ASK-A-GEOLOGIST by Jeff Wynn
What’s a real scientific paper look like? …Mapping Mount St Helens in 3D.
“Where is the Water and Where is the Hot Rock? Using Controlled-Source Audio-Magnetotellurics (CSAMT) to Map Beneath and Around Mount St Helens.”
What is “doing science” really like? I thought it might be helpful for readers to see what a scientific paper might actually look like: how science is done, how the information is conveyed to other scientists and ultimately the popular media. You can see here how we acquired the data, how we analyzed it, how we wrote the scientific paper… and how we revised it following reviews by other, independent scientists. The link below will take you to a PDF of the final product: a study to determine the 3-D interior of Mount St Helens and its surrounding region. This paper was published in the Journal of Environmental and Engineering Geophysics in 2016.
Some background might be helpful here: The eruption of Mount St Helens in 1980 drew a lot of attention because it involved the largest landslide in recorded human history. Volcanologists wondered: why didn’t the volcano just erupt upwards? Isn’t that what volcanoes do? The monstrous blow-out on the north flank devastated an area of 1,147km2 — that’s over 440 square miles! This doesn’t even account for the huge lahars that roared down the Toutle River and tributaries and nearly cut the I-5 corridor — the main economic lifeline of the Pacific Northwest between Portland and Seattle. The Army Corps of Engineers had to dredge the debris out of the Columbia River so that commercial freighters could reach Portland again. Ash reached the Tri-Cities area in central Washington State by mid-day. A meter of ash fell — collapsing roofs and darkening the sky so that the streetlamps came on a noon. Ash reached Denver by the following morning.
Experts thought there might be a reason for the spectacular edifice failure: the volcano was topped by a glacier. Water pore-pressure inside the edifice must have contributed to the over-pressure that blew out the north face of the volcano. But why not the south side? Can we acquire any evidence to help figure out what was/is going on?
MORE Background: Controlled-Source Audio-Frequency Magnetotellurics (CSAMT) is an ACTIVE geophysical method which uses variations of a portable transmitter’s magnetic and electric fields (generated with cable loops) to measure the electrical resistivity of the sub-surface beneath the Earth’s surface. Think: sending a wave-like signal downwards and seeing how it is changed coming back. The lower frequency elements “see” deeper. The “Audio” part means frequencies in the 20–10,000 Hz range.
Some interesting unwritten background about this paper: It represents well-data collected over two years, involving difficult walks over large distances in a heavily-dissected (lots of cliffs) terrain. This was followed by weeks of helicopter acquisition of CSAMT data… basically geo-electrical soundings that reached down a kilometer in depth, looking for electrically conductive stuff. Water with hot-leached ions in it is “conductive stuff,” for instance.
While each sounding cost less than a drill-hole, each one still required the four scientists and technicians to be sand-blasted by volcanic ash in the helicopter rotor-wash four separate times for each station — two in-and-out helicopter loads. The hardware we had weighed over 250 kg, so it was physically arduous work; with mobilization and battery-charging it involved two weeks of 14-hour days (we only get paid for eight hours). Nearly as arduous was the process of analyzing and converting the data into horizontal layers, generating 3-D maps of the results, and getting the final product peer-reviewed by knowledgeable scientists to make sure we didn’t make any gross mistakes. This took another year after the hard summer of data-acquisition.
The final results showed that there was not one, but TWO conductive aquifers involved: a shallow aquifer that derived from the crater glacier… punctuated by the hot magma still lying under the New Dome inside Mount St Helens crater. Then there was a vast regional aquifer far below this. From the CSAMT data we could see where the hot rock was: the furious boiling of glacier-water at its carapace made for a very conductive fluid that jumps out in the data. We could even see how fast the carapace on this magma was cooling over time as it solidified downwards!
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Here is a map of the volcano and the different data-stations:
Next, here is a figure showing how the phase-and-magnitude data from the deep CSAMT soundings were “inverted” frequency by frequency — that is, converted to conductivity-vs-depth (result on the left side):
The following figure is our final product: the 3-D image of the two different groundwater tables and where they finally met, at Coldwater Lake, seven kilometers distant from the volcano itself:
The crucial final figure showing where we collected the CSAMT data to identify the still-hot dacite magma body follows. Note that it is not possible to physically walk in most of this dangerous terrain — it is crevassed glacier and near-vertical rock — so helicopters had to be used extensively. Tx means the CSAMT transmitter, and Rx means receiver locations. The hot dacite magma body lies approximately beneath the words “2004–2008 Dome” below:
With this study, volcanologists now know a lot more about where the water lies under this volcano. That knowledge will be important for making predictions when (not if) Mount St Helens decides to erupt in the future.
If you want to grind through the entire manuscript, and see our efforts in more detail, you can read the actual peer-reviewed-and-published paper here: https://www.academia.edu/25587947/Where_is_the_Hot_Rock_and_Where_is_the_Ground_Water_Using_CSAMT_to_Map_Beneath_and_Around_Mount_St_Helens
