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. GROUND PENETRATING RADAR INVESTIGATION 1.0 INTRODUCTION Condor Earth Technologies, Inc (Condor) performed a Phase H Environmental Site Assessment at the request of the City of Stockton, Department of Housing and Redevelopment (DHR) for the properties (site) comprising the six-block area bounded by E Miner Avenue to the north, N Stamslaus Street to the west, E Weber Avenue to the south, and the Southern Pacific Railroad tracks to the east in the city of Stockton, California The ground penetrating radar (GPR) investigation was performed to garner additional information pertaining to the subsurface conditions at the proposed boring locations on selected private properties within the site The Phase II ESA fieldwork was performed between May 13 and September 16, 2002 A GPR investigation of selected portions of the site was conducted from May 13 to 29, and August 27 to 30, 2002 A total of 936 GPR data lines were collected from 28 private properties within the site The GPR investigation was performed to meet two objectives The primary objective of the investigation was to garner additional information pertaining to the subsurface conditions at the proposed boring locations on selected private properties within the site The secondary objective was to investigate certain properties or portions thereof for evidence of suspect environmental conditions, such as the potential presence of underground storage tanks (USTs) and other buried objects In addition, the GPR investigation was conducted to supplement other safety precautions regarding the location of proposed boreholes The GPR investigation provided additional assurance of the feasibility of drilling at a particular location for two reasons notification of USA does not generally result in identification of utility locations on private properties, and some utilities may be buried at depths greater than five feet bgs The following sections include a detailed description of GPR technology, an explanation of the investigation methodology and instrumentation, and a discussion of the investigation findings and results Figures referenced in this section are located in Appendix A of Condor's Phase II Environmental Site Assessment Report, Multi-Modal Station "Six-Block Project" Stockton, San Joaquin County, California dated January 6, 2003 1.1 BACKGROUND INFORMATION GPR systems generally consist of a transrrutting antenna and a receiving antenna that radiate and receive electromagnetic waves, respectively In many systems, a single antenna performs both functions The system records the travel time of the electromagnetic waves as they propagate through the ground and return to the instrument after the signals are reflected from underground features GPR impulse reflections from an underground feature indicate a change in dielectric properties across the interface of the feature Since dielectric property differences exist between air and earth materials, different types of earth materials, and even wet and dry soil of the same type, will generate radar impulse reflections in the radargram Manufactured objects are especially conspicuous in radargrams, as a significant difference in dielectric properties exists between earth materials and the steel walls of a UST, for example Additionally, buried objects like USTs, pipes, and drums are generally small features relative to the overall subsurface geology, and the reflection patterns from them can be distinct Processing the raw data often eliminates background noise and radar effects to generate a clearer representation of the underground features of the study area