Waveform tomography and its application at a ground water contamination site


This thesis develops and applies seismic waveform tomography to solve the unique problem of imaging complicated shallow sub-structures with high resolution. Shallow sub-structures are commonly characterized by seismic reflection/refraction imaging, georadar and seismic travel time tomography (e.g., Steeples, 1998; Carcione et al., 2000 and Azaria, 2002). Their resolving power or applicability is often limited. In contrast, waveform tomography, a full wave field inversion technique, resolves sub-structures at a resolution that is a fraction of the illuminating wavelengths. Forward modeling in waveform tomography is based on a finite difference solution to the acoustic wave equation in the space-frequency domain. During inversion for model parameters, the technique efficiently calculates the gradient of a misfit function with respect to model parameters by correlating back-propagated and forward modeled wave fields, avoiding the forbidding task of explicitly computing Frechet kernels. Part of this study compares travel time and waveform tomography in a synthetic cross-well test. The two tomographic approaches are found to be complementary if data contains no significant low frequency spectra. I then apply waveform tomography to two datasets from a ground water contamination site at the Hill Air Force Base (HAFB) to sample formation heterogeneities and to map the 3D geometry of a buried paleo-channel where DNAPLs (Dense Non-Aqueous Phase Liquids) were dumped. The first is a VSP-surface seismic experiment. The final velocity model from waveform tomography applied to the VSP dataset generally correlates well with lithology logs, depth migrated 2D/3D reflection data and a velocity model from 3D travel time tomography. Large velocity variations vertically and laterally (200m/s) occur in a distance as short as ∼1m. The model is interpreted geologically and petrologically. Scale features down to ∼1.5m were recovered. I then apply waveform tomography to 45 2D seismic profiles extracted from a 3-D surface seismic experiment at HAFB, and recover the 3D geometry of a buried paleo-channel acting as a trap for DNAPLs. By combining the identified cross-sectional geometry, the 3D geometry of the channel is reconstructed. The subsurface map could be used to plan injection/extraction well placements with good precision and low cost in the on-going ground water remediation program

Similar works

Full text

DSpace at Rice University

DSpace at Rice University

Provided original full text link
oaioai:scholarship.rice.edu:1911/18630Last time updated on 6/11/2012

This paper was published in DSpace at Rice University.

Having an issue?

Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.