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Geophysical UST Investigation AGS Project 13-084-ICA Linden,California variations (in millivolts) across the site (Figure 3). As part of the analysis, AGS looked for high- amplitude responses not readily attributable to known site features. Such responses are considered "anomalies"and are attributed to subsurface source bodies,which may include USTs, buried utilities, reinforced concrete foundations, and miscellaneous metallic debris. On the color contour maps, anomalies appear as "hot" (orange, red, and pink) colors representing areas with elevated EM61 measurements indicative of metallic objects. Because the EM61 was designed to produce a positive signal peak at the center of the metallic source body, it tends to produce anomalies with a shape and extent that approximates the footprint of the metal source object. Accordingly,AGS looked for rectangular anomalies with a footprint corresponding to the dimensions of the reported UST; however, it is worth noting that anomaly footprints are usually larger than that of the anomaly source body, and the footprint may also be distorted by nearby surface metal objects,underground utilities,and UST appurtenances. Anomaly amplitudes associated with USTs and similarly-sized metallic substructures depend on burial depth,but they are typically 200 millivolts(mV) or greater. AGS incorporated the site map into the EM61 contour map so that responses associated with surface metal objects and underground utilities could be identified and disregarded from consideration as a possible UST indication. As a further aid to the analysis, data profiles for each survey transect were prepared and inspected. The profiles are especially useful for assessing anomaly amplitudes and for identifying bad data caused by,say,a loose connection within the EM system or other type of equipment malfunction. 7.0 LIMITATIONS of GEOPHYSICAL LOCATING METHODS In general,a geophysical method's limitations for detecting a particular target are related to the target's size, burial depth, the amount of contrast in material properties between the target and surrounding material, and finally, the amount of interference from surrounding site features. For a target to be detected it must have sufficient size to reflect or otherwise disturb some the incoming energy used for detection. It also must have enough contrast with the surrounding material to reflect or otherwise disturb enough of the incoming energy so as to be detected. And, finally, it can't be buried so deeply that the reflected/disturbed energy is so dissipated that it is too weak to be detected when it returns to the surface. Weak energy returns during geophysical investigations are further exacerbated by ambient noise like that produced by natural and cultural features,such as utilities,fences,parked vehicles,vegetative cover,and debris. As stated above and shown on Figures 2 and 3, the Linden site contained buried utilities, and was bounded to the north by a building. These features produced electromagnetic interference that could have responses from buried objects,including a small UST. In addition soil conditions limited the GPR signal penetration to approximately 3 feet bgs;accordingly,objects buried deeper than 3 feet would not detected with GPR. p 5 ADVANCED GEOLOGICAL SERVICES