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Metal Detection(MD) I <br /> Methodolo¢v I <br /> This method uses the principle of electromagnetic induction to detect shallowlyburied metal objects <br /> such as USTs,metal utility conduits,rebar in concrete,manhole covers,and various metallic debris. " I <br /> This is done by carrying a hand-held radio transmitter-receiver unit above the ground and <br /> continuously scanning the surface. A primary coil broadcasts a radio*signal from a transmitter. This <br /> primaryradio signal induces secondary electrical currents in metal objects. These secondary currents I <br /> in turn a produce magnetic field which is detected by the receiver. <br /> Instrumentation I <br /> The MD instrument that we typically use for shallow subsurface investigations is aFisher TW-6 pipe <br /> and cable locator. This instrument is expressly designed to detect metallic pipes, cables, USTs, I <br /> manhole covers, and other large, shallowly buried metallic objects. The instrument operates by <br /> generating both a meter reading(unitless) and an audible response when near a metal object. The <br /> peak instrument response usually occurs when the unit is directly over the object. I <br /> Data Analysis <br /> The TW-6 does not provide a recordable data output that can be used for later computer processing. I <br /> Results are generally limited to marking the interpreted outlines of detected objects in the field and <br /> mapping their locations. I <br /> Limitations <br /> In general,the response of the MD instrument is roughly proportional to the horizontal surface area I <br /> of near surface buried objects (typically in the upper three or four feet). This relationship can be <br /> used to advantage in discriminating between metal debris,reinforced concrete pads,and pipelines. <br /> However,inthepresence ofabove groundmetal objects such as fences,walls,parked cars,andmetal <br /> debris, this is no longer valid. In some instances, the presence of such objects can make it very <br /> difficult to determine whether the instrument responses are associated with below ground targets or I <br /> above ground cultural features. When multiple sources are present it may not be possible to identify <br /> individual targets. Also,relatively large objects that have a limited horizontal cross-section such as <br /> well casing and fence posts are sometimes difficult to detect. <br /> I <br /> I <br /> A- 1 <br /> _ J <br />