Velocity imaging of the shallow subsurface using 3D inversion of VSP data to visualize a saturated portion of the vadose zone at Lawrence Livermore National Laboratory

摘要

A VSP study was conducted at an environmentally contaminated site in the southern portion of Lawrence Livermore National Laboratory to characterize the position and geometry of a perched aquifer thought to exist at this site. A triaxial geophone was clamped to the wall of a fiberglass-cased well thought to intersect the perched aquifer. For selected geophone depths (between 2 and 18 m), a sledgehammer source was used at positions to the west, south, and east of the deployment well, with 5-m spacing between shot points and a maximum horizontal distance of 30 m from the well. First-break traveltimes were identified in the data, and these were inverted using a proprietary three-dimensional velocity inversion algorithm. Inversion results indicate velocities increasing from approximately 300 m/s near the surface to approximately 1100 m/s at a depth of 20 m. A thin layer was identified with a velocity approximately 100 m/s faster than its surrounding layers at depth of 13-15 m. This layer appears to thicken to the northeast, and pinch to the southwest, by as much as 50% over a 10-m lateral distance. This high-velocity layer is interpreted to be the perched aquifer in question.