Two-dimensional inversion of refraction traveltimes by progressive model development

We develop a systematic approach for the analysis of 2-D refraction experiments using traveltimes that allows progressive improvement of velocity structure through a sequence from 1-D models to pseudo-2-D models, and then 2-D models. The approach consists of three steps. First, 1-D velocity models are constructed for each segment of the pro¢le using a genetic algorithm inversion, and then pseudo-2-D models are constructed using a turning point approximation. The purpose of this step is to provide an approximate image of 2-D velocity structure, and to infer the number and general location of layer boundaries. The second step uses 1-D layered modelling, again with a pseudo-2-D conversion, to generate a rough 2-D layered structure. The third step consists of smoothing the pseudo-2-D model in order to create initial models for use in 2-D inversion and the construction of a 2-D model using a Bayesian formulation of non-linear iterative inversion. All the steps exploit traveltime data for the ¢rst arrival and do not use any trial-and-error forward modelling. The progressive approach is e¤cient because the results of each step are employed as the initial model for the next step. The method is applied to real data from an along-strike experiment and also to a related synthetic example so that the quality of the solution can be judged. The results indicate that the method is robust, and this is con¢rmed by a further synthetic example that represents a survey across a trench and dipping subduction zone. The systematic approach to the inversion of refraction data enables a complex inversion to be undertaken in 1 or 2 days. The sequential approach allows the incorporation of additional information if desired.