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Geophysical UST Investigation AGS Project 13-084-ICA Linden,California applied in two ways: either directly by connecting the transmitter via a jumper wire to a daylighting portion of a target utility(a metal ground stake is used to complete the circuit),or indirectly,whereby the RD-8000 transmitter is placed on the ground surface within the search area and the signal is transferred to nearby utilities via natural electromagnetic induction. A third approach uses an inductive clamp to apply tracing signals to insulated wires or wires running inside conduit. The RD-8000 also detects metallic utilities by detecting radio signals traveling within them;the radio signals are ambient signals from distant sources(e.g.,a radio station transmitter)that are captured naturally by the utility,which acts as a buried radio antenna. 5.0 FIELD PROCEDURES First,AGS scanned the site with the M-Scope hand-held metal detector to look shallowly buried metal objects. AGS marked the locations of detected metal objects on the ground surface with spray paint. Then AGS scanned the site for buried utilities using the RD-8000 and marked the locations of detected utilities on the ground surface with spray paint. Next,AGS performed the EM61 survey by wheeling the EM61 instrument back-and-forth across the site along both north-south and east-west survey lines spaced approximately two feet apart. The EM61 was programmed to automatically obtain 4 readings per second,which approximately corresponds to one reading every foot along each survey line. Horizontal positioning data were obtained using a Trimble Pro-XR Global Positioning System(GPS)with the GPS antenna mounted over the center of the EM61 sensor coils using non-metallic straps and struts to minimize EM noise. AGS then performed the GPR survey using a special cart system to move GPR antenna across the ground surface in a grid pattern across the survey area. In general,AGS obtained GPR data along lines spaced 2.5 feet apart in the east-west direction and 5 to 10 feet apart in the north-south direction, taking particular care to scan across the pavement cut patches that indicated the UST location. After the EM61 and GPR data were obtained, AGS prepared a map of the investigation area using fiberglass tape measures for reference. In particular,AGS mapped the locations of the garage building and the buried utilities so that the associated EM61 responses would not be mistaken for UST indications.Overall,AGS obtained 1,050 line feet of EM61 data and 475 feet of GPR data for this investigation. 6.0 DATA PROCESSING AND ANALYSIS The M-Scope,RD-8000,and GPR data were analyzed in the field as the investigation progressed. The M-Scope and RF utility locators are designed to produce an audible tone when held near a metallic object; a detected object's location is then pinpointed by adjusting the instrument's sensitivity and monitoring the instrument readout to determine the"peak signal" location. For the GPR survey,AGS monitored the GPR data in the field and to look for definitive"upside-down U"reflection patterns,which would indicate a UST with near certainty. AGS took special care to obtain GPR data in areas where buried metal was detected. AGS then re-examined the GPR data upon returning to the office to look for weaker images that may have gone unnoticed in the field. EM61 data processing was done using the GEOSOFT Oasis montaj earth science software system. A GEOSOFT kriging algorithm was used to prepare color-filled contour maps showing EM61 response p 4 ADVANCED GEOLOGICAL SERVICES