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a"K4.k6:' ._ ""r,..,:•�" ,�� '" .�..Fes. ._:.N � _ a-- _ - - __ �`_ _ �.. � `�. su _ �ry ` Ae is Environmental , Enc. t 9 i r~ February 14, 1991 NORCAL Page 2 i .. Our scope of work also includes interpreting the GPR data and I U presenting our findings in a written report. GROUND PENETRATING RADAR Ground penetrating radar (GPR) is a method that provides a con- tinuous, high resolution cross-section depicting variations in rµ the electrical properties of the shallow subsurface. The method } is particularl:,- sensitive to variations in electrical conductivi- c ty and electrical permittivity (the ability of a material to hold a charge when an electrical field is applied) , In operation, the system continuously radiates an electromagnetic pulse into the ground from a transducer (antenna) as it is moved along a traverse. Since most earth materials are transparent to electromagnetic energy, only a portion of the radar signal is reflected back to the surface from interfaces representing varia- p" tions in electrical properties. When the signal encounters a ; metal object, .however, all of the incident energy is ref lec;ad. The reflected signals are received by the same transducer and are transmitted to a graphical recorder . The resulting records can Cprovide information regarding the stratification, the thickness and extent of fill material , the location of buried objects such as utilities, and changes in material conditions such as satura- tian and, .in some. ir?stancas, chemical differences (when the subsurface electrical properties are altered) . Generally, rela- tively high electrical conductivities reduce the penetration capability and limit radar performance. For this investigation, we used a Geophysical Survey Systems, Inc. SIR-3 Subsurface Interface Radar System equipped with a 500. pE; megaHertz (MHz) antenna. This antenna is near the center of the �p available frequency range and was used to provide high resolution at shallow depths. PROCEDURES We obtained OPR data by hand towing the 500 MHz antenna along the �! traverses shown on Plate 1 . Horizontal control along each tra- verse was established by electronically annotating the GPR re- cords at 5 foot intervals. These distances were determined from a fiberglass tape placed on the ground along the length of the — _— -- —traverse.— Following each traverse, we examined the GPR records for reflec tion arms indicative of buried objects. The locations of any inomallas we identified were then marked on the ground surface in the presence of Mr. Kimmerling, we subsequently performed amore thoroughexamination of the records in- the office, and plotted the locations of suspected buried objects on a computer drawn 1 plan view map. r: