Surface And Borehole Seismic Characterization Of The Boise Hydrogeophysical Research Site

We conducted borehole to borehole, borehole to surface, and surface seismic experiments to optimize data acquisition parameters, obtain a seismic velocity model, and to characterize seismic stratigraphic units in a shallow aquifer at the Boise Hydrogeophysical Research Site. The aquifer consists of coarse (cobble-and-sand) fluvial deposits underlain by clay at 18-2 1.5 m depth. We acquired data with a downhole seismic source (sparker), a sledge hammer source, a hydrophone string, a borehole geophone, and a surface geophone array to compare seismic signal quality and to place limitations on each seismic method. To fully characterize seismic reflections, the source-receiver geometry is an important parameter for both borehole and surface experiments. Direct arrivals and the presence of seismic reflections suggest a correlation between porosity changes and seismic velocities at the site. We have defined four seismic stratigraphic horizons that correlate with surface and borehole ground penetrating radar results, geophysical logs, and lithologic logs. These results provide an initial framework for hydrologic modeling. INTRODUCTION Imaging subsurface boundaries in the upper 20 m using seismic methods in a coarsegrained fluvial environment is often difficult. The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield containing 18 cored boreholes (4 inch diameter) that extend through a coarse-grained cobble-and-sand aquifer and terminate in a clay unit (for additional site details, see Barrash and others, 1999; Clement and others, 1999a). Numerous tests are underway to thoroughly characterize the three-dimensional distribution of lithologic, hydrologic, and geophysical parameters at the site. The overall goal of the project is to develop methods for mapping variations in permeability by combining information from hydrologic and non-invasive geophysical techniques. Large-scale permeability changes often occur at lithologic boundaries, and seismic velocity contrasts often occur at these same boundaries. To investigate the seismic character at this site, we conducted a series of borehole and surface experiments with different sourcereceiver geometries. Many seismic methods have been used to characterize shallow sites, including crosswell tomography (e.g., Hyndman and Harris, 1996), vertical seismic profiling (VSP) methods (e.g., Michaels, 1998; Milligan and others, 1997), and surface seismic reflection methods (e.g., Bachrach and Nur, 1998; Birkelo and others, 1987). Generally, crosswell seismic data are analyzed to generate a smoothed velocity map between boreholes based on first arrival picks. However, reflections in these data can identify discrete seismic boundaries. VSP data are often used to define an interval velocity profile at a borehole, but again reflections may also appear. Surface seismic reflection data is widely acquired to locate seismic boundaries across a site. Often these data suffer from poor resolution in the upper 20 m due to large velocity gradients (Miller and Xia, 1998) and interference from near-surface effects, including ground roll, guided waves, and the air