Detecting Perched Water Bodies Using Surface-seismic Time-lapse Traveltime Tomography

Applications of seismic time-lapse techniques generally are constrained to large-scale investigations associated with petroleum exploration and exploitation. There is growing interest in using geophysical methods to monitor near-surface phenomena, such as fluid flow in fractured or karstic bedrock, hydraulic infiltration, and anthropogenic manipulations during environmental remediation. Previous near-surface geophysical time-lapse studies have focused on electrical or electromagnetic (EM) techniques (including ground-penetrating radar) or borehole methods. To evaluate the utility of surface seismic time-lapse traveltime tomography, a site was monitored through time along a single 2D profile. The objective was to attribute increases in seismic P-wave velocity with the development of perched water bodies in the upper 4 m of the subsurface. The study was conducted in the Y-12 area of Oak Ridge National Laboratory in Tennessee, U.S.A., in conjunction with a broader multidisciplinary investigation on the fate and transport of contaminants. Because of previous anthropogenic alterations of the site associated with remediation efforts (e.g., replacing as much as 7 m of contaminated soil with poorly sorted limestone gravel fill during construction of a seepage basin cap), the near-surface hydrogeology was extremely heterogeneous and was hypothesized to have a large influence on differential infiltration, contaminant distribution, and contaminant remobilization. The seismic data were processed using a wavepath eikonal traveltime (WET) tomography approach, and a modified trend-analysis technique was applied to remove the larger spatial component associated with geologic variability. The final "residual" velocity-anomaly images were compared with wellbore hydrologic data and error analyses and were used to interpret the presence and geometry of perched water in the shallow subsurface. The study suggests that velocity estimates obtained from surface-seismic traveltime tomography methods are effective for indicating the spatial and temporal distribution of perched water bodies at the Oak Ridge site in the upper 4 m of the subsurface.