossed the drawbridge over the Alligator River. Being used to dry New Mexico, crossing water as far as we could see, with tiny sailboats dotting the horizon, was a real treat.   

Water! Crossing the drawbridge over Alligator River.

Following the recommendations of the Roanoke Island Visitor Center’s guide, upon reaching the seashore we made our first stop at the Bodie Island lighthouse.  The lighthouse was built in 1872 and has it’s original Fresnel lens. There used to be two other lighthouses further south but due to the changing shoreline those locations are now out at sea. The lighthouse was opened to the public in 2013 so Eric and I were able to climb the 200 plus steps of the metal spiral staircase, about 10 stories to the top, and see the great view with the bonus of a cool ocean breeze (it was hot inside the lighthouse while climbing the stairs, and in fact it was closed to climbers later in the day because it was to hot). Talking with one ranger, he told us he climbs the stairs about 24 times a day! While climbing the stairs, we speculated what it would have been like to build the staircase. Each metal step was bolted to the next and to slender metal posts coming down from the spiral handrail. I don’t know if it was, but the whole thing appeared to be free standing inside the tower.  We imagined the builders attaching the steps one to the next, slowly moving up one step at a time, standing on the step they’d just bolted on in order to install the next step, with nothing but increasingly higher empty space in front of them.

 

Bodie Island lighthouse.

Eric on the observation platform at the top of Bodie Island Light with the Atlantic Ocean behind him.

Looking down the spiral staircase inside the lighthouse.

From the lighthouse, we went across the road to Coquina beach.  We took our shoes off and climbed over the sand dunes to the water.  Nearing the top of the dune some dark sand mixed in with the light colored sand we’d started out in and the sand got quite hot. Our walk over the dunes turned into a dash down to lighter colored sand again where we threw ourselves down to get our blistered toes (really!) off the ground for a few minutes before continuing and finally cooling our feet in the ocean.

Eric’s feet, in the Atlantic Ocean for the first time, at Coquina Beach.

 

We became beachcombers for a while, rolling up our pants legs, walking along the tide line turning over washed up shells.  We were entertained by the sand crabs, peeping out of their holes, creeping up to the water’s edge, and then scurrying back as the surf washed in. We saw dolphins, sea birds, people fishing in the surf, and other people flying kites on the beach.

 

Sand crab on Coquina Beach, …

a dolphin (not shark), …

… kite flying over the beach.

The kites flying over the beach were a good introduction to our next stop, the Wright Brothers National Memorial at Kill Devil Hills. The excellent visitor center there walked us through Orville and Wilbur’s years of experiments leading to finally successful airplane flight, with letters, and excerpts from their notebooks, tools they used, photographs, and actual pieces of propellers and engine blocks. It was fascinating and really gave us a sense of their efforts and the magnitude of what they accomplished.  “They took what was just an idea and made it real”, as Eric put it.  When we saw the full size model of the plane, how fragile it was (the wing was so light and thin that to turn they used cables to pull on the end of the wing and actually deform its shape), and how exposed the pilot was, Eric said he didn’t think he would have been willing to donate his body to science like they did. We learned that Wilbur and Orville flipped a coin to see who would try the first run.

A model of the Wright Flyer at the Wright Brothers National Memorial at Kill Devil Hills.

The only things to hold the pilot on the plane were a couple of small wooden uprights, one on either side of the pilot’s body as he lay on the wing next to the engine, and a wooden strip to brace his feet against. The brothers had to figure out how to control the plane’s motion through the air, the yaw, pitch, and roll of the plane. They didn’t know if it would work. If it did, they didn’t know how high the plane would go. It was freezing cold in December with a 20 plus mile an hour wind blasting them with blowing sand. They tossed their coin in the air and then Wilbur laid himself across the wing between the little uprights – and took off!  

From the visitor center we walked across the now grassy area where the first flights took place and up to the top of Kill Devil Hill where a memorial tower stands.

"In commemoration of the conquest of the air by the brothers Wilbur and Orville Wright conceived by genius achieved by dauntless resolution and unconquerable faith." – Inscription around the base of the Wright Brothers Memorial Tower on Kill Devil Hill. The tower is 60 feet high.

 

I was curious about the history of the names for some of the places where we were, “Kill Devil Hills”, “Nags Head”, but I kept forgetting to ask local people that we met about it. Eric looked up Kill Devil Hills when we got back. Apparently, in the pirate and buccaneer times that the Outer Banks are famous for, the rum that was always on board the sailing ships was not very smooth and was referred to as “Kill Devil”. When a ship would run aground, local scavengers would loot the ships and frequently hide the stolen goods, including the rum, in the big sand dunes. These rum filled sand hills then became known as Kill Devil Hills. The story of Nags Head, again from that time, is that the locals would tie a lantern to a gentle horse, a “nag”, and walk it slowly back and forth along the shore at night. Ships at sea would think it was the light from a ship safely at anchor and head in towards it only to run aground on the shoals, thus providing more barrels of rum to stash in the sand dunes!

Eric and I had a nice late lunch at a local spot in Nags Head and then heading back inland, making one last stop in Roanoke. There we visited Fort Raleigh, which is where “The Lost Colony” was lost. In 1584, with England in competition with Spain in the New World, Sir Walter Raleigh led an expedition to find a place for a settlement, came across Roanoke Island and thought it looked pretty good. A first attempt at colonization in 1585 failed though, with the colonists basically hitching a ride back to England when Sir Francis Drake happened to sail by, but a second group of colonists arrived in 1587. They were headed to the Chesapeake Bay area and only intended Roanoke to be a brief stopping point, but their ship’s pilot refused to go any further, so they were stuck. They had increasing misunderstandings and hostilities with the local Indians, and their food was dwindling, so they sent their leader, John White, back to England on another passing ship to get them more supplies. Unfortunately England and Spain went to war just then and no ships could be spared for his return. John’s daughter, son-in-law, and granddaughter, “the first English citizen born in America”, were left in Roanoke though so he didn’t give up and he finally made it back three years later – only to find all the colonists gone with just the word “CROATOAN” carved on a tree. Croatoan is another outer bank island not too far south from Roanoke, 65 miles by car, and less by ship. We were told that John White tried to go on to Croatoan to look for his family and the other colonists “but a storm forced him to return to England”. And with that short statement the “The Lost Colony” disappeared into history and their fate is a mystery to this day. Our conclusion was, it was really quite easy to become lost back then. We also went by the waterfront in Manteo to look at the Elizabeth II, a sailing ship that represents those like the colonists traveled to North Carolina in, from England, in the 1580’s.

The earthen works fortification at Fort Raleigh, built by the 1585 expedition to Roanoke. We estimated it would be just about standing room only if all ~110 colonists were inside.

  

The “Elizabeth II”, at Roanoke Island Festival Park, represents a 16^th century merchant ship. At 69 feet in length, imagine sailing for three months from England to America on a ship like this with over a hundred other colonists plus the crew.

Eric and I had a great day exploring the coast of North Carolina. Back at the equipment center the next day, the shots had gone off well over night, so we were in hurry-up mode again offloading data. I thought our adventure was over, but when Team 6 brought their instruments back in for offloading, Jonathan said that Eric and I, connecting the hubs to the Texans, looked like the villains in a James Bond movie working on our evil plans for global mayhem!

“Texan” dataloggers offloading (aka SPECTRE tools for world domination?)

(Post by Bridget O'Neill)