The explosive material that’s being used to create “active seismic” activity around Mount St. Helens is a gel; it is pumped into the 80-foot-deep boreholes.
WOODLAND — When they set out to help chart the inaccessible regions far below a volcano, Nick Lock was swinging a pick and Leah Sabbeth was holding a GPS unit.
Lock didn’t have to dig very far Tuesday morning: just deep enough to bury a seismic instrument about the size of a water bottle.
Sabbeth didn’t have a tough navigational challenge: Her job was to mark the point where the package was planted.
And when up to a ton of explosives is detonated tonight, a geophone will read the tremor and feed the information to a cannister.
It will be a little bit of data about a very big subterranean system below Mount St. Helens, maybe the equivalent of one letter printed on one page.
But that process will be going on at more than 7,000 sensor sites — some near Vancouver — during the course of 23 separate explosions: That’s a lot of letters.
“If we put enough pages together,” Lock pointed out, “we’ll have a novel.”
The topic of this magma opus is the plumbing system under the volcano. The project is titled “Imaging Magma Under St. Helens,” or IMUSH. Researchers hope what they learn over the next couple of years will improve their ability to forecast volcanic eruptions.
IMUSH researchers are using three types of geophysical monitoring to create the images. Tuesday’s instrument installations were part of the active seismic phase: It’s called active because the scientists will be using explosives to create the seismic activity. Twenty-three boreholes have been drilled in a precise pattern in remote locations around the mountain; each borehole — 80 feet or so deep — is loaded with a 1,000-pound or 2,000-pound explosive charge. Lock and Sabbeth were among the volunteers who set out about 3,500 small sensors the last few days.
Researchers will begin the detonations tonight after a slight change of timetable. The first explosions were originally scheduled for Tuesday night and early this morning, but plans were set back a day. Another round of “shots” will be detonated in about a week, after the 3,500 seismic instruments have been repositioned — doubling the number of monitoring sites.
Alan Levander, the lead scientist for the experiment, compared the process with medical imagery — a CAT scan that “beams X-rays through you and out the other side,” he said. It’s done again and again and again, from different angles.
In this case, the imagery is created with seismic waves generated from different angles and depths, said Levander, a professor of earth science at Rice University in Texas.
A blast will be the equivalent of no more than a Magnitude 2 earthquake, and the area around the peak gets one of those a week, Levander said.
“We will know the time of the explosions,” said Eric Kiser, a post-doctoral researcher at Rice University. “We will know when they were recorded. We will use that travel time data to tell how fast or slow the waves are moving.
“Slow means warm material in the magmatic reservoir,” Kiser said Tuesday morning in the Woodland Intermediate School gym, a staging area and data hub for the project.
Each explosion will be logged by those 3,500 sensors arrayed within a circle that stretches from the Portland-Vancouver area to Mount Rainier — a diameter of more than 90 miles. Mount St. Helens, which is about 45 miles northeast of Vancouver, is in the middle of that circle.
The wider the ring of instruments, the deeper into the Earth the researchers will be able to look, Levander said. The more distant seismometers should give them a glimpse of the bottom of the magma pipeline.
Right now, data collected from ongoing monitoring has given scientists a pretty good idea of the subsurface to a depth of seven or eight kilometers — around five miles.
With luck, the scientists say, they will be able to extend that to 60 miles below the surface, where magma is generated.
“Theoretically, we will be able to see things 500 meters across,” Levander said. The process is not a quick one; it could be years before all the data are assembled and analyzed.
Last month, the focus was on another aspect of the research when teams installed 70 passive seismometers; they’re much more sophisticated than the instruments Lock and Sabbeth and dozens of other volunteers were installing this week.
The third phase of research, magnetotelluric monitoring, uses fluctuations in the Earth’s electromagnetic field to image the subterranean structure.
