High-resolution aquifer characterization using seismic cross-hole tomography: An evaluation experiment in a gravel delta
We performed field experiments at a well-studied aquifer test site to evaluate the geophysical method of seismic cross-hole tomography for high-resolution aquifer characterization. We address the challenge of finding meaningful relationships between seismic velocity and hydraulic conductivity at the field scale for hydraulic conductivity estimation. The resolution of seismic cross-hole data is limited by inversion ambiguities and diffraction effects, which are not accounted for using conventional ray-based inversion algorithms for velocity reconstruction. We investigate the impact of diffraction effects on seismic velocity inversion and implications for hydraulic modelling. Our ray-based seismic tomograms correlate well with sedimentary cores and hydraulic conductivity logs allowing us to interpret a zone model with distinct geophysical/hydraulic zones. We use this zone model to constrain seismic inversions using two approaches for forward modelling: ray tracing (infinite-frequency approximation) and wavefield modelling at a realistic frequency, which accounts for diffraction. We show that using the common ray tracing approach, seismic velocities in low-velocity zones may be significantly overestimated. Based on the available data, we observe a negative linear relationship between logarithmic hydraulic conductivity and seismic velocity. This relationship is used to parameterize hydraulic models based on seismic velocity models. Computer simulations of advective contaminant transport demonstrate that their results are extremely sensitive to an accurate determination of seismic low-velocity zones. We conclude that seismic tomography is indeed very useful for high-resolution aquifer characterization. However, an inversion approach that accounts for diffraction effects, a priori knowledge of the subsurface structure and additional well data should be applied for quantitative interpretations.
Becht, A, Bürger, C, Kostic, B, Appel, E and Dietrich, P. (2007) High-resolution aquifer characterization using seismic cross-hole tomography: An evaluation experiment in a gravel delta. Journal of Hydrology 336: p171-185
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