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InTransition Magazine : Transportation Planning, Practice & Progress

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Technical Toolbox

A Road Runs Through It

Using Geophysical Sensing in Transportation

By Jessica Zimmer

About Technical Toolbox

  • Technical Toolbox highlights innovations and emerging technologies making an impact in the transportation sector.

Arrowheads, glass bottles, human skeletons—when work starts on a road project, you never know what lies beneath. Geophysical sensing takes the mystery out of that history.

Geophysical sensing is the practice of using data-collecting instruments to map beneath the surface of land or water. At every step, archaeologists keep planners informed of the discovery of artifacts and sites. Their work helps engineers meet legal obligations and avoid physical and public relations debacles.


A worker uses a type of magnetometer to help excavate a church

property while creating a minimum disturbance as part of a

Maryland State Highway Administration project.

Transportation agencies are required by Section 4(f) of the Department of Transportation Act and Section 106 of the National Historic Preservation Act, both passed in 1966, to make all possible planning efforts to minimize the harm to property while working on road projects. This includes the removal or destruction of artifacts and historic structures in the proposed project area. In addition, state agencies are sometimes required by their own local regulations and state laws to identify and protect archaeological sites.

Archaeological excavations have been known to slow, alter and even stop transportation projects in their tracks. However, the steady improvement of geophysical sensing technology is today helping transportation agencies perform their work with greater speed and accuracy.

Bruce Aument, staff archaeologist of the Ohio Department of Transportation, said that geophysical sensing serves an important purpose.

“[Geophysical sensing] is a way of focusing the effort and ignoring the areas that don’t have any potential,” Aument said.

One of the largest projects currently utilizing geophysical sensing is the Columbia River Crossing. This project concerns the improvement of a section of Interstate 5 that crosses the Columbia River between Portland, Ore., and Vancouver, Wash.

“It is an incredible multibillion [dollar] project. The [two] bridges that cross the Columbia River are not meeting transportation capacity goals,” said Matthew Sterner, transportation archaeologist of the Washington State Department of Archaeology and Historic Preservation.

The two options to improve the flow of traffic in the area are to add a bridge for southbound traffic next to the existing two bridges or replace the two bridges with a new bridge downstream of the existing bridges. In addition, there will be construction to improve seven interchanges that feed traffic to the bridges.

One of the concerns in conducting geophysical surveys here is the area’s unique layered rock formations.

“On the south side of the Columbia River Valley, it goes down 250 feet. On the north side of the valley, 12,000 years [of history] is 6 inches down,” said Curt Peterson, a professor of geology at Portland State University who is assisting with the project.

Ground-penetrating radar (GPR) will be one of the tools used to locate archaeological sites. The GPR used for archaeological reconnaissance was refined from a version designed to map mines in the 1970s. It relies on two parts of a device that work in tandem. A large square box, the antenna, sends out electromagnetic waves in pulses into the soil. These waves reflect back to the antenna. Then the digital control unit of the device, which looks like a LCD screen, displays how far the waves penetrated. The peaks and valleys of the reflected waves reveal the presence of objects that got in the way of the waves.

Early crews for the Columbia River Crossing found that GPR had some technological limitations. There had been a previous disturbance in the area. Earlier road builders had cut away earth and laid down roadside debris near the river. GPR surveys could not see through the fill to possible sites of interest.

Heritage Research Associates, Inc. was the cultural resource management company employed to excavate the area. Rick Minor, senior archaeologist with the firm, said that the work will have to be conducted carefully because there are a variety of sites in the area.

According to Minor, there are three themes of archaeology around the site of the Columbia River Crossing. The south shore, which is in Oregon, has not been extensively excavated. It is likely to contain prehistoric Native American sites. The east side of the north shore, which is in Washington, is home to the Fort Vancouver National Historic Site. Fort Vancouver is the location of the original Hudson’s Bay Company trading post and a later U.S. Army post. Minor said that there may be prehistoric Native American sites under the historic sites on the east side of the north shore. Finally the historic city of Vancouver, Wash. is situated on the west side of the north shore. A number of historic sites in Vancouver should date between the late 1840s and early 1850s. These sites, which are located along the Columbia River, got covered over or abandoned as Vancouver expanded north, according to Minor.

Checking In at Historic Colorado Hotel

Geophysical sensing can also be used for smaller sites, as evidenced by the 1989 excavation of the Tremont House hotel in Colorado.

Colorado Assistant State Archaeologist Kevin Black said the hotel was excavated to advance road improvements in downtown Denver. A portion of heavily trafficked Speer Boulevard that crosses over some rail yards off the South Platte River was being reconstructed. The approaches to the overpass had to be rebuilt, Black said.

Colorado DOT

A view of the foundations of the historic Tremont House hotel in

Denver. The site was excavated with the help of ground-

penetrating radar in 1989 to advance a road project.

Archaeologists knew the general location of the hotel from old fire insurance records that indicated where the walls once stood. They used a proton precession magnetometer to measure the variations in the area’s magnetic fields. The basic concept of magnetometers were first developed in the 1830s, and later refined to allow surveyors to map distortions in a geomagnetic field caused by large objects of features. Here archaeologists used a proton magnetometer to locate the foundations of the hotel’s walls.

“The anomalies formed a really nice rectangular and square pattern of wall-like features in the exact area where the archaeologists felt the walls should be,” Black said.

Dan Jepson, senior staff archaeologist with the Colorado Department of Transportation, said the state did not change the plan for road improvements.

“The new paved roadway needed to go over the site. About 80 percent of the site is removed. The rest is still there under the median,” Jepson said.

Archaeologists were also eager to excavate the hotel because historical records indicated that the structure would be a significant discovery.

“Tremont House was a Gold Rush era hotel built in Denver in 1859, when Denver was still a frontier town. It was a hotel for 50 years, the first 50 years of the city of Denver,” Jepson said.

The hotel catered to the rich. This was proved by archaeologists’ recovery of many “swanky” artifacts—bone handle toothbrushes, fragments of fine china ceramic vessels and the butchered bones of bison and elk.

“[It appears] that people were going out to hunt them and bringing them back to be cooked in the hotel restaurant. To stay at the Tremont Hotel was living high,” Jepson said.

Ohio Native American Site Explored

Geophysical sensing has proven successful in finding prehistoric sites as well.

The Shriver Circle in Ross County, Ohio, is a prehistoric Native American earthwork, a man-made feature in the landscape created from earth or stone. The Shriver Circle is a raised, mounded outline of a circle shaped from dirt, measuring 1,200 feet in diameter.

In 2005 and 2006, the Ohio Department of Transportation (ODOT) decided to widen State Road 104 south of Columbus. The resulting excavation was to cut through the circle’s north and south ends. ODOT Assistant Environmental Administrator Paul Graham said the agency’s archaeologists had trouble locating the earthwork.

“We did two geophysical surveys, and then hand surveys, and we didn’t find anything. You picked up the signature, and you knew it was down there, but where?” Graham said.

At that point, Jarrod Burks, director of geophysical surveys at Ohio Valley Archaeological Consultants, Ltd., a cultural resources management company, entered the picture. Burks assisted ODOT archaeologists by using a magnetometer to determine that the earthwork was three to five feet under the surface of the ground.

“We saw a ditch dug into the ground and soil used to make an embankment,” Burks said.

Burks said ODOT’s archaeologists had trouble finding the circle because the ground had been disturbed.

“The circle is located near Camp Sherman, a World War I Army base,” Burks said. “They flattened out the mound and filled in the ditch so they could pull their artillery around out there.”

The trenches through the circle later cut by ODOT archaeologists did not uncover many artifacts.

“There were only a couple pieces of flint. The fill of the earthwork was very clean,” Graham said.

Yet the fill told a story.

“We saw that different-colored sandy soils were used to make the earthwork,” Aument said.

Different colored soils may signify that an earthwork has a ceremonial significance. Earthworks with levels of soil that range from red and yellow to white and black are present throughout much of the eastern U.S. Fills with different colored soils are not common in Ohio, Aument said.

“It is better documented in Illinois Hopewell sites,” Aument said.

Aument’s observation lends support to the idea that the Shriver Circle was important for religious reasons. The term “Hopewell” refers to a group of sites that typically contain artifacts dating between 100 B.C. and 500 A.D. The sites are thought to relate to religious rituals.

Technology Takes a Toll in Maryland

In one recent case, geophysical sensing was used to uncover the history of transportation.

Richard Ervin, archaeologist for the Maryland State Highway Administration (MDSHA), said the agency is currently working on a project involving ground-penetrating radar to locate the remains of a brick toll booth. The site is located along Maryland Route 108 in the town of Sandy Spring in Montgomery County.

The MDSHA is altering the road to create an intersection and take out a dangerous curve. According to Ervin, maps indicate there was a turnpike there during the late 19th century. At the time, such roads were common around the area of Baltimore and Washington, D.C., Ervin said. Turnpikes were models of early entrepreneurship.

“Private companies purchased the rights and either improved an existing road or built new turnpikes. Farmers paid a toll,” Ervin said.

Turnpikes were preferred because they had regular additions of gravel and were wider than other roads. In the 20th century, the turnpikes fell out of use.

“What ultimately happened was the companies weren’t able to keep them up with the toll payments. The roads were allowed to go into disrepair,” Ervin said. 

The current project to find the brick booth is part of an effort to record Maryland’s transportation history.

“It gives a lesson on the importance of the road network to the economy of a local town,” Ervin said.

Promise for the Future

The use of geophysical sensing, which began to be widely adopted by state transportation departments in the 1990s, appears to be on solid ground for the future.

Larry Conyers, associate professor of anthropology at the University of Denver, said that archaeologists now have many of the kinks of the technology worked out. Images produced by the devices used to be rough and therefore less accurate.

“[Now] all of the instruments have [imaging] software to go with them,” Conyers said. “With respect to what we can do with the equipment we already have, we are doing really well.”

Conyers said in the future, perhaps engineers could create a “superdevice,” a tool to make different technologies work together.

“Another thing that would help us greatly is if we could integrate all of the instruments with GPS, so we could use the data in real time. Then we could produce maps very quickly,” Conyers said.

Jessica Zimmer is a freelance writer based in Florida.

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