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1 * • 1 NORCAL rFor this investigation, we used a Geophysical Survey Systems, Inc. SIR-2 Subsurface Interface Radar System equipped with a 500 megahertz (MHz) transducer. This transducer is near the r center of the available frequency range and is used to provide high resolution at shallow depths. 1 Electromagnetic Line Location (EMLL) Electromagnetic line location techniques are used to locate the magnetic field resulting from r an electric current flowing on a line. These magnetic fields can arise from currents already on the line (passive) or currents applied to a line with a transmitter (active). The most common passive signals are generated by live electric lines and re-radiated radio signals. 1 Active signals can be introduced by connecting the transmitter to the line at accessible locations or by induction. r The detection of underground utilities is determined by the composition and construction of the line in question. Utilities detectable with standard line location techniques include any continuously connected metal pipes, cables/wires or utilities with tracer wires. Unless r carrying a passive current these utilities must be 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 detectable using standard electromagnetic 1 line location techniques include those made of non-electrically conductive materials such as PVC, fiberglass, vitrified clay, and pipes with insulated connections. 1 The induction mode is also used to detect buried near surface metal objects such as rebar, manhole covers, and various metallic debris. This is done by holding the transmitter-receiver unit above the ground and continuously scanning the surface. The unit utilizes two orthogonal 1 coils that are separated by a specified distance. One of the coils transmits an electromagnetic signal (primary magnetic field) which in turn produces a secondary magnetic field about the subsurface metal object. Since the receiver coil is orthogonal to the transmitter coil, it is 1 unaffected by the primary field. Therefore, secondary magnetic fields produced by buried metal will generate an audible response from the unit. The peak of this response indicates when the unit is directly over the metal object. r Our instrumentation for this investigation consisted of a Radiodetection RD-400 line locator and a Fisher TW-6 inductive pipe and cable locator. 1 1 r r