148 Seismic Imaging Here, we focus on two field examples that target shallow structures, the final results of which emphasize the advantage of applying hybrid seismic methods to provide more accurate geophysical models. The first example presents a refraction-reflection imaging strategy with the capability to evaluate reflectivity information from the acquisition surface. Depending on the minimum offset defined for the survey, standard reflection imaging techniques usually start to image the reflectivity parameter a few meters below the surface, therefore refracted arrivals are used to complete the reflectivity features for the shallowest structures. The procedure involves three steps: • construction of a depth velocity model from first arrival times, accomplished iteratively by tomographic inversion, • construction of a time reflectivity section from the reflected waves of direct and reverse shot gathers by classical reflection seismic processing. Generally, this is the most critical step in the imaging procedure, due to the low fold of reflection data, • extension up to the surface of the time reflectivity section by converting the shallowest depth velocity model to time reflectivity, associated with velocity contrasts in the subsurface. The time reflectivity sections require a factor scale before being gathered in a final time reflectivity section. As this hybrid approach has the capability to start imaging from the surface, it is a very useful tool for providing reflectivity information for targets located in the near and/or very near surface, which is often required for the monitoring of civil engineering structures, in environmental engineering studies and even archaeological exploration. The second example described in this chapter relates to another hybrid seismic strategy for refraction-surface waves imaging. When a compressional wave source is used in surface seismic surveys, more than two-thirds of the total seismic energy generated is usually imparted into Rayleigh waves, the principal component of ground roll. This hybrid seismic technique addresses this issue by combining information about the P-wave velocity provided by the refraction arrivals with the S-wave velocity distribution obtained from the surface wave data, also presented on the same field records. The velocity-estimation procedures include the following steps: • construction of a P-wave velocity model from first arrival times accomplished iteratively by tomographic inversion. A large range of initial models are used to estimate the sensitivity and depth of the investigation. The final P-wave velocity model is an average of all models satisfying the picked field data within a predetermined fitting level; • construction of an S-wave velocity model from the analysis of surface waves in the frequency-phase velocity domain. After field data windowing for the validation of a 1D model hypothesis; the experimental dispersion curve is easily identified in the f-k domain and the location of maxima energy can be picked.
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