Tomographic velocity model building of the near surface with velocity-inversion interfaces: A test using the Yilmaz model

Abstract

First-arrival traveltime tomography is a popular approach to building the near-surface velocity models for oil and gas exploration, mining, geoengineering, and environmental studies. However, the presence of velocity-inversion interfaces ͑VIIs͒, across which the overlying velocity is higher than the underlying velocity, might corrupt the tomographic solutions. This is because most first-arrival raypaths will not traverse along any VII, such as the top of a low-velocity zone. We have examined the impact of VIIs on first-arrival tomographic velocity model building of the near surface using a synthetic near-surface velocity model. This examination confirms the severe impact of VIIs on first-arrival tomography. When the source-to-receiver offset is greater than the lateral extent of the VIIs, good near-surface velocity models can still be established using a multiscale deformable-layer tomography ͑DLT͒, which uses a layer-based model parameterization and a multiscale scheme as regularization. Compared with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

Joint inversion of Rayleigh-wave dispersion and P-wave refraction data for laterally varying layered models

“…Using layers to build the near-surface model is motivated by the observation that many near-surface geologic features, such as the weathered zone and shallow stratigraphic bedding, resemble layers of variable thickness (Auken and Christiansen, 2004), and that layer parameterization can help in velocity model building in case of poor ray coverage (Liu et al, 2010).…”

3D Multi-scale Tomostatics using First Arrivals

“…After the work of Zhu et al (1992) and Docherty (1992), a widespread of the method is seen (e.g., Rajasekaran and McMechan, 1996;Zhang and Toksöz, 1998;Chang et al, 2002;Bergman et al, 2006;Zhu et al, 2008). In recent years we have published 2D applications of a new deformable layer tomography (DLT) in estimating near-surface statics using velocity models derived from inverting deformable velocity interfaces (Zhou et al, 2009;Liu et al, 2010). We have shown that in many areas the near surface velocities can be more suitably represented by the thickness-varying velocity layers than a regular mesh of velocity grids cells; in these cases DLT outperforms the traditional grid or cell tomography.…”