Detecting perched water bodies using surface seismic time-lapse travel-time tomography

Seismic timelapse methods have typically been constrained to large-scale geologic investigations associated with petroleum exploration and exploitation; however, there is growing interest in using geophysical methods for monitoring near-surface phenomena such as fluid flow in fractured or karstic geologic media, hydraulic infiltration, and near-surface anthropogenic manipulations during environmental remediation. To demonstrate additional utility of surface seismic time-lapse travel-time tomography for near-surface investigations, we monitored a site through time with the objective of attributing increases in seismic P-wave velocity with development of a perched water body in the vadose zone. Our study was conducted at the Oak Ridge National Laboratory, USA, during a multi-disciplinary investigation on the fate and transport of contaminants. Seismic data are processed using a wavepath eikonal traveltime tomography approach, and a modified trend analysis technique is applied to remove the regional velocity component. The final residual velocity anomaly images are compared to the wellbore hydrologic data and error analysis, and used to interpret the presence or absence of a perched water body. Our study suggests that velocity estimates, obtained from surface seismic traveltime methods, are effective for indicating spatial and temporal distribution of near-surface perched water bodies.