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Geophysical UST Investigation AGS Project 13-084-ICA Linden,California 2.0 RESULTS AGS detected a buried metal anomaly associated with two pavement cut patches; accordingly, AGS believes the anomaly represents the UST location. The anomaly footprint measures approximately 8 feet by 18 feet,which would equate to a 5,000-gallon UST;however,due to interference from buried utilities and the adjacent garage building,the anomaly exhibits a complex shape that does not readily indicate UST dimensions. In fact,the anomaly's bi-lobed nature suggests that two USTs might be present. AGS also detected a smaller area of more shallowly-buried metal within the large anomaly area. This smaller anomaly area measures approximately 4 feet by 7 feet, although it is worth noting that anomaly dimensions are usually larger than the footprint of the anomaly source object. A 4-by 7-foot anomaly represents a 500-gallon UST(or smaller),which is perhaps a more likely UST size considering the small garage building associated with the site. Investigation results are shown on Figures 2 and 3. Ground penetrating radar(GPR) was used to obtain more definitive information regarding UST size; however,no UST images were observed on the GPR records because the UST is buried deeper than the GPR signal penetration depth,which is estimated to be approximately three feet below ground surface at the Linden site. Several underground utilities were detected in the area,including natural gas,water,and telecommunications. 3.0 SITE DESCRIPTION The investigation was performed within a 40-by 80-foot topographically flat,"L-shaped"area alongside the building at the corner of SR-26 and North Market Street(Figure 2). Two small cut-patches indicating J former excavations were observed in the asphalt pavement,and AGS suspects that the target UST was encountered in one of the excavations. Although the site was free of surface obstructions,the building itself produced high levels of electromagnetic noise,as did the two underground utilities(natural gas and telecommunications)running through the survey area. 4.0 GEOPHYSICAL METHODS AND EQUIPMENT The geophysical investigation was performed using the following geophysical methods: • Time-domain Electromagnetics (EM), using a Geonics EM61 • Ground Penetrating Radar(GPR)using a GSSI SIR-3000 connected to a 400-MHz antenna • Electromagnetic metal detection(MD), using a Fisher TW-6 M-Scope • Radio-frequency utility locating using a RadioDetection Model RD-8000. Time-domain Electromagnetic Metal Detection (TDEM) The TDEM is a method commonly used to detect buried metallic objects at developed sites where surface metal objects, which can interfere with the geophysical measurements, are present. The TDEM instrument transmits a pulsed magnetic field, which causes (induces) small electrical currents (eddy currents) to flow through metallic objects near (below) the instrument. The strength of these eddy currents is measured by the TDEM receiver coil at a relatively long time after the magnetic field pulse subsides. This delayed measurement technique produces a reading that responds strongly to metal but p 2 ADVANCED GEOLOGICAL SERVICES