@nilsma

FAST -- Program package for First Arrival Seismic Tomography

. Rice University, Houston, USA, (1998)

Abstract

This program package is for 2D and 3D first arrival traveltime tomography. The models are parameterized on a uniform square grid (velocities specified at equal node spacing in the x,y,z directions). The inverse grid is cell-based with constant cell size in each direction, but the sizes may be different in the x,y,z directions. The node spacing used for the forward grid must divide equally into the x,y,z lengths of the model, and the cell size of the inverse grid must also divide equally into the x,y,z lengths of the model. The forward calculation of traveltimes and raypaths uses the Vidale (1988, 1990) scheme modified to handle large velocity contrasts according to the method of Hole and Zelt (1995). Sources and receivers may be anywhere in the model, although padding the model with at least a few nodes all around is a good idea to avoid having rays hit the edge of the model (and terminate). A point source is assumed. The tomographic method is regularized inversion incorporating a user-specified combination of smallest, flattest and smoothest perturbation constraints, the weights of each being allowed to vary with depth. The regularization is a jumping method in that the constraints are applied to the model perturbation with respect to a background model (usually the starting model). A starting model and iterative approach is employed in which new ray paths are calculated for each iteration. The sparse linear system of equations is solved using the LSQR variant of the conjugate gradient method described by Nolet (1987). An interface may be specified above which the model is held fixed, to allow either a layer stripping procedure or the consideration of marine data in which the interface is the bathymetric surface. The size of the velocity update at each iteration may be bounded according to user-specified limits. The forward modeling and most aspects of the inverse algorithm are described in Zelt and Barton (1998). The package also includes x-window graphics utilities for interactive plotting of ray paths, models, time files, interfaces, etc. and generation of postscript files (the same graphics package used by rayinvr). All codes are written in fortran except for the lowest level x graphics libraries which are written in C. The codes were developed on SPARC SUN workstations, but should be adaptable to any platform with fortran and C compilers without too much effort. It is not necessary to use the x graphics capabilities to compile or run the codes or create the postscript files.

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