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• NORCAL URS Greiner January 25, 1999 Page 2 South Area For the geophysical survey in the South Area, we used the GPR and EMLL methods. Magnetic ' interference caused by the overhead structure precluded the use of the VMG method in this area. We used the GPR to obtain shallow subsurface information that may indicate the presence of a UST. This method is typically unaffected by above ground metal objects. We ' used the EMLL method to detect near surface metal objects that may indicate an UST. Descriptions of the VMG, GPR, and EMLL methods are provided in Appendix A, of this report. DATA ACQUISITION Horizontal Control ' In both survey areas, site definition and data acquisition were based on a horizontal control grid. We used spray paint to mark the grid nodes on 10 by 10 foot centers. During data acquisition, measurement points between grid nodes were located by pacing. The location ' of the TC and IPC survey boundary, and the GPR traverses, is shown on Plate 1 . The specific locations of the grid nodes are not shown. ' Geophysical Survey In the North Area, we obtained VMG data at 5 foot intervals (stations) along south-north ' trending traverses spaced 5 feet apart. Following data acquisition, we downloaded the data to a portable laptop computer and produced preliminary contour maps in the field. We reviewed these maps for locations of VMG anomalies that may represent UST's. We then ' obtained GPR data over these anomalies along west-east and south-north trending traverses that ranged in length from 30 to 90 feet. The EMLL equipment was operated systematically over the entire survey area. tIn the South Area, we obtained GPR data along south-north trending traverses spaced 5 feet apart. These traverses ranged in length from 60 to 150 feet. The location of the GPR ' traverses are shown on Plate 1 . The EMLL technique was scanned along these same traverses. ' DATA ANALYSIS Computer Processing We down loaded the VMG data to a portable computer using the software package "Envimap" by Scintrex, Ltd. We then used the computer program "Surfer" by Golden Software to ' calculate an evenly spaced array of values (gridded) based on the observed field data. We also used Surfer to contour the gridded values and produce the VMG contour map shown on Plate 2.