Fuchun Gao

Waveform tomography at a groundwater contamination site: Surface reflection data

Wehaveappliedacoustic-waveformtomographyto452D seismic profiles to image the 3D geometry of a buried paleochannel at a groundwater-contamination site at Hill Air Force Base in Utah. The paleochannel, which is incised into an alluvium-covered clay aquitard, acts as a trap for dense nonaqueous-phase liquids DNAPLs that contaminate the shallowest groundwater system in the study area. The 2D profiles were extracted from a 3D surface reflection data set. First-arrival traveltime tomography provided initial velocity models for the waveform tomography. We inverted for six frequency components in the band 30‐90 Hz of the direct and refracted waves to produce 45 2D velocity models. The flanks and bottom of a channel with a maximum depth of about 15 m were well modeled in most of the 45 parallel 2D slices,whichallowedustoconstructa3Dimageofthechannel by combining and interpolating between the 45 image slices. The 3D model of the channel will be useful for siting extraction wells within the site remediation program.The alluvium that fills the channel showed marked vertical and lateral velocity heterogeneity. Traveltime tomography and waveform tomography can be complementary approaches. Used together, they can provide high-resolution images of complicatedshallowstructures.

Waveform tomography at a groundwater contamination site: VSP-surface data set

Application of 2D frequency-domain waveform tomography to a data set from a high-resolution vertical seismic profiling (VSP) experiment at a groundwater contamination site in Hill Air Force Base (HAFB), Utah, reveals a surprisingly complicated shallow substructure with a resolution of approximately 1.5 m. Variance in the waveform misfit function is reduced 69.4% by using an initial velocity model from first-arrival traveltime tomography. The waveform tomography model suggests (1) a low-velocity layer at 1 to 4 m depth, (2) a high-vertical-velocity gradient of 80 m/s/m on average, and (3) severe lateral variations — velocity contrasts as large as about 200 m/s occur in a distance as short as 1.5 m. The model is well correlated with lithologic logs and is interpreted geologically. A Q-value of 20 is estimated for the target area. The extreme lateral and vertical variations of the subsurface compromise many standard seismic processing methods.

Seismic velocity, Q, geological structure and lithology estimation at a ground water contamination site

A high resolution velocity model was obtained by applying waveform tomography to a VSP-surface dataset acquired at a ground water contamination site. This study estimates the Q value distribution of the target area by further iteration of waveform tomography. Large Q values (~50) are identified near one of the boreholes. The model correlates well with a depth-migrated image using the same dataset. Using HertzMindlin theory and Gassmann’s equation, a 1D velocity model derived from the 2D waveform tomography model is inverted to further characterize the material in terms of parameters such as porosity and degree of cementation. We found a large porosity layer in the depth range of 7-10 m and low cementation layers in the depth ranges of 2-3 m and 5-8 m.