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x - <br /> .fa <br /> Section 3 <br /> Field Investigation and Methodologies <br /> The following subsurface investigation techniques were implemented'. ground penetrat- <br /> ing radar (GPR), soil gas survey, soil horings, and subsurface soil sample collection and <br /> analysis. The purposes of these techniques are described below. <br /> Task Pur se <br /> GPR Attempt to locate the underground <br /> diesel tank without excavating <br /> Soil Gas Survey Qualitatively delineate the shallow <br /> vertical and lateral extent of contami- <br /> nation from the former gasoline tank. <br /> Assess the potential vertical and hori- <br /> Soil Borings and Sample zontal extent of contamination from <br /> the former gasoline tank and suspect- <br /> ed diesel tank. <br /> Ground-Penetrating Radar Survey <br /> A ground-penetrating radar (GPI:) survey was conducted at the East Miner Avenue <br /> property on April 29, 1991, by Norcal Geophysical Consultants, Inc. (Appendix B). <br /> GPR is capable of delineating subsurface structures (i.e., buried tanks) without the <br /> expense or inconvenience of an excavation. The purpose of the GPR survey was to <br /> attempt to locate a suspected 500-gallon underground diesel tank. <br /> The GPR equipment used for the survey was a Geophysical Survey Systems, Inc. SIR-3 <br /> Subsurface Interface Radar System equipped with a 500 megaHertz (MHz) antenna. <br /> The 500 MHz antenna is near the center of the available frequency range and was used <br /> to provide high resolution at shallow depths. <br /> Figure 3-1 shows the 12-foot by 15-foo: area that was traversed with the GPR equip- <br /> ment. The approximate depth of penetration was 4 fret below ground surf=ace (bgs). <br /> Raw data from the GPR instrumentation were examined during the survey. Distance <br /> intervals between traverses were decreased in areas where GPR anomalies were ob- <br /> served. The report by Norcal is attached as Appendix B. <br /> In order to ensure that the GPR equipment was operating; properly, test traverses were <br /> run over known buried water pipes. Anomalies characteristic of buried pipe were ob- <br /> RU <br /> _..____ _..._.. _____.... served during these test traverses. <br /> SF031605\RPi003.51 3-1 <br />