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Exploring CR’s ‘Beer Caves’ with Geophysics

Posted on February 8th, 2015
The electromagnetic terrain conductivity meter has a transmitter on one end and a receiver on the other, which creates an electrical field.

The electromagnetic terrain conductivity meter has a transmitter on one end and a receiver on the other, which creates an electrical field.

Iowa Geological Survey researchers are exploring the legendary “beer caves” near Interstate 380 in downtown Cedar Rapids. Using advanced geophysical technology, IGS researchers can detect and delineate the structures without ever venturing below ground level.

Geologist Jason Vogelgesang is leading the team of IGS researchers conducting the study. Along with IIHR Assistant Research Scientist Carrie Davis and Geological Technician Zachary Demanett, Vogelgesang conducted extensive studies of the area using electrical resistivity (ER) and electromagnetic terrain conductivity (EM) methods.

With this technology, they are able to model the subsurface to learn more about the location and the extent of the beer caves.

A Draft of History

View a slideshow of images of the Magnus Brewery and the current geophysical research

In the summer of 2014, heavy rains opened a sinkhole near the Seventh Street off-ramp in downtown Cedar Rapids. Iowa Department of Transportation (IDOT) bridge inspectors found the sinkhole and noted that it seemed to open into some sort of underground cavern. The inspectors lowered a camera into the void, and the resulting photos prompted further investigation.

After consultation with the Office of the State Archaeologist, it was determined that the sinkhole offered entry to one of the city’s long-abandoned beer caves.

A representative from the Office of the State Archaeologist briefly went into the caves and took photos. Photo by Marlin Ingalls

A representative from the Office of the State Archaeologist briefly went into the caves and took photos. Photo by Marlin Ingalls

Before Prohibition ended the party in the early 20th century, Cedar Rapids was home to two successful breweries. The Magnus Brewery was the largest west of the Mississippi River, and produced the locally popular Eagle brand beer. According to the Office of the State Archaeologist, the Magnus Brewery overlooked Cedar Lake from an impressive four-story structure built of Iowa limestone, with a magnificent bronze eagle perched atop the roof. The Williams Brewery was immediately adjacent.

Both breweries had closed by about 1920. The abandoned buildings were eventually deemed a hazard and torn down. Although above ground they were virtually erased from the city, one important facet of the beer business in Eastern Iowa has stubbornly refused to disappear — enormous underground barrel-vaulted masonry cellars or “caves” for beer storage and aging. Although the beer caves are now more than a century old and for the most part forgotten, they continue to be structurally sound. Skilled artisans, many of them European immigrants, constructed the caves in the tradition of similar French and German beer caves. Over the years, authorities partially filled in at least some of the caves in an effort to stop vandals, daredevils, and the homeless from occupying the caves.

Under the Interstate

When Interstate 380 was built in the 1970s, the beer caves once again came to light. The IDOT was required to log several borings, or drill holes, before construction. In the process, workers encountered voids that seemed to correlate with the old beer caves. The steel pilings that support raised sections of interstate ramps pierced the caves, but the road itself is safely supported by solid bedrock.

Once the interstate was completed, the beer caves once again slipped into obscurity. When the sinkhole appeared last summer, the IDOT contracted with the IGS to conduct an extensive geophysical investigation of the site to assist in determining the size, number, and orientation of the caves. According to the Office of the State Archaeologist, there may as many as 14 caves.

Vogelgesang explains that the geophysical tools his team uses don’t provide an exact image, but rather a model that can be combined with other known information to piece together what’s beneath the surface. ER is the most important tool for this study. Researchers pound 56 stainless steel stakes into the ground, connected by cables. They then send electric current through two of the stakes, which returns through up to eight of the other stakes. Each test takes about an hour and a half to cycle through all of the 56 electrodes.

The light dusting of snow caused no problems for the researchers or the geophysics equipment.

The light dusting of snow caused no problems for the researchers or the geophysics equipment.

“In the end, we produce a model of how the subsurface responds to the electrical charge,” Vogelgesang says. Since the caverns are assumed to consist mostly of open space, they should be more resistive electrically when compared to their surroundings, Vogelgesang explains.

EM, the electromagnetic terrain conductivity method, uses a 15-foot-long device with a transmitter at one end and a receiver at the other. The operator walks back and forth across the field site, holding the EM at about hip level. The EM device collects five conductivity readings per second, which are recorded with precise GPS locations. EM surveys can collect data up to 20 feet below the ground surface, and all the data is recorded on a field PC.

EM surveys provide a map view of what is below the surface; ER, on the other hand, offers a two-dimensional slices or three-dimensional blocks of what’s underground. “With both of them combined, we are able to get a more complete understanding of the subsurface,” Vogelgesang says.

Back at the office, analysis of the data is an intensive and time-consuming process. The IGS team will provide a report to the IDOT in mid-January. “Our portion of the project is intended to guide whatever decision the DOT ends up making,” Vogelgesang says. “We’re not part of the decision process. We provide the background data, the science, and the site survey.”

No Spelunking, Please

Vogelgesang, a geologist who has spent quite a bit of time in natural caves, says he’s not sure he’d want to go down in the beer caves. “If you did go down there, you’d just want to get back up as quickly as you could,” he says. The caves have also been picked clean of artifacts. “There are beer bottles from the Magnus Brewery all over Cedar Rapids, apparently, but none in the caves,” Vogelgesang says. “I’m told they’ve all been scavenged.”

IGS researchers typically use geophysical equipment to study shallow groundwater resources, but Vogelgesang says they are happy to help out with projects like this. “IGS has a strong service component,” he says. “We are thrilled to have the opportunity to work alongside the IDOT in this unique study.”

The IGS is now part of IIHR—Hydroscience & Engineering, a research institute and part of the University of Iowa College of Engineering.

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