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j <br /> Generally speaking,in a region with fairly uniform conductivity conditions the TC values will vary <br /> smoothly from one area to another. Under these conditions, contour lines are usually spaced far <br /> ' apart.In contrast, in those areas where lateral TC variations are stronger, the contours are closely <br /> spaced. In some cases the variations are so strong that the contours become highly contorted.These <br /> contorted contours may form roughly concentric circles suggestive of bull's-eyes, tightly wound <br /> loops and whorls that look like finger prints,or elongated parallel lines. Actual magnitude and shape <br /> 1fi of the contour lines is dependent on the how rapidly the conductivity of the subsurface changes and <br /> if there are any metallic objects that can affect the instrument readings. <br /> I= A series of generally parallelcontour lines typically indicates the source is an elongate object such <br /> . as a building wall,fence,or underground pipeline. If the parallel contours are more or less straight, <br /> then this indicates the object was oriented roughly parallel to the direction of the EM3I's coil boom <br /> during data collection. If the contour lines form a series of parallel, undulating contours (also <br /> referred to as a "herring bone" pattern), then this indicates the source was oriented roughly <br /> perpendicular to the EM31's boom during data collection. <br /> Roughly concentric circles are generally referred to.as monopoles. Monopoles that are roughly <br /> ' limited in extent to the data point spacing of the sampling grid are often caused by relatively small, <br /> Ft near surface metallic objects with limited cross-section. These typically consist of well caps,pull <br /> L boxes,balls of wire, etc. On the other hand,larger monopoles that extend across an area of several <br /> ' data points are typically associated with larger,deeper objects such USTs,concrete pads,backfilled <br /> zones, etc. <br /> Irregularpatterns of loops andwhorls are often indicative of several conductive objects beingpresent <br /> 1 with variable shape, size, conductivity, and distribution. These irregular TC patterns are the most <br /> `- difficult to interpret. Past experience has shown that such patterns are usually associated with debris <br /> fields,landfills,and demolition sites. <br /> Regardless of whether the contours form discrete monopoles,irregular patterns,or parallel lines,if <br /> there are no obvious nearby above ground sources that could cause such variations,then subsurface <br /> objects are suspected. TC contours are typically considered anomalous when differences larger than <br /> a few tens of milliSiemens per meter are displayed from one data station to the next. <br /> Limitations <br /> Buried ferrous in etal o bjects o ften p roduce l arge I ocalized v ariations,o r anomalies, i n t errain <br /> conductivity. As a general rule, anomaly magnitude typically decreases, and anomaly width <br /> increases, as distance (depth) to the source increases. This can make detection of small, deeply <br /> buried metallic objects difficult. In addition,the ability to detect a buried metal object is based on <br /> the intensity of these variations in contrast to the intensity of background variations. The intensity <br /> of background variations is based on the conductivity of the soil and the amount of above and below <br /> ' EF ground metal present within a survey area. Cultural features such as chain link fences,buildings, <br /> debris,railroad spurs,utilities,above ground electric lines,etc.typicallyproduce variations withhigh <br /> A-4 <br /> l:° <br />