A Layer-cell Tomography Method for Near-surface Velocity Model Building Using First Arrivals
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Pure and Applied Geophysics
A Layer-cell Tomography Method for Near-surface Velocity Model Building Using First Arrivals YUKAI WO,1 HUA-WEI ZHOU,1 HAO HU,1 JINGJING ZONG,1 and YINSHUAI DING1 Abstract—Variations in the topography and thickness of the near-surface low-velocity weathering zone negatively impact the quality of onshore seismic data and images. Such impact can be mitigated using accurate near-surface velocity models. We propose to estimate near-surface velocities using a layer-cell tomography method which sequentially combines the model parameterizations of multiscale deformable-layer tomography (DLT) and cell tomography. We first estimate the long-wavelength velocity variations using multiscale DLT, which maps the undulating geometry of the weathering base where the angular coverage of first-arrival raypaths is limited. Then we convert the solution model of multiscale DLT into a cell model, and further determine the shortwavelength velocity variations using cell tomography. We test the proposed layer-cell tomography method using synthetic datasets and a field dataset. The results indicate that the new method improves the resolution of the velocity model from the multiscale DLT, and reduces the artifacts in the velocity solution compared to those observed in the cell tomography method. Reverse time migration of the synthetic data shows that using the velocity model from the layer-cell tomography method gives more accurate images of reflectors. For the field dataset test, the layer-cell tomography method yields a high-resolution near-surface velocity model that could be validated by the improvements in the tomostatic correction results. Keywords: Near surface, tomography, inversion.
1. Introduction Strong variations in the topography and thickness of the near-surface low-velocity weathering layer distort the timing and amplitude of seismic waves. The complexity of the near surface remains a significant challenge for land seismic data processing and imaging. In a way, the near-surface heterogeneities act like a dirty glass blurring the seismic
1 Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77004, USA. E-mail: [email protected]
images of deeper strata. An effective way to solve this problem is to build an accurate near-surface velocity model, upon which the effects of the nearsurface complexities can be estimated and compensated for. Hence, first-arrival tomography has been applied for near-surface velocity model building (VMB) and tomostatic correction since the 1990’s (e.g., Docherty 1992; Zhu et al. 1992, 2008; Stefani 1995; Rajasekaran and McMechan 1996; Zhang and Toksoz 1998; Chang et al. 2002; Bergman et al. 2004). One of the challenges to near-surface VMB using first-arrival tomography is the unevenness of raypath coverage, which is crucial to the fidelity and resolution of the tomography results (Zhou et al. 2009). The distribution of first-arrival raypaths depends mainly on the acquisition geometry and the near-surface geology in terms of seismic velocity st
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