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CRC Environmental Risk Management, Inc. May 9, 1995 Page 2 METHODOLOGY NORCAL used the vertical magnetic gradient (VMAG), and ground penetrating radar (GPR) methods to obtain subsurface information. We used the VMAG method to detect magnetic gradients associated with buried metal. We performed GPR surveys to obtain subsurface information in areas where MAG data could not be collected because of interference from above ground structures. The VMAG survey area and the GPR traverses are shown on Plate 1 . Electromagnetic line locating (EMLL) techniques, combined with GPR traverses were implemented to investigate the five borehole locations shown on Plate 1 . We used a Scintrex ENVI-MAG to obtain the vertical magnetic gradient data. This instrument features a built-in memory that stores the vertical magnetic gradient and survey grid information. GPR data were collected with a Geophysical Survey Systems, Inc. SIR-3 Subsurface Interface Radar System equipped with a 500 megahertz (Mhz) antenna. Our EMLL instrumentation consisted of a Radiodetection RD-400 line locator and a Fisher TW-6 inductive pipe and cable locator. DATA ACQUISITION NORCAL collected VMAG data on 5 foot intervals along north-south trending lines spaced 5 feet apart. The length of the lines ranged from 15 to 160 feet. GPR data were collected along north-south trending traverses spaced five feet apart over most of the western portion of the site, as shown on Plate 1 . GPR data were not collected over the magnetic anomalies because of limited signal penetration. DATA ANALYSIS GPR Profiles We examined the GPR records for reflection patterns characteristic of UST's and underground utilities, as well as changes in reflection characteristics that may indicate variations in the site materials. These variations may be associated with previous UST's and utility trench excavations.