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iA Appendix A GEOPHYSICAL METHODOLOGY AND INSTRUMENTATION Ground Penetrating Radar (GPR) Ground penetrating radar is a method that provides a continuous, high resolution cross-section depicting variations in the electrical properties of the shallow subsurface The method is particularly sensitive to vanations in electrical conductivity and electrical permittivity (the ability of a material to hold a charge when an electrical field is applied) The system operates by continuously radiating an electromagnetic pulse into the ground from a transducer (antenna) as it is moved along the ground surface Since most earth materials are transparent to electromagnetic•energy, the signal propagates downward into the earth When the signal encounters an interface representing a change in electrical properties, a portion of the energy is reflected back to the surface where it is detected by the same transducer When the signal encounters a metal object, however, all of the incident energy is reflected Tne reflected signals are transmitted by cable to a recorder and are punted in cross-section form Depending upon depth and/or thickness the resulting records can provide information regarding the location of UST's, sumps, buned debris, underground utilities, and variations in the shallow site materials Typically, UST's result in a high-amplitude, parabciic reflections at least as broad as the UST itself • when the traverse is oriented perpendicular to the long axis of the UST Generally, electncally conductive materials such as clay, saturated silt or fill, and relrar can reduce the penetration capability and limit radar performance We used a Geophysical Survey Systems, Inc SIR-2000 Subsurface interface Radar System equipped with a 500 megahertz (MHz) antenna for this survey The 9-00 MHz antenna was used to provide the best balance Detween resolution and depth of penetration Electromagnetic Line Locating (EVILL The EMLL method is used to detect the electromagnetic fieid resulting from an electric current flowing on a line These fields can anse from currents already on the line (passive, or ambient) or currents applied to a tine with a transmitter (active) Tne most common passive signals are generated by live electric lines and re-radiated radio signals Active signals can be introduced by connecting the transmitter to the line at accessible locations or by induction The detection of underground utilities is determined by the composition and construction of tine line in question Utilities detectable with standard Ione location techniques include any continuously connected metal pipes, cables/wires or utilities with tracer wires Unless carrying passive currents these utilities must oe exposed at the surface or in accessible utility vaults These generally include water, electric, natural gas, telephone, and other conduits related to facility operations Utilities that are not aetectabie using stardard electromagnetic line location techniques include those made of non-electrically conductive materials sucn as PVC, fiberglass, vitrified clay, and pipes with insulated connections A-1