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5 m from the end geophone locations. The 3 lb hammer was used for the 0 and 1 m offset source and the 20 lbs sledge hammer was used for the remaining shot points. All field data was saved to hard disk and documented on field data acquisition forms. 3.3 Seismic Data Reduction and Modeling Seismic refraction data were modeled using the generalized reciprocal method (GRM), as outlined in Palmer (1980 and 1981), Lankston and Lankston (1986), and Lankston (1990). GRM is a seismic-refraction interpretation method designed to accurately map undulating refractor surfaces from in-line refraction data using both forward and reverse shots. The method is related to the Hales (1958) method and the reciprocal method (Hawkins, 1961). The first step in data processing consisted of picking the arrival time of the first energy received at each geophone (first arrival) for each shot point. The first arrivals on each seismic record are either a direct arrival from a compressional (P) wave traveling in the surface layer, or a refracted arrival from a subsurface interface where there is a velocity increase. First-arrival times were selected using the program FIRS TPIXTm, by Interpex Limited (1993). These first arrival times were saved in ASCII files containing each geophone location and its first arrival time. Errors in the first arrival times were quite variable with error generally increasing with distance from the shot point. First arrival picking errors probably averaged about 1 ms with error probably less than 0.5 ms at geophone locations near the shot point and up to 2 ms at distal geophone locations. A first arrival was not picked if the potential error was considered too great. The seismic refraction data were processed using the GRM computer program VIE WSEIS (Kassenaar, 1989-1992). For each seismic line the first arrival and elevation data files were entered into the program, and time-distance plots for the forward and reverse shots were generated. Forward shots are shot points where energy travels from geophone 1 to 24. Energy travels in the opposite direction for reverse shots and shots inside each spread (interior and center shots) have both forward and reverse components. The first arrival data for all the shot points were then assigned to the layer from which they were refracted. Two layers were assigned corresponding to a clayey fill / soil layer and a more consolidated soil unit. The travel time data refracted from the consolidated soil layer were then phantomed (shifted in time) to line up with the travel-time data associated with the zero-offset end shot, therefore forming a single travel- time curve for each refractor along the line. This method was employed for both forward and reverse shots according to the procedures outlined in Lankston and Lankston (1986) and Redpath (1973). After phantoming was completed, the GRM method was used to generate depth models for each seismic line. The MASW data was reduced using the software PICKWIN95 developed by Oyo Corporation and the following steps: Input seismic record into software Enter receiver spacing, geometry and wavelength restrictions, as necessary Apply wavefield transform to seismic record to convert the data to phase velocity — frequency space Identify and pick dispersion curve 6243 Versar 7 July 6, 2006