ARCHIVED REPORTS_PRELIMINARY ASSESSMENT_SITE INVESTIGATION REPORT

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The system operates by repeatedly radiating an electromagnetic pulse into the ground from a transducer (antenna) as it is moved along a traverse. Since most earth materials are relatively transparent to electromagnetic energy, only a portion of the radar signal is reflected back to the surface from interfaces representing variations in electrical properties. When the signal encounters a metal object, however, virtually all of the incident energy is reflected. The reflected signals are received by the transducer and are printed in cross-section form on a graphical recorder. Buried tanks, drums, and pipes often appear on the records as multiple inverted "V" images imbedded within the more or less horizontal banding produced by the hosting media. Depending upon depth of burial and/orthickness of the individual components of the overburden, the resulting GPR records can provide information regarding the location of UST's, sumps, buried debris, underground utilities, and variations in the shallow site materials. Generally speaking, electrically conductive non-metallic materials such as clay and saturated silt limit radar performance by damping the radar signal. Closely packed reflective objects, such as boulders, cobbles, and steel rebar in concrete generally limit radar performance because of signal scattering. For this investigation, we used a Geophysical Survey Systems, Inc. SIR-2 Subsurface Interface Radar System equipped with a 500 megahertz (MHz) transducer. This transducer usually provides both the resolution and depth penetration for characterizing the shallow depths. • DATA ANALYSIS Computer Processing We up-loaded the VMG data to a portable computer and converted them into a format suitable for contouring. The contouring program then calculated an evenly spaced array (data grid) of values based on the observed field data. Finally, these gridded values were contoured to produce the VMG Contour Map. Contour Map Interpretation Generally speaking, in a region with uniform magnetic conditions VMG values vary smoothly and produce maps with widely spaced contour lines. In areas where variations are strong the contours are closely spaced and are typically considered anomalous when there are no obvious above ground sources causing the strong variations. If the source of a particular anomaly is an isolated object or a group of closely spaced objects, the contours may form circular or elliptical closures. A large accumulation of buried objects may appear as a group of closely spaced, contorted anomalies or a single large anomaly. Elongated objects such as buried rails and pipes often give rise to elongated anomalies with their contour lines oriented parallel to the object, or as a string of circular or elliptical closures aligned along the axis of the utility. Actual anomaly magnitude and shape are dependent on the relative position and size of the buried objects with respect to the location of the data points. In general, anomaly magnitude will • decrease and anomaly width will increase as distance (depth) to the source increases. A-2