Metallic and Non-Metallic Pipe Locating
Buried metallic objects including utiltiy conduits, tanks, drums and vaults, can be located using various geophysical technologies. Utilities can be traced using pipe and cable locating equipment (RF/EM) by imparting a signal of known frequency through the cable or conduit and tracking the the strongest signal returns. The signal returns are represented graphically and/or audibly on the receiving instrument. Conduit composition is usually specific to the utiltiy, (e.g. copper for water, cast iron for sewer), and these metals transmit signals differently. Signal transmission and return through copper and cast iron are generally poor due to signal bleed and absorption. Making changes to the frequencies used to trace these conduits may strengthen the signal return, however, the use of a steel snake to trace more distal areas from the source often becomes necessary, especially when tracing drainage networks and sewers. The same procedure is used to impart a signal to the snake, (which is advanced after insertion into the conduit), and trace the utility along the trend of strongest signal return. Again, the most common problem encountered using this method (RF) is signal bleed into the surrounding conduit, resulting in poor signal return.
GPR is an effective tool used to located both metallic and non-metallic pipe and conduit. Return strength of the radar signal is dependent upon the type of substrate being traversed and the differences between the properties of the target (in this case, pipe and conduit) and the surrounding material. Text-book returns from targets that are composed of PVC, copper, cast iron, steel, ductile iron pipe and concrete can be realized where site conditions are ideal. High water and clay-sized particle content in the sediment are two major obstacles to strong radar-signal return. GPR traverses made in a direction perpendicular to the trend of the subsurface pipe, conduit or cable will reveal anomalies that occur at similar depths and positions (typically linear).
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