Abstract
3-D frequency-domain full waveform inversion (FWI) is applied on North Sea wide-azimuth ocean-bottom cable data at low frequencies (≤10 Hz) to jointly update vertical wave speed, density and quality factor Q in the viscoacoustic VTI approximation. We assess whether density and Q should be viewed as proxy to absorb artefacts resulting from approximate wave physics or are valuable for interpretation in the presence of soft sediments and gas cloud. FWI is performed in the frequency domain to account for attenuation easily. Multiparameter frequency-domain FWI is efficiently performed with a few discrete frequencies following a multiscale frequency continuation. However, grouping a few frequencies during each multiscale step is necessary to mitigate acquisition footprint and match dispersive shallow guided waves. Q and density absorb a significant part of the acquisition footprint hence cleaning the velocity model from this pollution. Low Q perturbations correlate with low-velocity zones associated with soft sediments and gas cloud. However, the amplitudes of the Q perturbations show significant variations when the inversion tuning is modified. This dispersion in the Q reconstructions is however not passed on the velocity parameter suggesting that cross-talks between first-order kinematic and second-order dynamic parameters are limited. The density model shows a good match with a well log at shallow depths. Moreover, the impedance built a posteriori from the FWI velocity and density models shows a well-focused image with however local differences with the velocity model near the sea bed where density might have absorbed elastic effects. The FWI models are finally assessed against time-domain synthetic seismogram modelling performed with the same frequency-domain modelling engine used for FWI.
Highlights
Besides compressional wave speed, imaging different earth properties (Poisson ratio, density, attenuation, anisotropy, etc.) from broadband long-offset seismic data is a topical issue in full waveform inversion (FWI) as these seismic properties are helpful to decrease the ambiguities about the rocks and fluid types and draw inferences on petrophysical attributes through downscaling approaches at mesoand microscales (Dupuy et al 2016)
This study presents a real 3-D data case study of multiparameter FWI where we jointly update the vertical wave speed (V0), the density (ρ) and the quality factor Q in the viscoacoustic vertical transverse isotropic (VTI) approximation and the 3.5–10 Hz frequency band, while the Thomsen’s parameters δ and are used in a passive way
We have shown with a real-data case study the feasibility of multiparameter frequency-domain FWI of stationary-receiver wideazimuth ocean-bottom cable (OBC) data for the reconstruction of the vertical wave speed, density and Q in the 3.5–10 Hz frequency band
Summary
Besides compressional wave speed, imaging different earth properties (Poisson ratio, density, attenuation, anisotropy, etc.) from broadband long-offset seismic data is a topical issue in full waveform inversion (FWI) as these seismic properties are helpful to decrease the ambiguities about the rocks and fluid types and draw inferences on petrophysical attributes through downscaling approaches at mesoand microscales (Dupuy et al 2016). In FWI, the sensitivity of the seismic response to a local parameter perturbation is represented by the wavefield emitted by the seismic source and scattered by this perturbation, referred to as the partial derivative wavefield (Pratt et al 1998). The secondary source formed by the perturbation has a radiation pattern, which depends on the parameter type and controls the amplitude versus scattering angle variations of the scattered wavefield. The radiation pattern is the only term in the gradient of the FWI misfit function that allows the inversion to discriminate the contribution of different parameter types in the seismic response. The spectral component (i.e. wavenumber vector) of the model perturbation constrained by a seismic event is related to the local wavelength and the scattering angle by k = 2ω cos (θ/2) (cos φ, sin φ) ,
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