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.