Multiparameter full waveform inversion of multicomponent ocean-bottom-cable data from the Valhall field. Part 1: imaging compressional wave speed, density and attenuation

Abstract

Multiparameter full waveform inversion (FWI) is a challenging quantitative seismic imaging method for lithological characterization and reservoir monitoring. The difficulties in multiparameter FWI arise from the variable influence of the different parameter classes on the phase and amplitude of the data, and the trade-off between these. In this framework, choosing a suitable parametrization of the subsurface and designing the suitable FWI workflow are two key methodological issues in non-linear waveform inversion. We assess frequencydomain visco-acoustic FWI to reconstruct the compressive velocity (V P ), the density (ρ) or the impedance (I P ) and the quality factor (Q P ), from the hydrophone component, using a synthetic data set that is representative of the Valhall oil field in the North Sea. We first assess which of the (V P , ρ) and (V P , I P ) parametrizations provides the most reliable FWI results when dealing with wide-aperture data. Contrary to widely accepted ideas, we show that the (V P , ρ) parametrization allows a better reconstruction of both the V P , ρ and I P parameters, first because it favours the broad-band reconstruction of the dominant V P parameter, and secondly because the trade-off effects between velocity and density at short-to-intermediate scattering angles can be removed by multiplication, to build an impedance model. This allows for the matching of the reflection amplitudes, while the broad-band velocity model accurately describes the kinematic attributes of both the diving waves and reflections. Then, we assess different inversion strategies to recover the quality factor Q P , in addition to parameters V P and ρ. A difficulty related to attenuation estimation arises because, on the one hand the values of Q P are on average one order of magnitude smaller than those of V P and ρ, and on the other hands model perturbations relative to the starting models can be much higher for Q P than for V P and ρ during FWI. In this framework, we show that an empirical tuning of the FWI regularization, which is adapted to each parameter class, is a key issue to correctly account for the attenuation in the inversion. We promote a hierarchical approach where the dominant parameter V P is reconstructed first from the full data set (i.e. without any data preconditioning) to build a velocity model as kinematically accurate as possible before performing the joint update of the three parameter classes during a second step. This hierarchical imaging of compressive wave speed, density and attenuation is applied to a real wide-aperture ocean-bottom-cable data set from the Valhall oil field. Several geological features, such as accumulation of gas below barriers of claystone and soft quaternary sediment are interpreted in the FWI models of density and attenuation. The models of V P , ρ and Q P that have been developed by visco-acoustic FWI of the hydrophone data can be used as initial models to perform viscoelastic FWI of the geophone data for the joint update of the compressive and shear wave speeds.

Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods

“…The latest tool for assimilating this information into seismic velocity models is fully three-dimensional (3-D) waveform tomography [e.g., Tarantola, 1988;Tromp et al, 2005;Chen et al, 2007aChen et al, , 2007bBen Hadj Ali et al, 2009a, 2009bChen, 2011;Fichtner, 2011;Lekic and Romanowicz, 2011;Liu and Gu, 2012;Colli et al, 2013;Fichtner et al, 2013;French et al, 2013;Prieux et al, 2013;Schiemenz and Igel, 2013]. Full-3-D tomography (F3DT) accounts for the physics of wave excitation and propagation by numerically solving the inhomogeneous equations of motion for a heterogeneous, anelastic solid [Olsen et al, 1995;Komatitsch and Tromp, 1999;Cui et al, 2010;Peter et al, 2011].…”

Time-lapse full-waveform inversion with ocean-bottom-cable data: Application on Valhall field

“…A permanent OBC system was installed in 2003 to enable frequently repeated time-lapse surveys to help manage the field. Due to the wide aperture and high quality of the surveys, numerous studies on 2D and 3D FWI use Valhall data (e.g., Sirgue et al, 2009;Prieux et al, 2011Prieux et al, , 2013Liu et al, 2013;Schiemenz and Igel, 2013). Barkved et al (2010) discuss the potential business impact of FWI and time-lapse FWI on Valhall, but technical details and comparisons between time-lapse FWI approaches were not presented.…”

A numerically exact local solver applied to salt boundary inversion in seismic full-waveform inversion

“…With the continuous increase in computational power and algorithmic efficiency, FWI is now often used in the field. An extensive collection of case studies is available, with some recent 3-D examples given by Plessix et al (2010), Prieux et al (2013) and Bansal et al (2013). A good overview of FWI, including references to more field applications, is given by Virieux & Operto (2009).…”