Category: Research

The construction of any large infrastructure project requires a strong foundation, especially one with the longest main span of any cable-stayed bridge in North America—namely, the Gordie Howe International Bridge over the Detroit River. More than a decade before ground was broken, careful siting of the bridge began to take place. By 2006 the list of possible crossings had been narrowed down to just two options.

Historical records from the early 1900s indicated that solution mining for salt had taken place on both sides of the river close to where the bridge was to be built. On the Michigan side, collapsed salt cavities caused sink holes located on nearby Grosse Isle. It was imperative that any salt cavities in the bridge construction area be found and avoided.

Seismologists Roger Turpening and Carol Asiala at Michigan Technological University were tasked by American and Canadian bridge contractors to select the best seismic method for searching for any cavities in the two proposed crossings—referred to at the time as “Crossing B” and “Crossing C”—and to interpret all resulting seismic images.

“Given the task to image a small target deep in the Earth, a seismologist will quickly ask two important questions: How small is ‘small?’ and How deep is ‘deep’? That’s because these two parameters conflict in seismic imaging,“ Turpening says.

“Seismic waves—vibrations of the Earth—are attenuated severely as they propagate through the Earth,” he explains. “Imaging small targets requires the use of high-frequency, seismic energy. When seismic sources and receivers are confined to the Earth’s surface, which is the usual case, waves must propagate downward through the Earth, reflect off of the target, and return to the surface. Soil, sand, and gravel in the surface layer overwhelmingly cause the greatest harm to image resolution, and the ray paths must pass through this zone twice.”

Turpening was one of the early developers of a technique called vertical seismic profiling, or VSP. “Seismic receivers are placed inside a vertical hole near the target. With the seismic source placed on the surface some distance from the hole, it’s possible to explore a region around the hole with ray paths that need to pass through the surface layer only once,” he says. “If the target is very important, we can drill a second hole and place the seismic source in it. Now we have even higher resolution because all of the ray paths are in the rock formations with low attenuation.”

The downside? “We can only make images of the region between the two holes. But if the target is extremely important in a limited area, we can use many boreholes and many images in the search. Given enough boreholes, a block of earth can be imaged with cross-well seismic reflection techniques.

To site the Gordon Howie bridge, Turpening and Asiala chose a frequency band of 100Hz to 2 KHz—much higher than could be used with surface sources and surface receivers—for surveys on both sides of the river. This yielded high resolution seismic images, crucial for detecting cavities—and indeed they found one—on the Canadian side.

“The high-resolution imaging made it easy for us to spot missing shale stringers in the B-Salt layer in that image,” says Turpening. “This made the final selection of the bridge location simple. We found the cavity between boreholes X11-3 and X11-4, thus forcing the Canadians to chose Crossing B.  Obviously, the Michigan group had to, also, choose Crossing B.”

On the US side of the river geologist Jimmie Diehl, Michigan Tech professor emeritus, provided corroborating borehole gravity data.

Michigan Tech volcanologist, Professor Simon Carn (GMES/EPSSI), is principal investigator on a new project, “Exploiting High-Cadence Observations of Volcanic Eruptions from DSCOVR/EPIC,” funded by NASA.

Carn and his team will use a satellite instrument, the Earth Polychromatic Imaging Camera (EPIC) onboard the Deep Space Climate Observatory (DSCOVR), which is parked in space a million miles from Earth.  EPIC provides global spectral images of the entire sunlit face of Earth, as viewed from an orbit around Lagrangian point 1 (L1)—the neutral gravity point between Earth and the sun.

“The unique feature of EPIC is that it can provide more satellite images per day of volcanic eruptions than other ultraviolet sensors we have used before,” Carn explains. “Our goal is to use this ‘high cadence’ imaging to improve understanding of volcanic eruption processes and impacts.”

Last Fall 2018, in an open-access article published online in the journal Geophysical Research Letters (GRL), Carn and his collaborators shared their first observations of volcanic eruption clouds from EPIC. The team developed and used an EPIC SO2 algorithm to detect every significant volcanic eruption since the DSCOVR launch in 2015.

“Although relatively small, these 16 eruptions, in places including Indonesia, Japan and Alaska (USA), have demonstrated EPIC’s sensitivity to moderate volcanic eruptions at a range of latitudes,” Carn noted. “EPIC should provide exceptional observations if still operational when the next major stratospheric volcanic eruption (VEI 4+) occurs.” VEI is short for Volcanic Explosivity Index. The team also demonstrated EPIC’s ability to track volcanic cloud transport on hourly timescales; a significant advance over low earth orbit UV sensors, such as the Ozone Monitoring Instrument, OMI—the visible and ultraviolet spectrometer aboard the NASA Aura spacecraft; and the Ozone Mapping and Profiler Suite (OMPS) on the NOAA polar satellite system.

“It is clear that the EPIC observations have great potential to provide new insight into the short‐term evolution of volcanic SO2 clouds, and also to enable more timely detection of volcanic eruptions. The potential value of frequent UV observations of volcanic clouds has been noted in the past, and with EPIC this has become a reality,” adds Carn.

Simon Carn has received multiple research grants totaling more than $2.8 million from NASA, the National Science Foundation, the National Geographic Society Committee for Research and Exploration, the Royal Society and the European Union. His research focus is the application of remote sensing data to studies of volcanic degassing, volcanic eruption clouds and anthropogenic pollution. His main focus: SO2, a precursor of sulfate aerosol, which plays an important role in the atmosphere through negative climate forcing and impacts on cloud microphysics.
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