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2 ELECTRICAL RESISTIVITY IMAGING SURVEY This section presents background information on the electrical imaging methods used to estimate thickness of the disposal area defined during the first phase of the geophysical investigation. A description of the geophysical methods used during this investigation, common applications of the methods, photographs of the instruments, and example applications are included in Appendix A. 2.1 2-D Electrical Resistivity Survey The 2-D electrical resistivity survey was conducted on May 24, 2006. The resistivity equipment used during this investigation consisted of an Advanced Geosciences, Inc. (AGI) SuperStingTm R8/IP memory earth resistivity meter with 56 internal electrode switches, electrode cables with 5 meter takeouts, and stainless steel electrodes. During a resistivity survey, electrical current is applied to a pair of current electrodes, and the potential difference (voltage) is measured between one or more pairs of potential electrodes. For a 2-D resistivity survey, the current and potential electrodes are generally arranged in a linear array. Common array types include pole-pole, dipole-dipole, Schlumberger, and the Wenner array. The Wenner array was evaluated during this investigation. The apparent resistivity is the bulk average of all soils and rock influencing the applied current. It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array being used, as well as electrode spacing. 2-D electrical resistivity lines RL-1 and RL-2 consisted of a single spread of 56 electrodes spaced 3.28 ft apart for a total line length of 180.4 ft. Electrode locations were measured using a 300 ft tape measure and marked with surveyor paint. The elevations of each electrode location were surveyed using a Nikon NPL 362 total station and prism. Maximum vertical errors from the leveling are estimated at about 0.1 ft. Endpoints of each resistivity profile were mapped using the Soklda Axis-3 submeter GPS and later plotted on the attached site map (Figure 1). Various functional checks and site-specific survey parameters were evaluated on site. Array type, number of electrodes, electrode spacing and other electrode parameters were programmed into the SuperSting 'rm R8/IP at the start of data collection. All resistivity data was stored in the internal memory of the SuperSting TM and downloaded to a laptop computer upon completion of the field investigation. Field data files were assigned a name that included array type and electrode spacing. Preliminary modeling of the field data was conducted in the field and completed in the office at a later date using procedures outlined in a following section. 6243 Versar 2 July 6, 2006