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generally due to changes in lithology, moisture, and/or fines (clay) content, whereas the in-phase <br />component is most sensitive to magnetic susceptibility, generally due to the presence of metallic <br />features in the subsurface, either ferrous or non-ferrous. Note that these are the primary sensitivities, <br />but that both components can be affected by buried metal or geologic features. <br />EM -31 data were recorded at a rate of 10 Hz, using a primary field at a frequency of 9.8 kHz, at an <br />antenna spacing of 12 feet. Global Positioning System (GPS) measurements embedded into the data <br />stream at a rate of 1 Hz to allow for precise data positioning. <br />The EM -31 is generally sensitive to a depth of approximately 10 to 20 feet. This sensitivity range, or <br />the "effective depth", is a function of the orientation and spacing of the transmitter and receiver coils, <br />the frequency of the primary field, and the bulk electromagnetic properties of the subsurface. It is <br />important to understand that each FDEM measurement is effectively a bulk response from subsurface <br />material within range of the generated electromagnetic field; therefore, FDEM results are indicative <br />only of lateral subsurface variations. In the case of the EM -31, depth/thickness of, for instance, <br />geologic anomalies, may be inferred with calibration to ground truth information (e.g., borehole data), <br />but only qualitatively. <br />The primary objective area consisted of approximately 3 -acres (Figure 4). The area of interest was <br />defined by a rectangle centered on a lineament of subsidence features and sinkholes approximately <br />400 feet long along an east -west strike, extending about 200 feet north and south. Data were collected <br />over this site at a nominal transect spacing of 5 feet in an orientation perpendicular to the strike of the <br />features of interest. <br />Figure 4. Site detail map showing data transects in white, and approximate area of interest of subsidence features <br />in red oval. <br />Project No. 6422A 4 Geophysical Report <br />