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3 SHALLOW SEISMIC SURVEY In-situ seismic measurements using the seismic refraction technique and the multi-channel array surface wave (MASW) method were made in or around the former drum disposal area at the National Guard Armory in Stockton. California on May 23. The purpose of this investigation was to provide shear wave velocity profiles to a depth of 10 meters (100 feet) at five locations on the site as shown on Figure 1. The seismic refraction survey was conducted to support the surface wave and electrical resistivity measurements. The primary objective to the seismic refraction survey was to determine if a higher velocity soil layer was present within the upper 30 ft, which may be associated with the lower vertical limit of the disposal fill material. 3.1 Seismic Methods Overview Seismic Refraction During seismic refraction surveys, acoustic energy is input to the subsurface by an energy source such as a sledgehammer impacting a metallic plate, weight drop, or explosive charge. The acoustic waves propagate into the subsurface at a velocity dependant on the elastic properties of the material through which they travel. When the waves reach an interface where the density or velocity changes significantly, a portion of the energy is reflected back to the surface, and the remainder is transmitted into the lower layer. Where the velocity of the lower layer is higher than that of the upper layer, a portion of the energy is also critically refracted along the interface. Critically refracted waves travel along the interface at the velocity of the lower layer and continually refract energy back to surface. Receivers (geophones), laid out in linear array on the surface, record the incoming refracted and reflected waves. The seismic refraction method involves analysis of the travel times of the first energy to arrive at the geophones. These first arrivals are from either the direct wave (at geophones close to the source), or critically refracted waves (at geophones further from the source). Multi-Channel Array Surface Wave The multi-channel array surface wave (MASW) method is in-situ seismic method for determining shear wave velocity (Vs) profiles [Stokoe et al., 1994; Stokoe et al., 1989; Park et al., 1999a and 1999b]. Surface wave techniques are non-invasive and non-destructive, with all testing performed on the ground surface at strain levels in the soil in the elastic range (< 0.001%). The basis of surface wave methods is the dispersive characteristic of Rayleigh waves when propagating in a layered medium. The phase velocity, VR, depends primarily on the material properties (Vs, mass density, and Poisson's ratio or compression wave velocity) over a depth of approximately one wavelength. Waves of different wavelengths, k, (or frequencies, f) sample different depths. As a result of the variance in the shear stiffness of the layers, waves with different wavelengths travel at different phase velocities; hence, dispersion. A surface wave dispersion curve, or dispersion curve for short, is the variation of VR with X or f. MASW testing 6243 Versar 5 July 6, 2006