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Burial Mapping: Using Subsurface Imaging

      
Date Published: 
March, 2004
Original Author: 
Scott Smith
General Engineering Geophysics
Original Publication: 
ICFM Magazine, March-April 2004

"My records are accurate and complete. I know exactly how many burials I have in my cemetery, and I know exactly where they are."

If you can say this with confidence, congratulations are in order, but many cemetery owners/operators are not as fortunate. Why? Because records are often inaccurate, incomplete, lost or simply nonexistent. Many older cemeteries have had several owners over the years, and in some cases, records did not get transferred.

The ramifications of not knowing burial locations can range from minor inconveniences to major problems: Undocumented burials can be discovered at awkward times, leading to uncomfortable situations with families or even legal trouble. Valuable real estate suspected of containing burials can go unused for fear of such an awkward discovery.

The situation can be remedied. Each cemetery is different, and mapping needs can vary. Where simple solutions such as probing are called for, they should be used, but high-tech methods are also available. Computer-Assisted Radar Tomography, or CART, can provide complete and accurate burial inventory records.

Seeing Below the Surface
Ground penetrating radar (GPR)-based CART is a three-dimensional subsurface imaging technology used to detect and map burials at a very high level of accuracy. Single-channel GPR (one transmitter and one receiver) has been used for decades in the geological and archaeological fields.

In the simplest terms, GPR works by transmitting radar waves into the ground, which then reflect off objects in the subsurface. The reflected radar waves are then detected by a receiver. The speed at which radar waves "bounce" off objects and return to the receiver are a function of the object's properties. The GPR data is interpreted to determine if there is a buried object which differs from the surrounding soils.

With single channel systems, this interpretation is a time-consuming and difficult process. CART technology takes subsurface imaging to a new level, one in which you can almost "see" underground. Because CART uses l6 channels, it beams radar waves at many different angles. When radar waves are reflected off an object and detected from different angles, it allows a three-dimensional view of the object. This allows the production of complete three-dimensional underground "CAT-scan" images of the cemetery subsurface.
This technology was initially developed for the underground utility industry. In fact, its first large-scale application was mapping the buried infrastructure surrounding the World Trade Center site following the September 11, 2001, collapse of the Twin Towers.

Because of its imaging capabilities, the technology is a natural fit for use in mapping burial vault locations. This GPR-based technology is being accepted in courts as a "best available technology" for vault mapping.

How It Works
So how does the process work? Let's take a look. First, it is important to remember the ultimate goal of most investigations, which is to create a detailed map showing the locations of burials.

You must have an accurate topographic survey map of the cemetery so that the "underground information" collected can be tied to above-ground data, which could consist of control points you establish or permanent cemetery fixtures such as row markers or sprinklers. Before any actual CART data is collected, the location of the CART system must be surveyed relative to the aboveground data.

Once the location of the CART system is established, underground data collection can begin. The CART system's starting point is marked on the ground with water-soluble paint or a flag.

The CART, which is approximately 8 feet wide, is then towed over the ground behind a small tractor, similar to the way a tractor would tow a bush hog-type mowing deck. The unit is towed at a walking speed, and a technician periodically marks the CART's path so that the tractor technician can make sure subsequent strips, called profiles, can be collected without leaving gaps. Think of it as how you overlap when you mow your lawn.

For tighter spaces, such as cemeteries with numerous headstones, there are narrower CART units that can be mounted on the front of a tractor.

All of the transmitter/receivers in the CART are connected to a laptop computer operated by the technician driving the tractor. A 360-degree survey prism is attached to a rod on the back of the CART, and the location of the CART unit is tracked by laser and continuously robotically surveyed during the collection of subsurface radar data.

This is how the locations of what is detected can ultimately be placed very accurately on a map. Each strip, or profile, collected by the CART is recorded separately, and all of the profiles are later stitched together by the software used to evaluate the data.

The CART system can cover an area up to 1.5 acres per day, so large cemeteries can be completely surveyed quickly.

The processing and interpretation of the CART data by geophysicists is obviously important. Because the data is collected as a three-dimensional "cube," each suspected burial can be evaluated from many angles. The software overlays the underground images into common drafting software such as AutoCAD and Microstation, allowing for great accuracy. Horizontal and vertical accuracy of CART data is typically within a few inches, but is dependent on local soil conditions.

At the end of the project, the cemetery owner receives a map that shows the location of detected burials. In some cases, they will be burials for which there is no cemetery marker. In other cases, the maps may show "marked" burials that are not aligned with their markers.

While it would be nice if this technology worked in every cemetery, the honest answer is that it does not. The effectiveness of GPR/CART technology can be limited in electrically conductive soils such as certain clays.

Conductive soils "absorb" radar waves, which limits the depth of radar penetration. This can prevent the CART from "seeing" deep enough into the subsurface to detect burials. However, not all clays are created equal, so the presence of clays does not automatically preclude the use of CART.

The most GPR/CART-friendly soils are sands, where penetration depths of more than 15 feet are not uncommon. Average penetration depths in most soils are usually up to 8 feet.

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