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t 1 • URS Corporation February 10, 2005 ' Page 2 ' values can be affected by both metallic and nonmetallic debris. IPC values are also affected by the electrical conductivity of the subsurface. However,they are particularly sensitive to the presence of both ferrous and non-ferrous metal objects to depths of approximately 7 to 10 feet. The effect that ' ferrous or nonferrous metal will have on the respective TC or IPC readings is dependent upon the depth and size of the metal object. We performed the EM survey using a Geonics EM31-DL ground ' conductivity meter connected to an Omnidata data recorder. VMG surveys are used to define localized magnetic variations that are caused by ferrous sources. For ' this survey, we used the VMG method to determine the locations of buried metallic (ferrous)debris within the suspected landfill. We used a Geometrics G-858 cesium vapor magnetometer to obtain the vertical magnetic gradient data, since it provides high sensitivity and rapid data acquisition. ' ER surveys are used to measure the electrical resistivity of the subsurface. Variations in the electrical properties may be caused by changes in subsurface materials due to landfill debris. For this survey,we ' used the dipole-dipole electrode configuration.The dipole-dipole array was chosen because it provides information on both the depth and lateral extent of subsurface electrical properties. We acquired the ER data using a Sting RI Resistivity meter with the Swift automatic multi-electrode system. Both ' systems are manufactured by Advanced Geosciences Incorporated. Descriptions of the EM,VMG,and ER methods are provided in Appendix A. ' Horizontal Control Prior to proceeding with the geophysical work, we established a survey grid to provide horizontal ' control. The survey grid was based on a rectangular coordinate system with the origin(OE,ON)located on the fence line to the southwest.We established the grid in the field using a fiberglass measuring tape and marking paint. The marking paint was used to mark the grid nodes every 20 by 100 feet on the ground. Wooden lath were used to mark the grid nodes every 100 by 100 feet. This grid was then used to guide the EM, VMG, and ER surveys. ' Following the geophysical investigations,we used a Trimble Pro XRS global positioning system(GPS) to survey the corners of the horizontal control grid. These coordinates were then used to convert the ' horizontal control grid to a California State Plane coordinate system, as shown on Plates 1 through 4. The GPS was also used to survey the perimeter fence, and the vapor monitoring wells along the perimeter. ' VMG/EM Surveys ' VMG data were obtained every 2 to 4 feet along southeast-northwest trending traverses spaced 10 feet apart,resulting in a total of 13,470 VMG measurement points. EM data were obtained every 10 feet