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The GPR system operates by repeatedly radiating an electromagnetic pulse into the ground from <br /> a transducer (antenna) as it is moved along a traverse. When the radar signal encounters an <br /> interface representing a change in permittivity (resulting in what is known as an impedance <br /> contrast) some of the electromagnetic energy is reflected back to the surface. Notably, when the <br /> signal encounters a metal object, virtually all of the incident energy is reflected. The reflected <br /> signals are received by the transducer, processed by the control unit, and then printed in cross- <br /> section form by a graphical recorder. The resulting records are then examined to determine the <br /> location and relative depth of buried objects. <br /> The effectiveness of GPR is affected by subsurface electrical conductivity and the contrast in <br /> electrical permittivity between native soils and buried objects. If the subsurface is highly <br /> conductive, the depth of investigation will be limited. If the contrast in permittivity between buried <br /> objects and the native soil is low, the objects won't be detected. If the contrast is high, too many <br /> objects may be detected. This would complicate the process of identifying objects (i.e. <br /> underground utilities) that are the target of the investigation. <br /> For this investigation, we used a Geophysical Survey Systems, Inc. SIR-2 Subsurface Interface <br /> Radar System, equipped with a 500 megahertz (MHz) antenna. This antenna frequency is near <br /> the center of the commonly available frequencies : 25, 100, 300, 500, 900, and 1,000 MHz). Of <br /> all the noted antennas, the 500 MHz antenna usually provides the best balance between target <br /> resolution and signal penetration for the expected targets at this site. <br /> A-2 <br />