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Geophysical UST Investigation AGS Project 13-084-ICA Linden,California very weakly to the electrical properties of the surrounding soil,thus making the EM61 a high-sensitivity metal detector. The EM61's sensitivity is enhanced by the use of two receiver coils; the second coil response can be used to reduce interference caused by nearby power lines and cultural objects such as buildings,vehicles,and chain-link fences. Ground Penetrating Radar(GPR) GPR uses radar technology to produce a graphical profile of the subsurface that shows soil layering and images of buried objects. GPR systems typically use a single transceiving antenna (one that both transmits and receives the radar signal)that is dragged along the ground surface. The antenna emits a radar pulse into the ground; some of the radar energy reflects off of interfaces between materials with different electrical properties(e.g.,soil and a UST)and returns to the surface where it is detected by the antenna and sent via the cable to a separate control unit where it is amplified and displayed on a computer screen as a vertical"wiggle trace,"which is a plot of the strength(amplitude)of the received GPR signal (i.e., the reflection) over time. Although the vertical scale of a GPR profile is usually considered as depth,it actually measures the travel time of the radar pulse from the surface to a reflecting interface and back to the surface. A subsurface profile is built as the antenna is pulled along the survey line and successive wiggle traces are recorded. GPR data are usually displayed as an array of closely-spaced traces; this procedure produces an image of the subsurface as the reflections(wiggles)on adjacent traces merge into coherent patterns. Soil layer boundaries appear as laterally continuous horizontal bands across a GPR profile. Buried objects appear as localized, high-amplitude (dark) reflection patterns. Buried pipes and USTs exhibit a characteristic"upside down U"hyperbolic pattern,which allows them to be readily identified on a GPR record. Buried refuse often appears as zones of chaotic reflection patterns that disrupt the horizontal layering on a GPR profile. Although GPR can be subject to significant investigation depth limitations, it is used for utility locating because it has the potential to detect non-metallic utilities, whereas RF and EM locating methods will detect only metallic utilities. Burial depths are determined by using calibrating GPR profiles with images objects buried at known depths. Culverts and storm drain pipelines observed in drop inlets are often used for this purpose. Electromagnetic Metal Detecting Electromagnetic metal detection (MD) is used to find shallowly buried localized metal masses (e.g., a buried manhole cover) that cannot be readily detected by RF locating systems. MD locating systems have a pair of wire coils(transmitter and receiver coils);the receiver soil is first"tuned"to a null position with respect to the magnetic field emanating from the transmitter coil. When the MD locator is held near a metal object,the magnetic field becomes distorted and the system is thrown"out of tune." MD devices are designed to emit an audible tone when they are out of tune,thus signaling the presence of a nearby metal object. RF Utility Locating Briefly,the RD-8000 system comprises a transmitter and a receiver;it locates energized electrical power lines by detecting the magnetic field associated with flowing electrical current. Additionally,the RD- 8000 locates metal pipes and unenergized cables by detecting the magnetic field associated with a tracing signal(a weak electrical current)that is applied with the system transmitter. The tracing signals can be p 3 ADVANCED GEOLOGICAL SERVICES