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Sunday, May 15, 2011

The Darker Side of Archaeology: Use of GPR at the L’Ambiance Plaza Disaster

L’Ambiance Plaza was a 16 story residential project that was under construction in Bridgeport, Connecticut at the corner of Washington Avenue and Coleman Street when its partial frame collapsed on April 23, 1987. There were 28 workers that were killed, caught between the sandwiched concrete floors of the collapsed building. The site was immediately swarmed by many of the construction workers from other projects in Bridgeport who tried to save trapped workers. From the start it was quite apparent that finding any survivors would be a miracle.

This photo taken at the scene of the disaster illustrates what kind of problems we faced using GPR.
(This photo taken at the scene of the disaster illustrates what kind of problems we faced using GPR.)

Shortly after the accident we received a phone call from the Bridgeport Police requesting us to come to the accident scene as soon as possible with our Ground Penetrating Radar (GPR) to join in the search for survivors. As soon as we arrived at the scene of the collapse we set up our GPR unit and quickly discovered that although our unit had the capability of penetrating several feet into the ground, it was unable to penetrate the concrete used in the construction of L'Ambiance Plaza.

The construction methods that were used totally defeated the GPR because the floors were reinforced with rebar and heavy wire mesh. It was impossible to penetrate the concrete and get any meaningful images as a result of the very strong echoes from the steel reinforcements.

Strangely, the next job we received was determining if there were sufficient reinforcements of rebar in a construction project in the neighboring city of Norwalk, Connecticut. In this case the GPR was effective because the rebar was far enough apart, so the solar radar beam did not pick up any conflicting echoes.

This brings up the question of where you can't use GPR, and there are several different natural conditions that can block the radar beam.

The GPR is virtually useless in the following conditions:
  • In concrete that is heavily enforced with rebar or wire mesh or any combination of both.
  • If the soil is contaminated with salt; the salt will have a tendency to break up the radar beam causing a very shallow penetration that at times can be measured in millimeters making the use of GPR virtually impossible.
  • The presence of excessive clay serves practically the same purpose since the penetration of the radar beam through clay can also be measured in millimeters.


(This shows a GPR unit at work at a forensic site)

Fortunately there are other places where GPR can be used to great effect. It can be used very effectively in penetrating solid stone. One of the early trials we conducted with ground penetrating radar was the Palisade Sill in New Jersey across the river from New York City. The radar beam penetrated to about 300 feet and showed the deposits of chromite that had settled through the molten basaltic magma from which the sill is composed.

(This is a readout showing five areas of interest that might be human remains. )

We have also used GPR to detect cavities in limestone underlying a stretch of Interstate 80 in western New Jersey. Some of these cavities were as small as a teacup and others were as large as a small house. The average depth of penetration in ordinary soil is about 12 feet, but that can very according to the amount of clay minerals the soil contains. The more clay that is present the less penetration.

GPR does not present a photo like image of conditions below the surface of the ground but rather a readout of geometric shapes and lines that require interpretation by a trained operator. It requires several weeks to train someone in the ability to properly interpret the readout from GPR on an environmental or archaeology site.

Author Bio John Carter is a geologist that paid his way through college as a commercial helicopter pilot in the NY Metro area. He has had extensive experience as a prospector, and operated an environmental consulting firm specializing in environmental site assessments and is certified by the State of Connecticut as an Environmental Analyst III. In his early life he worked as an experimental machinist on aircraft and electronic parts before receiving his degree as a geologist.

Read more: Ancient Digger Archaeology: The Environmental Site Assessment as an Archeological Study  
  1. L‘Ambiance Plaza, Wikipedia, http://en.wikipedia.org/wiki/L'Ambiance_Plaza
  2. GSSI, http://www.geophysical.com/?gclid=CNf24rf056gCFYfe4AoduGyGDg
  3. Personal Experience of John Carter
  1. Photot taken at the scene of the disaster illustrates what kind of problems we faced using GPR.
  2. GPR unit at work at a forensic site Photo by Vadams
  3. This is a readout showing five areas of interest that might be human remains. Photo by Matomj

1 Comment:

Mark Tery said...

Radar sound waves of high frequency will be discharged through the ground a few feet deep.
Ground penetrating radar

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