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Wright Environmental __ September 15, 1997 NORCAL Page 3 subsurface to a depth of about 10-15 feet. This measurement is sensitive to differences in the subsurface materials that may include earth materials and foreign debris. The IP component measurement is an indication of the electromagnetic signal and is more sensitive to metallic sources that TC. The EMLL equipment was used locate metallic pipelines. The GPR was used over a few representative variations to aid in further identifying possible sources. DATA ACQUISITION A reference grid, consisting of a northing and easting coordinate system, was established over each of the areas of investigation. We used fluorescent pink spray paint and a fiberglass measuring tape to mark a 20 ft by 20 ft grid over the accessible portions of each Parcel. This coordinate system provided horizontal control for all of the geophysical measurements. After the reference grid was established, we performed a reconnaissance scan of each area using EMLL equipment to detect the approximate routings of any underground utilities. This information is useful when interpreting the VMG and EM data. We then collected the VMG and EM data at 10 foot intervals along north-south trending traverses spaced 10 feet apart over the reference grids. GPR data were obtained over a few localized areas as necessary to obtain additional subsurface information regarding possible buried metallic sources. DATA ANALYSIS VMG and EM Data Processina At the completion of the investigations, we downloaded the VMG data, and the terrain conductivity (TC) and in-phase (IP) components of the EM data to a computer for processing. The computer processing included calculating an evenly spaced array of values (grid) based on the observed field data. These gridded values were then computer contoured to produce the Vertical Magnetic Gradient, Terrain Conductivity, and In-Phase Contour Maps shown on Plates 4-8. Contour Ma Interpretation Both above ground metallic sources as well as localized buried objects such as UST's, drums, or consolidated buried debris can be represented on contour maps by closely spaced closed contours. In addition, linear contours, or groups of contour closures forming a linear trend are often caused by laterally continuous features such as utility alignments. Characteristic contour trends that cannot be attributed to known above or below ground sources are considered anomalous and may be likely locations for buried objects or debris.