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Ground-penetrating radar used to prepare for archaeological dig at Amherst Historical Museum



Thursday, February 04, 2016
AMHERST — A high-altitude Peruvian rock shelter, Mayan ruins and caves in France are among the sites around the world where a three-wheeled device resembling an oversized tricycle has revealed what lurks below the ground’s surface.

While the lawn of the Amherst Historical Museum on Amity Street may not be the most unusual place that Peter Leach has brought his ground-penetrating radar, it was where the doctoral student in anthropology at the University of Connecticut spent his day on Monday.

“The idea is to use this to figure out how the entire property is laid out, slicing back through time,” Leach said.

Making 50-centimeter-wide swaths over much of the ¾-acre property, Leach began the process of discovering what might be hidden up to 6 feet underground, without the need to put any shovels in the lawns and gardens surrounding the mid-18th-century building.

This is the first step in an archaeological survey being undertaken by the University of Massachusetts Archaeological Services, hired after Town Meeting last spring approved spending $20,000 from the Community Preservation Act account to pay for the work at the museum, also known as the Strong House.

James Wald, president of the Amherst Historical Society, said the survey is part of good stewardship for the property and necessary preparation in case any work needs to be done on site, including potential repairs to the building.

Survey grid

Early Monday morning, Leach and Frederick “Tim” Barker, field director for UMass Archaeological Services, laid out a grid over the entire property, marking the edges of each area with baseline tape. This was time consuming, but essential to have a triangulated, rectangular grid so that an accurate map to accompany the data collected by the radar, Leach said.

“The whole idea is to fit a rectangular survey grid over an irregular property,” Leach said. “The tighter the grid, the better the data.”

Once done, Leach was meticulous as he guided the “GPR,” pushing it like a lawnmower west to east across the grass. He placed a small orange cone onto the baseline tape and moved it after each pass to ensure that he was mapping each piece of the property only once.

As the radar device sat flush with the ground and sent its signals into the soil, it delivered imagery to a screen near the handlebars. The image “looks like a Magic Eye at first,” Leach said, referring to the popular series of books featuring autostereograms which allow some people to see three-dimensional images.

The radar pulses that enter the ground respond differently depending on the materials they encounter. Based on how the energy is returned, and how long these pulses take to come back, Leach can quickly see whether there is compacted, undisturbed soil, or, more interestingly, if there is evidence of human activity, such as a stone wall, a foundation, pipes or debris.

As he viewed Monday’s data, Leach said it indicated the possible presence of a pipe.

Once this data is taken back to a lab, Leach will stitch the wavy lines seen on the screen into a three-dimensional cube, spending about eight hours manipulating the data using Geographic Information Services software to more clearly illustrate what is in the ground.

“My interpretations are where they are and what I think they are,” Leach said,

The results usually look so good that materials underground will be exposed, almost as if a backhoe had dug deep into the soil.

These will be presented to Barker, giving him both grid-specific coordinates and GPS coordinates to determine where he wants to dig next spring.

“It’s sort of like a treasure map, in a way,” Leach said.

Outhouses or privies can be some of the best places to search for, because they are where people would lose items from their pockets, such as coins, and were also used as places to hide valuables, Leach said.

Aerial drone

On Tuesday, Leach used a small aerial drone with a high-definition camera to photograph the topography to have one more record of the site.

Barker said the ground-penetrating radar results will supplement and expand upon the knowledge people already have about the property, including a study done about 25 years ago when the adjacent Jones Library underwent an expansion, a historic landscape report completed in 2002 and previous test pits, that each measured 50 by 50 centimeters, in 2011.

“They indicate there are buried archaeological resources here intact under the ground surface,” Barker said.

Barker said much of the museum property and the Strong House dates to the mid-18th century and has remained largely undisturbed, despite the development of downtown and increased commercial activity.

Barker said the information that Leach provides will be used to determine where to do the next archaeological excavation. Up to 9 square meters can be excavated in the spring.

What will be discovered is not known exactly, though records indicate several barns and wells were on the property. The most recent earlier digs unveiled refuse sheets where household garbage was disposed in the days before landfills and trash pickups.

Wald said he hopes that local schoolchildren can visit next spring’s dig to see firsthand what elements of the past are discovered.

Leach is excited by what might be found. “Every site is a continuing education,” he added. “Every site shows you something new.”

Scott Merzbach can be reached at smerzbach@gazettenet.com.