Crosswell seismic imaging in a contaminated basalt aquifer

Abstract

Multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at Idaho National Engineering and Environmental Laboratory. Most of these surveys used a high‐frequency (1000–10,000 Hz) piezoelectric seismic source to obtain P‐wave velocity tomograms. The P‐wave velocities range from less than 3200 m/s to more than 5000 m/s. Additionally, a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site. This source, known as an orbital vibrator, allows simultaneous acquisition of P‐ and S‐waves at frequencies of 100 to 400 Hz, and acquisition over larger distances. The velocity tomograms show a relationship to contaminant transport in the groundwater; zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows. The orbital vibrator data show high Vp/Vs values, from 1.8 to 2.8. In spite of the lower resolution of orbital vibrator data, these data were sufficient for constraining hydrologic models at this site while achieving imaging over large interwell distances. The combination of piezoelectric data for closer well spacing and orbital vibrator data for larger well spacings has provided optimal imaging capability and has been instrumental in our understanding of the site aquifer's hydrologic properties and its scale of heterogeneity.