A modified reconstruction algorithm for Fresnel volume tomography
Grandjean and Sage (2004) have presented a 2D seismic tomography software for estimating the velocity field using the first arrival traveltimes in paper ‘‘JaTS: A fully portable seismic tomography algorithm based on Fresnel wavepaths and a probabilistic reconstruction approach’’, Computers & Geosciences 30 (2004) 925–935. Unfortunately, the reconstruction algorithm includes errors in the paper. I correct the errors and present a modified reconstruction algorithm for Fresnel volume tomography.
A layer-stripping method for 3D near-surface velocity model building using seismic first-arrival times
In order to improve the efficiency of 3D near-surface velocity model building, we develop a layer-stripping method using seismic first-arrival times. The velocity model within a Common Mid-Point (CMP) gather is assumed to be stratified into thin layers, and the velocity of each layer varies linearly with depth. The thickness and velocity of the top layer are estimated using minimum-offset first-arrival data in a CMP gather. Then the top layer is stripped and the second layer becomes a new top layer. After removing the effect of the top layer from the former first-arrival data, the new first-arrival data are obtained and then used to estimate the parameters of the second layer. In this manner, the velocity model, being regarded as that at a CMP location, is built layer-by-layer from the top to the bottom. A 3D near-surface velocity model is then formed using the velocity models at all CMP locations. The tests on synthetic and observed seismic data show that the layer-stripping method can be used to build good near-surface velocity models for static correction, and its computation speed is approximately hundred times faster than that of grid tomography.
Local earthquake tomography between rays and waves: fat ray tomography
Abstract The limitations of ray-based forward solutions in seismic tomography are theoretically well known. To correctly represent the physical forward problem in seismic tomography, application of full three-dimensional (3-D) wave theory would be required. Up to now this is not possible for the size of a typical local earthquake study. With the concept of fat rays resembling the waves Fresnel volume, a more complete, physically consistent and accurate solution to the forward problem is available. In this paper, we present an approach to include fat rays in local earthquake tomography, called Fatomo. The comparative study with synthetic data and inversion results with Fatomo and a ray-based approach to local earthquake tomography, SIMULPS, reveals new insights into the role of resolution and model parameterization in local earthquake tomography. Intuitively expected effects of fat rays on resolution estimates, such as higher node sampling values and lower resolution diagonal element values for wider fat rays can be seen in the results. For ideal model parameterization, differences between fat ray and ray tomography are small. Our results document, however, that the influence of model parameterization is less critical for fat ray tomography than for ray tomography.