Finding the Forgotten, But Not Lost

Using Ground Penetrating Radar to Look Into The Past

It’s not uncommon for clients to come to Terracon to inquire about non-destructive ways to explore below the surface. For example, they may request an “X-Ray” of reinforcing steel in concrete, or the locations of underground storage tanks at an old gas station. Occasionally, however, there are requests to find more historically important items that may have been lost over time.

Terracon’s regional geophysicists can collaborate with local engineers and scientists to determine the best way to find items buried underground. Ground penetrating radar (GPR) is one of the tools in a near-surface geophysicist’s toolbox. GPR is the typical go-to for locating objects less than 10-feet in depth when prevailing soil conditions are conducive to this approach.

Locating History While Leaving It Undisturbed

Our Greensboro, N.C., office aided in locating unmarked graves at an existing cemetery. According to the client reports, the cemetery dated back to at least the 1800s. In a small corner of the cemetery there were only a few grave markers. The client expected more graves were present, but the number and location were unknown. The church community wanted to locate, remark, and memorialize the unmarked sites.

The Terracon team collected GPR data in a 10,000 square foot area with a cart-mounted antenna connected to a sub-meter GPS device. We collected the GPR data quickly and accurately, allowing individual anomalies consistent with an unmarked grave to be identified, and collect their precise horizontal locations.

What is a geophysicist?

A geophysicist is a scientist who studies a branch of geology that deals with the physical process and properties of the earth. This can include its electrical, gravitational, magnetism, and structural properties. Terracon’s geophysicists are typically near-surface geophysicists whom study the upper couple hundred feet of the earth.

We analyzed and processed the data in the office, which allowed us to identify more than 200 anomalies that could indicate an unmarked grave. In some areas, the anomalies did not appear consistent with typical cemetery layouts. Multiple graves appeared to be in close proximity and overlapping one another. While discussing the preliminary findings, the client revealed older members of the church had heard rumors that a nearby cemetery closed in the later 1800s. The cemetery reached out to the church in the 1800s to ask if it could relocate the graves to the church’s cemetery. If this did happen in the past, it could explain the cluster of anomalies.

GPR can be a great tool to quickly and accurately gather subsurface data, but it is not the single solution to all problems. Terracon’s geophysicists understand the objectives of the study, consider the subsurface conditions, and our client’s needs to determine which geophysical methods are best suited to meet project objectives. As with the cemetery, every project has its own challenges, and, with the assistance of an experienced geophysicist, our clients can get a glimpse into the past.

Common Uses for GPR

Locate

• Reinforcing steel
• Utilities
• Underground storage tanks
• Voids beneath concrete slabs
• Buried debris

Measure

• Depth to bottom of ponds
• Depth to shallow bedrock
• Concrete thickness