oa Inversion of Seismic Refraction Shootout First-Arrival Times using Simulated Annealing

The authors have developed a technique to invert first-arrival times using simulated annealing, and this technique will be utilized to analyze the seismic refraction shootout travel time data. the scheme is based on an extremely fast finite-difference solution of the Eikonal equation to compute the first-arrival time through the velocity models by the multistencils fast marching method. the core of the simulated annealing, the Metropolis sampler, is applied in cascade with respect to shots to significantly reduce computer time. Although the technique requires more computer time than some commercial packages, the technique offers several advantages. First, the Inversion technique does not depend on the Initial model, and this becomes important in regions where prior Information about subsurface profiles is not available. Second, rather than just one final model, simulated annealing provides a suite of final models clustering around the global solution and having comparable least-squared error. This provides an Inversion result by averaging all of these accepted models to mitigate the influence of noise and the non-uniqueness of the Inversion solutions. Last, the technique also can determine the uncertainties associated with inverted results. in cases where the Inversion results of subsurface formations are used for the design of engineering structures such as foundations, the uncertainty can be particularly useful in implementing the new load and resistance factor design (LRFD) methodology that can explicitly account for spatial variability and uncertainty in design parameters. the capability of this Inversion technique has been tested with both synthetic and real experimental data sets. the Inversion results show that this technique successfully maps 2-D velocity profiles with high variation. the inverted wave velocity from the real data appears to be consistent with cone penetration test (CPT),geotechnical borings, and standard penetration test (SPT) results