Quantitative imaging of complex structures from dense wide‐aperture seismic data by multiscale traveltime and waveform inversions: a case study

Abstract

A B S T R A C TAn integrated multiscale seismic imaging flow is applied to dense onshore wideaperture 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 finitedifference 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 highresistivity 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 wideaperture components of the data, allowing medium wavelengths in addition to the short wavelengths of the structure to be imaged.

Effects of Surface Scattering in Waveform Inversion

“…Brenders and Pratt (2007) also used absorbing boundary conditions when they inverted synthetic crustal-scale refraction data generated with a free surface by a third party, and they were able to image the model, which was not disclosed to them, with great detail. Operto et al (2004) and Bleibinhaus et al (2007) report good results, although they neglected the free surface condition in the inversion of data acquired in mountainous regions with irregular topography. However, Bleibinhaus et al also noted several artifacts in their waveform models, the source of which remained unclear.…”

“…In controlled source seismology the most popular forward solutions are finite difference frequency domain methods, mostly in a 2D viscoacoustic, isotropic implementation (e.g. Pratt, 1999;Hicks and Pratt, 2001;Operto et al, 2004;Ravaut et al, 2004;Operto et al, 2006;Bleibinhaus et al, 2007;Gao et al, 2007). The free surface is often ignored not only because modeling irregular topography is computationally extremely expensive, but also because modeling of free surface multiples requires extremely accurate background velocity and attenuation information, and also accurate correction factors for the geometric spreading of multiples, if the modeling is in 2D (Hicks and Pratt, 2001).…”

Skeletonized wave-equation inversion in vertical symmetry axis media without too much math

“…To avoid local minima in the misfit function, the data are lowpass filtered and then higher frequencies are gradually added to the traces to get more detail in the velocity model (Bunks et al, 1995;Operto et al, 2004). Various strategies are used for increasing the frequency content (Boonyasiriwat et al, 2009).…”

Effects of surface scattering in full-waveform inversion

“…For practical applications, severe simplifications must be employed to make the inversion feasible. In controlled-source seismology, the most popular forward solutions are 2D, isotropic, acoustic or viscoacoustic, and FD frequencydomain methods ͑e.g., Hicks and Pratt, 2001;Operto et al, 2004;Ravaut et al, 2004;Operto et al, 2006;Bleibinhaus et al, 2007;Gao et al, 2007;Malinowski and Operto, 2008͒. Validity of this approximation has been established by a study on a physical scale model by Pratt ͑1999͒.…”