A parameterization study for elastic VTI full-waveform inversion of hydrophone components: Synthetic and North sea field data examples

Abstract

Choosing the right parameterization to describe a transversely isotropic medium with a vertical symmetry axis (VTI) allows us to match the scattering potential of these parameters to the available data in a way that avoids a potential tradeoff and focuses on the parameters to which the data are sensitive. For 2D elastic full-waveform inversion in VTI media of pressure components and for data with a reasonable range of offsets (as with those found in conventional streamer data acquisition systems), assuming that we have a kinematically accurate normal moveout velocity (vNMO) and anellipticity parameter η (or horizontal velocity vh) obtained from tomographic methods, a parameterization in terms of horizontal velocity vh, η, and ϵ is preferred to the more conventional parameterization in terms of vv, δ, and ϵ. In the vh, η, and ϵ parameterization and for reasonable scattering angles (<60°), ϵ acts as a “garbage collector” and absorbs most of the amplitude discrepancies between the modeled and observed data, more so when density ρ and S-wave velocity VS are not inverted for (a standard practice with streamer data). On the contrary, in the vv, δ, and ϵ parameterization, ϵ is mostly sensitive to large scattering angles, leaving vv exposed to strong leakages from ρ mainly. These assertions will be demonstrated on the synthetic Marmousi II as well as a North Sea ocean bottom cable data set, in which inverting for the horizontal velocity rather than the vertical velocity yields more accurate models and migrated images.