Ground-Penetrating Radar in many environmental and civil engineering projects. In its basic form, Ground-Penetrating Radar equipment consists of a portable antenna and control unit. The Antenna is pulled along the ground surface over an area of interest and radar is transmitted into and reflected out of the ground based upon the conductivity of the subsurface materials encountered.

Several different antennas can be utilized depending upon the application and required depth of penetration. In typical Long Island sand, a 400 megahertz antenna can penetrate to depths ranging from 1 foot to 10 feet – the range most underground storage tanks and utilities are found within. A 300 megahertz antenna can penetrate depths of 15 feet or more and is suitable for surveying zones where debris piles, landfills or bedrock surfaces may be located. A small 1,000 megahertz antenna can locate re-bar or utility voids in concrete floors.

ACT employees have been trained and certified to use Geophysical Survey Systems, Inc. (GSSI) equipment.  ACT is one of the few consulting firms in the NYC Area who are certified to perform GPR.

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Uses of Ground-Penetrating Radar and Utility Surveys

A Ground-Penetrating Radar survey can be utilized as a stand alone investigation or preliminary to a Phase II Environmental Site Assessment. In environmental applications, Ground-Penetrating Radar can verify the presence of suspected buried structures and delineate the boundaries of known underground storage tanks and drainage structures (ie. leachpools, cesspools or drywells).

Previously excavated and backfilled areas such as former tank pits, landfills or other waste disposal locations can also be readily identified. In civil engineering applications, Ground-Penetrating Radar can identify the density of materials at various depths and locate the presence of voids beneath buildings or road surfaces. Ground-Penetrating Radar has even been used in archeological investigations to locate ancient Indian burial grounds.

Types of Ground-Penetrating Radar and Utility Surveys

All too many times, the sale of a property is delayed or even canceled due to last minute, unforseen issues. One of the most common issues is the possible presence of previously-unknown underground storage tanks. The possible presence of these tanks can be identified through many sources, such as fire insurance maps, public records or even hearsay.

In one such example, a 10,000 square foot commercial property in upstate New York was being purchased. A Phase I Environmental Site Assessment of the property identified a previous use as a service station. Due to this previous use, the closing of the property was delayed for 1 week and ACT was hired to identify whether or not any tanks were actually present.

ACT performed a survey of the property in one day utilizing a 500 megahertz antenna. A 5 foot by 5 foot grid was established over the entire property. The radar was set to penetrate to a depth of 8 feet. Four 1,000 gallon underground tanks were identified adjacent to the eastern wall of the building. The same day following the completion of the survey, ACT provided the verbal results to the client, who in turn went ahead with the purchase of the property. Prior to the date of closing, ACT provided the client with a letter report indicating the exact location of the tanks. The client purchased the property on a contingency that the seller remove the tanks. ACT was later informed by the client that the tanks were in fact removed and no contamination was found.

You never thought moving a piece of equipment 11 inches was so difficult. The motors which raise and lower the lift portion of the Triborough Bridge were going to be moved only 11 inches. In order to accomplish this, the motors had to be disassembled and then the floor of the motor rooms had to be demolished. However, the electrical lines to each motor were located within the 6 inch slab floor. Moreover, each elevator room was at the top of the bridge, over 200 feet above the Harlem River.

Utilizing a 1,500 megahertz antenna, ACT surveyed the floor of each room in two days. The radar was set to penetrate only to the depth of the bottom of the concrete floor. The location of the electrical lines were marked on the floor with spray paint. Now the client can safely demolish the floor of each room knowing the location of the electrical lines.

Unfortunately, void spaces beneath structures such as roads are mostly found something happens. One such situation was in Passaic County, New Jersey, where a portion of a public street was found to contain voids only after a parked automobile car sank 3 feet through the pavement and underlayment. The suspected culprit of the voids settling in the vicinity of 36 inch and 48 inch diameter storm drain pipes.

ACT was contacted to survey a 2,700 foot long, 15 foot wide portion of the street in the vicinity of both storm drain pipes. Utilizing both the 300 megahertz and 500 megahertz antennas, ACT identified all locations of suspect voids from immediately below the road surface to a depth of 10 feet. The location of each void was marked on the ground surface with spray paint and then later provided on a street plan furnished by the client. Following the survey, each void was grouted with cement.

A fuel oil release occurred at a property in southern Connecticut where the suspected bedrock surface was close to (less than ten feet) the ground surface. The Connecticut Department of Protection (CTDEP) required during one portion of the investigation of the release that the exact bedrock surface beneath the site be determined. The CTDEP wanted the bedrock surface determined for two reasons: 1) so that appropriate remedial measures could be evaluated and, 2) the direction of the bedrock slope in relationship to an on-site potable well.

Utilizing a 500 megahertz antenna, ACT performed a one day survey in an area of approximately 3,400 square feet. Following the completion of the survey, ACT was able to provide a letter report indicating that the depth to bedrock beneath the site ranged between six to seven feet and that the slope of the bedrock was toward the east-southeast. Based upon these results, the appropriate remedial measures were designed and implemented. The GPR survey also determined that the bedrock slop was away from the potable well.

A developer was constructing a resort and marina on an island in the Hudson River. A portion of the island was found to contain evidence of a former foundation and a fire pit. Additional research identified this portion of the island as a mid-1800 military fort and training ground. Immediately, the developer coordinated with archeological departments from local colleges to use this portion of the island as training grounds in archeological “digs”.

The developer contacted ACT to identify the extent of the former foundation and possible locations of additional fire pits or other possible dig locations. ACT performed a GPR survey over a three-day period and identified the foundation and numerous other possible dig locations. ACT provided the results of the survey to the client in the field as well as during a follow-up meeting, where a diagram depicting the results of the survey was also provided.