ALBERT

Notes: An integrated multiscale seismic imaging flow is applied to dense onshore wide-aperture seismic data recorded in a complex geological setting (thrust belt).An initial P-wave velocity macromodel is first developed by first-arrival traveltime tomography. This model is used as an initial guess for subsequent full-waveform tomography, which leads to greatly improved spatial resolution of the P-wave velocity model. However, the application of full-waveform tomography to the high-frequency part of the source bandwidth is difficult, due to the non-linearity of this kind of method. Moreover, it is computationally expensive at high frequencies since a finite-difference method is used to model the wave propagation. Hence, full-waveform tomography was complemented by asymptotic prestack depth migration to process the full-source bandwidth and develop a sharp image of the short wavelengths. The final traveltime tomography model and two smoothed versions of the final full-waveform tomography model were used as a macromodel for the prestack depth migration.In this study, wide-aperture multifold seismic data are used. After specific preprocessing of the data, 16 frequency components ranging from 5.4 Hz to 20 Hz were inverted in cascade by the full-waveform tomography algorithm. The full-waveform tomography successfully imaged SW-dipping structures previously identified as high-resistivity bodies. The relevance of the full-waveform tomography models is demonstrated locally by comparison with a coincident vertical seismic profiling (VSP) log available on the profile. The prestack depth-migrated images, inferred from the traveltime, and the smoothed full-waveform tomography macromodels are shown to be, on the whole, consistent with the final full-waveform tomography model. A more detailed analysis, based on common-image gather computations, and local comparison with the VSP log revealed that the most accurate migrated sections are those obtained from the full-waveform tomography macromodels. A resolution analysis suggests that the asymptotic prestack depth migration successfully migrated the wide-aperture components of the data, allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged.The processing flow that we applied to dense wide-aperture seismic data is shown to provide a promising approach, complementary to more classical seismic reflection data processing, to quantitative imaging of complex geological structures.